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 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 is expanding.

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 water vapor 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 unsaturated.

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.