Atmospheric pressure is defined as the pressure the atmosphere exerts on the Earth surface due to the weight of the Earth. At sea level, atmospheric pressure has a value of 101,000 Pa.

We do not feel atmospheric pressure because;

• The pressure inside our bodies (blood pressure) is always the same as the pressure outside our bodies (atmospheric pressure).
• Atmospheric pressure acts in all direction.

Factors Affecting Atmospheric Pressure

• Humidity:  Humidity is the amount of water vapor in the atmosphere. When humidity  increases  atmospheric  pressure increases and vice-versa.
• Height or Altitude: Atmospheric pressure decreases with increase in height or altitude, vice versa. The higher an object goes in the sky, the lower the atmospheric pressure exerted on the object. This explains why;

1. It is difficult to cook food at the mountain top than at the foot of the mountain. This is because, at the top of the mountain, atmospheric pressure is lower affecting the boiling point of water.
2. Passengers in a plane experience nose bleeding because atmospheric pressure has dropped meanwhile the blood pressure remains constant.

The existence of atmospheric pressure maybe demonstrated using the collapsing can experiment.

The Collapsing Can Demonstration ( Experiment)

The can is not collapsing because pressure outside is equal to pressure inside the can.

The can collapses because pressure inside has removed by the pump and the pressure outside.

Measuring Atmospheric Pressure

Atmospheric pressure can be measured using the instrument called barometer. It has basically two types, namely;

• Mercury barometer and
• Anerold barometer.

1. Mercury Barometer:

The mercury barometer consists of a glass tube containing mercury. One end of the glass tube is sealed and the other end is placed in a bowl of mercury. When the glass tube is inverted in a bowl of mercury, the mercury falls until it is about 760mm above level in the bowl.

The pressure at X due to the weight of column of mercury XY equals atmospheric pressure acting on the surface of the mercury in the bowl. XY measures pressure in mm of mercury (mmHg).

The disadvantages of a mercury barometer are:

• Mercury is very poisonous if eaten.
• The mercury barometer is not portable.

1. Aneroid Barometer:

Aneroid is a Greek word meaning “no liquid”. The aneroid barometer consists of a sealed ca from which air has been removed. I.e. The can is a vacuum. The can can contracts and expand with changes in pressure. When atmospheric pressure is high, the can contacts and when atmospheric pressure is low, the can expands.

The aneroid barometer can be used to predict high atmospheric pressure is associated to fine weather and low atmospheric pressure is associated to bad weather. The aneroid barometer is portable.