A collection of facts pertaining to weather

Earth Atmosphere
Weather and Clouds
Clouds
Wind
Beaufort Wind Scale
Wind Chill
Weather Chart terms
Form filling

Anecdotes.

1)	Red sky at night, Shepherd's delight
	Red in the morning, shepherd's warning

Most European weather comes from the west. Red sky at night means that the sun setting in the west is shining onto cloud in the east. These clouds have passed and as the sky is clear, good weather is coming. Red sky at morning means the sun rising in the east is shining on cloud in the west. These clouds are the approaching weather and so possibly mean rain is coming.

2)	Long foretold, long passed,
	Short notice, soon past.

This is indicative of how fast the wind is bringing the weather with it (I assume. Know better ??
mail me !

3)	Rain before seven,
	dry before eleven.

Usually, a weather front takes about 4 hours to pass over

4)	If the sun sets clear as a bell,
	Easterly winds you may foretell.
	If the sun behind a bank does set,
	Westerly winds you are sure to get.
	When the rain's before the wind
	Topsail halyards you must mind
	If the wind's before the rain,
	soon You'll make plain sails again.
	If the wind shifts against the sun,
	Trust it not, for back t'will run.

The sayings in item 4) I have no explanation for. Again, if you can elucidate, please
mail the site.

Atmospheric details

The Atmosphere is all around us.

The diagram indicates just how much of it there is !. There are five distinct layers. These are, in order of increasing height:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
5. Exosphere

At approximately 500km high, the exosphere merges into outer space.
Above the troposphere (ground level to 10km), the stratosphere reaches 50km above the earth surface. There is insufficient air to breathe and the temperature is constant at -60° Centigrade so no clouds form here.

The stratosphere is the area which keeps out the sun's harmful Ultra Violet radiation thanks to the Ozone layer at between 20 and 35 km. Without it we would either fry or grill to extinction. A sobering thought.

The troposphere is the only area of concern to us, the canoeist, as it is the only place where weather is found !!.

The air blanket, with its clouds in the lowest layer, keeps warmth in at night thus preventing too large a temperature gradient between day and night time. Clouds also contain water, essential to life and the basis of our sport (once the clouds have finished with it !)

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Weather systems

The break up of a spell of settled weather can be forecast by recognising very high level long wispy clouds, known coloquially as Grey Mare's Tails (Cirrus clouds). Similarly, At night in a beautiful clear sky, if you see the moon wearing a "Halo" of silvery grey light, expect rain through the night as the halo is formed by the action of moonlight diffracted through the atmospheric moisture.

Cloud formations at dawn and sunset give an indication of weather trends. Soft clouds indicate fine weather and light winds. Hard clouds indicate strong winds to come

Clouds are classified by height.

• High cloud - above 25000 feet.
• Low cloud - below 8000 feet.

The clouds you see give very good indicators of the weather both present and to come

Cirrus cloud. Grey mares tails. Ice crystal formations heralding the onset of a depression. Feather like in appearance indicating bad weather is coming from the west, particularily if they are headed north - south.
Cirrostratus cloud. Over 20000 feet and appears as a thin white veil of continuous cloud. If seen in the wake Cirrus, combined with a fall in barometric pressure, expect deteriorating weather.
Altostratus. Above 19000 feet Thickish grey continuous sheet of middle level cloud nicknamed "Watery sky". It is an indication of rain being on the way.
Stratus. Lowest clouds of all, associated with moist warm airstream and can bring fog.
Nimbostratus. 3000 feet, grey dark low intimidating cloud carrying continouos rain or snow, poor visibility accompanies these clouds.
Cumulus. These clouds are a welcome sight. Fluffy white light formed clouds formed by thermal convection. A promise of fair weather and good kayaking.

Barometers measure air pressure. Steady readings mean fairly settled weather conditions irrespective of whether pressure is low or high.
A slow rising barometer is an indication of good weather

A slow falling barometer is an indication of deteriorating weather conditions.
Rapid change of barometer reading indicates strong winds and when the barometer reading is dropping rapidly, stormy conditions are likely to follow

Average air pressure is 1013 mbar (760mmHg). Above this indicates High Pressure, and below indicates Low Pressure.

A few facts which help our understanding of weather

1. All air contains some water vapour.
2. Warm air holds more water vapour than cold air.
3. When warm air is cooled the water vapour contained condenses to water again.
4. The amount of water vapour contained within air is directly attributable to the surface it has travelled over. i.e. over water or land.
5. 2 main ways for air to cool:
   
   1. Air rises into the upper atmosphere, becomes less dense therefore spreading its heat over a larger area.
   2. Air comes into contact with something cold, i.e. land.
6. 3 ways cloud forms:
   
