Astronomija

Kolikšen je najvišji albedo zemeljske površine, obrnjen proti soncu, kdaj koli izmerjen?

Kolikšen je najvišji albedo zemeljske površine, obrnjen proti soncu, kdaj koli izmerjen?


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To spletno mesto (stran 26) trdi, da je poln sijaj na Luni 100-krat večji od sijaja polne lune na zemlji. Številka 100 je sestavljena iz razlike med velikostjo zemlje in lune, pomnoženo z razliko njihovih albedov. Zemlja je 13,45-krat večja od lune. 100, deljeno s 13,45, je enako 7,43. 7,43 pomnoženo z 0,12 (lune albedo) je enako 0,8916. Tako je moral izračunati z zemeljskim albedom 0,8916. Toda to bi lahko bil napačen izračun. Da bi to ugotovili, je treba zastaviti naslednje vprašanje:

Kolikšen je najvišji albedo zemeljske površine, obrnjen proti soncu, kdaj koli izmerjen? Upoštevajte, da vprašanje ne gre za albedo določenih delov zemlje, temveč za celotno stran zemlje, ki je obrnjena proti soncu.


Kateri izmed snega, trave in temne zemlje ima najvišji albedo?

Pravilen odgovor na to vprašanje je A, Snow. Albedo je vrsta merjenja in idejo o njem je predstavil Johann Heinrich Lambert. Uporablja se za merjenje difuznega odboja sončnega sevanja. Nima dimenzije in se meri na lestvici od 0 do 1.

Visok albedo pomeni, da se velika količina sevanja odbije na površini in se ne absorbira veliko. Sneg je največ, ker odbija približno 95% sevanja. Albedo je bistvena meritev na področjih, kot sta astronomija in klimatologija. Čeprav ne tako visoko kot sneg, druge površine z visokim albedom vključujejo led, pesek in tla.

C. Perez

Samo iz dneva v dan postajam boljši

Ali poznate izraz albedo? Če odgovorite z ne, morda ne boste razumeli, kako odgovoriti na to vprašanje. Albedo se nanaša na energijo sončne svetlobe, ki se vrne v ozračje. Obstajajo nekateri predmeti, ki imajo lahko nižji ali višji albedo, odvisno od tega, kako bi odražali sončne in rsquos žarke.

Od danega je tisti z najvišjim albedom sneg, kar pomeni, da je A pravi odgovor. Sneg je običajno bel in ponavadi odbija sončne žarke. To pomeni, da bo več energije, ki se bo vrnila v ozračje, ko bo naletelo na sneg, v primerjavi z dvema drugima izbirama.


Kolikšen je najvišji albedo zemeljske površine, obrnjen proti soncu, kdaj koli izmerjen? - astronomija


Luna, osvetljena z odsevano zemeljsko svetlobo

1. Tam je temna stran Lune.
Pogosta napačna predstava je, da je & ldquodark stran & rdquo Lune (to je stran, ki je obrnjena stran od Zemlje) tako poimenovana, ker z Zemlje ni vidna & mdashin z drugimi besedami, le metafora. Seveda vsi vemo, da oddaljena stran Lune ni vedno temna. Ampak obstaja zelo dober razlog, zakaj temu rečemo temna stran.

Približno 59% površine Lune je naenkrat vidno z Zemlje. To pomeni, da 41% površine Lune nikoli ni v popolni temi, ker je za tisti del Lune Zemlja vedno na nebu, vsako minuto dneva in noči. Med novo Luno, ko je bližnja stran obrnjena stran od Sonca, še vedno prejema veliko odsevane svetlobe od Zemlje. To je jasno razvidno na desni fotografiji. Ta fotografija je bila posneta v mraku. Temno modra območja v celoti osvetljuje zemeljska svetloba. Edini čas, ko se bližnja stran resnično zatemni, je med Luninim mrkom. Že takrat je Zemlje dovolj svetlobe, da jo s teleskopa vidi z Zemlje.

Po drugi strani pa se lahko Luna v nekaterih primerih resnično temni. Nobena svetloba z Zemlje je nikoli ne doseže. Med polno Luno, ko je obrnjena stran od Sonca, je oddaljena stran Lune res zelo temna. Ob polni luni je sonce (ali Zemlja) ne more doseči svetloba, toplota ali radijski valovi. Zato je res smiselno, da jo imenujemo & ldquodark strani Lune. & Rdquo

2. Ali je na Luni zemeljski vzhod?

Čeprav je ena stran Lune vedno obrnjena proti nam, vrtenje lune ni popolno. Z Zemlje je naenkrat mogoče videti približno 59% Lune. Dva koščka (4 & # 189% na vsaki strani) sta edina dela, ki sta kdaj videla Earthrises in Earthsets. V teh regijah se Zemlja komaj dvigne nad obzorje. Preostale regije vidijo Zemljo, pritrjeno na nebu, ali pa je sploh ne.

Ker se vrtenje Zemlje upočasnjuje, bo Zemlja sčasoma plimno zaklenjena z Luno. To pomeni, da bodo nekateri deli Zemlje videli Luno neprekinjeno, drugi deli pa je ne bodo videli nikoli. Vsi astronomi se bodo seveda preselili v del, ki nikoli ne vidi Lune (ob predpostavki, da Zemlje v tem času še ni požgalo sonce). Skupna teža njihove opreme bo porušila Zemljino ravnovesje in povzročila, da se Zemlja brez cilja vrne v vesolje. samo hecam se.

3. Kolikšen pritisk na Luno izvaja sončna svetloba?

Na Luni je na tone ton svetlobe & mdashliterally. Govorim o tlaku sevanja. Vsa svetloba izvaja fizični pritisk na vse, kar osvetli. Na odsevni površini sončna svetloba deluje 9,15 mikronetona sile na kvadratni meter. Če je svetloba absorbirana, je sila za polovico manjša ali 4,575 & mikroN / m 2. 7 km 2. -> Pomnoženo s skupno površino, obrnjeno proti soncu, in če luno obravnavamo kot ravno ploščo, ki absorbira svetlobo, to pomeni, da je skupni sevalni tlak (zelo približno) 43.175.000 newtonov ali 4880 ton tlaka (izmerjeno kot zemeljske tone). Ker Luna tehta 22 kg = 1,616e23 funtov -> 8,082 & krat10 19 ton, je to le 0,06 kvadriliontin (6e in minus17) Lunine mase, kar ni dovolj, da bi imelo opazen učinek.

4. Kako dolg je dan na Luni?

Dan na Luni, čas med enim in drugim sončnim vzhodom, je v povprečju 29,53 zemeljskih dni (nekoliko se spreminja). To je enako kot cikel faz Lune, ki je znan kot sinodični mesec.

A & ldquomonth & rdquo na Luni je za tisti del Lune, ki doživlja Zemeljske vstaje in Zemeljske zahode, prav tako 29,53 Zemeljskih dni. Zvezdniški mesec, čas, da dosežemo enak položaj glede na zvezde (uradno znan kot Mednarodni nebesni referenčni okvir), je 27.3217 dni. Zato se Luna v enem sončnem letu vrti okoli Zemlje 12,36-krat in v zvezdnem letu 13,37-krat. Hitrost Zemlje okoli Sonca je približno 30-krat večja od hitrosti Lune okoli Zemlje.

