Pressure and Mixing Ratio

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These panels depict sea-level pressure and mixing ratio. This chart is useful for finding high and low pressure systems. Its also useful for showing regions of moist air at the surface.

The solid white contours represent pressure contours (isobars) in millibars. The isobars have a contour interval of four millibars. The wind speed is directly related to the distance between the isobars. The closer the isobars are together, the stronger the pressure gradient, and the stronger the wind.

The color filled region on the background represent lines of equal mixing ratio. Mixing ratio has a contour interval of two grams per kilogram. Mixing ratio is another convenient way to express humidity, or atmospheric moisture. Mixing ratio is the mass of the water vapor in a specific volume to the mass of the remaining dry air. Another way to express that is Mixing ratio = mass of water vapor / mass of dry air. The units are grams per kilogram. Example: If you have a mixing ratio of 5, that means you have 5 grams of water vapor to every kilogram of dry air.

From the map above you can sometimes find warm fronts, cold fronts, and drylines. Fronts are usually located where the mixing ratio changes drastically over a short distance. A dryline is also an area where mixing ratio changes drastically over a short distance. During the day, temperatures are warmer west of the dryline and at night they are warmer on the east. Drylines can usually be found in the Plains during the development of low pressure systems.

For more information on how pressure affects the weather go to our forces and winds module.

12hr pres. change
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

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