# Atmospheric Variables

Variables that are used to describe the atmosphere are as

• Pressure
• Temperature
• Velocity
• Humidity

## Pressure

Pressure can be thought of in two way

• Pressure is the force exerted by all these air molecules colliding with a unit area of surface
• Pressure is the weight of air above you.

the 2nd definition also explains why pressure always decreases with respect to height.

The unit of pressure is N/m2 or Pascals (Pa). Pascal is a very small unit and meteorologists often express pressure in millibars

• Average Sea level Pressure= 101325 Pa or 1013.25 millibars

## Temperature

Temperature is often expressed as the mean kinetic energy of the molecules(which is related to the velocity of molecules but is not the same thing )

Kinetic energy is K.E = 1/2(mv2) so it depends not only on velocity but also on molecule mass.

All of these molecules have the same average kinetic energy but some are heavier than others

For example, the air is composed of different molecules (Oxygen, Nitrogen, Helium, Hydrogen) their molecular weight and velocity are

• Oxygen (molecular weight= 32 and velocity=478m/s)
• Nitrogen(molecular weight= 28 and velocity=511m/s)
• Helium (molecular weight=4 and velocity=1353m/s)
• Hydrogen(molecular weight=2 and velocity =1904m/s)

The unit of temperature can be expressed either in celsius or kelvin.

## Velocity

Here we are talking about the velocity of the wind not the velocity of individual air molecules. The wind is generally much slower than the motion of air molecules themselves.

Velocity is a vector so the wind has both speed and direction. The wind velocity has three components in the x, y, and z direction. But in wind energy, we consider mostly the horizontal motion of the wind and are rarely concerned with the vertical motions of the wind.

The unit is m/s

## Humidity

Humidity is the amount of water vapors in the air. water vapors does not play a huge role in wind energy.

Humidity= kilograms of water vapors/ kilogram of dry air

It is usually very small so we multiply by 1000 to get

Humidity= grams of water vapors/ kilogram of dry air