Physics Photo of the Week
The clouds in right-hand video clip shows stormy weather
approaching in contrast to the fair weather picture by Matt
Hansen. The clouds are moving southeast across the sky and are
constantly forming and moving along with the wind. Although some
clouds are evaporating at lower elevations, they do not evaporate at
the tops as in the fair weather clouds photographed by Matt.
These clouds are classified as nimbostratus and could potentially turn
into cumulonimbi or thunderstorm clouds. While nimbostratus
clouds do not always turn into rain, they did on this occasion.
Two types of weather systems
|Fair Weather Cumulus
Photo by Matt Hansen
Photo by Andrew Jones
Fair weather cumulus - by Matt Hansen
In the morning of April 27, 2007, I photographed cumulus humilis clouds
with a digital camera pointing the camera westward. Over a time span of
twenty minutes, a picture was taken every ten seconds. The photographs
were played together, allowing the development of these clouds to be
viewed quickly, similar to animation. Cumulus clouds form as water
vapor condenses from atmospheric pressure as it rises from ground-level
heat by the sun. These fair weather cumulus clouds show many
interesting characteristics that differentiate them from thunder clouds
(Cumulonimbus), and from the inclement weather clouds featured by
The general motion of the clouds is up and to the left, rolling with an
eastward moving wind and combining with smaller clouds as it moves
east. These motions are common to cumulus humilis clouds in the
mountains and are often viewed at an altitude around 6000 feet. The
lack of vertical development indicates that the atmospheric temperature
above the clouds does not significantly drop off. The cloud base and
below may be quite turbulent for pilots, causing them to seek a higher
Another interesting factor of this “movie” is the cloud that appears
suddenly in the left quadrant of the frame. At this point we are
viewing the point at which the water vapor is condensed and a cloud is
born. Other clouds seem to disappear or combine with the mass that is
forming with the larger cumulus humilis. Most of the motion is seen not
at the base of the clouds, in fact the base seems stationary and denser
than the upper parts where most of the motion is viewed.
Nimbostratus - by Andrew Jones.
These nimbostratus clouds are getting caught in an updraft. An
updraft occurs when hot air rising in the atmosphere mixes with the
less dense air already above it, and is therefore creating an updraft
among the clouds. A good example of an updraft can be seen in the
bottom left corner of the photo. Clouds are forming while
preexisting clouds are rolling southeast and eventually collide with
the forming clouds—a great way to develop big rain clouds.
Throughout the entire video they appear to be constantly growing in
mass—in the last frames they look capable of producing a thunderstorm.
One interesting aspect of this footage is to observe the clouds
rolling. In fair weather the clouds tend to just drift along the
sky and evaporate as they rise to higher elevation. These clouds,
on the other hand, tend to roll across the sky and don't evaporate at
their tops. The clouds are rolling in this picture because they
are caught in an updraft and there is a lot of wind moving around at
that elevation. The existing nimbostratus clouds roll upwards and
new nimbostratus clouds form at the base of the preexisting clouds to
develop a more massive cloud.
Matt Hansen and Andrew Jones
are students in Earth, Light, and Sky (Phy 121) at Warren Wilson
College. These photos and the descriptions are part of their
experimental projects for the class. D. Collins
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
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