When you read about any weather event, or perhaps even just a mundane daily weather report, you always hear the weather reporter referring to high-pressure and low-pressure areas. What are they and what does it mean for the weather? Keep reading to find out.

What is Air Pressure?

The terms high pressure and low pressure refer to air pressure, or the weight of the air. You might think air is weightless, but it’s not! Every square inch of planet Earth has around 14.7 pounds of air pressing down on it. This measurement is commonly referred to as pounds per square inch or psi.

Why Does Air Pressure Change?

There are a few things that can affect air pressure at any given moment. Air pressure tends to be higher when temperatures are higher. However, the way this affects our planet is actually the opposite. Because the hotter, more dense air rises, the result of the sun warming the air is actually that there will be lower pressure air closer to the earth’s surface.

When air is colder, it is denser and thus, heavier. In this situation, there will be higher pressure at the Earth’s surface.

Additionally, the density of the air affects the air pressure. Further from the surface of the Earth, higher in the atmosphere, the air has fewer molecules and is less dense, thus it has a lower psi than air closer to the surface. That is why it is harder to breathe at extremely high elevations – there is simply less air!

Finally, the air pressure on Earth is also affected by the planet’s tilt and spin. The earth spins faster at the equator than it does at the poles. Places near the equator are spinning at nearly 1,600 kilometers per hour. At the poles, the spinning speed is just a fraction of a mile per hour. Because of this, air currents do not simply go straight. The warm air from the equator rises and heads toward the poles. In the Northern Hemisphere, these currents go toward the right and in the Southern Hemisphere, they go toward the left. These winds help to create high and low-pressure systems that affect weather globally.

planet earth
Every square inch of planet Earth has around 14.7 pounds of air pressing down on it.

©iStock.com/Thaweesak Saengngoen

What Does High Pressure Mean For the Weather?

High-pressure weather systems are usually very stable. You can often expect to see sunny skies and clear days when there is high pressure. High-pressure systems push air away from them into lower-pressure areas.

What Does Low Pressure Mean For the Weather?

Low-pressure weather systems are less stable and often have inclement weather. Depending on the season and location that can mean rain, snow, thunderstorms, wind, and more. Low-pressure systems suck air from higher-pressure systems up into them.

Lightning striking during a thunderstorm.
Low-pressure weather systems are less stable and often have inclement weather such as rain, wind, snow and thunderstorms.

©iStock.com/solarseven

Examples of How High and Low Air Pressure Affect the Weather

There are many ways to show how these different areas of air pressure can interact with each other and affect our weather.

Hurricanes

Hurricanes start off as a low-pressure area that moves over warm ocean waters. It then sucks up the damp, warm air from the ocean’s surface. This air cools as it rises, and then forms thunderstorms and inclement weather. As the storm grows and keeps moving, it gets a circular pattern of movement that only gets stronger as more air gets sucked into the storm increasing the pattern and windspeed. Once the storm hits a windspeed of 74 miles per hour, it is officially a hurricane.

Hurricane seen from space.
Hurricanes are storms with sustained winds that reach 74 miles per hour or more.

©LiL SUS/Shutterstock.com

Polar Vortex

This winter weather phenomenon is also easily explained using areas of high and low air pressure. Both poles always have a polar jet stream. This is an area of high winds that rotate around the poles and typically has colder air on the side closer to the poles and warmer air on the other side. However, sometimes this high-pressure area becomes unstable and pushes the cold air out into lower-pressure areas, reaching as far south as Florida in some cases! When this happens the US gets an icy blast of cold temperatures.

What Else Does Air Pressure Affect?

Air pressure has an effect on the weather, but it can also affect our bodies! Changes in the local air pressure can cause your joints and other parts of your body to expand or contract. When there is lower air pressure you may notice conditions like arthritis or migraines are exacerbated.

Some avid fishing experts say that air pressure also affects any given fishing trip. They believe that fish do not come as close to the surface during times of high pressure and are more likely to bite when the weather is getting a bit stormy during low-pressure systems. Unfortunately, a beautiful sunny day is probably the most pleasant for a fishing trip!

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FAQs (Frequently Asked Questions) 

What is air pressure?

Air pressure is the weight of the air as it exerts pressure on earth.

What is the average air pressure on Earth?

Every square inch of Planet Earth has around 14.7 pounds of air pressing down on it. This measurement is commonly referred to as pounds per square inch or psi.

What makes air pressure change?

Temperature, air density, and the earth’s tilt and rotation can affect the air pressure.

How does high-pressure air affect weather?

High-pressure weather systems are usually very stable. You can often expect to see sunny skies and clear days when there is high pressure. High-pressure systems push air away from them into lower-pressure areas.

How does low-pressure air affect weather?

Low-pressure weather systems are less stable and often have inclement weather. Depending on the season and location that can mean rain, snow, thunderstorms, wind, and more. Low-pressure systems suck air from higher-pressure systems up into them.

How do hurricanes form?

Hurricanes start off as a low-pressure area that moves over warm ocean waters. It then sucks up the damp, warm air from the ocean’s surface. This air cools as it rises, and then forms thunderstorms and inclement weather. As the storm grows and keeps moving, it gets a circular pattern of movement that only gets stronger as more air gets sucked into the storm increasing the pattern and windspeed. Once the storm hits a windspeed of 74 miles per hour, it is officially a hurricane.

How do polar vortexes form?

Both poles always have a polar jet stream, an area of high winds that rotate around the poles and typically has colder air on the side closer to the poles and warmer air on the other side. However, sometimes this high-pressure area becomes unstable and pushes the cold air out into lower-pressure areas, reaching as far south as Florida in some cases! When this happens the US gets an icy blast of cold temperatures.

How does air pressure affect your body?

Changes in the local air pressure can cause your joints and other parts of your body to expand or contract. When there is lower air pressure you may notice conditions like arthritis or migraines are exacerbated.

How does air pressure affect a fishing trip?

According to some fishing experts, fish do not come as close to the surface during times of high pressure and are more likely to bite when the weather is getting a bit stormy during low-pressure systems.

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