One of the biggest weather trends for the past couple of months has been the occurrence and effects of upper level low pressure systems. These tough to forecast systems bring a mixture of sun, rain and storms to the lowlands and even late spring snow to the higher mountains.
Weather as we know it occurs in the atmosphere above us. All the air in the atmosphere has weight, and we call this weight atmospheric pressure. The higher the altitude the lower the pressure because there’s less air. The changes of pressure in the atmosphere above us determine the type of weather we see from the ground. This is what forecasters mean when they mention ridges of high pressure and systems of low pressure. These differences in pressure dance around and above each other in the skies above the earth all the time.
These differences in pressures are viewed similarly to topographical maps of terrain. The closer the lines are together the steeper the “pressure gradient” which is the rate that air is moving from one spot to another. This is also a way to forecast wind.
Forecasters use these maps made up of surface data to view and analyze the weather systems that are days ahead of reaching us and to determine their strength and path with computer weather models. However, upper level lows are not visible on the surface pressure map, because the disturbance is in the upper part of the atmosphere.
Upper level low pressure systems can be described as a rotating pocket of cold rising air moving above warmer air. These systems can be clearly seen in visible satellite imagery.
The upper levels of the atmosphere are harder for forecasters to analyze because of the lack of observations. In order to measure the pressure high up, they need to launch weather balloons, which is difficult when an upper level system is developing over the ocean. The data that we do collect from the upper levels can be seen on weather maps such as the 500mb viewed below (or 18,000 ft.).