Understanding Weather Maps, Pressure Systems, Air Masses and Fronts

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 your 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 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).

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 strong pressure gradients around them, strong winds.

Air Masses

Most weather occurs because of the impact of either hot (tropical) or cold (polar) air masses, or from the effects of their mixing together.

Air masses are simply three-dimensional parcels of air covering vast areas of the earth’s surface.

A specific air mass will generally have similar properties of temperature and humidity throughout, from the surface right up to the troposphere.

There are six basic types of air mass:

• Tropical continental (Tc) – hot and dry in summer
• Tropical maritime (Tm) – mild or warm and moist all year round
• Polar continental (Pc) – very cold and dry in winter
• Polar maritime (Pm) – cold and moist mainly all year round
• Arctic maritime (Am) – cold and moist in winter
• Returning polar maritime (rPm) – chilly polar air modified by warmer seas

Fronts

It wasn’t until the early twentieth century that it was recognised that many weather features occur through the interaction between contrasting air masses.

A Norwegian meteorologist named Bjerknes proposed that low pressure systems form where contrasting hot and cold air masses meet.

He suggested four stages of low pressure development:

Origin (Infancy)

A warm air mass meets a cooler air mass.

Maturity

Warm air is forced to rise over colder, denser air, creating a warm sector.

Occlusion

Cold air catches up with the warm air, lifting it above the surface and mixing it out aloft.

Death of the System

Temperature differences disappear and the weather system gradually breaks up.

Weather maps identify:

• Warm fronts — red semi-circles
• Cold fronts — blue triangles
• Occluded fronts — triangles and semi-circles together

Typical Weather Changes During a Low Pressure System

The Warm Front Approaches

High cirrus clouds appear first.

Temperatures rise slowly.

Pressure steadily falls.

Eventually rain begins.

The Warm Front Passes Over

Cloud thickens and lowers.

Continuous rain develops.

Visibility may become poor with drizzle and fog.

Pressure levels out before slowly rising again.

The Cold Front Passes Over

Towering cumulonimbus clouds develop.

Heavy rain, hail, and thunder may occur.

Temperature drops suddenly.

Pressure rises quickly.

Behind The Cold Front

Heavy rain clears.

Cloud breaks up.

Some sunshine may return.

Pressure continues to rise.

A few showers may remain but conditions become fresher and cooler.