What is Pressure?
Pressure is defined as the force exerted on the earth’s surface and other objects by air because air has weight.
On maps, places having equal atmospheric pressure are joined together by lines called isobars.
Factors affecting pressure:
- Altitude – when air descends, its pressure and temperature rise but when it ascends, it becomes less dense and it is pressure and temperature fall. In other words, pressure decreases as altitude increases.
- Temperature – when the temperature of the air increases, its pressure decreases, and when the temperature decreases, its pressure increases.
- Wind – it is known that wind blows from a high-pressure belt to a low-pressure belt.
- Rotation of the earth – owing to rotation of the earth, differences in pressure result in the deflection of the wind to the right in the northern hemisphere and to the left in the southern hemisphere.
There are four world pressure belts. These are:
- Doldrum or Equatorial low-pressure belt.
- Horse latitudes or Sub-Tropical High-Pressure Belt
- Temperature low-pressure belt
- Polar high-temperature belt
The distribution of pressure belt
- Doldrum or Equatorial low-pressure belt – this belt is found around the Equator between 5⁰ north and south of the Equator. Owing to intense heat, the pressure is low and all winds blow to converge in their belt.
- Horse Latitude or Sub-Tropical High-Pressure Belt – this is found in (sub-tropical latitudes 20⁰ – 30⁰ north and south of the equator. The pressure in this belt is high and winds tend to diverge and an anticyclone is formed.
- Temperature Low or Mid-Latitude Low-Pressure Belt – this pressure belt is found between 30⁰ and 60⁰ north and south of the equator. They are zones of convergence with cyclonic activities.
- Polar High (High-Pressure Belts) – these are found around the Polar Regions and planetary winds shift with seasons.
- Relative Humidity – Humidity is defined as a measure of the dampness o the atmosphere due to the presence of water vapor in the air. The actual amount of water vapor present in the air is called absolute humidity. It is expressed in g/m3
Relative humidity, therefore, is the ratio between the actual amount of water vapor and the total amount the air can hold at a given temperature. The relative humidity is expressed as a percentage.
If the air contains only half the amount it can hold, the relative humidity is 50%. When the relative humidity reaches 100%, the air is said to be saturated. At this stage, the air temperature is said to be at a dewpoint. Further cooling will condense the water vapor into clouds or rain.