Understanding Cloud Formation
1. Lapse Rate
The Earth's atmosphere is composed of several distinct layers; each
layer is defined by a change of sign in the "lapse rate". The lapse
rate is the change of temperature with height (G*).
There are many ways to express "height", but the two most basic ones
are geometric and pressure. Geometric coordinates use physical distance,
measured in kilometers (thousands of meters) or miles, and denoted as
"z". Pressure coordinates use atmospheric pressure to
characterize altitude, measured in millibars**, and denoted as
A positive lapse rate (G > 0) means
that temperature increases with altitude. This results in warmer air
(less dense) above colder air (more dense); a stable situation.
- * The Greek letter gamma (G) symbolizes
the change of temperature corresponding to a change in altitude. For
example, if the temperature drops 14° Celcius over a vertical
distance of 2 kilometers, G = -7°C/km.
This is called the lapse rate.
- ** A millibar is a unit of pressure and is equivalent to 100 Pascals
or 100 Newtons per square meter or 2.1 pounds per square foot. Standard
atmospheric pressure at sea level is 1013 mb, 101300 Pa, 101300
N/m², or 2116 lb/ft².
A negative lapse rate (G < 0) means
that temperature decreases with altitude. This results in colder air
(more dense) above warmer air (less dense); an unstable situation. The
more negative the lapse rate is (-9°C/km is steeper than
–7°C/km), the more unstable that layer is.
The lapse rate can change locally in a short time. To change the
lapse rate, alter the temperature at either the top or the bottom of the
layer, or both. For example, to make the tropospheric lapse rate more
negative (steeper and more unstable), increase the surface temperature
and/or decrease the temperature of the air aloft.
2. The Layered Atmosphere
There are 5 primary and distinct atmospheric layers discussed here:
the troposphere, stratosphere, mesosphere, thermosphere, and
magnetosphere. The boundaries between each of these layers are called
the tropopause, stratopause, mesopause, and thermopause, respectively.
Characteristics of each layer will now be briefly outlined…
a. Extends from 0 km to 11 km (0-7 mi) (1000-200 mb)
b. Temperatures fall from 17°C (63°F) at the surface to –50°C (-60°F) at the tropopause
c. the average tropopause height actually varies from 16 km (100mb) at the equator to 8 km (300mb) at the poles
d. The average lapse rate is –6.5 °C/km (-19 °F/mi)
e. 80% of the atmosphere’s mass is in the troposphere
f. all weather occurs in the troposphere (it has a negative lapse rate AND has moisture)
a. Extends from 11 km to 50 km (7-31 mi) (200-1 mb)
b. Temperatures increase from –50°C (-60°F) at the tropopause to about 0°C (32°F) at the stratopause
a. Extends from 50 km to 85 km (31-53 mi) (1-0.01 mb)
b. Temperatures decrease from 0°C (32°F) at the stratopause to about –90°C (-135°F) at the mesopause
c. Has a negative lapse rate, but no weather occurs there because there is no moisture
a. Extends from 85 km to 500 km (53-310 mi)
b. Temperature and pressure loose meaning here, there is very little atmosphere at this altitude, so very few air molecules. However, the molecules that are present are highly energetic, so in a sense, the temperature is very high (perhaps 2000°C), but
the pressure is very low (less than 0.01 mb)
a. Extends from 500 km (310 mi) to space
b. Again, temperature and pressure have little meaning here
c. Not a spherical shell, but more teardrop-shaped due to the solar wind pushing against it. This is the layer that protects the surface from the sun’s highly-energetic rays and particles.
Instability (a steep lapse rate) is naturally resolved by convection,
or turbulent overturning. Convection occurs in all fluids that are
heated differentially. In the case of the troposphere, the surface is
heated by the sun, and the air aloft is left to cool radiatively to the
overlying layers, so convection attempts to balance the temperature
difference (or equivalently, the vertical density gradient; because cold
air is more dense than warm air).
A "cell" of convection is characterized by a parcel of fluid that is
positively buoyant (less dense than its surroundings), causing it to rise
to some altitude where it is no longer positively buoyant (the parcel
that was once rising cools off enough to be neutral or even temporarily
negatively buoyant). In the atmosphere, this level is where the top of
the cloud would be found and is called the Equilibrium Level
In the case of the atmosphere, the surface air may not be positively
buoyant at first, but needs some forcing to get it there. If the parcel
of air reaches the height where it can rise without external forcing, it
rises freely up to the EL; the altitude at which this transition from
forced convection to free convection takes place is called the Level of
Free Convection (LFC).
Convection can be dynamically or thermodynamically forced. If forced
dynamically, some external "push" is applied to the atmosphere, such as a
cold front, warm front, dryline, thunderstorm outflow boundary, or any
other miscellaneous surface convergence line. If forced
thermodynamically, surface heating alone will initiate convection. If
the surface air reaches its "convective temperature" (TC), it
becomes positively buoyant and will rise freely.
4. Cloud Formation
The discussions of lapse rates and convection prepared the way to
introduce clouds. Clouds are merely visible tracers of atmospheric
convection, formed by the condensation of water vapor. Water vapor (the
invisible gaseous form of H2O) is found throughout the
troposphere, but is concentrated near the surface and is very sparse at
A useful quantity related to water vapor concentration is the
relative humidity (RH). It is related to the temperature (T) and the
dewpoint (Td). Dewpoint is a measure of the moisture content
of the air, comparable to temperature, which measures the heat content of
the air. Higher dewpoints indicate higher water vapor amounts. Relative
humidity becomes higher as the dewpoint approaches the temperature. In a
simplified sense, relative humidity increases to 100% when the dewpoint
reaches the temperature.
An example of identical RH at very different temperatures is the
following: if T=31°C (88°F) and Td=22°C
(72°F), then RH=60% (T, Td, or RH can be calculated using
equations not relevant to this discussion; see http://www.mcwar.org/humid.html
for an online calculator written by the author). The same RH can be
achieved for colder temperatures; consider T=7°C (45°F) and
Td=0°C (32°F), making RH=60% again. So, relative
humidity is exactly that...… relative. Dewpoint is used to express the
absolute humidity. A dewpoint of 21°C (70°F) feels "sticky"
anytime, anywhere; while a relative humidity of 60% may or may not feel
Water vapor condenses into water droplets when the air is near or at
100% RH. Since the troposphere has a negative lapse rate, air that
travels upward from the surface encounters colder air. The base of a
cloud forms when the ascending air's temperture equals the dewpoint at
that level. Lower cloud bases indicate that either the lower troposphere
is particularly moist, or the lapse rate is particularly steep (or a
combination thereof). The term for the altitude of the cloud base is the
Cloud Condensation Level (CCL).
In summary, a cloud is formed by the condensation of water vapor and
is merely a visible tracer of convection. It is bounded at the bottom by
the Cloud Condensation Level and at the top by the Equilibrium Level.
Clouds of many types (deep/shallow, narrow/broad,
precipitating/non-precipitating, low-level/high-level, etc) exist because
of the numerous atmospheric conditions that can exist.
Brian McNoldy for MESO, July 2001