What's up with the weather?

How to read weather maps.

What means what

• Weather maps are taken using the pressure which is found at ground level.

• For example the 1008 which is above the north island is the barometric pressure at ground level. The pressure and wind above this may be completely different.

Isobars.

• Those plain lines that curve across the map are called isobars (iso = equal, bar = pressure). They join together places with the same mean sea level air pressure (weight per square area of air above).

• Isobars measure barometric pressure.• They are only a smoothed out average!

Why isobars are important.

• Isobars can show the wind strength and high and low pressure systems.

• Isobars that have intervals a long distance apart show HIGH pressure systems with little or no wind.

• Isobars that have intervals that are close show LOW pressure they closer they are the stronger the wind.

Wind Direction

• Wind travels almost directly along isobars.• A high pressure system travels in an anti-

clockwise direction• A low pressure system travels clockwise.

Wind

• High pressure always moves towards low pressure systems therefore you get “windleaks”.

• Due to the wind rotating around the isobar a “spin-out” effect is created.

• This can mean that wind is stronger than the isobars show on a high and weaker in a low.

Highs

• A high or anti-cyclone has a central pressure over 1000.

• In the centre of the high winds are gentle.• Around the outside of a high winds are

stronger.• The bigger the high the slower it moves.

Lows

• A low or depression has a central pressure generally under 1000.

Fronts

• A cold front is the leading edge of a invading colder air-mass and is marked by a line with triangles pointing to where it is moving. Cold fronts push in underneath the warmer air ahead of them, forcing the warm air upwards and making cloud and areas of rain.

Warm front

• A warm front is the leading edge of an invasion of warmer air. Its surface position is marked by a line with semicircles pointing to where it is moving. The advancing warm air rises over a zone of retreating cooler air, making a cloud bank that slopes forwards from ground level upwards, often bringing prolonged steady rain.

Occluded front

• An occluded front or occlusion occurs when a cold front overtakes a warm front, so that all that remains of the original warm air is trapped above, where it cools making dense cloud and rain. It is marked by a line with triangles and semicircles on the same side, pointing to where the front is moving. As an occluded front passes by: any rain becomes patchy, wind eases, the rate of pressure fall may level outbut air temperature does not change much.

Stationary front

• A stationary front is one which has lost its impetus for movement, so that neither air-mass is making much progress. It is marked by a line with alternate triangles and semicircles on opposite sides ... the triangles protruding into the warmer air-mass and the semicircles protruding into the cooler air-mass. It takes a while for a stationary front to pass by: any rain clears only slowly and temperature and pressure do not change much.

How to read a map.

So what’s this?