We have a very simple forecast to finish off the month. A reasonably strong ridge of high pressure will park itself on the west coast keeping us dry and mostly sunny though Friday with a few clouds coming through the region over the weekend. Temperatures will rise to the mid-60s Wednesday and climb into the low 70s through Saturday, before trimming back to the mid-60s early next week.

500 mb

Nancy and Dave wondered what makes a high or low pressure system “big.”  The “size” or perhaps better put “strength” of a low and a high has to do with how low or high up in the atmosphere you have to go to reach the particular pressure surface, or how high or low the pressure is at the surface of the earth. Let me explain.

There are really two different ways that highs and lows are measured, depending on whether they are surface features or if they are higher up in the atmosphere.

Surface analysis

To map them at the surface, you simply measure the pressure at sea level (or measure the pressure at ground level above the surface, and correct to sea level) at various points on the map. Plot the pressures in millibars, and connect lines of equal pressures, much like you would when drawing a topographic map. Those lines are called isobars, and they define the surface highs and lows.

Tomorrow, I’ll talk about upper level features.