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Millibar vs. Feet

Millibar vs. Feet



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On the skew charts we have millibars on the left hand side of the chart,My question is how do we change Millibars ie 1000mb or 900mb to feet?

Nigel

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Ok, let's talk about the upper air. Nigel is referring to the Skew T chart. This is a graph of the atmosphere in the vertical direction (up!). You get this graph from a weather balloon which is released each day, twice a day, at the NWS office. If you've never seen a Skew T, go ahead and click the first link on the right to link up to today's chart. Skew T's are an essential tool in weather forecasting.

The atmosphere is made of layers, if we just took the weather balloon and went up, we can take the temperature at different heights at the atmosphere. The pressure at these heights changes all the time though. Pressure surfaces in the atmosphere are not constant like height is.

Pressure on the skew T is measured in millibars. One millibar is .02953 inches of mercury. You can convert between pressure units but you can't convert a unit of pressure to a unit of distance (height).

You can use the standard atmosphere chart to figure out a ballpark of what pressure levels equate generally to what level of height. Again, the atmosphere is always moving around like a liquid, so pressure surfaces change. We take a pressure reading all over the country to get an upper air chart, the link on the right will show you one. This 700 mb chart should have heights around 3100 meters on it. This means that the height of the atmosphere at 700 mb is DIFFERENT over different parts of the country. Or, you can think of it as where the pressure is measured to be 700 mb is at different heights in different cities. This is a very hard concept for a lot of people to understand. Think of slicing the atmosphere up in layers, the pressure in that layer is going to be different over say in Sandy than probably in St. George.

At any rate, you can't just go from millibars to feet. You can get a general idea using the standard atmosphere chart which is also on the right. Generally, a pressure of 700mb will correlate with a height of 3000 meters, you can convert meters to feet pretty easily. One meter equals 3.28 feet.

And a side note on the skew T while we're here. The red line on the right side of the graph corresponds with the temperature and the line on the left is the dewpoint. These readings are taken by a radiosonde (a weather sensor) attached to a weather balloon filled with hydrogen. Very cool.

Answered by KSL Meteorologist Dina Freedman.

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