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METEOROLOGY FOR SLACKERS
Or, is it worth skiving off tomorrow?
Meteorology for Slackers
Introduction
• Meteorology is a hideously complex subject, from
which we attempt to distil enough information to
equip us for the tribulations of gliding flight
• Thankfully we can disregard many of the more
complex atmospheric phenomena and concentrate
on what occurs in the lower levels of the atmosphere.
• With no other demands upon your time then simply
putting your head out of the window is the best
method!
Meteorology for Slackers
Introduction
The main factors influencing weather from
a pilot’s perspective are:
• Insolation
• The air mass and its recent history
– temperature
– humidity
• Air Pressure
– stability
Meteorology for Slackers
Coriolis
Coriolis effect determines the airflow about a
depression or anticyclone:
• HIGH pressure forms an ANTICYCLONE,
about which the wind rotates CLOCKWISE.
• LOW pressure forms a DEPRESSION
(which may be described as CYCLONIC)
and about which the wind rotates
ANTICLOCKWISE.
Meteorology for Slackers
Coriolis
This can be expressed as Buys Ballot’s Law:
In the Northern Hemisphere a person standing
with his back to the wind will have high
pressure to his right and low pressure to his
left.
ie Low on your Left
Meteorology for Slackers
Gradient Wind
• Balance of pressure gradient force, PG and
geostrophic, GF coriolis forces.
• Blows parallel to isobars.
• Measured at 1000 m.
• Usually clockwise of wind at lower altitude.
• At 50°, 4mb between Deal & Scillies = 10 Kt.
Meteorology for Slackers
Pressure Systems
High Pressure
• Air mass subsiding.
– Little/No convection cloud
.
• Inversions marked
– Thermals may be suppressed
– Turbulence at inversions.
– Wind on hill may be accelerated below inversion.
– Wind on hill be not generate lift if inversion below launch.
• Insolation unimpeded
– Occasionally,thermals can be surprisingly strong, but often short lived
– May become hot & humid
– Risk of CuNb if inversion(s) break
Meteorology for Slackers
Pressure Systems
Low Pressure
• Air mass rising.
– Significant convection cloud. (Cu, Cb, Ac, S, Sc)
– Wind on hill will generate lift
• Inversions weak/absent
– Thermals strong to cloudbase and beyond
– visibility good
• Insolation impeded
– Cloud cover may impede insolation after cumulus build up
– risk of spreadout - Cu merge to form S/Sc.
Meteorology for Slackers
Pressure Systems
Forecasts
Meteorology for Slackers
Pressure Systems
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Pressure Systems
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Pressure Systems
Meteorology for Slackers
Pressure Systems
Bracknell
Meteorology for Slackers
Pressure Systems
Meteorology for Slackers
Pressure Systems
Bracknell
http://meteonet.nl/aktueel/brackall.htm
http://www.itadvice.co.uk/weatherjack/charts-all.html
http://www.airworks.co.uk/links.htm
Meteorology for Slackers
Pressure Systems
Meteorology for Slackers
Pressure Systems
ExpertWeatherOnline
Isotachs:
http://expert.weatheronline.co.uk/is10_frame.htm
Meteorology for Slackers
Pressure Systems
Meteorology for Slackers
Pressure Systems
Meteorology for Slackers
Pressure Systems
Meteorology for Slackers
Pressure Systems
Meteorology for Slackers
Pressure Systems
Meteorology for Slackers
Satellite Imagery
http://www.metoffice.gov.uk/satpics/latest_uk_vis.jpg
Meteorology for Slackers
Satellite Imagery
http://www.metoffice.gov.uk/satpics/latest_uk_ir.html
Meteorology for Slackers
Satellite Imagery
http://130.92.54.203/rsbern/noaa/dw/realtime/current/n1bcurr.jpg
Meteorology for Slackers
Satellite Imagery
http://www.sat.dundee.ac.uk/abin/browseleaf
Requires registration
Meteorology for Slackers
Topography
Don’t forget topographic effects: eg in an easterly airflow the wind is accelerated
through the channel and also around the Wilmington/Beachy Head massif, such
that forecast wind strengths are often as much as a factor of two too low!
Meteorology for Slackers
Tephigrams
Is the day likely to be any good? Don’t forget about soundings!
