500 Millibar Level (approximately 18,000 feet)

500 Millibar Level (approximately 18,000 feet)

To most people this chart looks something like a bowl of spaghetti. At first glance the squiggly lines seem to wander all over the place, making little sense. Meteorologists call this the 500 millibar chart. Instead of plotting weather data such as temperature, wind, and moisture at a specified altitude it is plotted at a specified pressure - 500 millibars. The 500 millibar pressure reading is higher in some locations and lower in other. It just so happens that at 500 millibars half of the mass of the atmosphere is above 500 millibars and half is below it. Its altitude is roughly at 18,000 feet.

Pressure maps are similar to a topographical map that we use to show variations in the altitude of land. The highest points on the map above (marked by an "H") are like mountains - the 500 millibar pressure is found at a higher altitude than at the low points which are marked by an "L". The solid lines in between the Hs and Ls are the same as lines on a topographical map. Just like on a topographical map, the closer the lines are together the steeper the slope. The farther they are apart the flatter the terrain. In our case the winds are stronger where the lines are close together.

On this chart you are looking down over the North Pole. The flow at 500 millibars is meandering from west to east around the pole. This west to east flow is found generally north of 25 to 30 N latitude. As the flow dips southward cold air is flowing away from the polar areas. Where it turns back to the north warm air is moving away from the topics. This process is part of the Earth's natural temperature regulation.

Notice the dip southeastward in the flow from western Canada to the Upper Midwest. This is a new push of colder air headed toward the Upper Midwest. Areas of snow and blowing snow expected to cause blizzard or near blizzard conditions and drop temperatures considerably.  See www.weather.gov for more information on the latest forecast.

The winds aloft are strongest where the lines are closest together. When the lines are close together it is call a tight gradient. Where the lines are far  apart it is a weak gradient and winds are weak. It is the general west to east flow that brings periods of stormy and calm weather. It is the ever-changing flow that determines what the next few days will be like. More about that in another post.