a single cloud can drop rain snow sleet
or hail that's why you may have had the
experience of watching it rain on one
side of town while the Sun shines on the
other but weather isn't produced one
cloud at a time even an isolated
thunderstorm is part of a huge weather
pattern that changes with variations in
wind moisture temperature and air
pressure these weather patterns are
composed of enormous volumes of air that
carry heat and moisture from one region
of the world to another the driving
force for all this movement is energy
from the Sun without it the wind
wouldn't blow and water wouldn't flow
one of the most important details about
the sun's heating of Earth's surface is
that it's uneven just as the sun's
intensity varies from one time of day to
another it also varies from one region
to another with the equator absorbing
far more energy than areas near the
poles furthermore the heat the earth
absorbs doesn't stay in one place for
long it's transmitted from Earth's
surface to the surrounding air as the
air warms relative to its surroundings
it becomes less dense and begins to rise
creating an area of low air pressure
it's sort of an empty space or at least
an emptier space in the atmosphere in
response colder denser high-pressure air
rushes in to fill the low-pressure void
creating the movement of air we know as
wind even though it seems like air would
always mix easily volumes of air with
different temperature and humidity
characteristics often don't scientists
referred to a huge volume of air with
particular characteristics as an
air mass air masses tend to stay intact
and move independently in response to
winds and differences in air pressure by
themselves they can dramatically affect
the weather in areas they move into but
some of the most powerful storms arise
when two air masses meet the area where
two air masses meet is called a frontal
boundary or simply a front the
interactions along these lines which can
stretch for a thousand miles were more
can have a powerful influence on clouds
and weather for example when a cold
dense air mass slams into a warm humid
airmass it forces its way under the
warmer air pushing it up and causing the
warm air to lift and cool this causes
the moisture in the upper air to
condense into clouds and precipitate out
as rain or snow or even hail in some
thunderstorms the greater the
differences in temperature humidity and
air pressure between air masses the more
likely they are to produce severe even
deadly storms so even if we often care
most about the weather where we are
paying attention to the clouds and
atmospheric conditions on a much larger
scale can give us a heads up about the
weather we might expect in the hours or
days to come