Ask A Scientist© Weather Archive

> >>   name        Bob
> >>   status      educator
> >>   age         40s
>
> >>   Question -  What holds a cumulous cloud together as it travels with the
> >> air currents? I seem to recall that it is formed by rising warm air that
> >> has reached the dew point, starting at a particular altitude. I live in
> >> Cleveland, Ohio, and can often see Lake Erie "lake-effect" creating
> >> clouds as winds (especially NW winds) pass over the lake. Why doesn't the
> >> rising, moist air create one long sheet of cloud - more like a
> >> stratus-type? I think I have read that a cumulous cloud has rising air at
> >> its center and falling air currents at its sides. If that is so, then how
> >> can it possibly maintain that circulation pattern while passing over so
> >> many different types of surface terrain? Is the circulation pattern
> >> really that persistent in spite of attempts at disruption? Is there a
> >> different force at work that is keeping the clouds "together" and
> >> creating cumulous clouds with clear (sometimes predictably consistent)
> >> distances between adjacent, neighboring clouds? Shouldn't some other
> >> types of rising, falling, etc. air currents disrupt the cumulous cloud
> >> and cause it to run into neighboring clouds or pull apart as its
> >> circulation or the forces that maintain its integrity are disrupted?
> >
>
>Bob,
>
>A cumulous cloud is a "convective" cloud, meaning that it
>forms from rising air that is initially warmer than the
>surrounding air.  This air rises rapidly and cools as it rises,
>condenses, and forms a cloud.  These are often called cell
>clouds as they do not normally form a sheet (except in stratocumulus,
>altocumulous, and cirrocumulous) like stratus.  Stratus clouds are
>formed by the lifting of an entire layer of air.  The three
>cloud types that I named in parentheses combine the two
>processes (the layer is lifting and individual parcels of air
>are being lifted in the layer).  Sometimes rising air dominates
>and the cumulous clouds fill in and result in stratocumulous.
>As the cumulous cloud rises, the air around it must descend
>(assuming that the entire layer of air is not being lifted)
>to make way for it.  Thus, cumulous clouds are often well spaced.
>
>SODAR (SOund Detection And Ranging, using acoustics) and
>LIDAR (LIght Detection And Ranging, using lasers) remote
>sensing instrumentation show that on a cumulous cloud day, such as
>a nice day in summer) rising air from a large area of
>the surface is interspersed with falling air.  These rising and
>falling air volumes can be amazingly persistent and can move
>with the cloud.  A very small difference in temperature change
>with height can create either of these preferential areas.
>
>Terrain can indeed disrupt cumulous cell health.  Once formed, the
>cumulous cloud is it's own environment (a wet one) and will not
>dissipate unless the air that surrounds it drags it downwards
>(so that the air can warm above the dew point) or it is eroded
>by drier air from around it (called entrainment).
>
>As for the Lake Erie clouds, the different areas of warmer and cooler
>water (for different depths of water, etc.) act in the same way as
>the land, with preferential warm and cooler areas.  Cumulous cell
>development over the water usually is not as strong because the
>differences in water surface temperatures are not as great as those
>over land areas.  During winter, when water temperatures are low,
>you will see more stratus over any of the Great Lakes than during the
>summer; layer lifting is more common in winter.
>
>David Cook
>Argonne National Laboratory

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