What do the H, L and lines on a weather map mean?
H marks high pressure: sinking, settled air and usually fair skies. L marks low pressure: rising air, clouds and storms. The lines are fronts, the edges where a cold air mass meets a warm one, and that's where the weather changes.
A front can drop the temperature 15°F in an hour, with a gusty wind shift.
Behind a cold front: drier, cleaner air. Ahead of a warm front: damp and muggy.
Show me why1 min
Highs and lows
Air pressure is the weight of the air above you. Where air is sinking, it piles up and presses down harder: that's a high. Sinking air warms and dries out, so clouds evaporate and skies clear. Where air is rising, it presses down less: that's a low. Rising air cools, its water vapor condenses into clouds, and if there's enough of it you get rain or snow.
Wind is air flowing from higher pressure toward lower pressure. In the Northern Hemisphere it swirls clockwise out of a high and counterclockwise into a low, because Earth's spin bends its path. That swirl is why a high parked over Nevada sends desert air west to the California coast: a Santa Ana.
Go deeper2 more
The four kinds of front
- Cold front (blue line with triangles): cold air shoves under warm air and lifts it fast. Expect a short burst of showers or storms, a wind shift and a drop in temperature and humidity behind it.
- Warm front (red line with half circles): warm air glides up over cold air. Clouds thicken for hours, often with steady light rain, then it turns warmer and more humid.
- Stationary front (alternating red and blue): neither air mass is winning, so the boundary sits still and can bring rain to the same place for days.
- Occluded front (purple): a fast cold front has caught up with a warm front, a sign the storm is mature and starting to wind down.
A dashed line is a trough, a stretch of lower pressure without a sharp temperature change. In the desert Southwest a heat-driven trough sits over the hot ground much of the year.
Reading your own map
Find yourself, then the nearest H and L. Wind will be blowing from the high's side toward the low's side. If a cold front is upwind of you, expect it within a day or so; weather systems across the US usually drift west to east at 20 to 30 mph. Air Story's Zoom out chapter draws today's NOAA map around you, with your three-day air trail on top so you can see which high or low steered it.
Questions people ask
- Does high pressure always mean good weather?
- Usually clear and dry, but not always pleasant. A strong summer high can build a heat wave, and in winter a high can trap fog, haze and smoke under an inversion.
- Why is a cold front blue and a warm front red?
- The color shows which air is advancing: blue triangles point the way cold air is pushing, red half circles the way warm air is moving.
- How far ahead of a cold front does the weather change?
- The sharp change comes right at the front, often within an hour: a wind shift, gusts, maybe a line of showers, then cooler, drier air. Clouds can build a few hours ahead.