Cloud
-Mass of water in sky: a visible mass of water or ice particles in
the atmosphere from which rain and other forms of precipitation fall.
How Clouds are Formed: How to make a cloud in a bottle
Formation of Cloud
Air
Compression and Expansion
- When air is compressed, the
motion of gas molecules increases and the air temperature rises. The opposite
happens when air is allowed to escape. The air expands and cools. The expanding
air pushes on the surrounding air and cools by an amount equal to the energy
used up.
-Adiabatic
Temperature Changes
- Temperature changes that happen even though heat isn’t
added or subtracted are called adiabatic temperature changes. They
result when air is compressed or allowed to expand. When air is allowed to
expand, it cools, and when it is compressed, it warms.
-Expansion
and Cooling
- As you travel from Earth’s surface upward
through the atmosphere, the atmospheric pressure decreases. As a result, the ascending air expands and
cools. Unsaturated air cools at the constant rate of 10°C for every 1000
meters of ascent. In contrast, descending air encounters higher pressures,
compresses, and is heated 10°C for every 1000 meters it moves downward.
This rate of cooling or heating applies only to unsaturated air and is
called the dry adiabatic rate.
- If a parcel of air rises high enough, it will eventually
cool to its dew point. Here the process of condensation begins. From this point
on as the air rises, latent heat of condensation stored in the water vapor will
be released. This slower rate of cooling caused by the addition of latent heat
is called the wet adiabatic rate. Because the amount of latent heat
released depends on the quantity of moisture present in the air, the wet
adiabatic rate varies from 5–9°C per 1000 meters
Rising air cools at
the dry adiabatic rate of 10°C per 1000 meters, until the air reaches the dew
point and condensation (cloud formation) begins. As air continues to rise, the
latent heat released by condensation reduces the rate of cooling.
Processes That Lift Air
In general, air resists vertical movement. Air located
near the surface tends to stay near the surface. Some exceptions to this happen
when conditions in the atmosphere make air buoyant enough to rise without the
aid of outside forces. In other situations, clouds form because there is some
mechanical process that forces air to rise.
Four mechanisms that can
cause air to rise are
- Orographic
lifting
-
When elevated terrains, such as mountains, act as barriers to air flow,
orographic lifting of air occurs. The air cools adiabatically; clouds and
precipitation may result.
-Frontal
wedging
- Here the cooler, denser air acts as a barrier over
which the warmer, less dense air rises. This process, called frontal wedging. A front
is the boundary between two adjoining air masses having contrasting
characteristics.
Orographic Lifting and Frontal Wedging
-Convergence
- whenever air in the lower atmosphere flows together, lifting results. When air flows in from more than one direction, it must go somewhere. Because it cannot go down, it goes up. Convergence is when air flows together and rises ;and
-Localized convective lifting
- On warm summer days, unequal heating of Earth’s surface may cause pockets of air to be warmed more than the surrounding air. occurs where unequal surface heating causes pockets of air to rise because of their buoyancy.
Convergence and Localized Convective Lifting
10 Rare Cloud Formations - Top10Stuff
References:
Microsoft® Encarta® 2009. © 1993-2008 Microsoft
Corporation. All rights reserved.
http://mhs.wcpss.net/teachers/murphy/earthscience/textbook/este182.pdf
https://www.youtube.com/watch?v=9shBFnhg72k
