Highs and Lows


On the upper right side of our weather map, highs and lows are probably the most significant features. When you see H’s and L’s on the map, they are placed over the areas of lowest and highest pressure. Isobars, lines connecting constant surface pressure, are typically drawn at 2 or 4 millibar intervals. Air flows from areas of higher pressure to lower pressure, however due to the rotation of the Earth, the apparent Coriolis effect deflects air to the right in the Northern Hemisphere. Three forces explain the clockwise motion around a high and the counterclockwise motion around a low.



PRESSURE GRADIENT FORCE
This force is what causes air to blow from areas of higher pressure to lower pressure.

CORIOLIS FORCE
This force is caused by the rotation of the Earth about its axis.

FRICTION
By looking at the highs and lows on the upper right panel, information such as placement of troughs and ridges and also the positioning of fronts can be understood. By looking at spacing between isobars, you can determine the speed of the wind. The more concentrated they are, the greater your wind speed. Low (1) has a strong pressure gradient thus winds are strong, whereas Low (2) does not have as strong a gradient and the winds will not be as strong.