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Pressure /original

Pressure is the force that anything exerts on something it comes in contact with, either because of gravity pulling on it or other motion it 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.  The types of weather that occur may affect the types of field work that can be undertaken and can also influence the effectiveness of herbicide and pesticide treatments.

Graphical explanation of pressure
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

Two unequal columns of air becoming equal

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 we’re inhaling more water vapor and the excessive moisture in the air reduces our 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.


How does this relate to agriculture? 

In the atmosphere, if the air pressure is high, this generally means there are lots of sinking air molecules and the weather will be clear.  These favorable weather conditions are good for field work and spraying, but can also be associated with periods of drought if they last a long time.  If the air pressure is low, this generally means there are lots of rising air molecules and there is a chance of clouds and rain.  Some people complain that changes in air pressure can affect their moods and cause headaches and joint pain.  Pressure is also important in the maintenance of hydraulic equipment, where internal fluid pressures have to be carefully maintained to prevent damage and maximize efficiency.


Want to learn more? Humidity, Density, Temperature

Last modified date: Wednesday, August 4, 2010 - 4:38pm