Temperature — Page 62, Lesson 56

I want to walk you through temperature inversions and surface temperature variations. Let's start with inversions. Normally, temperature decreases as you go higher in the atmosphere. But when the temperature increases with an increase in height, that's called an inversion. You've already seen that at night we can expect an inversion just above the surface, but inversions can form in several different ways. One type is a Radiation Inversion. On a night with clear skies, radiation cooling of the ground causes the air right above the surface to become colder than the air above it, so temperature actually rises as you go up a short distance from the ground. That's a radiation inversion. Another type occurs due to turbulence. When we study cloud formation, you'll see that turbulence in the layer closest to the surface can create an inversion at a height of 2 or 3 thousand feet. At the tropopause — the boundary between the troposphere and stratosphere — instead of temperature remaining constant, it may show a slight rise for a few thousand feet. That's another inversion. Higher up in the stratosphere, temperature increases with height. In the International Standard Atmosphere, or ISA, from 20 kilometres to 32 kilometres, the temperature increases at a rate of 1°C per kilometre. Then there's a Subsidence Inversion. In a high-pressure system, air descends at the centre. As that air descends, it gets compressed and heated adiabatically — we'll cover that term in more detail later. The descending air becomes warmer than the air at lower levels, so you get an inversion where temperature increases as you go down through that descending layer. Now let's move to Surface Temperature. The surface air temperature measured in a Stevenson screen — that's the standard white louvered box that houses thermometers — is subject to considerable variations. These include Latitude Effect, Seasonal Effect, Diurnal Variation, and multiple effects due to cloud and wind. Let's start with the Angular Elevation of the Sun and the Latitude Effect. At the Equator, only a small area is heated by the sun's radiation, so that area receives the greatest heat per unit area. At the poles, the sun's rays spread over a much larger area, so there's the least heat per unit area. The actual distance of polar regions from the sun is only fractionally more than the distance from the Equator, so that distance effect can be ignored — it's the angle, not the distance, that matters. Now the Seasonal Effect. The Vernal Equinox — the spring equinox — occurs around 21 March, and the Autumnal Equinox around 21 September. At those times, the sun is directly over the Equator, and maximum heating occurs there. Around 21 June, the sun reaches its most northerly latitude — that's the Summer Solstice for the Northern Hemisphere — and maximum heating occurs in the Northern Hemisphere. But here's the key: the land and sea continue to heat up after that date, so maximum temperatures are found around late July or early August in temperate latitudes. Around 21 December, the sun reaches its most southerly latitude — the Winter Solstice for the Northern Hemisphere — and minimum heating occurs. But again, the land and sea continue to cool, so minimum temperatures are experienced around late January or early February in temperate latitudes. Finally, Diurnal Variation — the daily cycle. Note that this assumes clear skies, light winds, and no change in air mass. The sun is at its highest elevation at noon, but for two to three hours after noon, the earth is receiving more solar radiation than it is giving up as terrestrial radiation. So there's a balance between incoming and outgoing radiation that shifts during the day, and that's what drives the daily temperature cycle.