Kar zadeva leto dni: kot bi rekel Bill Clinton, je odvisno od tega, kaj & ldquoyear & rdquo je. Če z & ldquoyear & rdquo mislite na čas, da naredite popolno orbito okoli vsega, kar kroži okoli, bi to trajalo 29,53 dni. Toda to je zavajajoče, ker fizično ni bistvene razlike med Luno, ki kroži okoli Zemlje, in obratno. Oba krožita drug drugega (tehnično krožita okoli svojega središča mase). A & ldquoyear & rdquo na Luni, to je čas, potreben za obkrožanje Sonca, je približno enak enemu zemeljskemu letu.

5. Kako svetlo je na Luni, ko je Zemlja polna?

Lunin albedo je 0,12. Z drugimi besedami, Luna odseva 12% svetlobe, ki jo zadene. To pomeni, da je Luna, gledano z Zemlje, podnevi približno tako svetla kot asfaltna cesta. To pa ne pomeni, da je Luna 12% tako svetla kot Sonce.

Zemeljski albedo je 0,367. Zemlja je torej ne samo večja na nebu, temveč odseva trikrat več svetlobe na enoto površine. 2 = 122,1 milijona kvadratnih km Navidezna površina luninega diska = 9,48 milijona kvadratnih kilometrov. -> Na Luni se zdi Zemlja 3,67-krat večja po površini ali 13,46-krat večja od površine, kot se zdi Luna na Zemlji. Zato je Zemlja, ki jo vidimo z Lune, 13,46 in krat 0,367 / 0,12 = 41,2-krat svetlejša od Lune, ki jo vidimo z Zemlje.

Kako se to primerja s soncem? Sonce ima vizualno (navidezno) velikost & minus26,74. Vizualna velikost Zemlje, gledano s Sonca, je & minus3,86 [1,4]. Lunina vidna magnituda od Sonca je +0,21. Navidezna velikost Lune na Zemlji je & minus12,74.

Lahko uporabimo razmerje, ki pravi, da je ena velikost 2,512-krat večja svetlost. To je logaritemska lestvica, zato uporabljamo formulo Velikost = & minus2,5 log10 (f1 / f2). Iz te formule lahko izračunamo, da je Sonce 398.359-krat svetlejše od Lune, gledano z Zemlje, vendar le 9.673-krat svetlejše od Zemlje, če gledamo z Lune.

6. Kakšno vzdušje je na Luni?

Ker odbija manj svetlobe in absorbira trikrat več svetlobe na enoto površine kot Zemlja, je Luna (v povprečju) 16 & degC toplejša od Zemlje. To marsikoga preseneti, saj vsi vemo, da se lahko Luna resnično ohladi, ko se stemni. Del razloga je tanka atmosfera. Lunina atmosfera je samo 2 & krat10 & minus12 torr. Ker je 1 torr = 1,316 & krat10 & minus3 zemeljske atmosfere, je Lunina atmosfera 380 bilijonov krat tanjša od zemeljske.

Po podatkih NASA [2] je lunina atmosfera sestavljena iz naslednjih plinov:

Plin Odstotek od celotne vrednosti
Helij 25%
Neon-20 25%
Vodik (H2) 22%
Argon-40 19%
Neon-22 3.2%
Argon-36 1.2%
Metan 0.6%
Amoniak 0.6%
Ogljikov dioksid 0.6%

Odstotek ogljikovega dioksida v ozračju je na Luni dejansko 17-krat večji kot odstotek na Zemlji. Skupna masa Lunine atmosfere je le 25000 kilogramov ali 27,5 tone. To pomeni, da je 1011 kilogramov CO2 v luninem ozračju v primerjavi s 6.952.000.000.000.000 funtov v zemeljski atmosferi. Toda Luna je bogata s helijem-3, lahkim izotopom helija, ki postaja dragocen na Zemlji.

Odrasla oseba v mirovanju diha 7 do 8 litrov na minuto. To je enako 9 do 10 gramov zraka. Zato človek v petih letih vdihne in izdihne enako količino plina (glede na težo) kot celotno lunino ozračje. Povedano drugače, sedem milijard ljudi na Zemlji bi potrebovalo 271.428 let, da vsaj enkrat zadiha ves zrak na Zemlji. Če bi lahko dihali na Luni, bi vso atmosfero vdihnili v 0,0225 sekunde.

7. Ali je na Luni voda?

Da, vendar je količina zelo negotova. Obstajajo spektroskopski dokazi za nekatere hidroksilne radikale (& middotOH). Radarski dokazi sond NASA tudi kažejo, da bi lahko obstajala voda. Voda bi lahko obstajala le kot led, pod zemljo ali v stalnih sencah, ker bi zaradi nizkega atmosferskega tlaka izhlapel in močno ultravijolično sevanje razcepilo H2O v radikale & middotH in & middotOH.

8. Je Luna v Avstraliji obrnjena na glavo?

Ne, Avstralci so tisti, ki so postavljeni na glavo. Luna kaže pravilno pot.


Kako svetla je Luna?

Kako svetla je Luna? To je odvisno od številnih dejavnikov. Lunina faza je primarna. Drugi očitni dejavniki so razdalja Zemlja-Luna in Sonce-Luna ter prosojnost in izumrtje zraka. Albedo in njegove spremembe so zadnji glavni dejavnik, ki vpliva na lunino svetlost.

Vizualni albedo je opredeljen z "odbojnostjo površine planeta, lune, asteroida ali drugega nebesnega telesa, ki ne sveti z lastno svetlobo. Albedo se meri kot delež vpadne svetlobe, ki jo površina odbije nazaj v vse smeri. Popoln odsevnik ima po definiciji albedo enotnosti, torej vsa vpadna svetloba odbija telo, ki sploh ne odseva svetlobe, bi imelo albedo nič. " Pravzaprav realne površine nikoli nimajo albedov natančno nič ali ena, ampak nekaj vmes.

Astronomi so določili vizualne albede naših planetov. Med Nasinimi planetarnimi najdišči je najsvetlejša Venera z albedom 0,65. To pomeni, da se 65% dohodne sončne svetlobe odbija od z oblakom pokritega planeta. Preostalih 35% prispeva k toplotni energiji Venere. Merkur ima na 0,11 najnižji planetarni albedo. Zemljin albedo je 0,37, Mars je 0,15 Jupiter, 0,52 Saturn, 0,47 Uran, 0,51 Neptun 0,41. Plutonov albedo se giblje od 0,5 do 0,7.

