During World War I, long before there were computers, Lewis Richardson came up with the idea that we could forecast the weather by solving the basic equations of motion as applied to the atmosphere. To solve these equations would require hundreds of thousands, if not millions of calculations to adequately model the atmosphere. What Meteorologists do each day is look at these forecast models on various levels of the atmosphere. What is occurring here at the surface, is a direct reflection of what is occurring in the mid and upper levels as well. This surface map shows several things. Forecast precipitation in purple, atmospheric thickness/trough-ridge pattern ( yellow lines in the 500 range), and location of pressure systems. Thickness represents how warm or cold it is. Like metal, as the atmosphere heats, it expands. This tells us that higher atmospheric thicknesses means warming temps while lower thicknesses indicate cooling temps.

As we move up in the atmosphere, we use Milibars (mb) as a way to measure the pressure with height. 850 milibars is about 5,000 feet above the surface. The forecast model at this height shows us warm and cold airmasses, their movement, wind direction, wind strength, and the location of pressure systems in this part of the atmosphere. What Meteorologists really look at here is advection patterns. Advection means to transport from one location to another. As we look at wind direction and strength as indicated by the arrows, it tells us whether the movement will be transporting cold or warm air into a region. A process known as cold air advection and/or warm air advection. Remember as a warm front moves through, we experience warmer temps with warm air advection. After the frontal system and it's associated cold front move through, we see shifting winds and cooler temps with cold air advection.

About 10,000 feet above the surface shows us upward vertical velocities and how strong they are. This tells us specifically how strong the lifting of air is and where is it the strongest. This is important since rising air motion contributes to clouds forming and precipitation developing. As air rises, it cools. If it rises and cools enough, water vapor in the atmosphere will condense out into clouds. Remember low pressure areas create/contribute to rising air. That's why we usually associate unsettled weather with lows. Notice on this level of the forecast model that most of the upward air movement is very near and in association with low pressure systems. Note the yellows and reds, especially to the right of lows where warmer, lighter, less dense air is being driven north around the circulation and being forced to rise.

This level of the atmosphere gives us a very good look at the trough/ridge pattern. We look for long wave troughs and ridges, and also shorter waves. Many times we see these "upper level short waves" riding through the long wave pattern. These disturbances create upper level energy that contributes to rising air motion and the formation of clouds and precipitation. Specifically, we assess the strength of "vorticity" in the atmosphere. Vorticity is the "spin" of something. There are two types. Negative vorticity refers to a clockwise spin, associated with high pressure and ridges. Negative vorticity contributes to sinking air and a drying out of the atmophere. Positive vorticity is a counterclockwise spin, associated with low pressure and troughs. This positive vorticity contributes to rising air, lowering air pressure at the surface, and usually the formation of clouds and precipitation. Notice the upper level low over northern Mississippi, and a long wave trough over the Pacific Northwest with lows and plenty of vorticity from the northern Rockies into North Dakota.

This is the level most commercial airlines fly. It's also the best level to examine the location of the jetstream...the river of fast moving upper level winds that help transport high and low pressure systems across the country from west to east. Remember troughs are generally associated with cooler temperatures and unsettled weather, while ridges are conducive to warmer temps and fair, dry weather. This forecast map shows us the trough/ridge pattern and jet streaks. These streaks are locations of fast moving air embedded within the jetstream itself. Some of these jet streaks can move as fast as 150 knots and certain parts of them contribute to rising and sinking air. The locations that force rising air are associated with upper level divergence. Air that diverges in the upper level creates a vacuum at the surface and therefore must have air converging at the surface. This contributes to rising air, lower air pressure, and the formation of clouds and precipitation. Sometimes these streaks also aid in the development of severe weather because of the added "lift" in the atmosphere. Locations of a jet streak that promote sinking air have upper level convergence associated with them. Air that converges in the upper level must go somewhere...and it can't rise...so it's forced to sink. That creates a dry atmosphere and generally fair weather.