The 500 Millibar (mb) chart is a crucial tool for meteorologists in understanding and predicting weather patterns. Often considered the “middle” of the atmosphere, this constant pressure chart provides valuable insights into atmospheric dynamics. Let’s explore what the 500 millibar level signifies and how it helps forecast weather.
Decoding the 500 Millibar Height
The 500 millibar level typically resides between 16,000 feet (4,980 meters) and 20,000 feet (6,000 meters) above sea level. On a 500 mb chart, height contours depict the altitude at which the atmospheric pressure reaches 500 millibars. These contours are represented by lines connecting points of equal height. Understanding these lines is key to interpreting the chart. The values displayed on the chart omit the last digit (zero) for brevity. For instance, a value of “564” indicates a height of 5,640 meters (18,503 feet).
Vorticity and the 500 Millibar Level
The 500 millibar chart also illustrates atmospheric “spin,” known as vorticity. Absolute vorticity, a combination of shear and rotational vorticity, is measured at this level. It indicates the total spin of the air at a given location. Vorticity values on the chart are whole numbers, omitting the unit of 10-5/sec. For example, a value of “16” represents 16 x 10-5/sec. Higher vorticity numbers signify faster rotation.
Areas of maximum vorticity, called vorticity maximums, help pinpoint shortwave troughs embedded within larger atmospheric waves (longwaves). More critical than the vorticity value itself is the rate at which vorticity changes. Increasing vorticity downstream (ahead of) a maximum often indicates rising air and potential precipitation. Conversely, decreasing vorticity upstream (behind) a maximum suggests sinking air and fair weather.
Humidity at 500 Millibars
Moisture content at the 500 millibar level, depicted by relative humidity contours and shading, is another critical factor. While this level might have 70% relative humidity (indicated by light green shading), other atmospheric levels could have varying moisture content. This data, along with vorticity and height contours, contribute to a comprehensive understanding of atmospheric conditions and aid in forecasting.
Conclusion: The 500 Millibar Chart’s Significance
The 500 millibar chart provides a crucial vertical profile of the atmosphere. By analyzing height contours, vorticity, and humidity at this level, meteorologists gain valuable insights into weather patterns and potential for precipitation. This “middle” atmospheric layer provides a foundational understanding for accurate weather forecasting.
