Two weeks ago we had a rainy Monday, and the next day a visitor here asked an interesting question. He thought there was something wrong with his barometer - the instrument that measure air pressure - because during the rain it continued to show a high air pressure. And, he asked, isn't high pressure supposed to mean clear weather?

I checked the numbers, and indeed, the air pressure was relatively high during that rain, above 30.20 inches. Such pressures actually fall in the "fair weather" category on barometers that have descriptive words on them.

Now it is true that high pressure usually brings fair weather. When exceptions occur, it's often in winter when high pressure also means cold air. When warm moist air tries to move in, it rises up and over this heavier colder air, forming clouds and precipitation. That's usually what's happening when you get a rainy day with relatively high air pressure. One point to add, though - the air pressure usually decreases during the event - in this case, the rain - a sign that a low-pressure area is approaching. That's exactly what happened here.