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moist and stable at its origin,
becomes warmer and more
unstable as it comes South East
across the North Atlantic
(surface heating increases the
temperature). It is moist
because of surface evaporation,
leading to convective
cumuliform cloud. Any
instability is increased during
the day in Summer. In showers,
visibility will be poor, but will
improve as the air mass moves
inland as moisture is lost.
·  Tropical Maritime, moist and
warm, from tropical oceans. It
picks up moisture on its travels,
so relative humidity increases
and the air mass becomes more
stable as it moves North. In
Winter, you can expect
stratiform cloud and drizzle,
with poor visibility remaining
after cloud has been heated
away by the warmer land mass.
In Spring and early Summer,
you will get advection fog in
Western areas, particularly
Cornwall and Devon.
They can be modified if they move
over different areas.
Frontology
A front is a line of discontinuity, or a
narrow transition zone between air
masses where they are forced to mix,
even though they don’t want to. The
difference is usually in temperature,
but may be purely due to moisture
Weather 101
content. Fronts are always associated
with depressions, which are
sometimes referred to as a frontal
wave. Fronts will rotate
(anticlockwise) around the low.
Warm tropical air could be forced
over colder arctic air, for example,
because it is less dense and, if moist,
will form a typical cloud structure
that we on the ground can use to tell
when a front is coming. The name
of a front, that is, warm, or cold,
comes from whichever air mass is
overtaking the other, whereas the
type of weather you get is
determined by the stability and
moisture content of the warm air
mass (exam question). The actual
temperature is less important than its
relationship to that of the surface it
is passing over.
The Polar Front is an area where
south- and north-westerly airstreams
meet to form long series of
depressions, starting off the Atlantic
Coast of North America.
Frontogenesis is the term for the
forming of a front, and frontolysis the
one for its dissipation. The cold air
mass does not move at a stationary
front, and you get an upper front when
very cold air is caught on the surface
with the weather higher up.
The Warm Front
This exists where warm air overtakes
a colder air mass and is forced
upwards, meaning clouds. Its symbol
on a weather map, resembling beads
of sweat, is:
The frontal slope has a gradient of
somewhere between 1:150 and
1:200, although the clouds
themselves will be about 5 miles
high, starting with Nimbostratus at
more or less ground level, through
alto-stratus to cirrostratus (when
flying towards it, you would see the
clouds the other way round, of
course). Once you start seeing cirrus
clouds, you know that a warm front
is somewhere ahead, anywhere
between 300-600 miles away, or
nearly 24 hours at a typical speed of
about 25 kts, so have an overnight
kit if you have to wait it out (rain will
typically be 200 miles ahead). You
can use the typical slope figure to
figure out the cloud base in front of
the system. At 100 miles, it will be
2,640 feet, which comes from
1/200*100, making half a mile,
multiplied by 5280 (feet).
Clouds will appear in this order as
you fly towards a warm front –
cirrus, cirrostratus, altostratus,
nimbostratus and stratus.
The shallow slope ensures that
whatever is coming will last some
time, and you can expect the
pressure to fall, the cloud to get
lower, the wind to back and increase
in speed, rising humidity, bad
visibility, drizzle and rain, though
not necessarily in that order.
The freezing level will be lower in
front than behind, and the slope
means that freezing rain will be
falling on anything underneath (see
diagram above). Once you see ice
pellets, expect freezing rain next.
As the front passes, the rain will
stop, then become drizzle under an
overcast sky, and the wind will veer.
As humidity rises to saturation point,
102 JAR Private Pilot Studies
visibility will be poor. You will then
be in the warm sector, where
conditions will be more settled for a
few hours. For the exam, when
asked to predict the future position
of a warm front (and the type of
weather), use the direction of the
　
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