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The meaning and importance of 500 hPa charts and dotted lines superimposed on some synoptic charts. A page to help clear the mystery.
What are 500 hPa and thickness charts?
Many Web sites, including those on the An Essential Marine Forecast Page for Sailors, have charts labelled 500 hPa and Thickness. The latter is sometimes called Relative Topography. These charts are not of direct relevance to sailors unless they have a good knowledge of meteorological theory. The reasons for these particular charts to be issued at all is largely historical and dates back to the early days of being able to plot and analyse observations from balloons and aircraft. This coincided, more or less with the outbreak of World War II.
Surface pressure is about 1000 hPa (1000 mb), this is the total force (weight) of the atmosphere. The height at which the pressure is 500 hPa roughly divides in half the atmosphere vertically, half the mass of air being above and half below that height. In terms of height, 500 hPa is about 5,500 metres (18,000 feet) above the ground. The top of that part of the atmosphere in which our weather is formed is known as the tropopause and is at about 11,000 metres (35,000 feet). The 500 hPa level is, thus, effectively half way up the atmosphere as we know it.
The thickness, or relative topography, is the difference in heights between the level at which the pressure is 1000 hPa and that at which it is 500 hPa. This difference in height is a measure of the volume of the air between those two levels. Because volume varies with temperature, the thickness of the layer is a good indicator of the average temperature of the layer.
Why Are They Used
In the early days of numerical weather predictions computers were far too slow and small to attempt anything but the very simplest of atmospheric models. At that time, calculations could only be made at one level. Half way up in terms of both pressure and height of the atmosphere was, for various reasons, a good place to start..
Forecasters wishing to consider modifying Numerical Weather Predictions have conceptual models of atmospheric development. Until the development of NWP to its current standard, forecast charts were produced using these conceptual models. The behaviour of the 500 hPa level and the thickness were central to these concepts.
There is little for the sailor in these charts and I have left reference to them on the Essential Weather Sites for Sailors simply for those who have a meteorological background. One small way in which the charts may be of value is in the implied surface temperatures. Thickness values of 5280 metres (usually shown as 528 decametres) is usually regarded as the thickness below which any precipitation may be of snow. Thickness values exceeding 570 dm are usually associated with warm weather. Conversely, a thickness of 510 dm is getting very cold by UK standards.
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