Weather Maps & Weather Terms

Some key terms Air Pressure: the weight or pressure the air exerts on the ground. Pressure is measured in millibars (or kilopascals) and isobars are normally drawn at intervals of 4 millibars. Isobars: the lines on a weather chart that join points of equal pressure. Very much like contour lines on a topographic map.

Differing air pressure is what causes wind to blow When the isobar lines are close together, the wind is strong. When the lines are far apart, the wind is light.

L = Centre of Low Pressure The wind around a low pressure system blows counterclockwise and towards the centre. These are associated with:  Strong winds Rainstorms Cloudiness

H = Centre of High Pressure (Anticyclone) The wind around a high pressure system blows clockwise and away from the centre These are associated with: Nicer weather In winter it can sometimes mean frost Reduced chance of precipitation

What are Fronts? These mark the boundary between warmer air on one side and colder air on the other. The weather will change as the front passes over you Whenever 2 different air masses meet, there will be weather

Warm Front A warm front is the edge of a warm air mass.  Ahead of a warm front there is usually a wide belt of rain. Behind the warm front the temperatures increase and the rain gradually eases.  The side the semi-circles are on represent the direction in which the warm front is heading So this front is moving upwards

Cold Front A cold front is the edge of a cold air mass. It can move twice as fast as a warm front.  Ahead of a cold front there is a narrow rain belt. The rain can be torrential and wind speeds can be high. Behind a cold front the weather is brighter but the temperature is cooler. The direction the triangles point is the direction in which the cold front is moving.

Occluded Front This is formed when a cold front overtakes a warm front. Can mean thunderstorms are coming The passing of an occluded front usually brings drier air behind it

Stationary Front Warm air meets cold air, but neither is much stronger so they just sit there for days Usually leads to days of rain

3 Ways Precipitation Can Form

1) Orographic Precipitation This occurs when an air mass forced to rise over a mountain range. The air becomes cooled and precipitation takes place.

2) Convectional Precipitation This happens on a hot day when hot air rises and then cools The water vapour carried by the air condenses, producing heavy rain. Convectional rainfall is usually accompanied by a thunderstorm.

3) Frontal Precipitation This takes place at the front between a mass of warm air and a mass of cold air The water vapour in the warm air is chilled as it is pushed over the cold air and condenses to form clouds and rain.