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# Surface Analyses

The surface analyses show plots of surface station readings, lines of equal Mean Sea Level air pressure (called isobars), fronts (as drawn in by CMC meteorologists) and centres of high and low pressure. The graphical depiction of weather conditions used in the station plots follows the standards of the World Meteorological Organization.

## Upper Air Analyses

The upper air analyses depict the conditions at various standard altitude levels. They use a different convention than the surface analysis, and do need a little getting used to. The principle is this:

• Since air pressure diminishes with height according to a well-known mathematical formula, pressure can be used, instead of metres or feet, to measure altitude in the atmosphere.
• Moreover, exactly as air pressure changes slowly according to our position at the Mean Sea Level, the actual height of a constant pressure level changes slowly if we move horizontally.

Consider the chart obtained by measuring the air pressure at 5500 metres (the approximative height of the 500 hPa pressure level) at many points over Canada, and drawing isobars. While that chart would be a direct counterpart of the Mean Sea Level pressure analysis, it is actually easier, for technical reasons, to measure the height of the 500 Hectopascal pressure level at many points and then drawing lines of equal height. This is a little bit counterintuitive at first, but just remember that the two approaches are equivalent.

In addition to the main parameter of height, the upper air charts also contain secondary fields in dashed lines. Here is a brief explanation of each field:

• 850 hPa, 700 hPa: Secondary field is temperature in degrees Celsius
• 500 hPa preliminary: Secondary field is 1000 hPa to 500 hPa thickness. This is indicative of the average temperature of the column of air under the 500 hPa pressure level. Larger thicknesses indicate warmer air.
• 250 hPa: Secondary field is wind speed. This reveals the location of Jet Streams.

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