One of the most important things anyone wanting to understand the weather can learn is how to recognise the salient features shown on a weather map.

The first thing a meteorologist will draw up on his or her chart (although nowadays nearly all maps are generated automatically by computers) are lines of equal pressure from observations. These are usually in millibars (mb) or sometimes inches.

We call lines joining points of equal pressure isobars. They are helpful because by creating them we can start to identify areas of high and low pressure at both the surface and aloft, that control our weather pattern. Pressure values are generally corrected to Mean Sea Level Pressure (MSLP) so that a standardised level can be shown, otherwise the observers height would affect the readings, as pressure decreases with altitude quite substantially.

The chart below shows a map with pressure readings taken from stations on land in the United Kingdom and some from ships at sea. Can you locate the areas of high and low pressure on it? (Note: 01 = 1001mb etc)

We can see that the lowest pressure appears to be over the Northern Isles of Scotland, at around 998mb. The highest pressure appears to be near the Isle of Scilly, in the far south west, at around 1030mb. Already, knowing that higher pressure generally gives better weather, we have some idea that the best weather is likely to be in the south of the country.

In between the two extreme values we can plot isobars (usually at 4mb intervals). These are helpful because they can tell us something of both the direction and strength of the wind in any given location. Generally, where the isobars are closest together they will tend to indicate the areas of strongest winds. Conversely, the farther apart they are the lighter the winds. A pressure gradient zone will develop between areas of high and low pressure on the earth’s surface and this pressure gradient results in a net force that is directed from high to low pressure anywhere in the earth’s atmosphere, whether on or above the surface.

We can also tell something about the direction of the winds that occur as a result of this gradient force, when we have plotted the isobars on our chart. A nineteenth century meteorologist named Buys Ballot noted that the winds blow almost parallel to the isobars and devised the following simple rule: if you stand with you back to the wind in the northern Hemisphere, low pressure will always be on your left (in the southern Hemisphere the opposite will apply).

Areas of high and low pressure, air masses and weather fronts

High and low pressure areas

As previously mentioned, there are large areas of the earth’s surface where the winds are light and the weather generally fine as warm air gently descends to the surface.

As this warm air descends it expands but as there is a finite area that such airmasses can inhabit the air becomes ‘squeezed’ and (similar to the effect that occurs when we blow warm air into a balloon in a confined space) its pressure rises. This effect creates large areas of high pressure, which we call ‘anticyclones’. They can be identified on a weather chart as a large area of widely spaced isobars, with the winds gently blowing, clockwise, around the high pressure (in the Northern hemisphere; the opposite is true in the Southern hemisphere as a result of the earths rotation due to the coriolis force- see box insert for an explanation of why this occurs).

An area of low pressure (a ‘depression’) is basically the opposite of an anticyclone. On the weather map it shows us an area where the air is rising and cooling, hence contracting, and producing lower pressure than the surrounding areas. In the Northern Hemisphere winds blow around depressions in an anticlockwise direction. Areas of low pressure are usually associated with poor weather; cloud and rain and as a result of the strong pressure gradients that tend to occur around them, strong winds. We can see these on our weather map (image 2) as areas where the isobars are tightly packed together, such as over the North Sea.

Air masses

As mentioned initially, most of the weather that the world experiences at the surface occurs because of the impact of either hot (tropically based) or cold (polar based) air masses, or as a result of the effects of their mixing together. We call the areas where the air stagnates and acquires these basic characteristics source regions. These lie at the poles and the subtropics.

Air masses are simply three dimensional ‘parcels’ of air covering vast areas of the earth’s surface. A specific air mass will generally have very similar properties of both temperature and humidity throughout, from the surface right up to the top of the troposphere (the name we give to the three dimensional area, up to about 5-9 miles into the atmosphere, where most of the earth’s weather occurs). Where two air masses meet however, there is usually another vertically stacked transitional zone with quite large differences in temperature and humidity, within a relatively short distance. It is therefore easy to appreciate that as this zone of change passes over you, you may experience some rather rapid variations in the weather. The journey that an air mass takes as it is moved across the globe will often change its nature, slowly but surely. As a result air masses have distinct differences in their temperature, the amount of moisture they carry with them and more importantly perhaps, different rates of temperature change vertically and different levels of stability, all of which affect the weather they eventually give us.

An air mass can be rather cold or very warm, quite dry or rather moist, with different variations of these types, depending on the original source and subsequent journey. We need to remember that as well as moving horizontally however there are vertical changes that can occur to any air mass. For instance apply heat from below and a warm (tropical) air mass may become unstable. However, if it cools as it moves north (say up over the mid Atlantic Ocean from the warm tropics) then it may become more stable. Likewise, if a cold polar flow travels over cold dry surfaces the gradual chilling is also likely to stabilise it, however travel across warmer surfaces (perhaps relatively warmer seas like those around the British Isles) may make it more unstable.

