Thermodynamics

PRESSURE

Pressure is a force applied over an area. When discussing storms, air pressure can be thought of as the weight (the force due to gravity) of the air above a certain level. Pressure is dependent on temperature and density of the air, or the volume that a certain amount of air is contained in. This is known as the Ideal Gas Law. NOTE: Pressure decreases with increasing altitude

P=F/A

Figure 1. Atmospheric Pressure

IDEAL GAS LAW

The ideal gas law states that the pressure of a gas is directly related to it’s temperature and inversely proportional to the volume that a fixed amount of gas is held in.

PV = NRT

Why is this important? When temperature or density of air change there is a pressure difference. Since pressure is actually a FORCE applied over an area then from Newton’s second law we know that this force will cause acceleration of the air. This is how winds are created. We call this force the PRESSURE GRADIENT FORCE.

Figure 2. The net force in this diagram is the PRESSURE GRADIENT

FORCE

Question: What happens air in one region is denser than in another?

Answer: When there is a column of air that is denser than another we have a pressure difference. As density increases we know that, from the ideal gas law, pressure increases. This means we will have one column of high pressure and one of low pressure. From Newton’s 2^nd Law we will have an acceleration of air from the high-pressure column to the low-pressure column creating a wind.

Question: What happens when one region of air is heated to a warmer temperature than another region of air?

An increase in temperature will cause an increase in pressure in one area. This causes a pressure gradient force that will create a wind.

Air flows from HIGH to LOW pressure. How does this cause cyclones to form?

When one region of air is at a high pressure and another at a low pressure the pressure gradient force will cause winds to start to blow. These winds blow slightly across the lines of constant pressure due to the Coriolis force, creating spiraling storms. It is this same force that creates both large scale cyclones and smaller scale thunderstorms.

Atmospheric Pressure with Height

As seen in the diagram above, atmospheric pressure decreases with height. This can be modeled nicely by the magnet demonstration, shown in the section on demonstrations. This is because atmospheric pressure is related to the mass of the air above an area, and as you go further up in the atmosphere there is less air above you and therefore you experience a lower pressure. The blue triangle in the picture above represents Mount Everest, which has an atmospheric pressure of about half that at sea level.

NEWTON’S LAWS THERMODYNAMICS FORCES IN THE ATMOSPHERE CORIOLIS FORCE STORMS THE DEMONSTRATION HOME

Ó 2004 by Marisa Demers