Weather: Atmospheric Lapse rates

This might be deemed a bit of a 'dry' subject but it is important in weather prediction. As a hypothetical parcel of air cools at it rises in the atmosphere and warms as it descends in the atmosphere. This cooling and warming of the air is known as the lapse rate.

If you're having trouble thinking about a parcel of air rising, think of a invisible balloon of air rising. As it rises, it cools as it expands. If it begins to descend it will compress and the temperature will increase.



A parcel of dry air, that is with no moisture in it cools at a rate of 1 degree cels. per 100m it rises, the reverse applies if it drops in height. But the importance for weather is that as it rises it reaches what is known as a 'dew point' and that is when clouds are formed as the dry air can no longer its water vapour and it condenses.




There is also a measurement for air that is saturated with water has reached the dew point temperature and is carrying as much moisture as that parcel of air is capable of holding at that temperature. This cools at 0.5 degrees cels per 100m, but can vary with temperature. This measurement is known as the Saturated Adiabatic Lapse Rate.

The differences in rates are used to determine if the parcel of rising air will rise high enough for its water to condense to form clouds, and, having formed clouds, whether the air will continue to rise and form bigger shower clouds, and whether these clouds will get even bigger and form cumulonimbus clouds (thunder clouds).

Teaching this subject could be interlinked with past and current weather events such as the floods in Pakistan. It would be interesting for the students to try and find out the ALR's before the rains producing the floods or before a hurricane (see previous blog).