Meteorology for Slackers
Tephigrams
• Soundings help you to:
– Work out approximately what time the day will
kick off & when it will die.
– Know at which altitudes to expect maximum climb
rates.
– Be aware of any risk of over-development or
spread-out.
Meteorology for Slackers
Tephigrams
Lapse Rate
• For every 1000 feet gain of altitude, dry air
cools 3°C - The Dry Adiabatic Lapse Rate
• Saturated air cools by only 1.5°C/1000’ The Saturated Adiabatic Lapse Rate. This is
only an approximation - the rate changes
slightly with height.
Meteorology for Slackers
Tephigrams
Stability
• Air is stable if after being displaced vertically
it returns to its original altitude.
• Air is absolutely unstable if the
environmental lapse rate is greater then the
DALR.
Meteorology for Slackers
Tephigrams
How high is cloudbase likely to be?
A simple rule of thumb is:
(temperatures in °C)
400*(forecast temperature - dew point temperature)
= Cu base in feet
i.e.
cloudbase = 400(T-DP)
This does not hold true once the day starts to cool, but is
adequate for the main part of the day.
Meteorology for Slackers
Tephigrams
A more accurate
method of
determining cloud
base involves using
atmospheric
soundings, known as
Tephigrams.
The dry adiabat is the temperature line that dry air would follow
The saturated adiabat is the temperature line that saturated air would follow.
The SALR is a curve because the 1.5°/1000’ ‘rule’ is actually only an
approximation
Meteorology for Slackers
Tephigrams
Given a dew point of
19 °C* & a forecast
temperature of
25°C, follow these
two lines up their
respective adiabats
until they meet. This
is cloudbase.
*(ie the air contains sufficient water that once cooled to 19 °C it will start to condense
out.)
Meteorology for Slackers
Tephigrams
The Met office take
regular soundings of
the atmosphere and
produce an
environmental line
on the graph. This is
the actual
temperature at any
given altitude
inversion
Meteorology for Slackers
Tephigrams
Any thermal will continue to
rise until its temperature
is equal to that of the
surrounding air.
Thus our cloud will build
until the saturated
adiabat meets the
environmental line.
Without an inversion, the
thermal would rise to the
tropopause, becoming a
Cb.
inversion
Meteorology for Slackers
Tephigrams
A thermal rises because it
is warmer & thus less
dense than the
surrounding air.
The greater the difference,
the faster the rate of
ascent.
Any thermal will continue to
rise until its temperature
is equal to that of the
surrounding air.
Given this environmental
line
inversion
inversion
inversion
Meteorology for Slackers
Tephigrams
A thermal trigger temperature of 21°C is
needed before they will release but
they will stop at the top of the 500’
inversion.
At a trigger temperature of 24 °C the
thermal will rise quickly at first,
slowing towards 2000’ then
continuing slowly to the 3000’
inversion.
At a trigger temperature of 26 °C the
thermal will rise quickly at first,
slowing towards 3000’ then
continuing slowly to the 4500’
inversion.
Meteorology for Slackers
Tephigrams
Taking the last case , a trigger temperature
of 26 °C, plotting the difference between
the red & blue lines will yield the
anticipated thermal buoyancy vs.
altitude. Remember air has mass so
changes are not instantaneous
Inversion has stopped climb
Mildly buoyant, climb poor
Still fairly buoyant, climb good
Still very buoyant, acceleration moderating
Very buoyant, accelerating fast, hang on to
breakfast
Meteorology for Slackers
Tephigrams
Where can I get one?
• Noaa http://www.arl.noaa.gov/ready/cmet.html?
• Met Office - Herstmonceux or Camborne
Meteorology for Slackers
Tephigrams
Meteorology for Slackers
Tephigrams
Meteorology for Slackers
Tephigrams
Meteorology for Slackers
Tephigrams
Meteorology for Slackers
Tephigrams
Tephigrams in Summary
• A graph of air mass properties with height.
• In the real world plotted on a variety of
different axes
Meteorology for Slackers
Airmass
Is the day likely to be
any good?
What about the airmass?
Meteorology for Slackers
Airmass
Air masses affecting the British Isles can
be broadly categorised in terms of their
source and their path. This leads to
four possible types.
 Tropical maritime - warm and moist
 Tropical continental - warm and dry
 Polar maritime - cold and (fairly) moist
 Polar continental - cold and dry
• To these must be added another air mass - returning polar maritime - which
consists of polar air that has moved southwards over the sea and then turns
northwards and approaches the British Isles from the south.
