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It pays to understand the weather systems, particularly when out at sea.
Cyclones & Anticyclones
If you were to freeze the world and remove the clouds you would see a vast landscape of mountains and hollows, ridges, valleys and cols overlaid by two or three vast meandering rivers that in places would be narrow and obviously running very fast. Unfreeze this picture and you would see the air circulating around the centres of the hollows and mountains. Hence Cyclones and Anti-Cyclones.
A Cyclone (called a depression or low) would, in the Northern Hemisphere, rotate in an anti-clockwise direction and an Anti-Cyclone (or high) would rotate in a clockwise direction. In the Southern Hemisphere they would rotate in the opposite direction.
See Cyclone / Anticyclone Animated Display
Also in an anti-cyclone, the isobars will be further apart and often sandwiched between two depressions.
- Good weather is often associated with High Pressure (anticyclone)
- Bad weather is often associated with Low Pressure (cyclone),
In a newly formed depression (cyclone, or low) this spiraling motion is shallow and would extend upwards only 1,000m and would move 'downstream' being steered by the 'river' of air above it. In an older depression the spiral may be 10,000m to 15,000m and the 'river' would meander around it.
There is a direct relevance between a contour map of the earth with lines of equal height, and a weather map with its lines of equal pressure (isobars). The mountains are called highs and the hollows lows, the valleys troughs, and the ridges and cols keep their names.
In high pressures, there is a gradual descent of air, and in the low pressures the movement would be slightly faster and somewhat uneven. It is the ascent and descent of air that really makes weather. e.g. rising air cools, and as it cools the air condensates and clouds form and then rain falls when the dew point is finally reached. With descending (or subsiding) air, it warms as it falls, the cloud disperses and we have fine weather.
Understanding this means we can now understand the weather in ridges and troughs. Ridges have the same character as anti-cyclones with gradually subsiding air and fine weather, while troughs are characterised by rising air, cloud and rain. The ascent of air in a depression is often concentrated along the troughs; hence they are bands of thicker cloud and heavier rain with relatively bright weather either side. The heat energy that is released as the rain falls contributes to stronger winds in troughs.
The rate of movement of air up and down in lows and highs is so slow it is virtually impossible to measure directly. It amounts to a few metres a day - very different from the much more rapid upward motion found in cumulus and particularly in cumulonimbus clouds, where rising air currents at speeds of 60 knots have been encountered.
A particular form of trough is a front, a boundary between air masses of different characteristics. A front is only found in a trough of low pressure but the majority of troughs are not fronts!!!. Fronts are relatively rare events. In one month several may pass yet two months can also pass by without a single sighting.
On a weather picture, a Warm front has semi circles along it and a Cold front has Triangles.
Most common UK weather system is a depression and is born in the Atlantic along the line of the polar front. The warm air from south drives a wedge into cold polar air from the north, then the warm air is forced to rise over cold air cooling as it does.
It is the warm air cooling that forms water vapour (clouds), this is called a warm front (rain and high winds). The cold air tries to undercut the warm air forming a cold front (heavy showers and strong north westerlies).
There is more energy in a depression than in a hurricane but the hurricane is more concentrated.
The Life of a Depression
Depressions affecting Western Europe develop over the Atlantic where cold Polar air meets warm Tropical air, then moves East steered by the strong winds aloft. This boundary is known as the Polar Front as show in this picture.
The reason there is always cloud here is that the warm likes to rise over the denser cold air, and the cold air conversely pushes under the warm, lifting it off the ground.
If it is moving so that warm air is displacing cold air it is called a warm front . If the cold air is displacing warm, it is called a cold front . There is therefore warm air behind a warm front and cold air behind a cold front.
Eddies prefer to form close to the earth's surface with all the energy available, latent heat, stability etc, and the first sign of a new depression is normally a wobble in the surface wind blowing near the polar front.
Within a matter of hours we have an advancing boundary of warm air: The warm front, ahead of the new low, and an advancing boundary of cold air behind it. The cold front, each with it's typical cloud formations as shown below
NB Notice the wind arrows do not follow the isobars but actually point in by as much as 15 degrees as the air moves from high to low (the pressure gradient).
The following two pictures show a vertical cross-section through a stage 3's Warm and Cold front.
See Fronts Animated Display
As this low develops, warm air increasingly rises over the cold air ahead of it, and the cold air to the rear will increasingly undercut the warm air, in each case giving more cloud, more rain, and more latent energy to help drive the strengthening winds
Day 1
Day 1
The pressure falls, particularly along the fronts and in the region of the centre. Hence the pressure gradient increases and the wind strengthens. As the deep troughs develop we will always have stronger pressure gradients and therefore stronger winds that the shallow troughs. The new wind is always guided by the wind above it and may achieve speeds of 60 knots or more.
