What Is Atmospheric Pressure?
Atmospheric pressure — also called barometric pressure — is the force exerted by the weight of the air column above any given point. At sea level, the average pressure is approximately 1013.25 millibars (mb), or 29.92 inches of mercury. This value varies constantly depending on temperature, altitude, and atmospheric dynamics, and those variations are what drive nearly all weather changes you experience.
High Pressure Systems
A high-pressure system (also called an anticyclone) is an area where air pressure is greater than its surroundings. Air in a high-pressure center sinks and spreads outward near the surface. This descending air warms as it compresses, which inhibits cloud formation and precipitation.
What High Pressure Brings
- Clear, sunny skies
- Calm or light winds
- Low humidity
- Cold overnight temperatures in winter (due to radiative cooling with no cloud cover)
- Hot, dry conditions in summer when persistent
Low Pressure Systems
A low-pressure system (also called a cyclone) is an area where pressure is lower than its surroundings. Air converges toward the center and is forced to rise. As it rises, it cools and its moisture condenses, forming clouds and precipitation.
What Low Pressure Brings
- Cloudy skies and precipitation
- Stronger winds
- Warmer overnight temperatures (cloud cover traps heat)
- Severe weather potential when conditions are unstable
Why Air Moves the Way It Does
Air naturally flows from high to low pressure — similar to how water flows downhill. However, the rotation of the Earth introduces a force called the Coriolis effect, which deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This causes winds to circulate clockwise around highs and counterclockwise around lows in the Northern Hemisphere (and opposite in the Southern).
Reading a Barometer
A barometer measures atmospheric pressure. The trend — whether pressure is rising or falling — is often more informative than the current reading alone:
| Pressure Trend | What It May Indicate |
|---|---|
| Rapidly rising pressure | Clearing skies, improving conditions ahead |
| Slowly rising pressure | Gradual improvement, fair weather approaching |
| Steady pressure | Current conditions likely to persist |
| Slowly falling pressure | Gradual deterioration, rain possible |
| Rapidly falling pressure | Storm approaching, potentially severe weather |
Pressure and Altitude
Pressure decreases with altitude because there is less air above you. This is why weather reports adjust readings to sea-level equivalent values — it makes comparisons meaningful across different elevations. Pilots use pressure altimeter settings constantly to ensure accurate altitude readings regardless of current weather conditions.
Understanding pressure systems is one of the most foundational skills in weather literacy. Whether you're checking why it's sunny today or anticipating an approaching storm, atmospheric pressure is almost always part of the story.