When you look at a weather map on tv, you're typically going to see highs and lows, or areas of high pressure and low pressure. And you might know that air flows clockwise around Highs and counterclockwise around Lows. But do you know why? Think of a bike tire. When you pump up a bike tire so it's nice and firm, you've increased the pressure inside the tire. Let's say you open the valve. What happens to the air inside the tire? Of course, the air flows OUT of the tire. So air flows FROM high TO low pressure. Now let's apply this principle to a surface weather map. If you see a surface High and a surface Low on a map, you would assume the air would flow from the H to the L, in a straight line. But that is not the case. Air actually flows nearly parallel to the two pressure systems. But why? The answer is the fact that the Earth is constantly spinning on its axis (ie rotating). This rotation causes air to turn to the right. This "turn to the right" is caused by a force called the Coriolis Force. To see why this is the case, think of a record spinning on a record player. If you take a marble and roll it outward from the center of the record, its path will turn in the direction of rotation. The same thing happens as air flows from high to low pressure on the Earth's surface. It turns to the right.
-Kevin Coskren, ABC6 StormTracker WeatherTeam