   1. Slow lifting of warm air over the top of cold air.
      (A warm front)
   2. Rapid lifting within convection currents.
   3. Forced by landscape features such as hills/mountains.
      Termed Relief (Orographic) Rain
7. Not all cloud produces rain.
   Low, thin layer cloud will not produce rain but low heavy cloud which extends far upwards with ice crystals at the highest level, will produce heavy rain
8. Cold air is more dense than average, warm air less dense than average. This results in a particular volume of cold air being heavier than an equivalent volume of warm air; i.e. greater density
   (greater pressure).
9. Cold air
   High Pressure is mainly produced at the North and South poles. Warm air
   Low Pressure originates at the equator and other tropical areas.
10. This difference in pressure between hot and cold regions of the earth causes cold air (High Pressure) from the poles to be drawn towards the warm (Low Pressure) regions in an attempt to equalise the pressure differences. Where these moving airmasses meet is described as the "Polar Front" and covers the globe.
11. The rotation of the earth causes moving air to be deflected towards the right in the northern hemisphere and to the left in the southern hemisphere.
12. An airmass on the move is called an Airstream.
13. A high pressure polar airmass moving toward the equator will be a cold airstream which warms up at ground level. Warmed air rises inducing convection currents which produce uneven cloud cover.
    This is termed an "Unstable" airstream.
14. A subtropical airmass moving toward the pole (north or south) will cool down progressively at the surface, leading to condensation in the form of a blanket of low cloud, extreme cases of which lead to sea level cloud and extensive fog.
    This is termed a "stable airstream".
15. Often a warm airmass will penetrate the cold airmass of the polar front. This warm air ingress heralds the formation of a "Depression"
16. A depression during its progression, will be surrounded by cold air. The leading edge of the depression is termed the "Warm Front" and the trailing edge termed the "cold front". During the lifecycle of the depression the cold front eventually catches up with the warm front, squeezing the warm air away from the earth surface, heralding the end of the depression.
17. Most weather changes in our latitude are brought about by an endless succession of depressions with occasional high pressure systems in between.
    Depressing isn't it !
18. Depressions are of greatest significance to canoeists and other sea users due to the unpredicatability of associated windspeed and direction. In a severe depression Gale force winds (Force 8 and above) along with major changes in wind direction occur. The closer to the centre you are the worse the weather will be !.
19. The visible cloud signature for an approaching warm front can be seen in this diagram.
    Town A has clear, cold weather, with a few high cirrus clouds.
    Town B has thicker, lower clouds and the sky is grey.
    Town C has dark altostratus cloud with slight drizzle.
    Town D has nimbostratus clouds with heavy rain.
    When it stops raining it will be warm and humid. It will take 12 hours approximately for each town to experience the weather from the town before.
    
20. Signs of an approaching depression:
1. Cirrus clouds form Grey Mares Tails  The faster the cirrus, the stronger will be the depression with likelihood of high winds. 12 -24 hours before the depression hits. Barometer starts to fall.
2. Cirrostratus forms next. Barometer continues to fall. On a clear night a "Halo" can be seen around the moon. The wind will shift anticlockwise.
   back If the depression is severe, the cirrostratus thickens quickly into altostratus.
3. As cloud continues to thicken, Nimbostratus forms, decreasing visibility is evident, the cloud base becomes much lower and heavy rain falls. Pressure continues to fall and the wind continues to
   back
4. As the warm front passes over, the pressure stabilises and the wind will turnclockwise again.
   veer If the viewer is well away from the centre of the depression, the cloud may begin to break up again.
21. The visible cloud signature for an approaching cold front can be seen in this diagram.
    Town A has warm, moist air.
    Town B has large, cumulonimbus clouds with thunderstorms and torrential rain.
    Town C has rain from nimbostratus clouds.
    Town D has bright, cool air.
    It will take 12 hours approximately for each town to experience the weather from the town before.
    
22. Signs of an approaching cold front
1. Pressure may start to fall and wind may shift anticlockise a little.
   back These being the only sign of the impending front which will appear quickly.
2. Cumulonimbus cloud formations will bear down upon the observer caused by convection currents because cold air is sweeping in under warm air which lifts rapidly producing heavy showers and gusty squally conditions.
3. The next sequence of events is basically the reverse of a warm front. After the cumulonimbus, nimbostratus forms being followed by cirrus cloud as the front passes past.
   This all happens much more quickly than the passing of a warm front.
4. Wind veers, often sharply, and the pressure will start to rise. Clouds disperse and rapid improvement in visibility occurs.
   When a cold front passes at night, the "red sky at night" adage can be seen to work!.

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Wind

Sea breeze is a well known phenomenon which affects all coastal areas where typically we all go sea kayaking !
In the first of two images below, there is a wind depicted blowing onto the shore from seaward. This is caused when sunlight shines onto land. The land absorbs the heat of the sun causing air above it to heat up and rise upwards (Hot air rises). This updraught draws colder air in underneath it as it rises, the new cold air in turn being warmed by the land causes a cycle to occur, the net sea level effect of which is cold air being drawn onshore. This is noticeable to kayakers as an onshore breeze at sea level and is most noticeable on strongly sunny days.

The second of the two images depicts the opposite effect.
At night, heat is lost more rapidly from land than from sea so the net effect is that air above the water remains hotter than the land through the night so air rises above water drawing air from land out to sea. The kayaker notices this as an offshore breeze.

Wind directions

Northerly winds :

blowing FROM the North west through East, cause the barometer to rise giving less wind and drier weather.

Southerly winds :

blowing FROM the southeast through West, cause the barometer pressure to fall giving more wind and wet weather.

Southerly/Westerly :

(FROM the south west) winds in the UK are moist and warm due to having crossed the Atlantic Ocean where moisture has been absorbed into the atmosphere. On landing ashore the moisture precipitates as cloud or as rain.

Northerly/Easterly :

(FROM the north or east) winds generally bring cooler, clearer weather from the lands of the north.

Jet Streams

The Jetstream is a series of high altitude winds in both the northern and southern hemispheres. Both streams blow west to east right round the world and can be of speeds 100 to 300 kph.
The Jetstream travels westerly in our area (United Kingdom) causing depressions to move from west to east or north east; i.e. our weather comes from the west predominantly.

Below is a table of values that make up the Beaufort Scale

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A guide to wind chill

the table below can be used in conjunction with the above table to give a guide to sea condition and associated wind chill expected.

Windchill chart

Wind chill chart	Outside Temperature
Wind Speed (kph)	7°C	4°C	2°C	1°C	-4°C	-7°C	Wind Chill Temperature
8 (Force 2)	6°C	3°C	0°C	-3°C	-6°C	-9°C
16 (Force 3)	1°C	-2°C	-6°C	-8°C	-12°C	-16°C
24 (Force 4)	-2°C	-5°C	-9°C	-13°C	-19°C	-21°C
32 (Force 5)	-3°C	-7°C	-11°C	-16°C	-19°C	-23°C
40 (Force 5/6)	-5°C	-9°C	-13°C	-17°C	-22°C	-26°C
48 (Force 6)	-6°C	-11°C	-14°C	-19°C	-23°C	-26°C
56 (Force 6/7)	-7°C	-11°C	-16°C	-20°C	-24°C	-29°C

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Weather map - wind -

Isobars: are marked as black lines on a weather map indicating areas of equal pressure within a system. The closer the isobars are, the greater is the pressure differential, and the stronger the winds will be in that area.
You can see a daily isobar chart in the Glasgow Herald newspaper every day if you live in the author's area.

Glossary of terms as used by a weather forecaster in describing a days weather. The shipping forecast on Radio 4 uses the terminology listed.

Imminent :

You can expect the weather described to reach you within the next 6 hours.

Soon :

You can expect the weather described to reach you within the next 6 to 12 hours.

Later :

You can expect the weather described to reach you more than 12 hours away.

Back(ing) :

Wind direction will change in an anticlockwise direction.

Veer(ing) :

Wind direction will change in a clockwise direction.

Cyclonic :

Anticlockwise wind direction as associated with a LOW pressure system.

Anticyclonic :

Clockwise wind direction as associated with a HIGH pressure system.

Occlusion :

This is a specialised occurrance where two weather patterns collide.
More detail when I know it myself.

Squalls :

A ridge of high gusty wind which precedes a weather system where cold air blasts down from the front of a cloud layer as it approaches. These winds can be FIERCE.

Depression :

is the name given to a low pressure system in which bad weather is prevalent. Wind in a depression is cyclonic, anticlockwise in direction. The depression will travel reliably from south west to north east and the barometer will drop as it passes through where you are. Cirrus clouds initially formed will have changed to Cirrostratus, giving a halo around the sun/moon, followed by deepening cloud to stratus layer accompanied by freshening winds which back towards the south east. Stratus thickens to Nimbostratus with the wind veering South west again. By this time it is teeming down.A cold front often accompanies the tail end of a depression causing wind to increase violently and become gusty. An area to avoid by careful planning !.

Anticyclone :

is the name given to a HIGH pressure system in which good weather is prevalent In a high pressure system the isobars on the map will be well spaced out depicting light winds, few clouds and good visibility There is no defined direction attributable to an anticyclone, it drifts aimlessly. A down side of the clear skies is that the earth loses its heat through radiation rapidly which causes the air above to cool also resulting in the condensation of water vapour manifesting itself as FOG. Sea fog is caused when warm moist air leaves land, contacts the cool sea and condenses it's water vapour. On a warm day, passing through an area of sea fog can leave you surprisingly cold.

Line squalls :

are powerful squalls which can easily capsize the unwary water goer who is caught unprepared. Signs of approach are firstly a line of low black heavy clouds, usually from the North West, with a rapid series of gusts before the clouds arrive. Very strong winds, heavy rain or even thunder may be encountered as it passes.

Thunderstorms :

Air over rocky or sandy surfaces will heat rapidly in sunlight and rise through convection. Cooler air is then drawn in underneath to take its place and this spirals inwards. This causes a great deal of movement of air and subsequent friction between the thermal layers. Violent thermal effects which cause friction between the two air layers as they move create static electricity within the clouds, discharging as lightning with the associated noise as thunder. Winds are violent within this system and unpredictable in direction. Rain can also be very heavy in bursts.

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