Lapse rates
To understand more about the stability of air masses, we need to examine the temperature changes undergone by an air mass as it rises or sinks, which in turn relate to pressure changes. The simplest case concerns unsaturated air. Relationships between pressure and temperature lead to a simple linear relationship between temperature and altitude for rising or sinking air. This is known as the Dry Adiabatic Lapse Rate (DALR), and is equal to 9.8°K km-1. Air lifted up will cool at this rate due to reduction in pressure, air sinking will warm at this rate due to pressure increases. The word ‘adiabatic’ is derived from the Greek word for ‘impassable’, and it refers to a system which does not lose or gain energy. Thus, rising air is said to cool or warm adiabatically when its temperature changes are due entirely to pressure changes. In reality, some degree of energy exchange will always take place, but these are generally small on short timescales.
When condensation or evaporation occur in the air, however, lapse rates of rising or falling air differ from this value. As we have seen latent heat is released by condensation and consumed by evaporation. This alters the adiabatic lapse rate: because energy is released by condensation, rising air will cool more slowly if condensation is occurring. Thus, there is a smaller change in temperature with height than would be the case for unsaturated air. The modified lapse rate is termed the Saturated Adiabatic Lapse Rate (SALR). The varying amounts of water vapour that can be held in air at different temperatures means that the SALR is non linear. The SALR is lowest at high temperatures, because of much higher saturation mixing ratios: that is: greater amounts of energy are released at the vapour/droplet transition, therefore temperature changes with altitude are reduced. At low temperatures, the SALR is more similar to DALR: smaller amounts of moisture are available for condensation, so the modification of the lapse rate is less.
Adiabatic Lapse Rates are commonly different to the real vertical change in temperature, known as the Environmental Lapse Rate (ELR). The ELR is influenced by patterns of heating, cooling and mixing, and the past history of an air mass. Actual vertical temperature gradients in the atmosphere are thus highly variable, and can even show an increase in temperature with height, a situation known as a temperature inversion.
Conditions Determining Air Stability: The stability of air masses depends on the relative values of the ELR and the appropriate Adiabatic Lapse Rate.
Stability
Air is stable if the ELR less than the ALR. If, for any reason, a parcel of air is uplifted, it will cool to lower temperatures than its new surroundings along the ALR. Hence the air parcel will be denser than its surroundings and will tend to fall back to its original level. This situation is encouraged by a small ELR or a temperature inversion.
Instability
There are two cases:
Absolute instability
In this case, the ELR is greater than both DALR and SALR. Uplifted air cools relatively slowly, and will thus be warmer and less dense than its new surroundings. It will therefore tend to continue to rise.
Conditional instability
In this case, the ELR is less than the DALR but greater than the SALR. Air will be stable unless forced to rise to altitude where condensation occurs, whereupon spontaneous uplift will occur.
The tephigraph below shows the relationship between altitude and temperature on rising air.
To understand more about the stability of air masses, we need to examine the temperature changes undergone by an air mass as it rises or sinks, which in turn relate to pressure changes. The simplest case concerns unsaturated air. Relationships between pressure and temperature lead to a simple linear relationship between temperature and altitude for rising or sinking air. This is known as the Dry Adiabatic Lapse Rate (DALR), and is equal to 9.8°K km-1. Air lifted up will cool at this rate due to reduction in pressure, air sinking will warm at this rate due to pressure increases. The word ‘adiabatic’ is derived from the Greek word for ‘impassable’, and it refers to a system which does not lose or gain energy. Thus, rising air is said to cool or warm adiabatically when its temperature changes are due entirely to pressure changes. In reality, some degree of energy exchange will always take place, but these are generally small on short timescales.
When condensation or evaporation occur in the air, however, lapse rates of rising or falling air differ from this value. As we have seen latent heat is released by condensation and consumed by evaporation. This alters the adiabatic lapse rate: because energy is released by condensation, rising air will cool more slowly if condensation is occurring. Thus, there is a smaller change in temperature with height than would be the case for unsaturated air. The modified lapse rate is termed the Saturated Adiabatic Lapse Rate (SALR). The varying amounts of water vapour that can be held in air at different temperatures means that the SALR is non linear. The SALR is lowest at high temperatures, because of much higher saturation mixing ratios: that is: greater amounts of energy are released at the vapour/droplet transition, therefore temperature changes with altitude are reduced. At low temperatures, the SALR is more similar to DALR: smaller amounts of moisture are available for condensation, so the modification of the lapse rate is less.
Adiabatic Lapse Rates are commonly different to the real vertical change in temperature, known as the Environmental Lapse Rate (ELR). The ELR is influenced by patterns of heating, cooling and mixing, and the past history of an air mass. Actual vertical temperature gradients in the atmosphere are thus highly variable, and can even show an increase in temperature with height, a situation known as a temperature inversion.
Conditions Determining Air Stability: The stability of air masses depends on the relative values of the ELR and the appropriate Adiabatic Lapse Rate.
Stability
Air is stable if the ELR less than the ALR. If, for any reason, a parcel of air is uplifted, it will cool to lower temperatures than its new surroundings along the ALR. Hence the air parcel will be denser than its surroundings and will tend to fall back to its original level. This situation is encouraged by a small ELR or a temperature inversion.
Instability
There are two cases:
Absolute instability
In this case, the ELR is greater than both DALR and SALR. Uplifted air cools relatively slowly, and will thus be warmer and less dense than its new surroundings. It will therefore tend to continue to rise.
Conditional instability
In this case, the ELR is less than the DALR but greater than the SALR. Air will be stable unless forced to rise to altitude where condensation occurs, whereupon spontaneous uplift will occur.
The tephigraph below shows the relationship between altitude and temperature on rising air.