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Jun 26 2011
Last update:

synoptic weather charts

A Rough Guide to Understanding Pressure Charts (synoptic charts)

Pressure Chart - Midday

UK - Satellite Image

Pressure Chart - Midnight
 
Images from http://www.metoffice.com and http://www.weather.com

How to read & understand what the above charts are showing...

Pressure charts plot air pressure at a given altitude. eg: sea-level.
The charts indicate the strength of both high and low pressure areas (often marked with a H or L) and plot the lines of equal pressure (isobars).

A pressure, or synoptic, chart can tell a Meteorologist a lot about the weather situation at a glance - where high or low pressure systems are, how close together the isobars are, where and what type weather fronts they are.

From the above images you can see what the effects are from the isobars compared to the positions of the clouds...

High & Low pressure
High pressure (anticyclone) generally means the weather will be settled - often fine and cloudless sky. A high pressure is a section of air which is sinking. As air falls it warms, preventing clouds from forming, therefore areas of high pressure are often accompanied by fair weather.
In summer, they sometimes bring sea and coastal fog. In winter they may bring a lot of cloud or fog. Due to the temperature difference between Sea/Land and Air.
High pressure does not necessarily mean warm weather. A 'cold anticyclone' has cold air near the ground, where the cold air is cooling and subsiding. Another cold anticyclone is an area of high pressure sandwiched between two areas of low pressure. This is often found during British winters, results in fog and frost.

High pressure areas are normally larger and move more slower than low pressure systems.

Winds move around high pressure in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

A 'ridge of high pressure' - A ridge is an area of high pressure that does not have a closed circulation, it either extends from a high pressure or is sandwiched between lows. This often indicates settled weather.

a 'blocking high pressure' forces other weather systems to go round it. This gives some more unusual weather, either dragging air from further north or further south than usual.

Low pressure (depression) usually means the weather will be unsettled - often with plenty of rain (or snow) and strong winds or gales.
A low pressure is an area of air that is rising. Warm air near the surface will tend to rise, as it does it expands and cools forming clouds.

Winds circulate around low pressure in an anticlockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Note: Northern Hemisphere - when you stand with your back to the wind, you will have low pressure on your L/H side and high pressure on your R/H Side. Southern Hemisphere - it is the opposite.

They try to flow directly from a high to a low pressure, but due to the spinning of the earth and friction of the surface, flow around the pressure centres, following the isobars. The larger the difference in pressure the stronger the winds will blow.

The important relationships between isobars and winds are:
1. Winds blow in an attempt to combat the differences in air pressure.
2. They try to flow directly from a high to a low pressure, but due to the spinning of the earth flow around the pressure centres blows almost parallel to the isobars.
3. The closer the isobars (pressure gradient), the stronger the wind (directly proportional) and the bigger the waves being generated.

This does not apply in the tropics (Near the equator). For this reason, tropical meteorologists usually replace isobars with streamline arrows and isotachs (which indicate wind speed and direction).

Wind Direction is shown with arrows that have a series of barbs on their tails to indicate speed, as seen - Here
eg:
each Bft value is shown by adding ½ a barb to the pointer none, ½,1, 1½, 2 etc..
'L' would be indication of wind approaching from South at 2Bft
'F' would be an indication of wind approaching from North at 3Bft
4Bft would be 'F' with both barbs equal lengths.
8Bft would be indicated by 4 full length barbs.

Surface-level wind speeds are normally given for 10 metres above ground.

Gradient winds blow round the pressure systems along the direction of the isobars.

Wind blows from high to low pressure but due to earth rotation spirals out of high pressure regions into low pressure ones. You can also get strong winds from a weak low if it is next to a powerful high.

Weather Fronts
Warm front: indicated by red semicircles, which is the leading edge of a low pressure. These show warmer air moving in, and rain usually falls along the line of the front.
Cold front:- indicated by blue triangles, which is the trailing edge. These show colder air moving in - often with a change in wind direction to a more northerly direction. You can usually expect a band of rain, often followed by showers.
Occluded front:- indicated by both red semicircles and blue triangles. Again this normally means a band of rain is on the way.


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