How to Understand, Observe and Document Severe Thunderstorms Safely.
Your home weather station can reveal the secrets of the growth, behaviour and decline of thunderstorms.
Thunderstorms can, and do, occur at any time of year, night or day. All they require is moist air, instability, and a meansof lifting the air.
Air masses that have formed over oceans or seas contain plentyof moist air, and the major sources of moist air for most of the USA move landward from the Atlantic Ocean or Gulf of Mexico.
Instability simply means that if air is given some help to rise or fall, it will continue to do so independently of the initial cause.
A Classic Thunderstorm photographed
from a plane. Source NOAA Library
And the initial cause of rising air, or lifting mechanism,
can be convection caused by differential heating of the
land surface, or warm air being forced to rise over the
boundary between moving masses of air of different
temperature and moisture content, most frequently a cold
Of course, the absolutely indispensable feature of
- the rapid heating of air as lightning discharges
causes a shock wave, the sound of which we hear as
Single Cell Thunderstorms
Different parts of the earth's surface heat up at
different rates- for instance a ploughed field will heat
faster than one covered in vegetation, and land will
heat faster than water. Differential heating creates
thermals - rising columns of air - over the warmer
areas, and if the air is both moist and unstable a
single cell thunderstorm may develop.
Like all thunderstorms, single cell storms have the
potential to be dangerous, but in most cases they make
a bit of noise, bring some useful rain and temporary
relief from the heat, and decay quite rapidly.
On several occasions I have been fortunate enough to
watch a solitary thunderstorm form above me. The weather
had been hot and humid, with scattered cumulus but
mostly clear skies. Then one cloud, almost directly
above me, started to grow and darken - on this and
other occasions occasions the growing cloud was above
an area of bare rock which, being hotter than the
surrounding ground, probably was responsible for a
As the cloud grew, other small cumulus clouds surrounding
it shrank, as if their moisture was being transferred to
the building cloud. After a while, as the cloud spread
and grew, muffled thunder could be heard. Light rain
started, and it appeared that just a light shower would
be all that would happen, but the cloud continued to
build. It grew darker, and the rain became heavy. All the
while, it was obvious that the sun was shining brightly
elsewhere, and before long, the rain eased, the cloud
lightened, and soon disappeared.
In these circumstances, the storm stayed in the one
place, raining itself out rather than moving on. It is
quite likely that the rain, in cooling the bare rock
areas, removed the source of the thermal which started
the growth of the storm.
While these storms are usually mild, they can present
a few problems.
Another experience I had was while driving along a rough
track which required all my concentration. Most of the
time I was in mid afternoon sunlight, and it wasn't
until the first few heavy raindrops made craters in the
dust of the track that I realised that a small
thunderstorm had formed above me - not large enough to
block the sun half way down the western sky. Again,
although the initial rainfall seemed like a brief
shower, the storm was still growing and heavy rain
began and lasted about 40 minutes. It was enough to
produce flash flooding, where the small gully beside
the road changed from dry to a three foot flood of
turbulent muddy water within minutes. About 20 minutes
after the last of the rain the sky had cleared to
scattered fair weather cumulus, and there was no more
rain that day.
Quite often thunderstorms will form in clusters of cells,
which grow and dissipate at different rates while merging
together. These more active storms extend over larger
areas and can last for several hours. Highly active
Multi-cell storms can produce large hail, strong winds,
cloud to ground lightning, flash flooding, and isolated
Further up the scale of severity are storms which develop
in lines along a front. The strong lift along the front
provides the impetus for storm formation, which, once in
progress, can be self perpetuating as rain-cooled air
spreading out in front of the storm acts as a mini cold
front, lifting warm humid air above it to further feed
These squall lines can move very rapidly and last for
hours. During their life the damaging winds, heavy rain
can affect large areas.
strong straight line winds, including
may also form.
The junk yard dog of the thunderstorm world is the
supercell. It is a giant single cell storm which can last
for many hours. The energy involved is immense, as vast
amounts of humid air are drawn into the system, to return
to the ground as very heavy rainfall and large damaging
These highly active storms generate very strong winds,
and are typified by a strong, usually cyclonic rotation
pattern (anticlockwise in the northern hemisphere,
clockwise in the southern). Rotation frequently results
when winds at altitude are from a different direction
to those at the surface.
Supercell on Radar from 1945 Source; NOAA Archives
Rotation of the system can be seen in the clouds themselves and on radar. Under the right conditions tornadoes can form - almost all significant and highly destructive tornadoes are formed in supercells.
Depending on where you are in the USA, you can expect up to 100 days each year during which thunderstorms will form. With the exception of the west coast, almost everywhere will experience 20 or more days with storms, while 100 days with storms is to be expected in Florida.
So the chances are that several times a year a thunderstorm of some form will pass close to or over your home.
Unless you feel that just being in the path of a thunderstorm provides more than enough interest on its own, there are a number of things you can do to raise the level of your experience, particularly if your home weather station has good data saving capability.
Bear in mind that thunderstorms can be severe and destructive. I need all the readers I can get, so do take the time to check the weather warnings - a
is an excellent investment - and do whatever is necessary
to protect life and property.
If you can record wind speed, wind direction, and rainfall,
you have the opportunity to follow all facets of
thunderstorm growth, maturity and decline. What changes
do you see in temperature, humidity and air pressure as
the storm approaches and passes?
Is there any difference in the sequence between a single
cell thunderstorm and a squall line?
And what can you tell about wind speed and direction
during the various stages - how does this relate to peak
rainfall and thunder and lightning intensity?
If the storms are associated with frontal activity, is
your storm close to the front, or in advance of it?
There are many more possibilities in recording the passage
of a storm, including visual observation and linking of
your records to radar and satellite imagery.
Perhaps on some occasions you may be a little more
adventurous - these links will provide information on
Storm Spotting and Chasing. I'm still working on these pages - bookmark this page
and check again later.
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