What Is Atmospheric Pressure?
Atmospheric pressure — also called barometric pressure — is the force exerted by the weight of the air column above a given point on Earth's surface. At sea level, the atmosphere pushes down with a force of approximately 101,325 pascals (Pa), or about 14.7 pounds per square inch. This is defined as 1 atmosphere (atm), or roughly 1013 millibars (mb) — the unit most commonly used in weather science.
Pressure decreases with altitude because there is simply less air above you. This is why aircraft cabins are pressurized, and why climbers on high mountains experience altitude sickness.
High Pressure vs. Low Pressure: The Core Difference
The difference between high and low pressure systems is the foundation of almost all weather forecasting. Understanding it unlocks a huge amount of meteorological intuition.
High Pressure Systems (Anticyclones)
In a high pressure system, air sinks from the upper atmosphere toward the surface. As this air descends, it warms and dries, suppressing cloud formation. This is why high pressure systems are associated with:
- Clear, sunny skies
- Light or calm winds
- Dry conditions
- Settled, stable weather lasting several days
In the Northern Hemisphere, winds around a high pressure system rotate clockwise (anticyclonically). In the Southern Hemisphere, rotation is counterclockwise.
Low Pressure Systems (Cyclones/Depressions)
In a low pressure system, air converges at the surface and rises. As it rises, it cools, and water vapor condenses into clouds and precipitation. Low pressure systems bring:
- Cloudy skies and rain or snow
- Strong, gusty winds
- Stormy, unsettled weather
- Frontal systems (warm and cold fronts)
In the Northern Hemisphere, surface winds around a low pressure system rotate counterclockwise. In the Southern Hemisphere, they rotate clockwise.
How Pressure Gradients Create Wind
Air always moves from areas of high pressure toward areas of low pressure — just like air rushing out of a balloon. The steeper the pressure gradient (the more rapidly pressure changes over distance), the stronger the winds. Meteorologists display pressure on weather maps using isobars — lines connecting points of equal pressure. Closely spaced isobars indicate a steep gradient and strong winds; widely spaced isobars suggest gentle breezes.
Measuring Atmospheric Pressure
Pressure is measured with a barometer. There are two main types:
- Mercury barometer: The classic design, invented by Evangelista Torricelli in 1643. A column of mercury rises in a sealed tube based on the pressure of the atmosphere pushing down on a mercury reservoir. Still used as a reference standard.
- Aneroid barometer: Uses a small, flexible metal capsule that expands and contracts with pressure changes. More practical for home use. Most digital weather stations use electronic aneroid sensors.
Using Pressure Trends to Forecast Weather
A single pressure reading tells you less than the trend over time. Here's a practical guide:
| Pressure Trend | What It Often Indicates |
|---|---|
| Rapidly falling (>6 mb in 3 hours) | Strong storm or gale approaching |
| Slowly falling | Rain or unsettled weather moving in |
| Steady | Current conditions likely to persist |
| Slowly rising | Improving weather on the way |
| Rapidly rising | Cold, clear, but potentially windy conditions ahead |
A barograph — which records pressure continuously on a paper chart — is a wonderful tool for spotting these trends at a glance. Modern weather stations display equivalent trend data digitally.
Pressure and Human Health
Many people report that changes in barometric pressure affect how they feel — particularly those who experience migraines, joint pain, or sinus issues. While research is ongoing, some studies suggest that rapid pressure drops may trigger migraine episodes in susceptible individuals. Monitoring your local pressure trends may help you anticipate and prepare for these effects.