Or, is it worth skiving off tomorrow?

Or, is it worth skiving off tomorrow?

METEOROLOGY FOR SLACKERSMETEOROLOGY FOR SLACKERS

• 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 SlackersIntroduction

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 SlackersIntroduction

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 SlackersCoriolis

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 SlackersCoriolis

• 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 SlackersGradient Wind

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 SlackersPressure Systems

• 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.


Low Pressure


Forecasts





Bracknell



Bracknellhttp://meteonet.nl/aktueel/brackall.htm

http://www.itadvice.co.uk/weatherjack/charts-all.htmlhttp://www.airworks.co.uk/links.htm



ExpertWeatherOnline

Isotachs:

http://expert.weatheronline.co.uk/is10_frame.htm






Meteorology for SlackersSatellite Imagery

http://www.metoffice.gov.uk/satpics/latest_uk_vis.jpg

Meteorology for Slackers

http://www.metoffice.gov.uk/satpics/latest_uk_ir.html

Satellite Imagery

Meteorology for Slackershttp://130.92.54.203/rsbern/noaa/dw/realtime/current/n1bcurr.jpg

Satellite Imagery


http://www.sat.dundee.ac.uk/abin/browseleaf

Satellite Imagery

Requires registration










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!

Topography

Is the day likely to be any good? Don’t forget about soundings!

Meteorology for SlackersTephigrams

• 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.


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.


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.


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.


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 followThe 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


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.)


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


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


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.

inversion

inversion

inversion

Given this environmental line


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.


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 climbMildly buoyant, climb poor

Still fairly buoyant, climb good

Very buoyant, accelerating fast, hang on to breakfast

Still very buoyant, acceleration moderating


Where can I get one?• Noaa -

http://www.arl.noaa.gov/ready/cmet.html?• Met Office - Herstmonceux or Camborne






Tephigrams in Summary• A graph of air mass properties with height.• In the real world plotted on a variety of

different axes


Is the day likely to be any good?

What about the airmass?

Meteorology for SlackersAirmass

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.


Tropical Continental Air• Tropical continental air usually comes with south-

easterly 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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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.


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 north-

west 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.


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.


Polar Maritime Air

• The Scottish and Welsh mountains often shelter the eastern side of Britain, although, with a north-westerly 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.


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.


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.


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 north-westerly polar maritime airstream.


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.


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.


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.


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Or, is it worth skiving off tomorrow?