Treba je poudariti, da so ti planetarni albedi povprečja. Če za primer vzamemo Zemljo, se oblaki gibljejo od 0,4 do 0,8, sneg se giblje od 0,4 do 0,85, gozdovi se spreminjajo od 0,04 do 0,1, trava je približno 0,15 in voda se giblje od 0,02, sonce neposredno nad glavo pa do 0,8 pri nizki stopnji pojavnosti. Zemeljski albedo se torej spreminja in je odvisen od obsega oblačnosti, sneženja in vpadnega kota Sonca v oceanih. S povprečnim albedom 0,37 63% dohodne sončne energije prispeva k toploti našega planeta. Očitno je, da bi se, če bi se oblačnost močno zmanjšala, temperatura Zemljine površine povečala, kar bi prispevalo k drugim dejavnikom globalnega segrevanja, kot so količine toplogrednih plinov.

Povprečni vizualni albedo naše Lune je 0,12. Svetlost Lune se močno spreminja, ko se spreminja njena faza. V prvi in ​​tretji četrtini je vidna Luna 50-odstotno osvetljena s Soncem, vendar je njena svetlost le približno 8% polne Lune in # 2 712 povečanja. Lunin vizualni albedo na svojem osvetljenem segmentu se postopoma zmanjšuje, ko se kot med Zemljo in Soncem na Luni (fazni kot) poveča. Glavni razlog za to zmanjšanje vizualnega albeda z naraščajočim faznim kotom je večje ustvarjanje senc na nepravilni luninem površju in s tem zmanjšanje odbite svetlobe nazaj na Zemljo.

Grafični prikaz faznega kota glede na lunino svetlost

Na Zemlji nikoli ne vidimo popolnoma polne Lune, saj je dejanski fazni kot, ki ga vidimo, približno 5 stopinj. Z ničelnim faznim kotom bi bila Luna v Zemljini senci in doživeli bi popoln Lunin mrk. Apollonovi astronavti so poročali, da je resnična polna Luna približno 30% (0,2 magnitude) svetlejša od tiste, ki jo vidimo tukaj na Zemlji.

Torej, če ima polna Luna, kot jo vidimo na Zemlji, vizualno magnitudo –12,7, bi bila njena svetlost v prvi četrtini (fazni kot 90 stopinj) magnituda –10,0, zmanjšanje svetlosti 12x. Ker vidimo Luno na pol osvetljeno s Soncem v prvi četrtini, 6-kratno zmanjšanje svetlosti pomeni učinkovito zmanjšanje luninega albeda z, 12 na, 02.

Treba je poudariti, da ko dosežemo novo Luno, zemeljski sij postane dejavnik. Nekdo na Luni vidi "polno Zemljo", ko vidimo novo Luno. Kot je razvidno iz Lune, bi bila naša Zemlja videti približno 100-krat svetlejša od naše polne Lune. To je zaradi večje velikosti Zemlje in višjega albeda. Predstavljajte si, da ste na Luni in vidite poln zemeljski vzhod z magnitudo –17,7 z zemeljsko svetlobo, ki slabo osvetljuje in meče sence na lunino pokrajino.


ULTRAVIOLETNO SEVANJE

Prenos UV svetlobe v sistemu Atmosfera – Ocean

Če nas zanima predvsem prenos energije, zadostuje, da upoštevamo azimutno povprečno sevanje jazν(z,u),kje z označuje nivo v mediju (višina v ozračju ali globina na oceanu) in u= cosθ, θ je polarni kot. Primerno je sevalno polje razdeliti na dva dela: (i) neposredno sončni žarek, ki se ob prehodu skozi ozračje in ocean eksponentno oslabi, in (ii) difuzno ali razpršeno sevanje.

V skladu z enačbo [1] lahko prodor neposrednega sončnega žarka skozi ozračje zapišemo kot (spuščanje ν indeksa) jazsol(z)=jazsol(τ) (z)=F s e −τ s. Tukaj F s je sončno obsevanje (normalno na smer sončnega žarka), ki pada na vrh ozračja. Če je sonce blizu obzorja, zadostuje za uporabo geometrija ravnine, ob predpostavki, da sta ozračje in ocean stratificirana medija, pri katerih se optične lastnosti razlikujejo le v navpični smeri. Nato navpično optična globina τν je definirano kot τ ν (z) ≡ τ s μ 0 ≡ ∑ i ∫ z ∞ d z ′ k (ν, z ′) ali dτν(z)=−k(ν,z) dz, kjer je μ0= cosθ0in θ0 je kot sončnega zenita, kot je prikazano na sliki 4. V ravninski geometriji je difuzno sevanje v mediju opisano z

Slika 4. Shematski prikaz dveh sosednjih medijev z ravnim vmesnikom, kot je ozračje nad mirnim oceanom. Vzdušje ima drugačen lomni količnik (mr≈1) kot ocean mr=1.33.

Enačba [2] ima preprosto fizikalno razlago. Izraz na levi strani je sprememba sevanja po nagnjeni poti dz / u. Prvi izraz na desni strani je izguba sevanja iz žarka zaradi izumrtja. Drugi člen je dobiček zaradi večkratnega sipanja, kjer je normaliziran kotni prečni prerez, str(z,u′,u), daje verjetnost, da se svetloba razprši iz smeri u′ V smer u. Tretji je sončni psevdovir, sorazmeren z oslabljenim sončnim žarkom, ki 'poganja' difuzno sevanje. Namesto geometrijske razdalje dz, običajno je uporabiti nedimenzionalno optično globino, dτ (z)=−k(z) dz, kot neodvisna spremenljivka. Nato lahko enq [2] prepišemo na naslednji način (spustimo indeks ν):

pri čemer je albedo z enim sipanjem opredeljen kot a(t(z)) ≡σ (z)/k(z). Izvorni izraz S* (τ,u) je opredeljen spodaj.

Odsev in prenos površine

Za izračun difuznega polja sevanja je potrebno znanje o odbojnosti podzemnih površin kopnega in oceana. Tudi v plitvih vodah odbojnost dna oceana vpliva na difuzno sevanje v vodi in sevanje, ki zapusti vodno gladino. Odbojnost in prepustnost površine sta odvisna tako od vpadnih kotov kot odboja ali prenosa. Pri atmosferskem sevalnem prenosu se pogosto domneva, da podzemna ali oceanska površina odseva izhodno sevanje. Takšna površina se imenuje a Lambert reflektor, odbojnost pa se imenuje površinski albedo, ρL. Vendar je večina naravnih površin neambertovskih. Tako bi lahko sprostili predpostavko Lambertovega reflektorja in namesto tega uporabili funkcijo dvosmerne porazdelitve odseva, če je ta funkcija znana. Za sklopljeni sistem ozračje – ocean je bolje upoštevati dva sloja, enega za ozračje in drugega za ocean, vendar z različnimi lomnimi količniki. Osnovni sijaj, opredeljen kot Sem 2 kje m je dejanski del lomnega količnika, je ustrezna količina, če upoštevamo dva sloja z različnimi vrednostmi m. Če za preprostost predpostavimo, da je vmesnik raven in gladek (miren ocean), potem mora osnovni sij ustrezati Snelllovemu zakonu in Fresnelovim enačbam. Kot je prikazano na sliki 4, je sevanje navzdol porazdeljeno na 2πsr v ozračju bo omejen na stožec manjši od 2πsr (navedena na sliki 4 kot regija II), potem ko je bila prekinjena prek vmesnika v ocean. Žarki zunaj lomnega območja v oceanu so v celotnem odsevnem območju (na sliki 4 imenovano območje I). Razmejitev med lomnim in celotnim odbojnim območjem v oceanu je podana s kritičnim kotom μ c = (1 - 1 / m r 2) 1/2, kjer mr=mokn/mzrak je indeks loma na oceanu glede na zrak.

