Thursday, May 10, 2007

Temperature inverted

Two previous postings explained the temperatures of the lower atmosphere, first in terms of discrete entities (clouds, surface), then in terms of the lapse rate, the rate at which the temperature falls with altitude. Air temperature falls with altitude because some of the air's stored heat is converted to gravitational potential energy - or, equivalently, the air mass has to do work against gravity in order to rise. The condensation of water vapor complicates the details and ruins the simple heat content-temperature proportionality. But it doesn't change the essential point, that the temperature profile has a downward slope.

One more important qualification needs to be added to this picture. Depending whether it's ground or water and whether it's day or night, the temperature profile near the surface is often inverted. That means that the temperature rises with altitude in an inversion layer near the surface, then flips back to lapsing again at higher altitudes. Along with the inversion (where and when it exists) comes a downward heat flow near and at the surface, which is consistent with the temperature inversion: heat flows towards lower temperatures. Over water, this inversion is critical to enabling evaporation, which needs a constant source of heat flow downward into the water to "pay the heat of vaporization bill" that turns liquid water into vapor. Over land, the inversion keeps the ground warmer at night than would be otherwise. It also frequently leads to fog. The associated downward heat flow is all radiative, since convection can't happen if the temperature profile is upwardly-sloped: there is no bouyancy.

The inversion phenomenon arises from the different speeds at which water, air, and land both heat up and cool down. Water is the slowest, land the fastest, and air in the middle. An earlier posting touched on these differences in connection with the forces that drive climate.

Evaporation is one result. Another is an interesting complementarity - but not full symmetry - between land and convection on the one hand, and ocean and evaporation on the other.

Land and convection: Inversion by night. Land heats up more quickly than air during the daytime. With the ground hotter than the air above it, strong upward convection results. Late summertime afternoon thunderstorms, sometimes with spectacular clouds rising into the stratosphere, are the most extreme sign of this. At night, however, the ground cools off faster than the air above it, and an inversion layer forms. Heat flows down from the air into the ground. Such inversion layers (and any associated fogs) typically disappear in the early morning, as the Sun starts to heat the ground again.

Water and evaporation: Inversion by day. Inversion develops during the daytime - the air heats up faster than the water - and peaks late in the daytime and early evening. At night, the ocean retains more heat than the air and cools off more slowly. The inversion disappears, only to reappear again the next day. The coolness of the daytime ocean air is one sign of the great heat sink that drives evaporation. Some evaporation happens by direct radiation from the Sun and clouds. But most of it is due to the downward heat flow in the atmosphere's lowest layer.

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