Meteorology
Moisture in the Atmosphere
Humidity
 Humidity – amount of WATER VAPOR in the air.
When an object (including air) is heated, it EXPANDS.

The expansion is creating “empty” space that can be filled with water vapor.
Water holding
capacity

AIR TEMPERATURE
Relative Humidity

Relative Humidity – a comparison of the actual amount of WATER VAPOR in the air to the
amount it can HOLD at a given temperature.

Expressed as a PERCENTAGE
Effect of Temperature on Relative Humidity
 Relationship: as air temperature INCREASES, and the amount of moisture remains the same,
Relative
Humidity
relative humidity will DECREASE.
AIR TEMPERATURE
Warm Air & Relative Humidity
Air warms and
expands
Warmer air has a
HIGHER water vapor
holding capacity
As air temperature DECREASES, its ability to hold more water
DECREEASES, and its relative humidity INCREASES.
Effect of Time of Day on Relative Humidity

If relative humidity decreases as air temperature increases then:
o
The highest relative humidity will occur at the COOLEST time of day
 5:00am
o
The lowest relative humidity will occur at the WARMEST time of day.
 2:00pm
Dew Point Temperature
 Dew Point Temperature – The temperature to which air must be COOLED to reach
SATURATION.
o
If the air temperature and dew point temperature are EQUAL, the relative humidity is
100%.
o
If the air temperature falls BELOW the dew point temperature, CONDENSATION begins.
Sling Psychrometer
 Consists of two thermometers. One is the dry thermometer (dry-bulb) and the other has a wet cloth
on the end (wet-bulb).
 The sling psychrometer is swung around and the wet cloth dries (evaporation) if the air is NOT
saturated.
 Evaporation absorbs heat and is a COOLING process so the wet-bulb temperature should always be
EQUAL TO or LOWER
than the dry-bulb.
Relationships
 The drier the air, the MORE evaporation will occur resulting in GREATER cooling. In turn, the
difference in temperature between the dry-bulb and wet-bulb will be GREATER.
 The more humid the air, the LESS evaporation will occur resulting in LESS cooling of the wet-bulb
thermometer. In turn, the difference in temperature between the dry-bulb and wet-bulb will be
SMALLER.
 At saturation, the temperature difference between the dry-bulb and wet-bulb would be ZERO.
Cloud Formation

Clouds - Tiny droplets of liquid water or tiny ice crystals suspended in air.

Conditions needed for cloud formation:
1. MOISTURE in the air
2. COOLING air temperature
3. Condensation Nuclei – AEROSOLS or DUST particles in the atmosphere which provide a
SURFACE for water molecules to condense on
Cooling in the Atmosphere
 As air RISES, the atmospheric pressure surrounding the parcel of air decreases. Therefore, the
parcel of air EXPANDS in volume.
 As it expands, it becomes COOLER.
 When the temperature of the air falls BELOW the dew point temperature, the water vapor will
CONDENSE and a cloud will form.
Examples of Condensation – Dew
 Dew is MOISTURE that CONDENSES at the GROUND level.
 The layer of air within about a meter above the ground experiences rapid changes in TEMPERATURE
from day to night. Therefore the temperature may be COOLER on the ground than the air above.
 Consequently, the air temperature close to the ground may cool to BELOW the dew point
temperature for CONDENSATION to form.

Moisture, therefore, is observed on grass and on the surface of
some objects such as


with metal surf
Condensation – Frost

Frost is observed as ICE crystals mostly on grass. It forms when dew FREEZES because the air
temperature falls BELOW the level necessary for freezing (from liquid to solid form of moisture).

Frost may also form under the process of DEPOSITION when water vapor is cooled so rapidly it
forms ice (from gas to solid form of moisture).
Condensation – Fog

Fog can be described as a CLOUD forming very close to or at ground level. It requires air with
sufficient MOISTURE to cool or to be cooled to a temperature to achieve CONDENSATION.

Fog is more common in VALLEYS and near CREEKS or STREAMS. It also most commonly occurs
overnight and during the morning.