High pressure:

Generally associated with a region of sinking or descending air.  This air is compressed as it sinks and is warmed slightly.

Clear skies & fair weather

Low pressure:

Generally associated with a region of rising or ascending air.  This air is expanded as it rises and is cooled slightly.

Cloudy skies & rain

Pressure Gradient Force (PGF)

• Directed from high pressure to low pressure
• Magnitude is equal to the change in pressure between 2 points divided by the distance between the 2 points;
Size of PGF=(Pressure decrease)/(distance)
• PGF in the vertical direction is directed upward since the pressure in the atmosphere decreases as altitude increases.  This is generally balanced by the weight of the air above that point (pulled downward by gravity).
• When the vertical PGF is equal in magnitude to the weight of the air above, there is no vertical movement of the air.  This is called hydrostatic equilibrium.
• PGF in the horizontal direction:
• On a sea level pressure chart the direction of the horizontal PGF is perpendicular to the isobars and from high to low pressure (from larger numbers to smaller numbers).  The magnitude is larger where the isobars are closer together, than where they are farther apart.
• On a constant pressure chart (i.e. the 500 mb map handed out in class) the direction of the horizontal PGF is perpendicular to the lines of constant height and from high to low heights (from larger numbers to smaller numbers).  The magnitude is larger where the contours are closer together, than where they are farther apart.

• Coriolis Force (CF)
• Caused by the rotation of the earth.  This force is only apparent to observers that are attached to a coordinate system that rotates with the earth.  An observer not rotating with the earth would not see this.  That is why the Coriolis force is often called a pseudo force.  None-the-less, we are rotating around with the earth and we will see the results of an object moving in this rotating coordinate system as the Coriolis force.
• Characteristics
1. It is independent of the direction of travel and acts perpendicular to that direction: Toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere.
2. Strongest at the poles and decreases toward the equator where it is zero.
3. Is proportional to the speed of the object
4. Is a deflective force, so it affects the direction and NOT the speed of an object.