lecture 7 - atmospheric moisture and precipitaiton

Question 1

saturation vapour pressure - SVP

Answer 1

As air temperature increases the quantity of water vapour increases in the atomosphere.
However, there is a maximum amount of water vapour that can be held by the atmosphere for any given temperature (saturation vapour pressure - SVP).

Question 2

Specific humidity (q)

Answer 2

compares the actual mass of water vapour held by a parcel of air to the total mass of the air parcel (grams of water vapour per kilogram of air - g kg-1).
Specific humidity therefore is a measure of how much water vapour is available for precipitation.

Question 3

Saturation specific humidity (qs)

Answer 3

the maximum mass of water that can be present at a given temperature.

Question 4

Dewpoint temperature

Answer 4

is the temperature to which air must be cooled for saturation to occur.
At dewpoint temperature, the air is saturated (holding the maximum amount of water vapour), and if further cooling continues, the water vapour will begin to condense.

Question 5

frost point

Answer 5

If the dewpoint temperature is below freezing, then it is usually referred to as the frost point

Question 6

Relative Humidity

Answer 6

Relative humidity compares the amount of water vapour present with the maximum amount that the air can hold at that temperature (expressed as a percentage).
Relative humidity = water vapour content / water vapour capacity x 100

Question 7

adiabatic process

Answer 7

When a gas is allowed to expand, its temperature drops. Conversely, when a gas is compressed, its temperature rises.
Heating or cooling that results solely from a change in pressure is an adiabatic process.

Question 8

dry adiabatic lapse rate

Answer 8

When a parcel of unsaturated air is displaced vertically, then its temperature changes at the dry adiabatic lapse rate (DALR).
Under these conditions, as the air parcel rises through the atmosphere it will cool at a rate of 10 °C per 1,000 m (10 °C 1,000 m-1). If it sinks, it will warm at the same rate.

Question 9

lifting condensation level

Answer 9

The altitude at which a rising air mass becomes saturated (100% humidity) as a result of adiabatic cooling is called the lifting condensation level.

Question 10

saturated (wet) adiabatic lapse rate (SALR).

Answer 10

More energy is released through adiabatic cooling than is gained by condensation, so the air will continue to cool as it rises.
The release of latent heat slows the rate of cooling; this slower rate of cooling for saturated air is called the saturated (wet) adiabatic lapse rate (SALR).
The SALR ranges from 4 to 9 °C 1,000 m-1.

Question 11

cloud condensation nucleus (CCN

Answer 11

Most cloud particles form around a tiny centre of solid matter, called a cloud condensation nucleus (CCN).
Cloud condensation nuclei have diameters in the range 0.1 to 1 mm.

Question 12

Two major classes are recognized on the basis of form:

Answer 12

stratisform, cumuliform

Question 13

stratiform

Answer 13

clouds are extensive, layered clouds which generally cover large areas

Question 14

cumuliform

Answer 14

clouds are associated with rising parcels of air and are distinguished from stratiform clouds by their stronger vertical development and globular, puffy appearance.

Question 15

fog

Answer 15

is simply a layer of stratus cloud at or very close to the surface.

Question 16

polluted fog

Answer 16

smog — can cause lethal respiratory problems.
Radiation fog, is formed at night when the temperature of the near-ground air layer falls below the dewpoint.
Valley fog is similar to radiation fog in that cold air drains downward and collects in low-lying areas, chilling the overlying moist air.

Question 17

precipitation fog

Answer 17

forms when rain falls into cold air and evaporates; it often occurs along warm fronts

Question 18

steam fog

Answer 18

forms when water evaporates from a warm body of water and condenses in the cold air above.

Question 19

advection fog

Answer 19

forms when a warm, moist air layer moves over a cold surface.

Question 20

sea fog

Answer 20

is caused when warm, moist tropical air is cooled as it passes over cold ocean currents that carry water north from the southern oceans surrounding Antarctica.