|CHANGES IN THERMO VARIABLES|
AS A PARCEL RISES
METEOROLOGIST JEFF HABY
When considering an adiabatic process a rising parcel of air always cools. If it is unsaturated it
cools at the dry adiabatic lapse rate (10 C/km). If it is saturated it cools at the dry adiabatic lapse
rate minus the amount of heat added due to
latent heat release of condensation. If the latent heat
release is small, such as in cold air, then the wet adiabatic lapse rate will be close to the
dry adiabatic lapse rate (i.e. 8 C/km). If the latent heat release is large, such as in warm
saturated air, then the wet adiabatic lapse rate will be around 4 to 6 C/km. An important point to
make is that the wet adiabatic lapse rate is not constant because as a parcel of saturated air
rises it is cooling and the colder the air gets then the less latent heat it can release. Once a parcel
rises into the upper troposphere it is so cold that the wet adiabatic and dry adiabatic lapse rates
will approach each other.
PARCEL IN GENERAL
A parcel of air expands and becomes less dense as it rises. This occurs because the air pressure lowers
around the parcel as it increases in altitude. The volume of the parcel increases since it
The temperature of a rising parcel always cools even though it is becoming less dense.
The mass of the parcel remains constant since adiabatic theory assumes no exchange in mass
between parcel and environmental air. The only exception to this is the consideration of
rain leaving a saturated parcel once it is condensed out.
UNSATURATED PARCEL OF AIR
As an unsaturated parcel of air rises no latent heat is released thus the cooling is at the
dry adiabatic lapse rate. The dewpoint decreases by 2 C/km as unsaturated air rises. For example
if the temperature is 20 C and the dewpoint is 10 C, after rising 1 km the new temperature will
be 10 C and the new dewpoint will be 8 C.
The mixing ratio remains constant in a rising parcel of unsaturated air. This occurs because the
amount of moisture in the parcel remains constant. Mixing ratio is defined as mass of
divided by mass of dry air. Since the mass of water vapor remains constant then the mixing ratio
stays constant. Why does dewpoint decrease though if the amount of moisture is the same? Dewpoint
decreases because lowering pressure has an influence on the thermodynamic variable of dewpoint that
does not occur with mixing ratio.
The saturation mixing ratio value decreases as air rises since the temperature is decreasing. While
mixing ratio remains constant the saturation mixing ratio decreases.
The relative humidity of a rising unsaturated air parcel increases. This is because the temperature
and dewpoint values are converging towards each other (the temperature is cooling more than the
dewpoint thus as they approach each other the relative humidity increases). The relative humidity
will continue to increase until it reaches 100%. At this point the parcel is no loner
SATURATED PARCEL OF AIR
As a saturated parcel of air rises condensation is taking place. The condensed moisture leaves the
parcel as rain. The relative humidity remains 100% as the saturated parcel rises. The temperature
and dewpoint fall together at the same lapse rate as the saturated air rises.
Since rain is falling from the parcel it leaves less moisture in the parcel as it continues
to rise. This causes the mixing ratio (note mixing ratio is equal to saturation mixing ratio
since temperature is equal to dewpoint in saturated air) to decrease. Both the dewpoint and
mixing ratio decrease as saturated air rises. The dewpoint lapse rate is equal to the
wet adiabatic lapse rate in saturated air.