Introductory Agrometeorology/Atmospheric Pressure

Objectives of the chapter

• To understand:
  • pressure gradient of isobar
  • measurement of atmospheric pressure

Atmospheric pressure

The atmospheric pressure is the weight of the air, which lies vertically above a unit area centered at a point. The weight of the air presses down the earth with the pressure of 1.034 gm / cm2. It is expressed in millibar (mb) equal to 100 N/m2 or 1000 dynes/cm2. Unequal heating of the earth and its atmosphere by the sun and rotation of the earth bring about differences in atmospheric pressure.atmospheric pressure eats medrick

Isobars

The distribution of pressure is represented on maps by ‘isobars’. Isobars are defined as the imaginary lines drawn on a map to join places having the same atmospheric pressure.

Pressure gradient(PG)

Wind flows from high pressure to low pressure area. Pressure gradient=pressure difference/shortest distance between two places So, pressure difference between two different place to their shortest or perpendicular distance is pressure gradient. More pressure gradient, more wind flows or pressure gradient is the rate of change of pressure per unit horizontal direction.

Pressure gradient is also called as barometric slope, expressed in millibar per 100km. or millibar per degree latitude.

• Unit of pressure gradient = mb/100km = mb/0latitude
• Isobar drawn closely - steep pressure gradient (strong pressure gradient – high wind velocity)
• Isobar drawn apart – weak pressure gradient (weak Pressure gradient – light wind)

Instruments used to measure air pressure

• Mercury barometer
• Aneroid barometer
• Barometer
• Microbarograph
• Micro-barovarigraph

Vertical variation of air pressure

As elevation increase, weight of air above decreases. In ground the air is highly compressible and denser. As we go up, the thinner it becomes and air molecules most diffused which results less pressure and inter molecular space become greater.

-Rate of pressure varies with height is not constant -Every 5 km up pressure reduced by ½ approximately. -At Mount Everest the pressure is 2/3 of x if pressure at sea level is x.

Pressure systems of the world

The shape of the earth is not uniform and subjected to uneven distribution of solar radiation, when it revolves around the sun. The uneven distribution of solar radiation over different regions of the globe leads to contrast in surface air temperature. This results in variations of surface atmospheric pressure systems, which are known as standard atmospheric pressure systems / belts. There are altogether seven alternating low and high pressure belts on the earth’s surface. They are as follows:

1. Equatorial trough of low pressure (between 5°N and 5°S)
2. Subtropical high pressure belt (Northern hemisphere) (25° and 35°N)
3. Subtropical high pressure belt (Southern hemisphere) (25° and 35°S)
4. Sub polar low pressure belt (Northern hemisphere) (60° and 70°N)
5. Sub polar low pressure belt (Southern hemisphere) (60° and 70°S)
6. Polar high (Northern hemisphere)
7. Polar high (Southern hemisphere)

The equatorial region receives more solar radiation and thus the surface air temperature is high, which creates lighter air near the ground compared to higher latitudes. The above condition leads to low atmospheric pressure over the equatorial region while sub tropical high pressure belts develop in both the hemispheres between 25 and 35 degree latitudes due to relatively low surface air temperature. It is due to low solar radiation received due to inclined sun’s rays over the subtropical region when compared to the equatorial belt. Like wise alternate low and high atmospheric pressure belt systems are developed across the globe from the equator to the poles.

1 atm. = 760 mm of Hg =1 bar

=101.325 Kp

=1.302 kg/cm²

1 kg/m² = 9.807 N/m² =9.807 Pascal