CHAPTER 4
THE THUNDERCLOUD
4.1 THERMAL DYNAMICS
On a warm and sunny day heat is absorbed by the earth’s surface
Causing both water vapour and hot air to rise to higher atmospheric
Levels. The warmer the air the more water vapour it can hold before
Saturation. The secret of a thunderstorm and its devastating power
Lies in the amount of water vapour present. The latent energy of water
Vapour is the key factor in thunderstorm formation and electrification.
At the beach most of us have experienced the sensation of cold after
Getting out of the water. The explanation is that water takes away
Heat from the body surface at a rate of about 540 calories or 2250
Joules per gramme when it evaporates. When water condenses to form
Drops, the same amount of energy, in the form of heat, is released.
When a cloud is formed by warm humid air reaching higher and cooler
Levels, condensation will create small drops between 5 – 10 microns
Dia. During condensation latent heat is released warming the
Surrounding air which will rise further (the hot air balloon effect) to
Considerable heights pulling more humid air up from below to take its
Place. A chain reaction starts when humid air is fed in from below
Pumping more and more energy into the cloud in the form of heat and
Convection. A cloud might grow up to 60,000 feet or 20 km in height in
58 ATMOSPHERIC ELECTROSTATICS
Very narrow columns. Updraft velocities can easily reach 30 m/s. Each
Column is called a cell and a thundercloud usually consists of several
Cells. Each cell has an average life time of 30 minutes. When a cell
Reaches a maximum altitude it is in a so-called mature stage, at which
Time the cloud top usually flattens out to form the typical anvil shape
Often associated with thunder heads. The flat top is believed to occur
When a cloud reaches
the stratosphere or the boundary in the
Atmosphere where air temperatures begin to rise with altitude which
Will cause the cloud to evaporate again. In the growing stage of a cloud,
Drops increase in size as they reach higher levels and eventually
Become heavy enough to overcome the updraft velocities of the
Convection. At this point rain or precipitation starts to fall down and
The cell has reached its dissipating stage. At greater latitudes,
Thundercloud precipitation without exception always reach
Temperatures below freezing. Hail and sleet are formed which often
Fall down and reach ground before melting. The cold precipitation will
Cool the air in the cell which becomes heavy and begins to move
Downward as well. The cold heavy air might cause considerable down
Drafts which, when reaching ground, supply the outward rush of cool
Air often felt as a relief to most of us after a hot day. When
Precipitation and down drafts occur the cell is said to be in its
Dissipating state and although some lightning activity is still going on
Its life is nearly over. New cells are formed adjacent to the old ones and
Often while observing their growth one can see small bubbles or turrets
Developing above and around each cell. The thunder cloud described
Is a typical heat cloud or isolated storm. Also common are lines of
Thunderstorms or squall lines. Lines of thunderstorms are formed
When a cold front wedges in under the warm humid air mass along a
Warm front and squall lines can extend for several hundred miles. The
Energy source is the same, namely condensation of water and release of
Latent energy. Thunderstorms along a squall line often seem to lean
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