There are six basic types of air mass that we regularly experience and we will consider how a few of them affect our weather in the British Isles below.

They are:

  • Tropical continental (Tc) –hot and dry in the summer

  • Tropical maritime (Tm) – mild or warm and moist all year round

  • Polar continental (Pc) – very cold and dry in the winter

  • Polar maritime (Pm)- cold and moist mainly all year round

  • Arctic maritime (Am)- cold and moist in the winter

  • Returning polar maritime (rPm) –chilly polar air much modified by the warm sea

Fronts

It wasn’t until the early twentieth century that it was recognised that the development of a lot of weather features occur through the direct result of the interaction between contrasting air masses. A Norwegian named Bjerknes founded a school of Meteorology in Bergen and persuaded the Norwegian government to install a network of surface observing stations to provide data for his meteorological studies. As a result in 1918 he published a theory that accommodated the development and decay of low pressure systems across the middle latitudes of the globe.

He suggested that where the contrasting hot and cold airmasses meet in these latitudes low pressure centres are formed, along with a series of zones and boundaries, that initially separate the air masses and then eventually mix them together. Whilst this may process may sound like common sense to us today it was a revolutionary idea when first postulated. However, further study of the processes of weather, shown by using regular weather maps, convinced others that he was absolutely correct.

Bjerknes suggested that a four stage process for the development of low pressure areas as they moved relative to the earth’s surface and coriolis force and called the zones of interaction ‘fronts’, a term derived from an analogy to the Great War’s zones of conflict.

 

The first stage he suggested (see diagram) was its origin (or infancy). A warm air mass, such as tropical maritime or tropical continental mass meets a cooler air mass, such as polar maritime or polar continental.

The second (maturity) occurs as the warm air in the tropical airmass is forced to rise, spiralling up (in an anticlockwise manner) over the denser colder airmass. As a result of this process a three dimensional zone or sector of warmer air develops.

The third phase (occlusion) occurs as the air masses then occlude and the warm sector disappears. This occurs because of the differential speeds at which warm and cold air are able to travel across the earth’s surface. The colder denser air is able to nose more rapidly into the lighter warmer air, typically at speeds of around 30 knots. However ahead of it the warmer air is over riding rather colder denser air, ‘flattening’ it still further as it does so, but the process is a slower one, typically only 20knots. Eventually then the cold air catches up with this warmer air, lifts it up above the surface and gradually mixes it out aloft. This area is called an occlusion or occluded front.

We now add and identify these areas on our weather map as warm fronts, shown as bold lines with red semi-circles or humps; cold fronts, as bold blue lines with triangles; the direction of the hump or point of the triangle indicating the direction in which the front is moving. We also identify areas where the process is more or less complete as the occluded front, showing both triangles and humps side by side.

The final process, the death of the system occurs as with all the warmer air having risen and cooled, the differences in temperature have been equalled out, and the occluded front then gradually breaks up.

Such processes often occur not by themselves but in ‘families’. Once the process starts it may well continue whilst the conditions are right and we see chains of depressions with their associated fronts, strung right out across the Atlantic; a major spawning ground for this type of weather process, especially in the winter. Such charts in winter, with three or four depressions strung out across the Atlantic are the basis of our classic British ‘wet and windy’ winter weather pattern.

The classic low pressure system will always produce the same sequence of weather changes as it passes across you.

High cirrus clouds may occur in long feather-like streaks (‘mares tails’) which is the warm air approaching aloft at around 20,000 ft (6500m). As the warm air lowers, the temperatures slowly rise, and pressure steadily drops. Eventually it starts to rain.

The warm front passes over

The amount of cloud increases and the cloud base keeps falling as does the pressure. Altostratus and then nimbostratus clouds develop. The overcast dull grey sky produces continuous rain.

 Eventually the warm air reaches the surface and the rain may lighten or stop. In winter especially, drizzle may fall with poor visibility and fog. Pressure levels out but then as the cold front approaches, starts to slowly rise.

The cold front passes over

 Large, towering cumulonimbus clouds develop, rising up to 25,000 ft, as the cold front itself passes over. This is the area where the cold air is cutting under the warm air. The process produces instability with heavy downpours of rain and sometimes hail and thunder. The temperature falls suddenly and pressure may rise quickly for a time.

Behind the cold front

 The heavy rain stops and the clouds break up, perhaps allowing some sunshine again in daytime. The skies can look dramatic as the cumulonimbus cloud is revealed as it moves away. The pressure continues to steadily rise. A few showers may occur from some puffy cumulus clouds but it is generally fine and notably fresher and cooler.