3
$\begingroup$

I live in the Northern Emisphere. One of the things I have been taught when I was in high school is that winter is colder than summer because in that season we're at an unfavorable angle and nights last longer than days, despite Earth being closer to the Sun during winter months.

Does this really mean that on the Equator, where day and night are of the same length all year long, winter is the hottest season*?

Does this also mean that Winters in the Southern hemisphere are colder than our Summers and their Winters are hotter than or Summers (or that they would be if not for the Gulf Stream, air currents and other temperature-changing effects)?


*I can think of a different reason that would make equinoxes hotter (more sunrays per surface area when the sun hits from above all day long) but I have no idea of which effect would be greater.

$\endgroup$

migrated from physics.stackexchange.com Apr 2 '17 at 13:48

This question came from our site for active researchers, academics and students of physics.

  • $\begingroup$ Circulation of heat, weather patterns and Ocean Temps might make slightly different seasonal variation for equatorial countries, but in theory, your thinking is correct. Equatorial countries get about 6% more energy from the sun around December 20 than around June 20. About 3% more over the Winter season to Summer season. $\endgroup$ – userLTK Apr 2 '17 at 15:00
  • $\begingroup$ One additional reason it might be warmer (at least in daytime temperatures) during the winter season in the tropics and nearer both solstices nearest the equator: the migration of the ITCZ (source page). Days with less cloudiness and rain should equate to warmer high temperatures. But of course, it's more complex in the end. $\endgroup$ – JeopardyTempest Apr 4 '17 at 12:10
6
$\begingroup$

Does this really mean that on the Equator, where day and night are of the same length all year long, winter is the hottest season?

By "winter", I assume you mean December, January, and February. The answer is "No!"

Insolation varies on an annual basis outside of the tropics, one maximum and one minimum every year. This is not the case in tropical regions, where available insolation has two local maxima and two local minima every year. This effect is greatest at the equator. The graph below depicts available insolation at increments of 30 degrees latitude, from the equator to the north pole.

Plot of available insolation at various latitudes as a function of time of year

Note that at the equator, available insolation achieves local maxima at the two equinoxes and local minima at the two solstices. The reason is that the Sun is directly overhead at local noon on the equinoxes but is 23.5 degrees from vertical at local noon on the solstices. Sunlight has to travel through more air at the solstices than at the equinoxes.

The slight variations in insolation at the equator are easily overcome by climate. Equatorial regions tend to have wet seasons and dry seasons. When these occur depends much more on wind patterns than it does on aphelion / perihelion.


Does this also mean that Winters in the Southern hemisphere are colder than our Summers and their Winters are hotter than or Summers?

The answer is once again "No". The slight variation in insolation due to the Earth's eccentric orbit is once again easily overcome by other phenomena. The driving characteristic for this part of the question is that the northern hemisphere has much more land mass than does the southern hemisphere. This means that except for polar regions, southern hemisphere seasons tend to be more moderate than do northern hemisphere seasons.

$\endgroup$
  • 1
    $\begingroup$ Can you clarify: is this insolation (incoming solar radiation) at the top of the atmosphere (TOA)? It would appear so or the signal wouldn't be so smooth as it would be modulated by clouds, but it's not stated explicitly in your answer. $\endgroup$ – gerrit Apr 3 '17 at 10:13
  • $\begingroup$ I never noticed some mothns spelled "Jason" and... is it really Winter here while it's Summer at the South? I wonder what season it is at the parting line but I guess it's "wet" or "dry". $\endgroup$ – Zachiel Apr 3 '17 at 17:34
  • $\begingroup$ @Zachiel -- The seasons in the southern hemisphere are opposite those in the northern hemisphere. Axial tilt is the driving factor of the seasons. $\endgroup$ – David Hammen Apr 4 '17 at 6:23
  • $\begingroup$ @DavidHammen I know that, I just thoought that winter was not "the cold season" but "the season from December to March" $\endgroup$ – Zachiel Apr 4 '17 at 17:43
  • $\begingroup$ Note that despite the sunlight travelling trough more air when the sun is not at zenith, the main reason why the radiation onto a horizontal plane is less at lower angles is not the atmospheric absorption. In each hemisphere's winter, the same amount of radiation reaching the top of the atmosphere spreads over a larger area. A quantitative study could start with this radiation parametrization scheme: Eq 10 (dx.doi.org/10.1175/…) originating from (dx.doi.org/10.1175/…) $\endgroup$ – Lukas Apr 7 '17 at 14:35
3
$\begingroup$

The mean temperature on the equator for three locations (South America, Asia, Africa) show that while there is a small seasonal variation, the timing of the peak is not the same at every point. This is presumably because other atmospheric effects dominate the climate at the equator - not just the distance to the Sun:

enter image description here

(Data extracted from Wikipedia article) If you wanted the effect of the solar irradiation alone - the distance from aphelion to perihelion varies from 0.9832899 AU to 1.0167103 AU (about a 3% variation). Since solar irradiance goes with the inverse square of the distance, you expect the solar intensity to change by 6%, with the "hottest" moment coming in early January.

A simplified black body model has the Earth's temperature responding according to the Stefan-Boltzmann law - which would make the temperature increase with the fourth root of the incident power, or 1.5%. For a mean surface temperature of 27°C at the equator (300K) that would be a 10K change...

But a "simple black body model" is not at all how Earth works - so we're back to "all the other effects" completely dominating the seasonal variations.

As a reference, you might want to consult this link which possibly contains more information that you would ever want...

$\endgroup$
  • 1
    $\begingroup$ Other atmospheric effects: such as circulation and latent heat (clouds and rainfall). Using models, one could still calculate how large the effect due to proximite of the Sun is; I would imagine it is in the order of some tens of millikelvins. $\endgroup$ – gerrit Apr 2 '17 at 17:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.