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All weather-related clouds form in the
troposphere, the lowest layer of Earth's atmosphere. This generally happens
when one or more lifting agents causes air containing invisible water vapor to
rise and cool to its dew point, the temperature at which the air becomes
saturated. The main mechanism behind this process is adiabatic cooling.
Atmospheric pressure decreases with altitude, so the rising air expands in a
process that expends energy and causes the air to cool, which reduces its
capacity to hold water vapor. If the air is cooled to its dew point and becomes
saturated, it normally sheds vapor it can no longer retain which condenses into
cloud.
The altitude at which this begins to
happen is called the lifted condensation level, which roughly determines the
height of the cloud base. Water vapor in saturated air is normally attracted to
condensation nuclei such as salt particles that are small enough to be held
aloft by normal circulation of the air. If the condensation process occurs
below the freezing level in the troposphere, the nuclei help transform the
vapor into very small water droplets. Clouds that form just above the freezing
level are composed mostly of supercooled liquid droplets, while those that
condense out at higher altitudes where the air is much colder generally take
the form of ice crystals. An absence of sufficient condensation particles at
and above the condensation level causes the rising air to become supersaturated
and the formation of cloud tends to be inhibited.
There are three main agents of
vertical lift. One comprises two closely related processes which work together.
Frontal lift and cyclonic lift occur when stable or slightly unstable air,
which has been subjected to little or no surface heating, is forced aloft at
weather fronts and around centers of low pressure. Cloud droplets form when the
air is lifted beyond the condensation level where water vapor condenses on
so-called nuclei; (small particles) that grow to a size of typically 0.002 mm
(.00008 in). In a cloud the droplets collide to form larger droplets.
These larger droplets remain aloft as
long as the drag force of the air dominates over the gravitational force for
small particles. If the cloud droplets continue to grow past this size, they
become too heavy to be held aloft as the gravitational force overcomes the
atmospheric drag, and they fall from the cloud as rain.
When this process takes place just
above the freezing level, the vapor tends to condense into supercooled water
droplets, which with additional lifting and growth in size, can eventually turn
into freezing rain. At temperatures well below freezing, the vapor desublimates
into ice crystals that average about 0.25 mm in length. Continuing lift and
desublimation will tend to increase the number of ice crystals which may
combine until they are too heavy to be supported by the vertical air currents
and fall out as snow.
Another agent is the buoyant
convective upward motion caused by significant daytime solar heating at surface
level, or by relatively high absolute humidity. Air warmed in this way becomes
increasingly unstable. This causes it to rise and cool until temperature
equilibrium is achieved with the surrounding air aloft. If air near the surface
becomes extremely warm and unstable, its upward motion can become quite
explosive resulting in towering clouds that can break through the tropopause or
cause severe weather.
Strong convection upcurrents may allow
the droplets to grow to nearly .08 mm (.003 in) before precipitating as heavy
rain from an active thundercloud. More occasionally, very warm unstable air is
present around fronts and low-pressure centers. As with non-frontal convective
lift, increasing instability promotes upward vertical cloud growth and raises
the potential for severe weather.
A third source of lift is wind
circulation forcing air over a physical barrier such as a mountain (orographic
lift). If the air is generally stable, nothing more than lenticular cap clouds
will form. However, if the air becomes sufficiently moist and unstable,
orographic showers or thunderstorms may appear.Windy evening twilight enhanced
by the Sun's angle, can visually mimic a tornado resulting from orographic lift