Ker sevalno polje v oceanu poganja sončno sevanje, ki prehaja skozi vmesnik zrak-voda, velja enačba [3] tudi v oceanu, vendar so sončni psevdoviri drugačni. V ozračju, ki ga imamo

kjer je prvi člen običajni psevdo-vir sončnega žarka, drugi člen pa zaradi odboja, ki se pojavi na vmesniku, ki je sorazmeren ρs(−μ0mr, koeficient zrcalnega odboja. Psevdo-vir v oceanu je le oslabljeni sončni žarek, ki se je lomil skozi vmesnik:

kjer je T s (- μ 0 m r) prepustnost žarka skozi vmesnik in μt je kosinus sončnega zenitnega kota v oceanu, ki je povezan z μ0 po Snell & # x27s zakonu μ t ≡ μ t (μ 0, m r) = (1 - (1 - μ 0 2) / m r 2) 1/2. Z uporabo ustreznih psevdovirov za ozračje (S zrak * (τ, u)) in ocean (S ocn * (τ, u)) lahko eqn [3] zdaj rešimo v skladu z mejnimi pogoji na vrhu ozračja in dna oceana. Vendar pa moramo poleg tega ustrezno upoštevati odsev in prenos skozi vmesnik tako, da to zahtevamo Sem 2 izpolnjuje Snell-ov zakon in Fresnelove enačbe.


5. Analiza in razprava podatkov

5.1 Dnevno povprečno Albedo

5.1.1. Primerjava med postajami

Postaje Maks. α Min. α
Neumayer 0.85 0.93 0.79 0,82 b b Albedo povprečje 4 dni, naštetih v tabeli 1.
0,86 c c povprečje Albeda v oblačnih dneh brez sneženja.
0,88 0,89 d d Povprečje Albeda v oblačnih dneh s sneženjem.
Hells Gate 0.72 0.89 0.54 0.73 0.73
Reeves Névé 0.84 0.92 0.80 0.81 0.88
Dome Concordia 0.80 0.82 0.80 0.80
  • skoraj v vseh oblačnih dneh se je zgodilo sneženje.
  • b Albedo povprečje 4 dni iz tabele 1.
  • c Povprečje Albeda v oblačnih dneh brez sneženja.
  • d Povprečje Albeda v oblačnih dneh s sneženjem.

[36] Snežne razmere so bile pri HG drugačne. Visoke razlike v albedu je dobro opisal Casacchia et al. [2002], ki je analiziral spektralni albedo različnih vzorcev snežne površine na tem območju. Ugotovili so tri različne vrste površin z različnim albedom: (1) viseča snežna površina, (2) ledena površina s snežnimi vključki in (3) gole ledene površine. Poleti pri HG se pogostost katabatskega vetra zmanjšuje v primerjavi s stanjem pozimi in spomladi [ Bromwich, 1985]. To vodi k zmanjšanju sezonske srednje vrednosti hitrosti vetra in k zmanjšanju pogostosti visečih snežnih dogodkov. Rusin [1964] je ugotovil, da so poleti na obalnih območjih, ki so izpostavljena katabatskemu vetru, razmeroma redki in šibki snežni dogodki povezani le s ciklonskimi vetrovi. Velika količina sončnega sevanja, Ts blizu tališča, in razmeroma miren veter spodbujata veliko metamorfizem snežne odeje in zaporedno preoblikovanje v led. Casacchia et al. [2002] je opazoval tri različne tipe površin na različnih mestih na območju HG, vendar tukaj predstavljene meritve kažejo, da so se te tri značilnosti površine poleti pojavljale tudi na določenem mestu. V celotni terenski kampanji se je α spreminjal od 0,54 do 0,89, najvišji α je bil opažen v dneh, ko je zapadlo sneženje, najnižji pa je ustrezal goli ledeni površini. Med močnim vetrom je gladkost golega ledu preprečevala kopičenje visečih snežnih kristalov. Nasprotno pa je šibki veter in Ts v bližini tališča naklonjen sneženju kristalov v led med sneženjem in kasnejšim kopičenjem snega. V bližini poletnega solsticija so metamorfizem snega, sublimacijo in taljenje okrepile velike količine Swv. Ko se je snežna plast tanjšala, je ledeni led vedno bolj vplival na površino in tako povečal površinsko absorpcijo sončnega sevanja. Velike spremembe albeda, ki jih povzročajo razlike v značilnostih površja, prevladujejo nad vplivom oblaka na albedo, zato med povprečno α ni očitne razlikekl in povprečni albedo oblačnega neba (αov) v celotni kampanji (tabela 2), čeprav je v večini oblačnih dni snežilo.

[37] Pri RN, tako kot pri NM, je bilo območje variacije α skromno: najnižje vrednosti so se pojavile v jasnih dneh, α pa se je v oblačnih dneh povečala za 0,07 (tabela 2). Sneženje je bilo v vseh oblačnih dneh, razen v enem. Pri razlagi podatkov o albedu je treba biti še posebej previden pri RN, ker je površina navzdol nagnjena proti jugovzhodu (približno 2–3 °) in pod instrumenti podlaga sastruga. Iz razprave v 3. poglavju predlagamo, da takšna usmeritev pobočja ne vpliva na povprečni dnevni albedo, vendar je treba zaradi prisotnosti sastruge pričakovati pristranskost pri vzorčenju. Podrobnosti o pristranskosti so obravnavane v oddelku 5.2.

[38] Kampanja v DC je bila zelo kratka, vremenske razmere so bile na splošno dobre in αkl je bila dva jasna dneva 0,80. Kljub kratkosti merilnega obdobja menimo, da opažanja albeda odražajo tiste razmere, značilne za visoko planoto poleti. Snežnih padavin je namreč malo in izredno nizke temperature zavirajo hitro preobrazbo snega. Kljub temu, kot je razloženo v poglavju 2.1, se poleti pojavljajo nekatere metamorfne spremembe snega tudi pri zelo nizkih temperaturah zaradi visoke količine Swv doseže površje. To bi lahko pojasnilo nekoliko nižjo vrednost αkl na visoki planoti v primerjavi z vrednostmi na NM. Tudi če sta bili obe površini zasneženi, je bil v NM neprekinjeno obnavljan fin sneg, medtem ko je bil v DC sneg star vsaj 7 dni (ne vemo natančne starosti, saj pred začetkom kampanje nihče ni bil tam). Poleg tega je delež dohodne svetlobe v bližnjem infrardečem območju večji pri enosmernem toku, saj je količina atmosferske vodne pare nizka v primerjavi z NM (glej oddelek 2.3). Zato tudi ob enakih snežnih razmerah na površju sledi Aoki et al. [1999] bi pričakovali, da pri DC αkl bi bil za 1% nižji kot na obali.

