Weather part two

In the previous article I explained how the sun generates the earth’s weather by heating the planet unevenly, why we have seasons and why the Canadian Rockies are largely to blame for the large number of low pressure systems that hit the UK on a regular basis. In this issue I want to look at the weather we can expect when a depression and it’s associated fronts crosses over the UK, how we can recognise them on a weather chart and how we can predict their movements over the following days. I assume that the reader understands some of the terms from part one.

A depression is an area of low pressure that brings with it unsettled weather in the form of fronts (the barrier between warmer and colder air). The fronts force air to rise and cool down resulting in cloud formation; the higher the clouds the more streaky they become as they become ice crystals. The bigger the difference there is between the warm air to the south of the polar front and the cold air to the north of it, the deeper the depression will be and the more unsettled and windy the weather will be. The amount of rainfall is dependent on the amount of moisture that the depression has picked up and that depends on where the low pressure been circulating and what it is currently circulating over. Unfortunately for the UK it most frequently approaches from the atlantic so it has plenty of potential to pick up moisture! The cloud and ensuing precipitation is not uniformly distributed around a depression and the most significant rainfall always occurs along the warm and cold fronts.

Approaching warm front - As the warm front approaches, the temperature starts to rise, and the pressure steadily falls (there is less air sitting your head as air rises). The clouds at the warm front progress from thin, high-level cirrus clouds up to 1000 kilometres ahead of the surface position of the front,  to low dense stratus and cumulous clouds. The rain can extend 160-320 kilometres ahead of the surface front. High cirrus clouds are a good indicator that a front is near by but not that it is going to pass overhead. You will need to watch the development of the clouds over time or look at a weather map.

In the warm sector, (the bit between the fronts) -After the warm front passes, the pressure stabilises and a section of warmer and more humid air occurs. The amount of cloud is reduced as the clouds start to thin out and the precipitation is reduced. In a classic depression the weather is generally fine, with a little stratus or stratocumulus.

Approaching cold front - As the cold front approaches pressure begins to fall rapidly, but rises steadily as the cold front passes over. The air temperature starts to drop and large, towering, cumulonimbus clouds develop producing heavy downpours of rain and fierce squalls, sometimes with hail and thunder.

After the cold front -As the cumulonimbus clouds move away the pressure continues to steadily rise. A few showers may occur from some small cumulus clouds, but it generally becomes fine and cool behind the cold front.

Occluded front - The characteristics of an occlusion i.e. where all the warm air is lifted above the cold air, are similar to those of a cold front in that the rain belt is narrow.

High-pressure or Anticyclone sytems

A depression occurs because the the air is rising thereby creating low pressure, but at other places on the earth, warm air is cooling and falling, often over very large areas (over 1000 sq km).  This falling air means that there are more air molecules at the earths surface and hence an area of high pressure called an anticyclone is created. The air in high pressure systems spread outwards at the earths surface - in a clockwise direction in the northern hemisphere.  If we consider that rising air cools and forms clouds as the water vapour becomes droplets of water, we can reasonably deduce that sinking air does the opposite i.e. it warms and inhibits cloud formation.  Because of this anticyclones usually bring prolonged warm dry, sometimes cloudless or hazy weather in the summer...But not always!  When anticyclones are circulating over the sea (picking up moisture), the weather can vary from fine and sunny to overcast cloud. This cloud may be thick enough to give drizzle and may fall low enough to produce fog. If the moist air is pushed up over mountains, it can also rain. This happens most often during spring and is least frequently in autumn.

The absence of clouds has a significant effect in its own right. Under a cloudless sky it is beautifully sunny by day but freezing cold at night because there are no clouds to prevent the heat escaping. Because they contain lots of air molecules anticyclones are slow moving and often ‘sit’ over an area for a long time. This high pressure blocks the movement of depressions (blocking high) pushing them to the north of the UK or down over Spain.

How do depressions disappear?

Because anticylones contain a lot of air molecules (high air pressure) and because nature is always trying to even everything out (but can’t due to the input of the heat from the sun) the air rushes from areas of high pressure to areas of low pressure. This is what meteorologists called ‘filling’ of a low and it is this that results in low pressures weakening and eventually disappearing.

How to interpret weather charts

The lines on a weather chart that look like the contours on a map are called ‘isobars’, but instead of representing a height above sea level they represent an area of the same air pressure. 

