When watching forecasts, the public have become familiar with terms like: high and low pressure systems, cold and warm fronts and the jet stream. Depending on your region, you may hear about Sea Breezes if you live near the ocean, lake breezes, if you live near a lake or even see a map depicting velocity during a severe weather event. One of the most important forecasting tools for explaining a long-term forecast often goes overlooked — this is called Geopotential Height, which can is often styled as a height anomaly. The anomaly describes a departure from average or usual occurrence at this level.
The layer of the atmosphere called the troposphere is where most of the weather occurs on the planet. This layer ends at the tropopause, which is on average 36,000 feet high. The reason this would be an average is because the atmosphere fluctuates based on warm and cold regions and is not actually a steady layer throughout. This case goes back to the general rule of science that heat rises and cold sinks — the best application of this rule is in the atmosphere!
Atmospheric pressure is commonly measured in millibars (mb) in forecast models. Standard pressure at the surface (ground level) is defined as 1013.25mb, but we see this change all the time. For example, a high-pressure system event could bring this value up to 1030mb or a low-pressure system can bring this value below 1000mb. While these values do not account for extremes, they provide example for changes at the surface. The jet stream is another subject that is discussed as a forecasting tool in broadcasting that shows a source region for warm or cold air and how it is expected to flow. This area is found around 250mb, which on average is about 35,000 feet high. Recall that pressure levels in atmospheric sciences refer to different heights in the atmosphere that are used to dictate the activity at the surface.
Right between the surface level and the jet stream is the 500mb height (between 16,000 and 20,000 feet in the atmosphere). This area is perfect to provide a look at the source regions of an air mass and can assist in long-term temperature forecasts (7-14 day timeframe). Height anomalies appear in red or blue color shades that determine the intensity. For instance, the darker the red color appears then the higher the height will be. The darker the blue, the lower the height will be. As one could imagine, the red correlates to warmth and the blue correlates to cooling.
Photo: An example of a 500mb height anomaly map from the GFS Forecasting Model depicting areas of colder than average temperatures (in blue) and areas of warmer than average temperatures (in red). (Courtesy of Pivotal Weather)
These maps are best related to temperature probability maps that are provided by the National Weather Service (NWS) depicting an outlook of 6-10 days or an outlook of 8-14 days. While the timeframe can change for specific events like a rain shower, this displays an expectation of which part of the country will see warm or cold temperatures.
Photo: An example showing the 6-10 day temperature outlook for the United States, which depicts the surface conditions with a similar color coding to 500mb height anomaly maps. (Courtesy of the National Weather Service)
Considering that these maps help us track air masses through the atmosphere, we can look at a source region of the air as it approaches your area. This will help distinguish potentially drastic changes. This photo depicts the common air masses found in the United States:
Photo: Air Masses in their common source region with abbreviations over North America. (Photo courtesy of Pennsylvania State University)
For a worldwide view, we can see these source regions globally in this photo:
Photo: Air Masses from their source region across the world. (Image courtesy of Bahrain Weather)
The positioning of multiple air masses can lead to prolonged periods of heat or warmth for a region because of a block. Strong areas of high pressure will keep another air mass ‘blocked’ in to a region where it can’t get out and lead to periods similar-occurring weather patterns for multiple days. These drastic periods can be seen distinctly on a map of 500mb height anomalies that gives a view of where the air mass would be coming from and can aid in determining what is happening at the surface.
Multiple forecasting tools exist and the 500mb height anomaly offers further confirmation of long term possibilities for temperature trends on your area. The usual forecasting topics heard above the surface involve the jet stream, but there are multiple levels between them that can help put the pieces together for a more confident forecast.
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©2019 Meteorologist Jason Maska