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 When
delivering a weather forecast, the broadcaster pays particular
attention to whether the wet weather is rain or showers, but
what is the difference? Many people think that showers are
shorter or heavier than rain, but this is not always the case.
The real difference is simple - it just depends on the type
of cloud it comes from.
Rain comes from 'stratiform'
clouds, layered clouds that often cover the whole sky. They
are generally a uniform blanket of cloud, so rain is often
a steady blanket too. It can stay around for many hours and
then it is called 'persistent'. Rain does have exceptions
though and can be more fragmented to give 'outbreaks of rain'.
On the other hand,
showers come from 'convective' clouds. These are the fluffy
'cotton wool' cumulus clouds, or the larger, more threatening
cumulonimbus clouds. Cumulonimbus clouds often give thunder
and lightning, as well as a shower of rain, or hail. Both
these clouds are generally quite well spaced out, which is
why showers are often described as 'isolated' or 'scattered'.
Being so isolated, it is virtually impossible to determine
exactly where a shower will form. This is a problem for forecasters
because between the showers there is often beautiful blue
sky.
 Cumulus
clouds are formed by the air mixing, which is often caused
by the earth's surface heating the air near to it. This causes
the air near the ground to rise, and hence mixes it. During
the summer, the sun heats the ground daily. As it heats up
it can trigger off enough mixing to form convective clouds.
This is why showers often form during the day in summer, and
die away over night.
During the winter it
is the sea that heats the air, as sea keeps its summer heat
better than the ground. This is why in winter most showers
form over the sea. If there is a wind, these showers can be
pushed onto the coast. This is why tornadoes in this country,
which come from cumulonimbus clouds, have been spotted in
coastal resorts.
One thing that is consistent
for both rain and showers is the shape of the raindrop. The
familiar teardrop is not actually correct. Raindrops are actually
shaped more like a hamburger bun, no matter what cloud it
came out of.
Other features in
the Weather Basics series
Air Pressure
Clouds
Day and Night
Fog and Mist
Fronts
Frost
High Pressure
Humidity
Jet Streams
Lightning
Low
Pressure
Mirages
Northern Lights
Thunder
Watercycle
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