Physics Photo of the Week

May 4, 2007

Two types of weather systems
Large file - please have patience! Large file - please have patience!
Fair Weather Cumulus
Photo by Matt Hansen
Nimbostratus
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 Andrew Jones.

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 elevation.

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.

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. 

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


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.


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