Hello readers, last week we have discussed Atmosphere, today we will revise it’s Pressure and it’s belts, which led to the movement of air.
Air Pressure is also known as Atmospheric Pressure. Let’s take a picture of pressure of air. 🙂
What is air pressure?
Air Pressure is a total weight of a mass of the column of air above per unit area at sea level. It is maximum at sea level. The cause of of air pressure is the uneven heating by the sun.
- Air pressure is measured in terms of height of mercury in the glass tube, which is usually done with the help of an instrument called a barometer.
- The standard air pressure at sea level is 1013.25 millibars (mb).
- The imaginary line, joining the place of equal pressure at sea level is called Isobars.
Air pressure decreases with increases altitudes (height of place from sea levels) at the rate of 3.4 mb per 600 feet, but this rate of decrease is confined to the altitude of a few thousand only.
Air Pressure – Causes movement of air
The main cause of Earth’s winds is uneven heating by the sun. This uneven heating causes changes of atmospheric pressure and results in blowing of winds.
- Always keep in mind that “winds blow from areas of high pressure toward areas of low pressure”.
- The difference of pressure between any two places is called pressure gradient.
Types of Air Pressure
They are generally divided into two categories:
- High Pressure also called as high or anticyclone.
- Low Pressure also called as low or cyclone or depression.
Air Pressure Belts
The air pressure is divided into different belts, i.e. these are those which have consistently high or low pressure. The belts tend to move with the seasons, toward the equator in winter and toward the poles in summer, thus causes movement of air.
On the basis of mode of genesis, pressure belts are divided into two categories:
- Thermally induced pressure belts ( equatorial low pressure belt and polar high pressure belt)
- Dynamically induced pressure belts (subtropical high pressure belt and sub polar low pressure belt)
Let’s take a look on these belts.
Equatorial low pressure belt – It is located on either side of the geographical equator in the zone extending between 5 degrees north and 5 degrees south latitudes, but this zone is not stationary because there is a seasonal shift of this belt with the northward and southward migration of the sun.
- There are light, feeble and variable winds within this convergence belt. Because of the frequent calm condition this belt is called as belt of calm or doldrum.
Sub Tropical High Pressure belt – It extends between the latitudes of 25-35 degree in both hemispheres. It is important to note that this high pressure belt is not thermally induced because this zone besides two or three winter months, receives high temperature throughout the year.
- The convergence of winds at higher altitude above this zone results in the subsidence of air from higher altitudes. Thus descent winds resulted in the contraction of their volume and ultimately causes high pressure.
- This zone of high pressure is called horse latitude because of the prevalence of frequent calms.
Sub polar low pressure belt – It is located 60-65 degree latitudes in both hemispheres. The low pressure belt does not appear to be thermally induced because there is a low temperature throughout the year and as such there should have been high pressure belt instead of a low pressure belt.
- The surface air spreads outward from this zone due to rotation of the earth and low pressure is caused.
- The sub polar low pressure belt is more developed and regular in the southern hemisphere while it is broken in the northern hemisphere because of over dominance of water in the former.
Polar High Pressure belt – High pressure persists at the poles throughout the year because of the prevalence of very low temperature all the year round. There is thinning out of layers of air due to the diurnal rotation of the earth as the earth spreads outward due to this effect is overshadowed by the thermal factor and hence high pressure is produced due to very low temperature.
With this we have completed Today’s topic. Hope you have learned today.
We will post Quiz on Air Pressure – The Cause Behind Movement of Winds in Today’s Cracking Session.
Please give your feedback in comments, we are ready to serve better 🙂
Subscribe and Share !
Keep Visiting !
Happy Learning !