Unit 2: The atmosphere                                                     1   2   3   4   5

2. Atmospheric pressure and humidity

2.1. Atmospheric pressure

Air has weight.

The pressure it exerts on Earth’s surface is called atmospheric pressure.

It is caused by gravity.

a) Units of measurement

Pressure is a magnitude. The international unit for pressure is the Pascal (Pa).

When we refer to atmospheric pressure we normally use

the atmosphere (atm), the millibar (mb) or the hectopascal (hPa).

b) Factors provoking variations of atmospheric pressure

      - Altitude

       Atmospheric pressure decreases when altitude increases.

       This means that the higher the altitude, the lower the pressure. 

       This is due to the lower layers supporting the weight of the upper layers

       and they are compressed.

       The gases are compressed and denser near the surface. It provokes

       that the atmospheric pressure on the top of a mountain is less

       than at the sea level.

         Normal pressure is the pressure at sea level (1 atm or 1,013 hPa).

             - Pressure above this is high pressure

             - Pressure below this is low pressure.

      - Temperature

       Atmospheric pressure decreases when temperature increases. 

       This means that the higher the temperature, the lower the pressure. 

       Air moves because the Sun heats it. Hot air rises because its density

       is lower and the colder air moves in to take its place. This is the origin

       of wind.

               - Low pressure area (depression or cyclone).

               In depressions the pressure decreases towards the center.

               The air, light and hot, moves from down to up.

               The ascendant air is moist. It provokes unstable weather,

               because when this water vapour gets colder, it condenses

               and provokes precipitations.

            - High pressure area (or anticyclone).

               In anticyclones the pressure increases towards the centrer.    

               The air, heavy and cold, moves from up to down.

               The descendent air is dry. It provokes stable weather,

               because clouds do not form.

The air moves from the areas of high pressure to the areas of low pressure,

causing air currents or winds.

2.2. Atmospheric humidity

Air contains water vapour because of evaporation (from the surface of seas,

lakes and oceans) and transpiration (from living beings, especially plants)

Humidity is the amount of water vapour that the air contains.

It varies depending on the place, the time of day and the temperature.

Hot air usually contains more water vapour than cold air.

If air is completely dry, its humidity is zero. If it is saturated then it is 100%

and if it contains half the possible water vapour then humidity is 50%.

2.1. Answer the following questions about atmospheric pressure:

       a. What is the atmospheric pressure?

       b. Why does atmospheric pressure decrease when we climb a mountain?

       c. How does temperature affect atmospheric pressure?

       d. What measurement unit do we use to refer to atmosphere pressure?

       e. Where do we find normal pressure? What is its value?

2.2. The following sentences are wrong. Correct them:

       a. Humidity is the pressure that the air exerts over the Earth’s surface.

       b. The atmospheric pressure is lower at sea level than at the top

           of a mountain.

       c. The air is saturated of water vapour when it has 80% of humidity.

       d. The wind goes from low pressure areas to high pressure areas.

       e. Anticyclones provoke unstable weather, precipitations and low

           temperatures.