Sea level pressure map
Pressure is the weight of the air above you. To compare places at different altitudes, forecasters convert it to what it would be at sea level. This map shows that value in hectopascals, hPa, with thin isobars drawn every 4 hPa. The data comes from the NOAA GFS model on a 0.25 degree grid.
Average sea-level pressure is about 1013 hPa. Values well below that mark low-pressure systems, where air rises, clouds build and rain is likely. Values well above it mark highs, which usually bring clear and calm weather.
Isobars work like contour lines on a hill. The closer they are together, the steeper the pressure change and the stronger the wind. In the northern hemisphere the air circulates anticlockwise around a low and clockwise around a high; in the southern hemisphere it is the other way round.
The animated wind is drawn on top, so you can watch the circulation form. Press play on the timeline to see lows deepen and move over 48 hours, with a new forecast run four times a day.