Atmospheric Pressure and Measurement

• Atmospheric pressure varies with height.

Atmosphere

• The atmosphere is a blanket of gases surrounding the Earth's surface.
• The atmosphere is one of the four major components of the Earth ecosystem.
• The remaining three major components of the Earth ecosystem are biosphere, hydrosphere, and lithosphere.
• Life cannot exist without the atmosphere.
• Major gases present in the ecosystem include nitrogen, oxygen, and argon.

Air Pressure

• Air has mass, therefore it has weight.
• Air pressure is a force exerted in all directions due to the weight of the air above it.
• Atmospheric pressure is the weight of a column of air in a unit area from mean sea level to the top of the atmosphere.
• Atmospheric pressure is expressed in various units.

Measurement of Air Pressure

• Atmospheric pressure is measured using a barometer.
• Meteorologists use millibars (mb) to measure air pressure.
• 1 millibar equals the force of one gram on a square centimeter, or the weight of a column of mercury 75 cm high.
• 1000 millibars is equal to the weight of 1.053 kilograms per square centimeter.
• Normal pressure at sea level is about 76 centimeters (1013.25 millibars).

Vertical Variation of Pressure

• Pressure decreases rapidly with height in the lower atmosphere.
• Air pressure at the height of Mt. Everest is about two-thirds less than at sea level.
• The decrease in pressure with altitude is not constant.
• Factors controlling air density, temperature, water vapor, and gravity do not allow for a simple relationship between altitude and pressure.
• Atmospheric pressure decreases on average about 34 millibars every 300 meters of height.
• The vertical pressure gradient force is balanced by a nearly equal but opposite gravitational force.
• Air is denser at the surface due to gravity, hence it has higher pressure.
• Air pressure is proportional to density and temperature, so a change in either will cause a pressure change.
• Pressure decreases with height and varies from place to place at any elevation.
• Variation in pressure causes air motion or wind from high to low pressure areas.
• Rising pressure indicates fine, settled weather, while falling pressure indicates unstable and cloudy weather.

Horizontal Distribution of Pressure

• Distribution of pressure is studied by drawing isobars at constant levels
• Small differences in pressure significantly affect wind direction and velocity.
• Isobars connect places with equal pressure.
• The effect of altitude on pressure is eliminated by reducing measurements to sea level for comparison.
• Isobar spacing indicates the rate and direction of pressure changes, i.e. the pressure gradient.
• Close isobars indicate a steep or strong pressure gradient.
• Wide isobars indicate a weak gradient.
• Pressure gradient is the decrease in pressure per unit distance in the direction of the most rapid pressure decrease.
• There are seven identifiable zones of homogeneous horizontal pressure regimes, or pressure belts, on the Earth's surface.

Pressure Belts

• Equatorial Low-Pressure Belt or Doldrums: Lies between 10°N and 10°S latitudes, varying in width between 5°N and 5°S and 20°N and 20°S.
• The Equatorial Low-Pressure Belt is also known as the Doldrums due to calm air movements.
• The position of the Equatorial Low-Pressure Belt varies with the sun's movement.
• Subtropical High-Pressure Belt or Horse Latitudes: Extends from near the tropics to about 35°N and S.
• The Subtropical High-Pressure Belt contains subsiding air that is warm and dry; most deserts are present along this belt in both hemispheres.
• Sub-Polar Low-Pressure Belt: located between 45°N and S latitudes along with the Arctic and Antarctic circles (66.5° N and S latitudes).
• The Sub-Polar Low-Pressure Belts are not very well pronounced year long because temperatures are low.
• The Sub-Polar Low-Pressure Belts in the southern hemisphere surround the periphery of Antarctica and are not as well differentiated.
• Polar High-Pressure Belt: small and extends around the poles.
• The Polar High-Pressure Belt is situated around the poles between 80 – 90° N and S latitudes.
• The lowest temperature can be found over the poles within the Polar High-Pressure Belt.