Meteorology for Slackers
Airmass
Tropical Continental Air
• Tropical continental air usually comes with southeasterly or southerly airstreams.
• It originates in North Africa and often travels over the
Mediterranean Sea, Spain and France before reaching
the British Isles. In summer, even easterly winds from
central Europe or the Ukraine could be included in
this category, as the continent becomes so hot at this
time of year.
• The air picks up some moisture over the
Mediterranean/ Bay of Biscay, but overall the air tends
to be quite dry and the skies are typically cloudless.
Meteorology for Slackers
Airmass
Tropical Continental Air
• Strictly speaking, an air mass cooled from below on
its northward journey should be stable.
• Sometimes, however, moisture may have found its
way to medium levels in the atmosphere.
• Then, if there is a layer of unstable air and a trigger
to set off convection, altocumulus castellanus
clouds can develop, looking like turrets.
• These are often the forerunner to tremendous
thunderstorms, which can occur by day or night.
Meteorology for Slackers
Airmass
Tropical Continental Air
• The majority of tropical continental airstreams give
a marvellous heat wave (in summer)
• The lack of moisture usually causes the visibility to
be good. However, in the air there may be desert
dust, fine soil or pollution particles, which can lead
to moderate visibility (often described as 'heat
haze').
• Also, the cloudless sky sometimes looks milky
because of pollutants.
Meteorology for Slackers
Airmass
Polar Continental Air
• A polar continental air mass originates in
Scandinavia or Russia, and the air mass reaches
the British Isles when north-easterly or easterly
winds become established.
• This tends to occur when there is a high pressure
area somewhere to the north of the British Isles,
often over Scandinavia itself.
• Polar continental air masses mainly affect the
British Isles during the winter half of the year.
Meteorology for Slackers
Airmass
Polar Continental Air
• Temperatures in polar continental air masses are
below average in winter, except perhaps to the lee
of mountains. In summer, however, the
temperatures tend to be above average.
• The moisture content is low in these air masses,
especially when they take the short sea track in the
Calais/Dover region. This leads to clouds being
generally well broken, and so the weather is fine
and sunny.
Meteorology for Slackers
Airmass
Polar Continental Air
• Air that has crossed the North Sea between
Denmark and Scotland is said to have taken a long
sea track. It therefore collects more moisture and
clouds tend to form during its journey over the sea.
Consequently, it is cloudy in eastern districts (with
perhaps drizzle or snow flurries), but further inland
there tends to be a mixture of cloud and sunshine.
• Visibility varies, generally being very good when air
comes from Scandinavia, but moderate or poor
when the air originates in the industrialised regions
of central or eastern Europe.
Meteorology for Slackers
Airmass
Polar Continental Air
• Even in April or May, the North Sea is cold and
does little to modify the air mass, apart from adding
a little unwelcome moisture. Southern England is
particularly chilled by polar continental air masses.
Further north the airstream is less cold and the
wind is less strong.
Meteorology for Slackers
Airmass
Tropical Maritime Air
• Tropical maritime air usually approaches the British
Isles from the south-west.
• Its source region is the subtropical Atlantic Ocean,
typically the Azores area, although occasionally it
may come almost directly from the Caribbean.
• During its passage across the Atlantic, the air is
cooled from below as it passes over a progressively
cooler ocean, and so it becomes more stable.
• While it cools down, little of its moisture is lost. It
therefore reaches south-west England or western
Ireland almost saturated, giving dull, warm, overcast
weather.
Meteorology for Slackers
Airmass
Tropical Maritime Air
• On the coasts, sea fog is common in these tropical
maritime south-westerlies.
• However, if the cloud base of the stratus or
stratocumulus is several hundred feet, sea-level sites
may be saved from the fog, but on rising ground and
hills there may be fog and drizzle.
• Bodmin Moor, Dartmoor, Dyfed, western Ireland and
western Scotland can be shrouded in mild, damp
conditions whether it be winter or summer.
Meteorology for Slackers
Airmass
Tropical Maritime Air
• Further inland, in the summer half of the year at least,
the low stratus may be burnt off by the sun and it
could turn out to be quite warm, though still humid.
• In the lee of hills or mountain ranges, the clouds
sometimes break up and there is a lot of sunshine.