[39] Robustnost primerjave albeda med postajami je očitno omejena z instrumentalnimi napakami. Piranometri, uporabljeni na različnih lokacijah, so pokazali različne natančnosti, v najslabšem primeru pri enosmernem toku pa je negotovost zaradi eksperimentalnih napak verjetno večja od razlike med α tam in na drugih postajah. Kljub temu se nam je zdelo koristno, da v primerjavo vključimo tudi manj natančne meritve, čeprav bodo za dokončno potrditev nekaterih rezultatov morda potrebna nadaljnja opazovanja.

5.1.2. Parametriranje dnevnega povprečnega Albeda pri Hells Gate

[40] Iz mesečnih povprečnih vrednosti albeda, o katerih poroča Rusin [1964] na Mirnyju, postaji na vzhodni antarktični obali, lahko sklepamo, da so površinske razmere podobne tistim v HG poleti: povprečni mesečni albedo se je z 0,80 oktobra 1956 zmanjšal na 0,69 novembra in na 0,61 januarja 1957. Med različnimi kampanjami na Antarktiki je Bintanja in van den Broeke [1995] , Bintanja et al. [1997] in Bintanja [2000] je pridobil 42-dnevni albedo 0,56, 432-dnevni albedo 0,55 in 37-dnevni albedo 0,58, 0,65 in 0,68 na različnih lokacijah nad območji modrega ledu v deželi Dronning Maud. , med 1150 in 1310 m n.v. Spodnji povprečni albedi, ki so jih avtorji dobili, v primerjavi z 0,72, opaženimi pri HG, so bili najverjetneje posledica modro ledene površine z manj pogostimi snežnimi padavinami (regija je približno 300 km od obale). Na Zahodni Grenlandiji (1155 m n.v.) poleti Greuell in Konzelmann [1994] je pokazal spremembe v α, podobne tistim, opaženim pri HG. Podobno območje vrednosti je poročal Brock in sod. [2000], ki temelji na opazovanjih nad starim snegom poleti v Haut Glacier d'Arolla v Švici.

[41] V krajih, kjer se na površini pojavljajo velike spremembe, kot so te, je za sevalne modele izjemnega pomena, da reproducirajo spremembe v α zaradi dramatičnega vpliva na proračun površinskega sevanja. Za uporabo v modelih morskega ledu in snežne odeje je bilo predlaganih več parametriziranj albedo. Nad morskim ledom je albedo poleti zelo spremenljiv: zaradi taljenja snega / ledu, sneženja, nastajanja talilnih ribnikov in zmrzovanja se lahko albedo spreminja v območju, ki je celo večje od tistega, opaženega pri HG [ Grenfell in Maykut, 1977 Perovich et al., 2002]. Pri parametrizaciji albeda morskega leda je albedo izražen kot funkcija ene ali več naslednjih spremenljivk: temperatura zraka, Ts, vrsta snega / ledu, del oblačnosti, globina snega in debelina morskega ledu [ Curry et al., 2001]. Med temi parametri je edini, ki je na voljo pri HG, Ts. Na sliki 5 je α narisan v primerjavi s povprečno dnevno vrednostjo Ts: dnevi z jasnim nebom so označeni s kvadratki, dnevi s sneženjem s trikotniki, drugi dnevi pa s točkami. Iz velikega razpršenosti podatkov na sliki 5 je razvidno, da samo Ts lahko razloži le skromno količino variance v α. Kljub temu smo opazili, da je bila α na splošno visoka ob sneženju, večinoma nad 0,7, Ts pa je bila v večini primerov le nekaj stopinj pod tališčem. Vse ostale dni se je zdelo, da se α spreminja v dveh različnih načinih, odvisno od temperature. Za Ts & lt 269 K je bilo povprečje α 0,76, medtem ko je bilo za 269 K & lt Ts & lt 273 K povprečje α 0,61. Odvisnost albeda od Ts je povezana z dejstvom, da ko se Ts približa 273 K, se rast snežnih zrn, taljenje snega in vsebnost tekoče vode na površini močno povečajo, s čimer se zmanjša snežni albedo. Ko je bila povprečna dnevna vrednost Ts nad 269 K, je v večini primerov prišlo do taljenja snega v najtoplejših urah dneva, sicer je Ts dosegel vrednosti nad 271 K, kar je omogočilo pojav talnega taljenja [ Liston in sod., 1999]. Verjamemo, da je nenaden padec α za Ts & gt 269 K v glavnem posledica nastopa taljenja na površini / površini.

[42] Rezultati, pridobljeni na HG, predstavljajo zanimive analogije z modelom, ki ga je predlagal Ross in Walsh [1987] za prikaz nihanj arktičnega poletnega snežnega albeda (slika 5). Njihov prag Ts je bil 268 K namesto 269 K, njihove vrednosti albeda za Ts pa so bile nižje od praga in pri 273 K 0,80 oziroma 0,65, torej 5% višje kot v tem primeru. Opazili so tudi linearno znižanje albeda med 268 K in 273 K, kar ni jasno razvidno iz podatkov HG, ki jih je vseeno premalo, da bi izključili linearno razmerje. Parametriranje Rossa in Walsha so potrdila tudi opazovanja snežnega albeda, izmerjena na štirih ruskih postajah v obdobju 1978–1983 [ Roesch et al., 1999, slika 6]. Podatki o albedu so se gibali okoli 0,8 za Ts & lt 268 K in pokazali približno linearno zmanjšanje v območju 268 K & lt Ts & lt 273 K.

5.2. Dnevni cikel Albeda

5.2.1 Učinki sončnega kota zenita in metamorfizem snega

[43] Dnevni cikel albeda z jasnim nebom pri HG je bil močno odvisen od površinskih razmer. Na sliki 6 so predstavljene krivulje albeda v jasnem nebu za površine s svežim ali visečim snegom in za gol led. Ta dva primera predstavljata skrajnosti, med katerimi so padle vse druge krivulje, opažene na različnih stopnjah snežne preobrazbe. Vrednosti Albedo so narisane samo za z & lt 80 °, saj je za višje vrednosti z merilne napake so bile večje, količina Swv je izredno nizka v primerjavi s tistim v preostalem delu dneva. Na levi strani slike so jutranje vrednosti, na desni pa popoldanske vrednosti. Pred tremi od štirih jasnih dni na HG je 1-2 dni pred tem zapadlo sneženje. Albedo svežega snega je bil v teh treh dneh visok in se je le malo spreminjal, čez dan pa se je postopoma zmanjševal.

