Weather
High vs Low Pressure: What Each Means for the Weather
High pressure usually brings clear, calm weather; low pressure brings clouds, wind and rain. Here is why, with the numbers that separate them and how to tell which one you are in today.
High pressure (above about 30.20 inHg / 1023 hPa) means sinking air, which usually brings clear skies and light winds. Low pressure (below about 29.80 inHg / 1009 hPa) means rising air, which builds clouds, wind and rain. The direction the pressure is moving is often more useful than the number itself.
Side by side
| High pressure | Low pressure | |
|---|---|---|
| Sea-level reading | above ~30.20 inHg (1023 hPa) | below ~29.80 inHg (1009 hPa) |
| Air motion | Sinks and spreads out | Converges and rises |
| Sky | Clear or few clouds | Cloudy, often precipitation |
| Wind | Light, sometimes calm | Stronger, gusty near fronts |
| Northern Hemisphere spin | Clockwise | Counterclockwise |
| On a weather map | Blue H | Red L |
| Typical season effect | Hot summer days, cold clear winter nights | Storms, mild cloudy winter days |
Why sinking air clears the sky
Air that sinks is compressed by the growing weight above it and warms up. Warmer air can hold more water vapor, so clouds evaporate. That is why a strong high can give a week of blue sky.
The flip side: highs trap whatever is near the ground. In winter they bring hard frosts; in summer they can bring heat waves and haze.
Why rising air makes weather
Air that rises expands as the pressure around it drops, and expanding air cools. Once it cools to its dew point, vapor condenses into cloud droplets. Keep lifting and the droplets grow into rain or snow. Low-pressure systems are giant machines for lifting air, which is why they carry fronts, storms and most of our precipitation.
Wind follows from the pressure difference itself: air flows from high toward low, and the tighter the gap between the two on a map, the stronger the wind.
Reading the trend
- Falling steadily: a low or a front is coming. Expect cloud to thicken and wind to pick up within 12–24 hours.
- Falling quickly (more than 3.5 hPa, about 0.10 inHg, in three hours): stormy weather is close.
- Rising after rain: the front has passed; skies clear, often with a cooler, gusty day.
- High and steady: settled weather continues.
The live reading shows today’s value and the change over the last three hours. For the full rules see how to read a barometer.
Not just weather
Pressure also changes with altitude: there is far more difference between Miami and Denver (about 5 inHg in absolute terms) than between a high and a low at the same place (rarely more than 1.5 inHg). That is why weather pressure is always given at sea level. See air pressure and altitude.