Answer to "pressure (barometric)" question

BAROMETRIC PRESSURE -- Fernando Sanchez of Bogota, Columbia asked "How can the barometric pressure be lower, at the same instant, in Miami than in Bogota (about 9000 ft above sea level)? Readings from the Weather Channel on Monday 3 at 11:30 am showed the following: Miami 30.08" of mercury and Bogota 30.38" of mercury.

Also, why don't barometer readings change a lot here? Because of this, I can't seem to use my barometer for weather forecasting. Why?

Barometric (or atmospheric) pressure is simply a measure of the amount (weight) of air above a point. Nearest the ground, the entire atmosphere is above you. As you go higher, some of that atmosphere lies below you, so pressures get lower. The same thing happens when one goes diving. The deeper one goes, the more water lies above you, hence, the greater the pressure.

In the case of mountains, like in the Bogota area, the mountain takes the place of the atmosphere. Since you are almost 2 miles above sea level, you have lost about 1/4 of the atmosphere. The actual barometric pressure in Bogota, therefore, is probably only around 22" - 23" of mercury.

If meteorologists were to use the actual weather station pressure values in analyzing "surface" weather maps, the pressure pattern would vary with the terrain and the analysis would simply look like the mountains. So, meteorologists routinely convert weather station pressure values to sea level and then analyze sea level pressure maps. To do this, they have to place a fictitious atmosphere between the weather station and sea level to convert all pressure data to sea level.

Because temperature is related to the pressure of the atmosphere, meteorologists have to assume a temperature value between the weather station and sea level. If the air is warm (and the air less dense), then the pressure change between sea level and the elevated weather station is less; it's cold (more dense air), then the pressure chnage will be more.

In the case of Bogota, around 5 degrees North latitude, I would suspect that temperatures used to compute the "fictitious" low level temperature profile would have been relatively warm. This would mean that the "fictitious" pressure addition would have been less than for a colder atmosphere. In places like Alaska or Colorado during the winter, the additional pressure factor would be much larger. That's why there is sometimes a very high high pressure system over the Rocky Mountains during the winter.

All that said, I did some further checking. On December 6, for example, the following pressures were reported at the Weather Channel web site (Note: this data is reported by the individual weather stations and simply posted by The Weather Channel):

Bogota - 30.23"

Dallas, TX - 30.31"

North Carolina - 29.71"

Miami, FL - 29.98"

Yet, according to sea level pressure weather maps I viewed on the web, the pressure in Bogota should have been below 29.90". Weather map analysis uses all data in an area to portray the pressure pattern; it would appear that Bogota's data was discarded from this analysis. This suggests that the mathematical formula or data used in the sea level pressure reduction procedure at Bogota may be in error. In fact, I think a colder temperature value for the "fictitious" atmosphere may have been applied. I suggest that you contact someone in the weather department or at the airport there and ask them to review this situation.

You may also want to keep track of the daily pressure values at Bogota and other places to verify my hypothesis and some of the following ideas. The pressure in the tropics (with the exception of tropical storms and hurricanes), simply doesn't change a lot from day to day. Certainly the day to day change is much less than in places like Miami, FL; Chicago, IL; or Santiago, Chile. So, while your pressure (corrected to sea level) should vary from about 29.8" - 30.1", Miami's pressure might change from 29.6" to 30.5". In higher latitudes, variations from 29" - 31" are not uncommon. And it is for these middle to higher latitude locations that the words on the face of a barometer (and the associated barometric forecasting rules) were developed. Unfortunately, you'll have to develop a set of rules specific to your area.

Take a look back at the Question