Lapse Rates

• Adiabatic (vertical) movement of air caused by uneven land relief or differences in surface temperature contribute to localised weather conditions.
• Moisture content of air is a key factor affecting its rate of temperature decrease as it ascends. The nature and stability of an air mass/parcel of ascending air is determined by the relationship between three lapse rates of air outlined below:

 

Environmental Lapse Rate (ELR) = rate of air temperature decrease with altitude.

 

Dry Adiabatic Lapse Rate (DALR) = rate of cooling of dry air.

Saturated Adiabatic Lapse Rate (SALR) = rate of cooling of saturated air.

 

When rising air reaches condensation level it becomes saturated, a process that involves the release of latent heat that in turn retards the speed of temperature decrease as the air continues to rise. It depends on the airs carrying capacity of water vapour (greater at warmer temperatures e.g humid tropics). Therefore, latent heat energy release reduces with height as less water vapour condenses.

The relationship between the ELR, DALR and SALR influence the temperature and subsequently the density and buoyancy of an air mass.

The graph below presents the different changes in air temperature lapse rates for dry and saturated parcels of air compared to the environmental lapse rate.

 

 

This graph is available at: http://www.revisionworld.co.uk/node/7805

 

 

This simplified diagram is potentially useful for A Level Geography. More in-depth information about the stability of air along with this diagram is available at:

http://eesc.columbia.edu/courses/ees/climate/lectures/atm_phys.html

Comparing the properties of two different air masses