The question is deceptively simple, but as you'll see, the answer isn't. The author is an instrument-rated pilot who flies out of Gaithersburg, Maryland. He also holds a degree in meteorology.
May 1, 1998
Beautiful, fluffy onesdark, roiling, malevolent ones; they add
immeasurably to our natural landscape. For non-aesthetic reasons
they are important, too. They help regulate the earth's energy
balance, by reflecting and scattering solar radiation or absorbing
the earth's radiated infrared energy. But they also visually indicate
physical processes which shape the weather that we fly throughnot
the least of which is this thing called atmospheric stability.
It's part of what makes those clouds that perhaps some small person
has already asked you aboutor that perhaps you yourself have
How they're formed
You undoubtedly know that clouds form when air rises and cools.
When a blob of air goes up into an area of less pressure, it cools.
When an unsaturated volume of air cools on ascent, without any
energy being added or removed, it follows what is known as the
dry adiabatic lapse ratewhich is approximately 9.76 Centigrade
degrees per Km (or about 5.33 Fahrenheit degrees per thousand
feet). When it reaches its dew point temperature, the rising parcel
is no longer unsaturated. Water begins to condense. As it does
so, it releases copious amounts of heat.
Water in fact has a very
high "latent heat." Just as it takes large amounts of
energy to boil water, large amounts are given up when it returns
to the liquid statefrom about 541 calories/gram at 100 degrees
C up to about 597 calories per gram at zero degrees C. So some
of that cooling off due to its ascent is offset by the latent
heat released by the condensing water vapor. The air parcel then
cools off at a lower ratecalled the moist adiabatic rate. This
rate varies with temperature, and is on the order of 6 Centigrade
degrees per Km, or about 3.3 Fahrenheit degrees/1000 feet.
in addition to having a high latent heat, water also has a high
specific heat. Just within the liquid phase, it takes plenty of
energy to raise or lower the temperature of a given amount of
waterwitness the pronounced climatological effects wherever
there is a large body of water nearby!)
You may also know that several mechanisms can cause air to rise
and clouds to form: surface convective heating; orographic lifting;
convergence of air masses; and uplift along fronts. What you may
not know is that even if none of these things occurred, the fact
that a parcel of air might pick up a greater amount of moisture
than the surrounding air contains will itself make it rise. This
is because the same total number of water molecules (in vapor
form) are about one-third lighter than the identical combined
number of molecules of nitrogen, oxygen, and other gases in their
proper proportions; i.e., air. In reality, even super-steamy jungle
air will only hold about 6% of water vapor by weight, so even
very humid air will only be about 2% lighter (one-third times
0.06) than absolutely dry airbut it's enough to do the trick.
This is one of the larger cogs (the biggest of course being solar
heating) in the driving mechanism behind the hydrologic cycle!
The air isn't always rising, thoughor if it is, the amount of
lifting changes. The reason why hinges on the concept of atmospheric
stability. If rising air is colder than surrounding air, it will
sink back down: the air is said to be stable because it "resists"
displacement. If rising air is warmer than the surrounding air,
it will continue to rise (unstable air) until it's not.
When rising air is (or would be) colder than its environment at
all levels, it is considered absolutely stable. Such is the case
when the environmental lapse rate is less than either the dry
or moist adiabatic lapse rates. Picture a nearly-homogeneous air
mass where temperature drops little with altitude: In this case,
if a parcel is forced to rise, it will always be cooler and heavier
than the air around it, and it will spread out horizontally if
it can't "get" back down. Any clouds will be thin, spread
out, and have flat tops and basesin other words, stratus clouds.
The atmosphere becomes more stable if air aloft warms (either
by sinking and compressing, or via advection of neighboring warmer
air)or if surface air cools (by nighttime radiational cooling,
advection of colder air, or contact with a cold surface). Incidentally,
that is why you see the most hot air balloons early in the morning,
when the lowest surface temperatures are recorded. (The extreme
case of this of course is the inversionwhen temperatures rise
Don't forget lapse rate!
When the atmosphere's temperature profile shows instead a rapid
dropfor example anything greater than even the dry adiabatic
lapse rateall air parcels (even dry ones) will not cool as quickly
as the ambient air, and once they "get the chance" to
go up, they will always be hotter and lighter than the air around
them, and they keep going up. This is an absolutely unstable atmosphere.
It is characterized, of course, by cumuliform clouds. This steepening
of the environmental lapse rate occurs when air aloft gets colder,
or when the surface becomes warmer (by daytime radiative heating,
advection of warm air, or conductive heating from below). Does
this sound like the familiar summer afternoon thunderstorm scenario?
When the lapse rate is between the moist and dry adiabatic rates,
things get interesting. An unsaturated parcel rises (for whatever
reason) and cools, first at the dry adiabatic rate. This is greater
than ambient, which therefore makes it colder, heavier, and thus,
stablethat is, until it reaches its condensation level. Here
the air is 100% saturated (i.e., at its dew point). Now above
this the air cools at the moist adiabatic rate. Due to the release
of latent heat, it cools more slowly than the air around it, which
makes it warmer, lighter, and thus unstable. This is conditionally
unstable airwhich depends on how humid the air is and at what
point it becomes saturated.
Wasn't that easy?
This is enough for Meteorology-On-Daddy's-Lap 101. Atmospheric
stability does figure prominently in our considerations of weather.
You have most likely heard of the National Weather Service's Composite
Moisture Stability Chart, issued twice daily: It features its
own panel, showing the "lifted index", which reflects
the stability of the air over the continental US. (Those glider
pilots among you will no doubt have heard of it!) I hope this
has given youor that it will allow you to give someone you knowa
better intuitive understanding of the physical processes behind