Lapse Rates

As meteorologists we have to study the atmosphere in three dimensions or, I suppose, if you include time, probably four dimensions. Suffice to say that it is a very challenging exercise that is now primarily left to very fast super computers. Nonetheless the forecaster still has to study the properties of the atmosphere to decide whether or not the computer models are giving good advice.

One of the fundamental aspects of weather forecasting is how stable or unstable the atmosphere is. If it is very unstable then thunderstorms or tornadoes can easily form, whereas if it is very stable nothing of the sort would happen.

All around the world meteorologists fly helium filled balloons to measure, amongst other things, the temperature of the air at different heights above the ground. Satellites also do this by looking down towards the Earth.

The temperature profile that we get from this is known as the environmental lapse rate, and as I have said, does vary from day to day.

As we ascend from the ground through the troposphere, the part of the atmosphere in which we live, the temperature normally decreases.

The laws of physics tell us that if we move air up through the atmosphere it will expand and cool down at a fixed rate of 3 degrees Celsius for every 1000 feet, or 10 degrees Celsius for every kilometre it rises. This happens continually as the sun heats the ground or the wind pushes air across a mountain range.

On a clear, cold morning with blue skies, the sun gets to work. Once air near the ground becomes warmer than the surrounding air it starts to rise. If as it rises and cools down it is still warmer than the surrounding air, it continues to rise.

...saturated air cools at half the rate of dry air...
As the ascending air cools down it gradually becomes more humid and can eventually saturated thus forming clouds. If the air continues to rise by being warmer than its surroundings, then the saturated air cools at half the rate of the dry air, since the process of water vapour changing to water droplets releases latent or stored heat.

A weather forecaster uses a three dimensional map of the atmosphere, and calculates the temperature at which air will start to rise. The next step is to work out how far the air will rise and whether it will be far enough for the air to become saturate and thus form clouds. Then if clouds do form what sort will they be, and hence what type of weather will we get.

So the stability of the air above us is of vital importance to a forecaster when deciding whether it will be a sunny day, and whether or not thunderstorms will develop in the afternoon to possibly give flash floods.

And, oh by the way, the temperature of the air already in the atmosphere is continually changing with time just to make matters that little more complicated.

Three-dimensional chess is easy by comparison.