Understanding weather fronts
Check the charts for great shots.
Storm Depression
The letter ‘L’ marks two low-pressure
systems, or depressions. The curved lines are isobars that link areas of equal
atmospheric pressure. The big depression is a storm: pressures have fallen to
953hPa at the centre with tightly-packed isobars.
Types of front
Warm fronts are lines with red
semi-circular flags. Cold fronts have sharp-edged blue flags. Purple lines with
both flags are occluded fronts where the warm and cold fronts have merged and
the cold air has lifted the warm air off the ground.
Warm front cross-section
Warm tropical air (left) collides with cold
polar air (right) at the warm front (red line). The warm air rises, cools and
clouds form along the front. Stratus clouds generally form featureless layers.
Nimbostratus and stratus produce rain and snow.
Cold front cross-section
Cold air (left) spiraling around the storm
depression catches up with the warm tropical air (right), and forces it off the
ground at the cold front (blue line). The rising warm air often forms
cumulonimbus. Visibility is poor beneath low cloud.
What’s that cloud?
Discover how to spot the most common types
with our handy guide
Cirrocumulus
Small round puffs of high cloud found in
rows. Large numbers produce ‘markerel skies’ because they look like fish scales
Altocumulus
Grey-white flat clouds often arranged in
regular rolls, stripes or clumps. Bigger than cirrocumulus, they partially hide
the sun
Mammatus
Rare clouds shaped like cows’ udders. The
most impressive form at the rear of huge cumulonimbus clouds
Lenticular
Streamlined UFO-or disc-shaped clouds found
on calm days downwind of hills or mountains. They rarely produce rain
Cirrus clouds
Thin and wispy high clouds found mainly in
fair weather. Sheet-like cirrus can create ice halos around the sun
Cumulonimbus
Dark thunderclouds, sometimes with a top
shaped like a blacksmith’s anvil, which billow up to 10 miles into the
atmosphere
Stratus clouds
Characterless layers associated with
overcast skies. Stratus often produces drizzle. Nimbostratus brings rain or
snow
Cumulus
Cotton ball-shaped clouds with sharp
outlines. Fair-weather cumulus have flat tops. Those with cauliflower-shaped
tops develop into cumulonimbus
Will it snow?
Snow presents great photo opportunities,
which get better if you’ve prepared your kit for a wintery shoot. Weather
charts showing ‘thickness’ can help you anticipate snow perhaps five days in
advance. Thickness typically appears as dashed lines (red on the chart below),
is measured in tens of metres, and gives a rough measure of the warmth of the
atmosphere. Lower values mean the air is colder and denser, and snow is more
likely. In northwest Europe, rain and snow are equally possible when thickness
is 522 to 538. Snow is rare when thickness is more than 540, but likely to fall
when thickness is less than 519.
In the weather chart below, snow may fall
over mid-Sweden, north of Norway and off the southern tip of Greenland.
Thicknesses are between 510 and 528, raising the possibility of snowfall.
Clouds could form because air is warming and rising along the occluded fronts
(thick lines with black flags) of passing low-pressure systems (‘L’ shapes).
Snow is more likely in a colder atmosphere
because it can reach the ground without melting. Snowflakes form when ice
freezes around dust and other particles in clouds, then the crystals collide
and stick together. The flakes get too big and then fall towards the Earth’s
surface. If they tumble through cold air, they reach the ground as snow. They
can drop perhaps 300m (985ft) through warm air before the water turns into
rain.
Snow expected
You can predict snow using a map of mean
sea level pressure (MSLP) and thickness.
