S'COOL: Measuring pressure - Details

The answer is that these maps, and your daily weather report, use pressure corrected to sea level pressure. That is, they report what the pressure would be for your location if some of that dirt and rock below you were turned into air, and you were at sea level. This method of presentation takes out the effect of topography (how high the ground is), and highlights the things that are important for weather. Mainly, that is that wind blows from high pressure to low pressure. If they did not correct to sea level, it would appear that air should always be rushing in towards the Rockies to fill in that low pressure region. To do that, however, the air would also have to move uphill (against gravity). The correction to sea level pressure accounts for this effect, and gives meteorologists maps on which they can identify "real" high and low pressure areas.

If you live higher than sea level, you might see this difference in your pressure measurement, depending on what kind of instrument you have, how it was calibrated, or perhaps what source you obtain pressure from. I recently spent some time in Colorado, and noted that the TV stations report sea level-corrected pressure, while the local university weather station reports actual local atmospheric pressure. In that case, the difference is quite substantial: ~840 vs ~1020 mb! In most places, the difference is not that great.

We would like to get all pressure measurements in sea level-corrected form. However, if you would rather report local pressure, that's fine too. Just tell us that in the comment area.