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This chapter explains how surface weather observations are plotted on weather maps using the Surface Station Model. It covers decoding sea level pressure data, reading wind speed and direction, and the use of isobars and isotherms. The relationship between winds and pressure patterns is explored, as well as the use of pressure as a vertical coordinate in meteorology.
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Surface Station Model Used to plot surface weather observations on weather maps It shows:
Decoding Sea Level Pressure Data If coded SLP is greater than 500: Put a 9 in front of the 3 digit coded SLP Insert a decimal point between the last two digits Add units of mb Example: coded SLP = 95 6 Decoded SLP = 995.6 mb If coded SLP is less than 500: Put a 10 in front of the 3 digit coded SLP Insert a decimal point between the last two digits Add units of mb Example: coded SLP = 052 Decoded SLP = 1005.2 mb Reading Wind Speed and Direction Meteorologists want to know what direction the wind is coming from, so wind direction always indicates the direction that the wind is coming from.
Isotherm – contour line of constant temperature Temperature gradient – change in temperature over a given distance Isodrosotherms – contour lines of constant dewpoint temperature How do the areas of warm temperature compare with dewpoint?
Typically we use height (altitude) as a vertical coordinate in everyday life. Since pressure always decreases with height, and above any given spot on the earth each height has a unique pressure we can also use pressure as a vertical coordinate. Why do meteorologists use pressure as a vertical coordinate?
Pressure surface – an imaginary surface above the ground where the pressure has a constant value How does the pressure change, at an elevation of 3000 m, as you move from left to right in this figure?
Commonly Available Constant Pressure Maps Pressure Level Approximate Altitude (ft) Approximate Altitude (km) 850 mb About 5,000 ft About 1.5 km 700 mb About 10,000 ft About 3.0 km 500 mb About 18,000 ft About 5.5 km 300 mb About 30,000 ft About 9.0 km 250 mb About 35,000 ft About 10.5 km 200 mb About 39,000 ft About 12.0 km Constant pressure maps are typically available twice per day at 00 and 12 UTC. Why are these maps only produced twice per day? Upper Air Station Model What are the differences between the surface and upper air station model?
Sample 850 mb Constant Pressure Level Map What data are plotted on this map? What atmospheric features can we see on this map?
30 0, 250, and 200 mb Constant Pressure Level Maps Sample 300 mb Constant Pressure Level Map What data are plotted on this map? Isotach – contour line of constant wind speed What atmospheric features can we see on this map? Jetstream – band of strong winds that circle the earth at mid-latitudes Jet Streak – region of exceptionally strong winds embedded in the jetstream
