Atmospheric pressure is the force from the applied from the weight of the atoms that make up the atmosphere. A barometer is used to measure atmospheric pressure, which is typically given in millibars or inches of mercury. The figure on the left shows how a simple barometer works; the mercury column is what is measured in inches for pressure. When pressure drops the mercury column drops, when pressure rises the more mercury is pushed into the column making it rise. A simple conversion can be used to go between inches of mercury and millibars:
- Convert between inches of mercury and milliabars:
- 5 inches of mercury = _______________________ millibars
- 1004 millibars = _______________________ inches of mercury
- 28 inches of mercury = _______________________ millibars
- 1040 millibars = _______________________ inches of mercury
Aneroid barometers, image on the right, have a partially evacuated chamber that changes shape with changes in pressure. On the face of the barometer different pressures are associated with different weather conditions.
- Complete the below table:
|Condition||Pressure Range (millibars)|
In order to “predict” the weather you must look at pressure changes over time. A simple way to see this is to map pressure and draw isobars (lines of equal pressure).
- Draw isobars at a 5 millibar interval on the map below.
- Place an L in the isobar loop for the low pressure system, and place an H in the isobar loop for the high pressure system. Thinking logically, what direction does wind blow; from high pressure to low pressure, or from low pressure to high pressure? Why?
- Assuming for right now that the pressure gradient is the only factor affecting wind direction, draw arrows at points E, F, G, H, I and J on the mapindicating the direction the wind is flowing at each point.
- Wind is the horizontal movement of air as a result of pressure differences. What is the pressure gradient in millibars per kilometer from points A to B and C to D?
A to B: _________________________
C to D: _________________________
- How does wind relate to pressure gradient? Which set of points (A to B, or C to D) has faster wind?
The rotation of the Earth causes moving masses (air in the atmosphere, water in the oceans) to be deflected from a linear (straight line) path; a phenomenon known as the Coriolis effect. In the Northern Hemisphere deflection is to the right, and in the Southern Hemisphere moving masses are deflected to the left.
- Using this new knowledge draw arrow for wind in the scenarios below. (Hint: the arrows should not go straight into or out of the pressure cells, the arrows should be straight lines)
- On the map on page 2 draw new arrows at points E, F, G, H, I, and J for the wind direction accounting for the Coriolis effect.
- Meteorology: Moisture, Pressure, Wind
Dew point graph page 90 (Easchiff, Link to original, CC-BY-SA-3.0), United States Map page 96 (Strafpeloton2, Link to original, CC-BY-SA-3.0), Aneroid Barometer (Langspeed / Saperaud, Link to original, CC-BY-SA-3.0), Stratocumulus Cloud (Matany, Link to original, CC-BY-SA-2.0), Stratus Cloud Photo (Famartin, Link to original, CC-BY-SA-3.0), Fog Photo (Ian Furst, Link to original, CC-BY-SA-3.0), Cirrocumulus Photo (Biswarup Ganguly, Link to original, CC-BY-SA-3.0), Cumulus Photo (Michael Jastremski, Link to original, CC-BY-SA-3.0), Nimbostratus Photo (Simon A. Eugster, Link to original, CC-BY-SA-3.0), Cirrus Photo (Nissim Angdembay, Link to original, CC-BY-SA-3.0), Altostratus (Simon Eugster, Link to original, CC-BY-SA-3.0)