[44] Krivulje albedo izhajajo iz prekrivanja odvisnosti albeda od z ter glede velikosti in oblike zrna snega. Prej je minimalni albedo popoldan opazoval Liljequist [1956] v Maudheimu, avtor Dirmhirn in Eaton [1975] nad različnimi ledeniki in z McGuffie in Henderson-Sellers [1985] pri Resolute, Kanada. Kot so predlagali ti avtorji, je zmanjšanje albeda tekom dneva morda posledica metamorfizma snežnih zrn, ki ga povzroča ogrevanje površine. Pod jasnim nebom, ko se Sonce približa obzorju in se temperatura zniža, lahko na subnetu s sublimacijo nastanejo kristali, sčasoma pa se ledeni kristali lahko kondenzirajo v zraku in padejo na površje. Kristali povečujejo površinski albedo, dokler z spet zmanjšuje in s povečanjem Swv metamorfizem reciklira. 28. novembra na HG se je največji Ts popoldne gibal okoli tališča, medtem ko je bil 1. in 2. decembra največji Ts 271 K oziroma 269 K. In all three cases there were the conditions for a rather pronounced snow metamorphism, which evidently predominated over the albedo dependence on z in the afternoon (Figure 6). On 15 January snow drifting occurred, as it appeared from the strong wind (between 10 and 15 m/s) recorded by an automatic weather station (“Sofia”) located about 10 km upwind from HG. Due to the high reflectivity of the small crystals deposited at the surface when the wind set down, the albedo was high and similar to the albedo of fresh snow. Again, albedo decreased during the day, with a rate slightly faster than on days with freshly fallen snow.

[45] The daily curves of bare ice albedo show a much steeper decrease during the morning, from 0.8 to 0.54, and a constant albedo in the afternoon (for z < 80°). Other studies (see section 2.2) have already shown that the albedo dependence on z is steep for hard erosional snow and weak for fine-grained snow. Although we cannot totally exclude the presence of a thin layer of frost on the dome of the pyranometers, the total absence of symmetry between morning and afternoon is probably again attributable to the daily cycle of melting and refreezing/frost crystal formation at the surface. McGuffie and Henderson-Sellers [1985] observed a similar albedo curve at Resolute on 27–29 May 1970. For z < 80°, albedo decreased from 0.7 in the early morning to about 0.5 at noon, and remained approximately constant throughout the afternoon. Although they did not specify the surface temperature and the snow conditions during those days, they attributed the albedo variation to the diurnal deposition and evaporation of a hoar-frost coating on the snow surface.

[47] To examine the daily albedo curves over highly reflective snow, we compare in Figure 7 the averaged daily cycle of clear-sky albedo at HG, DC, and NM when z < 80°. At HG the mean curve is calculated only for those four clear days with fresh or drifting snow at the surface. For each station, the mean daily cycle was obtained by interpolating the albedo values of the single days at fixed values of z at 1° interval. The number of days used in the calculation of the average is marked in brackets. Standard deviation of the averaged clear-sky albedo for z < 70° was practically zero at DC, lower than 0.03 at NM and reached the maximum value of 0.05 at HG in the late afternoon, as can be deduced from Figure 6.

[48] The trend in albedo already observed at HG is also present at NM and DC. On each clear day studied and in the daily average for each station, albedo decreased gently from early morning to late afternoon, approximately at the same rate for all stations. During the clear days examined here, maximum Ts were 245 K and 270 K at DC and NM, respectively.

[49] To investigate the reasons for the observed albedo curves, at first we considered the factors that may have caused the albedo measured to be different from the true surface albedo. In section 4 we mentioned that shadows compromised the albedo measurements until 7:40 ST (at elevation 22°), but, after this time, we estimated that the effect of shadows was in the noise level of the other uncertainties. The shadows at NM and HG were much smaller, and their effect on the measured albedo was considered insignificant.

[50] As discussed in section 4, other geometric factors that can affect the daily albedo cycle are surface tilting and the presence of three-dimensional features near the measurement stations. However, we know that all three stations were located over flat and rather uniform snow far from obstacles. If the stations were over small slopes, we would conclude from the curves in Figure 6 that the direction and magnitude of the inclinations were the same for all three sites, which is highly improbable.

[51] The fact that all the instruments at the four stations observe the same daily albedo cycle, although they have different accuracy and operate in different environments, allows considerations on the reliability of the measurements. We noted in Figure 2 that ERjaz is dependent on z. Nevertheless, since the daily albedo cycle observed is not correlated with the daily change in z, the shape of the albedo curves cannot be caused by the daily variation of ERjaz. Therefore we believe that the errors described in Figure 2 mainly give an idea of the confidence we have in the absolute values of albedo, while the hourly albedo variations and differences among the albedo curves at each single station are less affected by measurement errors.

[52] Liljequist [1956] undertook a detailed study of the clear-sky albedo at Maudheim, a Norwegian base located on the ice shelf, about 115 km to the west of NM and about 5 km from the coastline. The meteorological and surface conditions were very similar to those of NM. Liljequist observed that in November the diurnal cycle of clear-sky albedo was symmetric around noon, with albedo decreasing with decreasing z, while in December and January albedo was rather asymmetrical, as shown in Figure 7. The method used by Liljequist to calculate the averaged albedo curve differed from the one adopted in the present study. He concluded that, even if no melting was ever observed, metamorphism into larger snow grains occurred during the central hours of the day. When z increased and temperature decreased, refreezing and/or crystal formation by sublimation formed a layer of highly scattering grains at the surface. This explanation appears realistic also for the albedo curve observed at NM, where the maximum Ts during conditions of clear sky was below 270 K and melting is seldom observed throughout summer [ König-Langlo and Herber, 1996 ].

[53] The extremely low Ts at DC clearly allowed only a slow metamorphism to occur in the snow, even if the daily temperature excursions were very large (Table 1). During the coldest hours, strong temperature and humidity inversions developed at the surface layer. Rusin [1964] observed rapid development of frost crystals during the coldest hours on the Antarctic Plateau, followed by sublimation of the rime due to the radiative heating during the warmest hours. This may explain the daily albedo cycle observed at DC. Yamanouchi [1983] showed a decreasing albedo during the day at Mizuho Station (2230 m a.s.l.), on East Antarctica, even after flattening the surface to remove the sastrugi. He argued that the albedo curve could be caused by the sastrugi that remained around the flattened area. However, the effect observed by Yamanouchi could probably be due to the daily metamorphism of the snow surface, as also suggested by McGuffie and Henderson-Sellers [1985] . During December, surface conditions at Mizuho Stations were between those at NM and DC: the daily mean albedo was never lower than 0.8 [ Yamanouchi, 1983 ], and the daily mean surface temperature was between −263 K and −248 K [ Yamanouchi and Kawaguchi, 1984 ].

[55] In the study done by Carrol and Fitch [1981] , the daily albedo cycle has been presented as a function of z, thus averaging over morning and afternoon values. Presumably, it is also because of this that the albedo variation during the central hours of the day was obscured, and albedo varied only for z > 70°. Rusin [1964] averaged all the clear-sky albedo observations as a function of z, without morning/afternoon distinction and without discriminating between the various surface conditions. He obtained a curve in which albedo decreased with decreasing z, from 0.83 at z = 80° to 0.74 at z = 45°. The changing rate of albedo was intermediate between the two morning slopes obtained at HG for the extreme situations of fresh snow and ice. At Terre Adelie, Wendler and Kelley [1988] presented a clear-day albedo curve that gently decreased with decreasing z, probably calculated by averaging morning and afternoon values, and they mentioned no asymmetry between morning and afternoon. However, in that location it is possible that crystal formation during the coldest hours and successive sublimation in the warmest time of the day did not occur, due to the continuously blowing strong katabatic wind (the station was on a slope at an altitude of 1560 m). The observations of Kuhn et al. [1977] at Plateau Station show that the monthly averaged daily variation in albedo was symmetric around noon, when (and only when) the surface irregularities were leveled by drifting or falling snow. Since their albedo trends resulted from monthly averages and most probably included both clear and cloudy days, it is not possible to make a direct comparison with our data.

[56] The measurement errors at DC were probably larger than at the other stations. Nevertheless, we believe that the daily albedo curves presented here are reliable and physically explainable.

5.2.2. Sastrugi Effect at Reeves Névé

[57] Figure 8 shows the effect of sastrugi on the daily albedo cycle at RN. Each of the seven lines plotted in the figure corresponds to a clear day. A difference can be seen between the curves measured on days before and after 1 December 1994, when the structure supporting the radiometers was rotated 77° counterclockwise.

[58] The difference between the two groups of curves in Figure 8 arose from new positioning of the albedometer with respect to a sastruga, about 20 cm high and 1 m long, present on the snow surface just below the instrument. There were no other large sastrugi in the 2- to 3-m 2 field of view below the albedometer, which was at about 1 m above the surface. Since during most of the campaign the wind was from the same north-nortwest direction, the sastrugi field did not undergo significant alterations. This was also confirmed by the fact that the daily albedo trends in the days before rotation of the structure were very similar to each other, as were the albedo trends after rotation (Figure 8).

[59] In addition to the presence of sastrugi, the surface at RN showed a 2–3° down slope toward the southeast. Measuring the albedo from horizontally leveled sensors, the tilting alone would produce an apparent albedo higher when the Sun is downhill (during morning) and lower when the Sun is uphill. The data, however, showed the opposite trend: evidently the sastruga effect dominated over the slope effect. Prior to rotation of the mast, the daily mean clear-sky albedo was 0.01 higher than after rotation (Table 1). The RN data demonstrate how sensitive albedo measurements can be to surface inhomogeneities.

[60] The average daily variation in albedo presented by Kuhn et al. [1977] over several months displayed quite asymmetric trends, with albedo minima occurring sometimes in the morning or afternoon, depending on the geometrical shape of the features formed at the surface by the action of wind and drifting snow. The authors concluded that the effect of large three-dimensional surface features on the monthly mean albedo was of the order of 1%.


Vsebina

Despite its size, Hygiea appears very dim when observed from Earth. This is due to its dark surface and its position in the outer main belt. For this reason, six smaller asteroids were observed before Annibale de Gasparis discovered Hygiea on 12 April 1849. At most oppositions, Hygiea has a magnitude that is four magnitudes dimmer than Vesta's, and observing it typically requires at least a 100-millimetre (4 in) telescope. However, while at a perihelic opposition, it can be observed just with 10x50 binoculars as Hygiea would have a magnitude of +9.1. [15]

On 12 April 1849, in Naples, Italy, astronomer Annibale de Gasparis (age 29) discovered Hygiea. [16] It was the first of his nine asteroid discoveries. The director of the Naples observatory, Ernesto Capocci, named the asteroid. He chose to call it Igea Borbonica ("Bourbon Hygieia") in honor of the ruling family of the Kingdom of the Two Sicilies where Naples was located. [17]

In 1852, John Russell Hind wrote that "it is universally termed Hygiea, the unnecessary appendage 'Borbonica' being dropped" [17] (as well as the final "ia" in favor of just "a"). The name comes from Hygieia, the Greek goddess of health, daughter of Asclepius (Aesculapius for the Romans). [18]

Based on spectral evidence, Hygiea's surface is thought to consist of primitive carbonaceous materials similar to those found in carbonaceous chondrite meteorites. Aqueous alteration products have been detected on its surface, which could indicate the presence of water ice in the past which was heated sufficiently to melt. [18] The primitive present surface composition would indicate that Hygiea had not been melted during the early period of Solar System formation, [18] in contrast to other large planetesimals like 4 Vesta. [ navedba potrebna ] . However, observations in 2019 suggest Hygiea had suffered a head-on collision which had disrupted it, with its re-accretion resulting in its present spherical shape. No deep basins are visible in the VLT images, indicating that any large craters must have flat floors, consistent with an icy C-type composition. [6]

In images taken with the Very Large Telescope imager in 2017, a bright surface feature is visible, as well as at least two dark craters, which have been informally [a] named Serpens and Calix after the Latin words for snake and cup, respectively. [7] Serpens has a size of 180 km, Calix is about 90 km in diameter. [7] [6]

Hygiea is the largest of the class of dark C-type asteroids that are dominant in the outer asteroid belt, beyond the Kirkwood gap at 2.82 AU. [20] Hygiea has an average diameter of 434 ± 14 km. [6] While early studies (Ragazzoni et al., 2000) have found a noticeably oblate shape with a semimajor axis ratio of 1.11 (much more than for the other objects in the "big four"—2 Pallas, 4 Vesta and the dwarf planet Ceres), [21] [18] recent results indicate that Hygiea is more spherical, with an axis ratio of 1.06, [b] consistent with a MacLaurin ellipsoid. [6] Aside from being the smallest of the "big four", Hygiea, like Ceres, has a relatively low density, which is more comparable to the icy satellites of Jupiter or Saturn than to the terrestrial planets or the stony asteroids. [22]

Although it is the largest body in its region, due to its dark surface and farther-than-average distance from the Sun, Hygiea appears very dim when observed from Earth. In fact, it is the third dimmest of the first twenty-three asteroids discovered, with only 13 Egeria and 17 Thetis having lower mean opposition magnitudes. [23] At most oppositions, Hygiea has a magnitude of around +10.2, [23] which is as much as four orders fainter than Vesta, and observation calls for at least a 4-inch (100 mm) telescope to resolve. [24] However, at a perihelic opposition, Hygiea can reach +9.1 magnitude and may just be resolvable with 10x50 binoculars, unlike the next two largest asteroids in the asteroid belt, 704 Interamnia and 511 Davida, which are always beyond binocular visibility. [15]

A total of 17 stellar occultations by Hygiea have been tracked by Earth-based astronomers, [25] [26] including two (in 2002 and 2014) that were seen by a large number of observers. The observations have been used to constrain Hygiea's size, shape and rotation axis. [27] The Hubble Space Telescope has resolved the asteroid and ruled out the presence of any orbiting companions larger than about 16 kilometres (9.9 mi) in diameter. [28]

Orbiting at an average of 3.14 AU from the Sun, Hygiea is the most distant of the "big four" asteroids. It lies closer to the ecliptic as well, with an orbital inclination of 4°. [18] Its orbit is less circular than those of Ceres or Vesta, with an eccentricity of around 0.12. [1] Its perihelion is at a quite similar longitude to those of Vesta and Ceres, though its ascending and descending nodes are opposite to the corresponding ones for those objects. Although its perihelion is extremely close to the mean distance of Ceres and Pallas, a collision between Hygiea and its larger companions is impossible because at that distance they are always on opposite sides of the ecliptic. [ navedba potrebna ] In 2056, Hygiea will pass 0.025 AU from Ceres, and then in 2063, Hygiea will pass 0.020 AU from Pallas. [29] [ failed verification ] At aphelion Hygiea reaches out to the extreme edge of the asteroid belt at the perihelia of the Hilda family, which is in a 3:2 orbital resonance with Jupiter. [30]

As one of the most massive asteroids, Hygiea is used by the Minor Planet Center to calculate perturbations. [31]

Hygiea is in an unstable three-body mean motion resonance with Jupiter and Saturn. [32] The computed Lyapunov time for this asteroid is 30,000 years, indicating that it occupies a chaotic orbit that will change randomly over time because of gravitational perturbations by the planets. [32] It is the lowest numbered asteroid in such a resonance (the next lowest numbered being 70 Panopaea). [32] [33]

Hygiea has a rotation period of about 13.8256 hours, determined from observations with the VLT in 2017 and 2018. [6] Its single-peaked light curve has an amplitude of 0.27 mag, [7] which is largely attributed to albedo variations. [6] As of September 2019 [update] , the direction of Hygiea's rotation is not known, due to a twofold ambiguity in lightcurve data that is exacerbated by its long rotation period—which makes single-night telescope observations span at best only a fraction of a full rotation—but it is believed to be retrograde. [18] Lightcurve analysis indicates that Hygiea's pole points towards either ecliptic coordinates (β, λ) = (30°, 115°) or (30°, 300°) with a 10° uncertainty. [34] This gives an axial tilt of about 60° in both cases. [34]

Hygiea is the main member of the Hygiean asteroid family that constitutes about 1% of asteroids in the main belt. [ navedba potrebna ] The family was formed when an object with a diameter of about 100 km collided with proto-Hygiea about 2 billion years ago. Because the impact craters on Hygiea today are too small to contain the volume of ejected material, it is thought that Hygiea was completely disrupted by the impact and that the majority of the debris recoalesced after the pieces that formed the rest of the family had escaped. Hygiea contains almost all the mass (over 98%) of the family. [6]


Fairbanks vs. Van Nuys Edit

Okay, the Fairbanks section says:

According to the National Climatic Data Center's GHCN 2 data, which is composed of 30-year smoothed climatic means for thousands of weather stations across the world, the college weather station at Fairbanks, Alaska, is about 3 °C (5 °F) warmer than the airport at Fairbanks, partly because of drainage patterns but also largely because of the lower albedo at the college resulting from a higher concentration of spruce trees and therefore less open snowy ground to reflect the heat back into space. Neunke and Kukla have shown that this difference is especially marked during the late winter months, when solar radiation is greater.

..but I read once that some climatologists said that if they plant about 10,000 trees in the ever-sunny steaming-hot San Fernando Valley of Los Angeles, that it could lower the average temperature of the Valley by several degrees, as the Valley's concrete sidewalks are reflecting the sunlight and heating up the air for several meters above the surface. So- which is right? or, is it that the trees will provide shade, heating up themselves to provide cooler air at the surface? I'm confused. - Eric 00:10, 25 December 2006 (UTC)

It's not albedo, Eric. While a small amount of the energy is used in photosynthesis, evapotranspiration would be the major cause of the cooling. Concrete and asphalt pavement reflect relatively little, the heating of the air is mainly due to conductive transfer. It takes about 600 calories of heat energy to change 1 gram of liquid water into a gas.(DW 20061231) 68.151.34.107 21:45, 31 December 2006 (UTC)


a characteristic of the reflecting properties of the surface of a body: the ratio of the flow of radiation scattered by a surface to the flow falling on that surface. There is true albedo (or diffuse albedo, Lambert albedo, the coefficient of diffuse reflection) and apparent albedo (or the brightness factor). True albedo is the ratio of the flow scattered in all directions by a plane element of a surface to the flow falling on that element. Apparent albedo is the ratio of the brightness of a surface illuminated by parallel bundles of rays to the brightness of an absolutely white surface (that is, a surface for which the ratio of the brightness to the illumination does not depend on the direction and for which the true albedo equals one) perpendicular to the illuminated beam.

In astronomy the concept of albedo is generalized and is considered as a characteristic of a non-self-luminous heavenly body as a whole. Spherical albedo (Bond&rsquos albedo) is the ratio of light flow scattered by a body in all directions to the flow falling on the body. Geometric albedo is the ratio of the mean brightness of an object given a phase angle of zero to the brightness of an absolutely white plane screen perpendicular to the sun&rsquos rays, placed in the same point and visible under the same solid angle as the object. Illustrative albedo differs from geometric albedo in that the mean brightness of an absolutely white sphere is the standard, rather than the brightness, of a planar screen.

Integral (energetic) albedo is also considered, for a whole flow of radiation, as are monochromatic albedo (in monochromatic light) and albedo in various regions of the spectrum, such as ultraviolet, visual, and infrared. The true visual albedo of the earth&rsquos surface varies from 0.03 ( a water surface) to 0.9 (fresh snow, clouds). The albedo of vegetation in the visible range of the spectrum is 0.1&ndash0.3 and reaches 0.9 in the infrared range. The spherical visual albedo of the earth, determined according to the earthlight on the moon, based on measurements with artificial cosmic bodies and also according to the calculation of the heat balance of the earth, is close to 0.45. The albedo of the planets and satellites lacking an atmosphere (Mercury, the moon) is usually low and close to 0.07 the albedo of planets with a dense cloud atmosphere (Venus, Jupiter, Saturn, Uranus) is close to 0.6 the albedo of Mars is approximately 0.15.


Anthroposphere – Human portion of the biosphere

Earth at night
Graphic: NASA

Humans are a force of nature. No single species in the history of life can claim such a mantle. Yet, humans are a part of the biosphere. Because of our outsized impact, it is useful and perhaps even appropriate to consider the human element of the planet independently from the other spheres. Thus, the anthroposphere. The human impact on the planet goes well beyond altering the atmosphere and climate, leading some scientists to consider whether the Earth itself has indeed entered a new geological epoch. As of this writing, the Anthropocene Working Group, a subcommittee of the Quaternary Working Group of the International Commission on Stratigraphy has officially voted to recommend several things. First, that the Anthropocene should be treated as a chronostratigraphic unit for the geologic time scale (Series/Epoch level). Second, that the mid-20th century should be the time marker for its start. If ratified by the International Union of the Geosciences, the Holocene Epoch will have ended around 1950 and we will be living, officially, in the Anthropocene.


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