Being out on Dartmoor means many different things to people, but for me I like a mixture of
physical effort and also quiet times appreciating the area I’m in. Sometimes this is at the belay of a
rock climb after completing a hard section or having a drink stop whilst out running cross country on
the moor.
It’s at these times that I find myself not looking at hand holds, or where my feet are going to tread
next but at the greater expanse of the moor and the sky above it. Whether I’m rock climbing in good
weather or running in inclement conditions I enjoy keeping an eye out on what’s happening around,
and above me.
Climbing above the clouds, Chamonix.
Watching and trying to identify the clouds feels like a natural thing to do at these moments.
Appreciating the different forms clouds take and attempting to predict what the weather has in
store is a skill that is easily lost in our modern lifestyles, but put your geeky cloud spotting hats on,
and you’ll find yourself learning more about the weather without trying.
In the last article we looked at summer showers, and keeping an eye on those
building cumulus clouds. This time we’re looking at a longer sequence of weather, the passage of a
low, or “depression”.
In the UK a Low pressure or “Depression” system sweeps across the Atlantic picking up plenty of
moisture on it’s way, ready to give Dartmoor a good soaking when it hits the south-west. You can
nearly always associate a “Low” with wind and rain. Typically a series of fronts lie within a
depression, which are simply boundaries between warm and cold air. Warm air of course, rises, and
as it does so the moisture laden air turns into clouds and therefore rain.
Note the low slope angle of the warm front, everything happens gradually.
The warm front usually arrives first, and helpfully gives us plenty of warning with its clouds. The
warm air of the front pushes over the cold air it’s replacing very gradually and the first wispy clouds
can appear a day or so in advance of the front itself. “Trace in the sky the painters brush, the winds
around you will soon rush”
Wispy Cirrus clouds over Bellever Tor, Dartmoor.
These wispy clouds are known as Cirrus clouds and are right at the upper limit that clouds form,
around 20,000ft – 40,000ft, and they are indeed frozen ice particles rather than water vapour like
the lower cloud varieties.
After these first Cirrus clouds we get a pattern of gradually lowering clouds as the front approaches.
The Cirrus spreads out and covers more of the sky, giving it a milky veil.
Thickening Cuirrus Cloud over Denali, Alaska. The weather came in pretty bad the following day, we were stuck at high camp for 3 days.
This “Cirrostratus” thickens and lowers turning into “Altostratus” when it blocks out the sun giving
that overcast, rains on its way look.
In fact rain is on the way, and pretty soon too. As the Altostratus thickens light rain starts to fall and
gets heavier as we get the final cloud in the warm front sequence, the Nimbostratus. Nimbus means
rain in Latin, and the Nimbostratus produces lots of it, for hours sometimes.
Getting damp on Dartmoor, below Hay Tor.
This is the classic sequence of a warm front. The high cloud spreading, lowering and thickening over
the course of a day or more giving prolonged rain (courtesy of the nimbostratus), and as the front
passes the cloud thins and breaks up into fluffy cumulus again, and we are standing on a now
warmer Moor.
A cold front is quite different than a warm front. Whereas the warm front is a long drawn out affair
lasting several hours a cold front is a short sharp shock, delivering heavy downpours followed by
sunshine a fresh, clear sky.
Note the much steeper angle of the Cold front, everything happens much quicker than a warm front.
A good warning cloud for an approaching cold front is the classic Cirrocumulus and AltoCumulus,
known as a Mackerel Sky.” Mackerel sky, Mackerel sky, not long wet, not long dry”.
Pre-Frontal clouds coming from the West over Hameldown, Dartmoor.
A cold front is much steeper than a warm one and causes the air to rise much more rapidly creating
strong winds and large towering clouds which produce these sharp, short lived showers.
Thunderstorms are not uncommon at the leading edge of a cold front. A few heavy showers
followed by more cumulus clouds and a slightly cooler feel in the clear skies typify the cold front.
A dramatically clearing cold front in the Lake District, the weather front is moving right to left in the image. It turned into a great day.
We often see these two classic sequences occur one after the other, but at other times we can get a
mixture of the two, known as an occluded front.
A warm front moves slower than a cold front and so the cold front gradually catches up and the two
fronts merge, squeezing the warm air upwards into a sandwich between two areas of cooler air.
These occluded fronts are harder to identify by a quick glance at the sky because they are indeed a
mixture of the two fronts and a great deal of variables exist that affect their characteristics.
Taking only a quick glance at the sky will tell you little about the weather to come, but over a few
hours you can see the clouds constantly changing, giving you a better idea, and if your lucky enough
to be out when a big low is sweeping across the country, don’t worry about the rain. You might see
some good clouds.
Crepuscular rays shining through some Cumulus Clouds, Dartmoor.
