Pressure Systems, Winds and Local Circulations
Pressure and wind basics
Wind blows from high to low pressure, deflected to the right in the northern hemisphere (left in the southern) by the Coriolis effect. Geostrophic wind flows parallel to straight isobars; gradient wind accounts for curvature. Friction near the surface slows the wind and turns it toward low pressure. The closer the isobars, the stronger the wind. In the northern hemisphere, low pressure is anticlockwise (cyclonic) and high pressure clockwise (anticyclonic).
Local circulations
Learn the named local winds and their causes. Sea breezes blow from sea to land by day as land heats; land breezes reverse at night. Valley breezes blow upslope by day, anabatic, and downslope at night, katabatic. Foehn or chinook winds are warm dry downslope winds on the lee side of mountains; the bora and mistral are cold downslope winds. Mountain waves, with lenticular cloud and severe turbulence, form in strong cross-mountain flow. The trap is the direction: candidates often reverse sea and land breezes, or forget that the land breeze is the nocturnal one.
Worked example
A synoptic chart shows a low of 985 hPa with tightly packed isobars near a coastal airport, and the forecast mentions a strong pressure gradient. Expect strong winds, with coastal gales likely. If the isobars were widely spaced around a 1025 hPa high, expect light winds and, in winter, radiation fog potential in moist valleys.