When studying contrails, you quite often come across diagrams and charts where instead of the altitude in feet or meters, we are given a pressure value in mb (millibars, sometimes also labeled hPa, which is exactly the same unit)
For example, on the excellent site earth.nullschool.net, we have:
Converting between mb and altitude is a slightly complex formula* you can't do in your head. Here's a spreadsheet that does it for a variety of values:
Source: https://docs.google.com/spreadsheets/d/1nDolj1BIv4HcyLYMA32_53S8-BKPu29Fc4cfYK_lC1s/edit?usp=sharing
But in order to more easily use the various sites that use mb/hPa, it's useful to remember a few things.
It's not exact. Pressure varies with the weather, so the conversion is only an approximation. It's generally good to within about 100 feet though.
Higher pressure is lower altitude. So the smaller the mb, the higher we are in feet.
1000 mb is about sea level. The international standard is 1013.2, but "about 1000" is easier to remember.
300 mb is about 30,000 feet. An easy one to remember, most contrails in temperate areas like the US (excepting Alaska) happen above 30,000 feet, so pressure under 300 mb is generally where contrails will form. However, they can form at higher pressure (lower altitude). The key factor is really temperature.
250 mb is about 34,000 feet. This is a very typical cruise altitude. It's also the only cruise altitude that earth.nullschool.net gives as an option. It's a little harder to remember than 300/30000, but it's a nice round number for the middle of the cruise altitude range.
150 mb is about 44,000 feet. This is a useful upper range for possible commercial planes, which will rarely fly about that level.
100 mb is about 52,000 feet. Pretty much a hard limit for the majority of planes, so you can usually discount any mb that's in two digits
Other than nullschool, a great place to use this is NASA's contrail forecast page:
http://cloudsgate2.larc.nasa.gov/cgi-bin/site/showdoc?docid=33&cmd=latest
Here the range is 400mb (23,500 feet) to 125mb (48,000 feet). The color indicates the likely altitudes where contrails will from. Notice the difference between north and south. Lots of green over the US, indicating around 250mb (34,000 feet), the sweet spot contrail altitude.
And of course you will see it a lot on weather charts, for example:
source: http://weather.rap.ucar.edu/upper/
This is a 500mb RH and temperature chart. The green indicates above 70% RH, but since it's only 500mb (around 18,000 feet) we know immediately most of this area will not be cold enough. However look at the -40 line near the top, it's possible some areas of Alaska could see low altitude contrails today.
Regarding units, we are discussing atmospheric pressure here, and most people will be familiar with this as the pressure at ground level, and using the change in that pressure to forecast changes in the weather (higher pressure is clear, lower pressure means rain, very roughly). So you might have seen a barometer like this:
Notice there are two scales, the outside one goes from 28 to 31, and represents inHg (inches of mercury). The inside one goes from 950 to 1050, and represents mb (millibars).
Different units are used both in different situations, and in different parts of the world. In aviation we use inHG (the 28-31 range) to use in calibrating altimeters. In meteorology we more commonly use mb (millibars). But in countries that use the metric system (most of the world) it's common to use the term hPa (hectopascals), which is exactly the same thing as a millibar.
There's one more common unit used, millimeters of mercury, or mmHg, which is just a metric version of inHg, but unlike the mb/hPa, the units are different. Here's all three units on one Barometer:
And where does this "inches/mm of mercury" come from? Well early barometers use a three foot long glass tube inserted into a reservoir of mercury. Not very practical.
* Formula to convert from millibars to feet:
(1-(millibars/1013.25)^.190284))*145366.45
For example, on the excellent site earth.nullschool.net, we have:
Converting between mb and altitude is a slightly complex formula* you can't do in your head. Here's a spreadsheet that does it for a variety of values:
Source: https://docs.google.com/spreadsheets/d/1nDolj1BIv4HcyLYMA32_53S8-BKPu29Fc4cfYK_lC1s/edit?usp=sharing
But in order to more easily use the various sites that use mb/hPa, it's useful to remember a few things.
It's not exact. Pressure varies with the weather, so the conversion is only an approximation. It's generally good to within about 100 feet though.
Higher pressure is lower altitude. So the smaller the mb, the higher we are in feet.
1000 mb is about sea level. The international standard is 1013.2, but "about 1000" is easier to remember.
300 mb is about 30,000 feet. An easy one to remember, most contrails in temperate areas like the US (excepting Alaska) happen above 30,000 feet, so pressure under 300 mb is generally where contrails will form. However, they can form at higher pressure (lower altitude). The key factor is really temperature.
250 mb is about 34,000 feet. This is a very typical cruise altitude. It's also the only cruise altitude that earth.nullschool.net gives as an option. It's a little harder to remember than 300/30000, but it's a nice round number for the middle of the cruise altitude range.
150 mb is about 44,000 feet. This is a useful upper range for possible commercial planes, which will rarely fly about that level.
100 mb is about 52,000 feet. Pretty much a hard limit for the majority of planes, so you can usually discount any mb that's in two digits
Other than nullschool, a great place to use this is NASA's contrail forecast page:
http://cloudsgate2.larc.nasa.gov/cgi-bin/site/showdoc?docid=33&cmd=latest
Here the range is 400mb (23,500 feet) to 125mb (48,000 feet). The color indicates the likely altitudes where contrails will from. Notice the difference between north and south. Lots of green over the US, indicating around 250mb (34,000 feet), the sweet spot contrail altitude.
And of course you will see it a lot on weather charts, for example:
source: http://weather.rap.ucar.edu/upper/
This is a 500mb RH and temperature chart. The green indicates above 70% RH, but since it's only 500mb (around 18,000 feet) we know immediately most of this area will not be cold enough. However look at the -40 line near the top, it's possible some areas of Alaska could see low altitude contrails today.
Regarding units, we are discussing atmospheric pressure here, and most people will be familiar with this as the pressure at ground level, and using the change in that pressure to forecast changes in the weather (higher pressure is clear, lower pressure means rain, very roughly). So you might have seen a barometer like this:
Notice there are two scales, the outside one goes from 28 to 31, and represents inHg (inches of mercury). The inside one goes from 950 to 1050, and represents mb (millibars).
Different units are used both in different situations, and in different parts of the world. In aviation we use inHG (the 28-31 range) to use in calibrating altimeters. In meteorology we more commonly use mb (millibars). But in countries that use the metric system (most of the world) it's common to use the term hPa (hectopascals), which is exactly the same thing as a millibar.
There's one more common unit used, millimeters of mercury, or mmHg, which is just a metric version of inHg, but unlike the mb/hPa, the units are different. Here's all three units on one Barometer:
And where does this "inches/mm of mercury" come from? Well early barometers use a three foot long glass tube inserted into a reservoir of mercury. Not very practical.
* Formula to convert from millibars to feet:
LaTeX:
\[(1-(millibars/1013.25)^{.190284}))*145366.45\]
Last edited:
