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How come it’s coldest when we are closest to the sun and warmest when we are furthest away?
We are closest to the Sun (Perihelion) on Jan 4th, and furthest from the sun (Aphelion) on Jul 4th.
The heat we receive from the sun is in inversely proportion to our distance from it.
When we are closest to the sun, in January, we receive 353.25 w/m2 irradiation (measured over the surface of the earth, not as the earth as a disk). When we are furthest away, Jul 4th, we receive 330.50 w/m2 irradiation.
Yet when we receive the most heat from the sun, the mean global temperature is at its lowest.
Month, mean global temp deg C
Jan 12
Feb 12.1
Mar 12.7
Apr 13.7
May 14.8
Jun 15.5
Jul 15.8
Aug 15.6
Sep 15
Oct 14
Nov 12.9
Dec 12.2
We are coolest when we are receiving most energy. How does this happen?
Annual: Irradiance vs mean temperature as a chart.
http://farm7.static.flickr.com/6033/6235945346_b54...
Sources:
Mean temperature by month: http://www.ncdc.noaa.gov/cmb-faq/anomalies.php
Perihelion/Aphelion irradiance: http://www.climates.com/SPECIAL%20TOPICS/GW/earths...
Earth distance from the Sun: http://ssd.jpl.nasa.gov/horizons.cgi#top
Edit Red Rose.
Re "That's only for the Northern Hemisphere". No, the monthly temperatures I used are the GLOBAL Annual Monthly mean for 1901 - 2000. See the first link in "sources", about 2/3 of the way down the page. This is the mean combined Land + Sea Surface.
Edit:
I have it now; thank you for those that pointed me in the right direction. But it has raised another more perplexing question which I'll ask about at the bottom of the explanation.
This phenomenon is caused by 2 things. The emissivity (proportion of irradiated energy that is absorbed by a material instead of being reflected) of the ocean is about 0.5 whereas the emissivity of land can be up to about 0.9. So irradiated land will absorb far more energy than irradiated ocean. This absorbed energy is radiated back to the atmosphere as IR. So the atmosphere is heated much by IR more over land than it is over sea.
So if you imagine the sun travelling over the tropic of Cancer you can see that far more energy is re-radiated to the atmosphere from the land there than at the tropic of Capricorn. Of the 168 w/m2 that strikes the earth’s surface, land can absorb up to 151 w/m2 whereas the ocean will absorb about 84 w/m2, a difference of 67w/m2. The difference between irradiation at perihe
I got cut off. The energy difference between perihelion and aphelion is only 23 w/m2 so is dominated by the NH Land mass.
The curious thing is this though... So the NH land mass makes the earth warmest in June. But why is the Mean Sea Surface Temp also warmest in June? The SH is 80% ocean NH 60% ocean. Why isn't the SST be highest in Jan/Feb?
8 個解答
- BaccheusLv 710 年前最愛解答
The average global atmospheric temperature is warmest during the northern hemisphere summer because most of the earth's landmass is in the northern hemisphere. Land warms faster than water, so the atmosphere is warmed more.
- Ottawa MikeLv 610 年前
I haven't looked it up and thus have no source but I have an educated guess.
The Northern Hemisphere had a much higher land to ocean ratio than the Southern Hemisphere. As you know, the oceans act as a temperature moderator. So the summer to winter temperature swings in the Northern Hemisphere are greater and likely drive the global average into the trend that you indicated.
I'll even go out on a limb and say that when the eccentricity of the Earth's orbit changes and January is no longer the month when Earth is closest to the Sun, then the global average will vary even more than it does now and this might be the trigger for an ice age (i.e Milankovitch cycle for orbit eccentricity).
- Red RoseLv 710 年前
That's an average. Cooler temperatures in Nov - March are only for the Northern Hemisphere. The Southern Hemisphere is the opposite, with the warmest temperatures occurring at perihelion and the coolest at aphelion.
The Earth's orbit is near-circular, the difference in distance is tiny compared to the average distance from the Sun. Remember also that climate is complicated - ocean currents carry heat energy around and have a massive effect on climate. The energy received from the Sun is absorbed differently by different types of land and by water, and moved around a lot, so it is not a simple case of energy received immediately translating to a change in temperature. There can be a delay of weeks or months.
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Edit: yes, I realised that after submitting my answer, so I already edited it to correct myself.
Edit 2: I haven't misunderstood anything, I simply made my answer quite simple and straightforward - I saw no need to overcomplicate things.
Apology accepted :) :) :)
- DarwinistLv 610 年前
Of course, you are talking of the planet as a whole; RR seems to have slightly misunderstood the question.
The answer would have to be due to the uneven distribution of land between the hemispheres. Land masses show a greater annual temperature variation than oceans so, when the northern hemisphere has its summer season, the greater land mass in that hemisphere causes average GLOBAL temperature to rise. In the NH winter, the opposite happens.
The Earth does get more sun in NH winter, that's true; the orbit is changing though, and there will come a time when the two effects are in phase and we will get more extreme NH summers and winters.
This is due to precession and is one of the Milankovitch cycles. From memory, this cycle takes around 26000 years. In combination, the Milankovitch cycles are responsible for the ice ages and interglacials that dominate our climate over periods of 100 000 years or so.
http://en.wikipedia.org/wiki/Milankovitch_cycles
Edit: I see RR has spotted and corrected his/her error in the time I took to type. Apologies to RR.
Edit: Dave, your follow up question seems to be incomplete. It looks interesting though, hopefully you can either finish it here or put it as a separate question. Meanwhile, here's a star.
Later: That's an interesting question and I have no idea why that should be so. If I was to guess, it would be that heat is exchanged at the air/ocean boundary. Heat flows from warmer to cooler so if (in NH summer), more warm air from the land comes into contact with cooler water, this will raise the average SST.
In NH winter, the reverse would happen because the air from the land is now cooler than the SST. The average SST probably doesn't change that much between summer and winter.
But, as I said, that's just a guess. It could be completely wrong!
- Jeff MLv 710 年前
Aside from those already mentioned dealing with more land in the northern hemisphere I think I saw it listed before that the ocean has the ability to store heat energy for a longer time period which is one of the reasons why the warmest part of the day isn't when the Sun is directly over head but is instead a few hours afterward.
- 10 年前
Maybe it's a bit like how seawater seems to get warmer well in to the night and then becomes quite cold at around ten a.m before startign to warm up again.
It takes some time to absorb the heat and some time to dissipate it.
- RioLv 610 年前
The numbers given are a global means. The distinction between land and water are of little consequence considering temperature moderators... regional or hemispheric would be the exception. The largest being water the second being the atmosphere which interacts with water: http://science.nasa.gov/earth-science/oceanography...
The rate, duration and forcing of energy exchange between each moderator is extremely complex. Though some will tell you otherwise: http://en.wikipedia.org/wiki/Thermal_flywheel_effe...
- Grandpa MacLv 410 年前
All those other theories are quite interesting and may have some merit, but the PRIMARY reason those of us in the Northern hemisphere are warmer when we are further away is because the angle the sun hits us is more direct in the summer more than offsetting the very small difference in distance. Conversely, those in the Southern hemisphere are warmer when they're closer because they get more direct sunlight then.