CLOUDS
Various types of cloud formation exist. A common catalyst to the cloud formation process, known as condensation, is when the heat from the sun warms the earth’s surface.
Air in direct contact with earth’s surface heats up, becomes less dense and starts to rise. As warm air rises it expands and cools adiabatically. An ‘adiabatic process’ is one with no loss nor gain of heat to a volume of air.
For example, when air is forced upward over a ridge, the upward-moving air encounters decreased atmospheric pressure. It expands and cools. A sinking parcel of air compresses and heats.
- If the air is not saturated with water vapour, the process is called dry adiabatic.
- If an ascending airmass reaches saturation, the addition of latent heat from condensing moisture will partially overcome the cooling due to expansion. The saturated adiabatic lapse rate is therefore smaller.
LATENT HEAT is the heat energy, measured in calories, absorbed or released when water changes from one state to another. There are 3 states of water.
- Water vapour (gas)
- Water liquid (cloud, mist, fog, rain, etc.)
- Water solids (ice)
When water changes to a higher energy state, i.e., from ice to liquid to vapor, it absorbs/uses latent heat energy (from the surrounding atmosphere).
When it moves to a lower energy state, i.e., from vapour to liquid to ice, it releases latent heat energy (into the surrounding atmosphere/properties)
This transfer of energy is significant in certain forms of airframe ice formation.
Cool as can’t hold as much water vapour as warm air. This is related to an airmass’ relative humidity, which ultimately depends on temperature. As the air rises, it cools and expands.
Water vapour condenses onto tiny particles floating in the air. These particles are known as condensation nuclei and contains aerosols such as salt and dust. When enough vapour condenses around the nuclei, a cloud droplet is formed. When billions of these droplets come together they become visible as cloud. Sometimes these water droplets combine to become larger drops or crystals. When large and heavy enough, they gradually fall from the sky as rain.
Clouds are categorized by height and appearance. There are four main groups of clouds:
‘NIMBUS’ added as a prefix or suffix indicates that a cloud is rain or snow bearing.
These four groups are further subdivided with the following prefixed names according to the level of their base above mean sea level (MSL):
- ‘CIRRO’, or high-level clouds: cloud base > 16,500 — 20,000 ft.
- ‘ALTO’, or medium-level cloud: cloud base > 6,500 ft.
- No prefix, for low-level clouds: cloud base <6,500 ft.
For cloud formation to be possible, the following properties must exist:
- Moisture present in the air.
- A lifting action to cause a parcel of air to rise. The four main lifting actions are:
a. Convection
b. Turbulence
c. Frontal
d. Orographic - Adiabatic cooling of the rising air.
If a parcel of air containing water vapour is lifted sufficiently, it will cool adiabatically, and its capacity to hold water vapour will decrease.
i.e., cooler air supports less water
Therefore, its relative humidity increases until the parcel of air cools to its dew point temperature, where its capacity to hold water vapour is equal to that which it is actually holding, and the parcel of air is said to be saturated.
i.e., its relative humidity is 100 procent.
Any further cooling will cause some of the water vapour to condense out of its vapour state as water droplets and form clouds. Further, if the air is unable to support these water droplets, then they will fall as precipitation in the form of rain, hail, or snow.
Provided that the properties for a cloud to form are present, the height of a cloud base is determined by the difference between the dew point temperature and the ground temperature.
The difference divided by the appropriate lapse rate per 1,000 ft. will determine the height of the cloud base.
The higher the moisture content present in the rising air, the higher is the dew point temperature, and the less the difference between the surface and the dew point temperatures, the lower is the cloud base. Thus the amount of moisture content in the air is a determining factor of the cloud base height.
Cumulus clouds have flat bases due to the uniform decrease in temperature of the dry adiabatic lapse rate (DALR). They have round and uneven tops because of the uneven decrease in the environmental lapse rate (ELR) temperature and difference magnitudes in movement of the rising air inside the cloud.
If the ELR is high (i.e., the temperature of the surrounding air reduces more quickly with height gained than the rising DALR/SALR parcel of air) then this parcel of air will always be warmer than the surrounding (ELR) air. Therefore, it will be lighter and thus will keep rising.
This air is said to be ‘unstable’ and will produce cumuliform clouds.
Airflow rises over mountains due to orographic uplift and cools adiabatically. If it cools below its dew point temperature, then the water vapour will condense out and form clouds, either as lenticular clouds, often on the hillside when there is a stable layer of air above the mountain, or as cumulus or even cumulonimbus clouds when there is unstable air above the mountain.
