About pressure maps and synoptic weather

Surface pressure maps show atmospheric pressure at mean sea level (MSL), usually in hectopascals (hPa) or millibars (mb). Lines of equal pressure are called isobars; they are typically drawn at 2, 4 or 5 hPa intervals. High pressure (anticyclone) is associated with sinking air, often clear skies and light winds; low pressure (cyclone/depression) with rising air, clouds and precipitation. Pressure patterns drive wind and weather on synoptic scales (WMO; NOAA).

Wind blows approximately along isobars in the free atmosphere (geostrophic balance), with low pressure to the left in the Northern Hemisphere (Buys Ballot's law). Tighter spacing of isobars means stronger pressure gradient and stronger winds. Fronts (cold, warm, occluded) often lie in troughs of low pressure and mark boundaries between air masses. National meteorological services and agencies like NOAA's Weather Prediction Center produce and update these analyses (NOAA WPC; WMO).

The maps on this page may be sourced from NOAA or other providers. They are useful for understanding large-scale weather; for local forecasts and warnings always refer to your national meteorological service.