The Jet Stream is the name of the air systems that blows from USA to England with speeds of up to 250 kph. |
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Day 2-3
The distortion of the surface wind field extends steadily upwards. As the upper wind field becomes distorted the depression slows down and typically turns to the left - but this depends on what the uppers where initially doing! At the surface for a given pressure gradient, the cold air moves faster than the warm air, the cold air behind the cold front gradually pushes the warm air upwards and out of the way.
The cold front catches up with the warm front and we are left at the surface with an occluded front or occlusion. The vertical slope of this occlusion will depend on whether the cold air behind the cold front is colder or warmer than the cold air originally ahead of the warm front.
The main troughs of the depression will continue to coincide with the fronts, but subsidiary troughs of low pressure will develop, particularly in the cold air on its left hand side. These will be characterised by bands of thicker cloud and probably showers. Between the subsidiary troughs minor ridges may occur characterised by mainly clear skies. |
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Day 3 - 6
After 3 to 4 days of growth and development the depression will be large and slow moving with its centre usually somewhere between Iceland and Norway as this picture shows.
The fronts become increasingly twisted around the centre and the cold front will trail away to the south-west or west, slotting into a trough which becomes shallower as you move away towards higher pressure. Typically the next stage is for a small wave depression to form on this cold front (which is still the polar front) usually between 300 and 500 miles front the centre of the parent low. It starts life just as the parent low did but frequently runs into the parent low and merges with it. Sometimes the new wave depression will deepen even more than the parent low, and take up a position to the south or south-east of the original centre.
Depressions over Time
Front |
Stage |
Wind |
Cloud |
Rain |
Visibility |
Pressure |
Dew Point |
Warm Front |
Front approaching |
increases and backs |
sequence of cirrus, cirrostratus, altostratus, nimbostratus and stratus |
becomes heavier and more continuous |
deteriorates slowly as rain gets heavier |
falls at increasing rate |
- |
| As it passes |
veers |
nimbostratus |
stops or turns to drizzle |
deteriorates |
stops falling |
rises |
| In warm sector |
direction steady |
stratus, stratocumulus |
occasional drizzle or light rain |
moderate or poor, fog likely |
falls if depression deepening otherwise it is steady |
little change |
Cold Front |
Front approaching |
backs and increases close to front |
stratus and stratocumulus thickening to nimbostratus |
heavy rain near the front |
moderate or poor, perhaps fog |
- |
- |
| As it passes |
sudden veer often with squall |
cumulonimbus |
heavy rain, perhaps hail and thunder |
poor in rain |
sudden rise |
sudden fall |
| In cold air behind it |
probably backs a little then direction steady, stronger and gusty |
often total clearance, cumulus develops |
usually fine for an hour or two then showers |
very good |
rise gradually, levels off |
little change |
The Passing of a Depression
(click picture to enlarge it)
If a low pressure was passing over us at point A we would experience:
| Point A |
Point B |
Point C |
As Warm Front Approaches
- Wind increasing and backing
- Cloud building and thickening
- Rain becoming heavier
- Air pressure falling
- Visibility deteriorating
Warm Front Passes
- Nimbostratus clouds
- Rain turns to drizzle
- Air pressure steadies
- Wind veers
- Visibility is poor
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In the Warm Sector
- Wind steady
- Pressure steady
- Occasional showers
- Poor visibility.
As the cold front passes
- Wind very squally and veers
- Cloud thickens
- Heavy rain
- Air pressure falls near the front then suddenly rises
- Visibility poor
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Behind the cold front:
- Strong and gusty wind
- Clear skies for a while
- Air pressure steady
- Visibility good.
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Depressions pass at different speeds usually from 6 to 24 hours. It is usual for the wind to die down immediately after the rain squall only to rise again as the next one approaches. As the cold front passes the temperature drops dramatically.
Winds tend to blow anti clockwise around the centre of the depression in the Northern Hemisphere and clockwise in the Southern Hemisphere. Therefore winds can be predicted: Start off North Westerly, Then move to Westerly, Then South Westerly, South, and finally South Easterly.
Wind Aloft & Wind Sheer
Sometimes the wind aloft varies greatly in direction and speed with the wind on the ground. This gives us two very simple rules (the following applies in the Northern Hemisphere, the opposite applies in the Southern Hemisphere:
A wind veering with height means advancing warm air and normally deteriorating weather.
A wind backing with height means advancing cold air, and normally improving weather.
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