Pressure is the force exerted over a given area or object, either because of gravity pulling on it or other motion the object has. Molecules in the air produce pressure through both their weight and movement, and this pressure is connected to other properties of the atmosphere.
Why do I care? Atmospheric pressure determines the types of weather that are likely to occur.
Figure A. Image from NASA.
Pressure is force exerted over a given area. In the atmosphere, the molecules in the air apply pressure to everything on earth, including us. For instance, individual molecules in the air push against tiny areas on the top of our head. The force that air exerts is called air pressure. The more air molecules there are above you, the greater the force they exert, so the greater the pressure.
Pressure is important because it is related to volume, density, and temperature. In the atmosphere, warm surfaces can heat the air above them, causing the air to become less dense and to rise. This can eventually result in clouds and precipitation in the areas of rising motion, such as in the center of low pressure systems. High pressure in the atmosphere causes the air to compress and sink, leading to clear skies and calm conditions.
Air Pressure
Figure B: Equilibrium of Pressure/Temperature
Air pressure is the mass of air above a given area. The atmosphere thins rapidly as you move toward space. This causes the air pressure to decrease rapidly as well. Air pressure is affected by both temperature and moisture. Cool air is denser than warm air. Moist air is actually less dense than dry air. This makes it easier to lift and form clouds. On humid days the air feels heavier because more water vapor molecules are inhaled and the excessive moisture in the air reduces the ability to sweat effectively. The easiest way to relate air pressure to weather is to think of air as being in columns.
Say there is a cold air column next to a warm air column, but they both have the same air pressure, the cold air column is denser, so it will have a smaller volume and be shorter than the warm air column. This explains why the tropopause is higher over warm areas like the tropics and lower over cold areas like the poles. In an attempt to equalize temperature, volume, and pressure, air is transported from the warmer column to the cooler column. This causes an initial rise in air pressure in the cold column and a decrease in air pressure over the warm column. Once the temperatures are equal, the heights, volumes, and pressures will become equal, too. This transport of air drives weather.
Want to learn more?
Humidity, Density, Temperature
Links to National Science Education Standards:
7th grade science: 7.E.1 : Understand how the cycling of matter (water and gases) in and out of the atmosphere relates to Earth's atmosphere, weather and climate, and the effects of the atmosphere on humans.
Earth Science: EEn.2.5.2 : Explain the formation of typical air masses and the weather systems that result from air mass interactions.
Activities to accompany the information above:
Activity: Cloud in a Bottle (Link to online tutorial.)
Instructor Set-up Instruction
Student Activity: pdf document
Description: This activity will demonstrate the direct affects of pressure and temperature on cloud formation.
Relationships to topics: Temperature, Pressure, Humidity, How Clouds Form
Activity: The Weather (Link to original activity)
Student Activity: pdf document word document
Description: This activity focuses on observing atmospheric conditions over a 4-day period. Students take observations for temperature, air pressure, wind speed and direction, and record sunrise and sunset. The measurements the student records are then compared with a NC State Climate Office station's measurements.
Relationships to topics: Temperature, Pressure
Activity: Building a Weather Instrument
Instructor Set-up Instructions
Student Activity: pdf document
Description: This activity will assist students in learning how to build their own instruments.
Relationships to topics: Pressure
Activity: Renewable Energy: Wind (pdf version of original activity.)
Description: This activity focuses on the relationship between barometric pressure, wind speed and wind direction. Students will use these parameters to analyze surface maps of the United States and form a relationship with the patterns that will show up on the map.
Relationships to topics: Pressure, What Drives Weather, Coriolis Effect, General Circulation of the Atmosphere