• Favoured locations like north Somerset, North Wales,
Northumberland and the Moray Firth can bask in
spring-like weather on a January day.
Meteorology for Slackers
Airmass
Tropical Maritime Air
• In a tropical maritime air mass, the nights are mild
and damp, especially in mid-winter.
• In December and January the overcast skies result in
there being little variation in temperature between
day and night.
• However, if there are light winds and clear skies, fog
may form inland overnight.
Meteorology for Slackers
Airmass
Polar Maritime Air
• Polar maritime air is the most common type of air
mass affecting the British Isles.
• The air has its source in the Canadian Arctic or the
Greenland area.
• It reaches the British Isles from the west or northwest after having swung around the western side of
a depression.
• As the cold air travels over the relatively warm sea,
it is warmed from below and becomes unstable.
Meteorology for Slackers
Airmass
Polar Maritime Air
• Unstable airstreams tend to produce convection,
and so cumulus clouds, cumulonimbus clouds and
showers are likely in polar maritime air.
• Other characteristics of the air are that it is cool
(especially in summer), fairly moist and associated
with good visibility.
• In winter, most of the convection is initiated over
the Atlantic, and showers hit the coasts, spreading
inland if the winds are strong.
Meteorology for Slackers
Airmass
Polar Maritime Air
• The Scottish and Welsh mountains often shelter
the eastern side of Britain, although, with a northwesterly wind, some showers sneak through the
Cheshire Gap to reach Birmingham and perhaps
London.
• With a westerly wind the winter showers can cross
Glasgow and central Scotland to reach Edinburgh
and Fife; others travel up the Bristol Channel to
affect Cardiff and Bristol.
Meteorology for Slackers
Airmass
Polar Maritime Air
• In spring and summer, convection clouds tend to
be set off inland by daytime heating.
• Now, the shelter of the western mountains is less
important, and showers or short-lived
thunderstorms can occur almost anywhere.
• At night the clouds disperse.
Meteorology for Slackers
Airmass
Polar Maritime Air
• After a low has crossed eastwards over the British
Isles, winds 'veer' (a clockwise change in wind
direction) to a northerly point, and true arctic air
may reach us.
•
This is sometimes referred to as arctic maritime
air. It is similar to polar maritime air but tends to be
more unstable, colder and drier.
• Consequently, showers of rain, snow, sleet or hail
often occur on northern coasts and over high
ground.
• Elsewhere there tend to be clear skies.
Meteorology for Slackers
Airmass
Returning Polar Maritime Air
• Returning polar maritime air, like polar maritime air,
originates in polar regions, but travels southwards
before turning north towards the British Isles.
•
The classic returning polar maritime airstream
occurs when a large depression is situated
somewhere to the north-west of the British Isles.
• Normally, once the associated weather fronts have
passed through, the British Isles are left in a northwesterly polar maritime airstream.
Meteorology for Slackers
Airmass
Returning Polar Maritime Air
• However, if the air reaching the British Isles has
travelled around the southern edge of the
depression and the winds are between south and
south-west, the air is designated as returning polar
maritime.
• The air is originally cold, but as it takes a long sea
track southwards across the Atlantic, the lower
layers become warmer, more moist and more
unstable.
• However, as it returns northwards, the lower layers
are cooled and become more stable.
Meteorology for Slackers
Airmass
Returning Polar Maritime Air
• This mixture of a stable layer near the surface and
an unstable layer aloft can lead to a wide variety of
weather.
• On exposed coasts and hills, the combination of
high moisture content and low-level stability can
lead to stratus clouds and hill fog.
• Sometimes, however, the unstable layer leads to
the formation of cumulonimbus clouds and
showers (and occasionally thunderstorms).
• Further inland a mixture of weather can occur stratus lifts and disperses and then suddenly gives
way to a heavy shower.
Meteorology for Slackers
Airmass
Returning Polar Maritime Air
• South-west England and Wales usually have the
first taste of a returning polar maritime airstream;
such airstreams are especially common in autumn.
• Further north and east, with some shelter from the
mountains, conditions tend to be better.
• East coast areas may well be quite warm, with only
broken convection clouds.
• At night, these areas are usually clear, dry and
cool.
• Moisture contents are quite high, especially near
southern coasts, but the clean air usually means
good visibility.