Lapse rates
Describe the changes in temperature with height through the lower layers of the atmosphere. They are often shown
in diagrammatic form to show atmospheric stability or instability.
Environmental lapse rate:
This is the expected decrease in temperature with height through the lower atmosphere, approximately 6.5 degrees
per 1000m. It varies according to height, time of year, and over different surfaces.
Adiabatic lapse rate:
Used to explain what occurs as a parcel of air rises, decreases in pressure and temperature, but increases in volume.
The conditions are reversed if a parcel of air moves towards Earth.
Adiabatic lapse rates can be either dry or saturated.
Dry adiabatic lapse rate (DALR):
This is the rate a parcel of air cools at as it rises (or warms if falling) if condensation does not occur. The rate is
approximately 1 degree per 100m and is shown on the diagram beneath:
Saturated Adiabatic Lapse Rate (SALR):
The rate at which air cools if it has risen sufficiently to reach dew point temperature, and condensation occurs. The
rate of cooling is slower than the DALR because of the release of latent heat. The SALR varies from 4 degrees per
1000m to 9 degrees per 1000m but the average is 5.5 degrees. The reason for the variation is because of the ability
of warmer air to hold more moisture, and thus release more latent heat after condensation.
Stability
This is where a parcel of air rises and cools at a quicker rate than the air surrounding it. The parcel of air is colder and
denser than its surroundings so cannot rise further and sinks. Condensation and then subsequent rainfall does not
occur. Stability is most commonly associated with high pressure and anticyclones.
The diagram below shows a graph of stable air conditions: (The DALR lies to the right of the ELR)
Instability
On warm summer day's high levels of insolation can create high surface temperatures. The air above such localised
surfaces is then heated by conduction, leading to a high lapse rate. The air rises and cools less quickly than its
surroundings. If it remains warmer than its surroundings, the air parcel will continue to rise. If dew point is reached,
clouds and thunderstorms may result.
Instability is shown in the diagram below:
Unstable air:
When lifting stimulus (ground heating) is relaxed, air continues to rise. Vertical currents dominate. Cumulonimbus
clouds and thunderstorms.
SALR:
If rising air is saturated, its temperature falls at about 6 degrees per 1000m. Latent heat is released due to
condensation, so decrease is slower.
ELR:
Temperature of air decreases with height at an average rate of 6.5 degrees per 1000m.
DALR:
When a mass of air rises, its temperature decreases adiabatically. The air expands and loses heat energy as it rises
and if it is unsaturated, it loses 10 degrees for every 1000m of ascent.
Conditional instability
This occurs where the continuation of air rising to form condensation, clouds and precipitation, is conditional on
something (for example, mountains and fronts). The state of instability is based on the air being forced to rise
initially.