Knowing basics can help you interpret map
As a meteorologist looking at a weather map, whether it be a chart of atmospheric pressure, contours of temperature, or just a jumble of weather symbols, the overall picture is usually crystal-clear to me. Sometimes I take that knowledge for granted, however, and forget that not everyone makes immediate sense of that kind of information. What I consider the basics may look like a foreign language to some, so I thought I would cover some of these basics to help you understand what you might see on your local weather map on any given day.
Two letters that are very common on weather maps, are the big blue H and the big red L. The H stands for high pressure and the L stands for low pressure. I joke with school kids that the pressure I’m describing is not the type of feeling they have before a big test they didn’t prepare for. I’m talking about air pressure, which is the force of air molecules as they move around, colliding with each other and surrounding objects. On a map of sea level pressure, the H merely marks the center of highest pressure, relative to the levels around it.
To understand how these symbols can help you gauge what type of weather to expect, you should know that air likes to move from higher to lower pressure. This is easily proven by blowing up a balloon. Air molecules inside the balloon are confined to a limited space – they are more crowded in there – and as such there are more collisions going on between molecules and with the surrounding balloon.
The air is under higher pressure inside the balloon than outside of the balloon, so if you don’t tie off the end of the balloon, the air will come rushing out. Because air likes to move toward lower pressure, the location of the big red L on the weather map usually indicates where there will be some stormy or at least unsettled weather. As air rushes toward the low from all sides, it converges (comes together), and with no where else to go, it is forced to rise. Rising air is necessary for cloud formation and precipitation.
With surface high pressure, air is moving away from the center of the high. As air near the surface spreads out away from the center of the high, air from above moves down to fill the void. In other words, you have sinking air, which suppresses cloud formation and precipitation. Though there are many exceptions (who said meteorology was simple) an L on the map over North Idaho does not bode well for outdoor recreation. We usually refer to low pressure areas as storm systems. An H overhead usually means quiet weather – though not always sunny, especially during the winter months.
You may often hear the term ridge of high pressure. That term is used because on certain weather charts that meteorologists use, the contour lines will form a hill, or a ridge where the area of high pressure is. Likewise, an area where the contour lines dip indicates an elongated area of low pressure, or a trough. The difference in pressure between the highs and lows is what causes the wind to blow. The stronger the difference in pressure – called the pressure gradient – the stronger the winds are.
Though low pressure areas are usually the “bad guys” of weather, there are a couple of instances when lingering high pressure is not so great either. In the winter, extended days under high pressure lead to air stagnation and a decrease in air quality. In the summertime, high pressure parked over one part of the U.S. for long periods can lead to or worsen drought conditions.