Atmospheric Pressure

Constantly we are under pressure from the large amounts of air surrounding us. The pressure resulting from all of this air is called atmospheric pressure. Gases making up our atmosphere cover us entirely and depending on many factors (including altitude and even weather) the pressure can vary in different areas. As you rise in altitude, less gas is pressing against you so the pressure drops. When you reach a lower altitude, there is more gas pressing against you so the pressure increases. As you rise in altitude, the air pressure decreases and you may notice that your ears pop. This popping is the pressure inside your head equalizing to the pressure outside.

Atmospheric pressure can be measured using a barometer. This instrument is composed of water, air, or mercury encased in a glass tube to measure the atmospheric pressure. Mercury is so heavily used, that barometric pressure measurements are often given in inches of mercury (inHg), or how much pressure it takes to raise one inch of mercury. When atmospheric pressure is applied to the glass tubing, it causes the fluid to rise or drop. As the pressure rises the mercury rises, and as the pressure drops the mercury drops. Atmospheric pressure is measured by looking at the fluid with relation to the etches or dashes on the tube, which are in increments of inches. Barometric readings are converted from inches of mercury (inHg) into millibars (mbar). Pressure is measured in bars and the atmospheric pressure fluctuates heavily on the milli (0.001) level. As a result, pressure is measured in millibars.

A barometer can aid those interested in the weather. Usually, an atmospheric pressure drop indicates a storm may be coming. When a barometer is used in addition to other instruments, such as an anemometer, short-term weather predictions can be made.

Barometric measurements can be measured in either measurments of pascals (such as kilopascals (kPa)), inches of mercury (inHg), or millibars (mbar). To convert from kPa to inHg to mbar, use the following information:

1 inHg = 33.86 mbars
10 mbars = 1 kPa


You hear that the pressure is 29.9 inHg. What would this reading be in millibars?


inHg to mbar

inHg to mbar

inHg to mbar


Data on atmospheric pressure from the site is provided in measurements of kilopascals. When collecting data from the data exporter you notice the barometric reading is 99.870 kPa. What would this be in mbars?

kPa to mbar

kPa to mbar


Complete the following conversions with the provided information from the GRIDc site (


26.5 inHg =


_______ mbar


998.75 mbar =


_______ inHg


100.25 kPa =




1004.57 mbar =


_______ kPa


Go to and use the data exporter to gather information on barometric readings throughout the day. Using the barometric pressure option in the annex field, choose a few readings and practice converting them.



Barometric pressure kPa

Barometric pressure mbars

Barometric pressure inHg


119.360 kPa

1193.6 mbars

35.25 inHg

















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