High pressure or anticyclone - An anticyclone is drawn as a large area of widely spaced isobars, where the pressure is higher than surrounding areas.

Low pressure or depression - The classic representation of a depression is concentric rings around the crest of the ripple (Rossby wave) – like contours around a mountain.  The isobars often form a distinctive ‘thumb-print pattern with low pressure at the centre. The lower the pressure, the closer together the isobars will be to represent the deep depression. This is the same way that a steep mountain has more contour lines packed together. Close isobars mean the wind is moving very quickly from the high pressure to the low pressure. Winds blow anticlockwise in low-pressure areas their strength shown by the distance between the isobars.

Weather fronts – Cold fronts are marked by blue triangles on the line marking the boundary between the warmer air in front and the colder air behind. Warm fronts are marked by blue semi circles on the line marking the boundary between the colder air in front and the warmer air behind.The side of the front, which has the 'humps' or 'triangles' indicates the direction in which the front is moving. A front is marked by a single line marking its position on the earth’s surface. An occluded front has both triangles and bumps.

Is the occurance and subsequent movement of high and low pressure systems predictable?

Depressions unfortunately occur in 'families'. It seems that once a ‘Rossby’ wave has occurred, others follow in its path. The resulting depressions migrate across the Atlantic along the ‘Polar Front’ towards the UK. Sometimes as many as four or five mature depressions may make their way across the Atlantic, before a ridge of high pressure builds up over Europe to prevent any more advancing over the UK. alternatively the polar front disappears when there is not enough difference between the cold air to the north and warm air to the south.

The movement of a depression is not easy to predict, but using the following principles will help you:

  • The future movement of a depression is usually an extension of its previous track.
  • Depressions tend to move from areas of increasing pressure to areas of decreasing pressure.
  • The centre of the depression will move parallel to the isobars in the warm sector.

·      Depressions tend to move around large stationary high pressure areas

·      A depression with an occluded front tends to move to the left of its track.

·      High pressure systems have no definite path of travel and may linger for several days before their power is dissipated

Thunderstorms and Hurricanes

There are two weather occurances that don't involve the same principles as depressions or anticyclones - thunder and lightening. When parts of the earth are being quickly warmed large cumulous clouds can occur in isolation, created by the huge amount of warm air rising. These large clouds generate a lot of energy as they grow and may result in torrential downpours when they suddenly release the rain contained in them. This rainfall is circulating with the violent winds and as it rises it can freeze - fall- rise and refreeze resulting in tiny pellets of ice. When the particles of ice become heavy enough they fall as hailstones. If it’s hailing then you are under a large cumulous cloud then you must be very careful because it  can also create thunder and lightening. Lightning causes the thunder by the extreme heating and expansion of the air along it’s path.

Lightning kills around 73 people per year in the USA and every person in Britain has a one in two million chance of being killed by lighting each year.

Lightning is a large electrical spark caused by the movement of electrons in cumulonimbus clouds. The fact that 90% of it travels from cloud to cloud (sheet lightning), means that 10% is coming earthwards! It heads earthwards when the electrical differences between the cloud and the ground become large enough.  It can strike from great distances, up to 50 km from its origin.

 A simple way to estimate the distance of a thunderstorm and whether it is getting closer is to use the speed of sound. The speed of sound in air is 335 metres per second, which means that it travels about a kilometre in just over three seconds. To determine the distance between the lightning strike hit and your position, count the number of seconds between the lightning flash and the thunder and divide by three.


A hurricane forms when the warm sea heats the air above quickly enough for it to create a large rotating storm centred around an isolated area of very low pressure. The rising air is replaced by cool air around it and the rotation of the earth causes the rising column to twist. The ensuing vilolent storm may be 10km high and 650km wide and the winds can be more than 250km/hour. As it travels across the ocean, a hurricane can pick up as much as two billion tons of water a day and if the heat released by an average hurricane in one day could be converted to electricity, it could supply the United States with electricity for six months. Hurricanes occur between July and October in the Atlantic, Eastern and Western pacific north of the equator and off Australia and in the Indian Ocean between November and March. Although the UK is not affected by hurricanes it is sometimes receives the effects of the dying remnants of hurricanes that resemble very large polar front depressions.

In the final article on weather I will look at how the major mountains of the world create their own weather, make depressions worse and what happens as winter approaches.



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