Decoding the Weather Forecast
Weather maps can look complicated, but they use a standard set of symbols to show you what's happening in the atmosphere. Understanding these symbols can help you predict the weather in your area.
Pressure Systems
Air in the atmosphere has pressure. This pressure isn't the same everywhere, and the differences in pressure are what drive the wind and create weather systems.
High-Pressure System (H): This is an area where the air pressure is higher than the surrounding areas. Air in a high-pressure system sinks and flows outwards in a clockwise direction (in the Northern Hemisphere). Sinking air warms and dries out.
Weather: High-pressure systems generally bring clear skies, fair weather, and calm conditions. They are often called 'fair-weather systems'.
Low-Pressure System (L): This is an area where the air pressure is lower than the surrounding areas. Air flows inwards towards the center of a low in a counter-clockwise direction (in the Northern Hemisphere) and then rises. As the air rises, it cools and the water vapor in it condenses to form clouds.
Weather: Low-pressure systems are associated with clouds, precipitation (rain or snow), and stormy weather.
Fronts
A front is the boundary line where two different air masses meet. Most of the 'interesting' weather happens along these fronts.
Cold Front: A cold front is shown on a map as a blue line with blue triangles. The triangles point in the direction the cold air is moving.
Process: A cold, dense air mass pushes into a warmer air mass, forcing the warm air to rise rapidly.
Weather: This rapid lifting creates tall cumulonimbus clouds, leading to thunderstorms, heavy rain, and sometimes severe weather. After the front passes, the temperature drops quickly.
Warm Front: A warm front is shown as a red line with red semi-circles. The semi-circles point in the direction the warm air is moving.
Process: A warm, less-dense air mass slides up and over a colder air mass.
Weather: This gentle lifting creates widespread, flat stratus clouds. Warm fronts typically bring steady, light rain or snow for an extended period. After the front passes, the temperature rises.
Stationary Front: A front that is not moving. It is shown as an alternating blue and red line with triangles on the blue side and semi-circles on the red side. It often brings many days of clouds and precipitation.
Occluded Front: A complex front where a cold front overtakes a warm front. It's shown as a purple line with both triangles and semi-circles.
Other Symbols
Isobars: These are lines on a map that connect points of equal air pressure. When isobars are close together, it indicates a large pressure difference and strong winds. When they are far apart, winds are light.
Precipitation: Rain is often shown as green shading, snow as blue or purple, and mixed precipitation as pink.