Using barometric pressure to forecast the local weather.
Forecasting the weather is a complicated subject that occupies books! For the sake of simplicity and a place to start, we will learn to make a forecast by using four different measurements from our homemade weather station:
· Barometric pressure
1. Barometric pressure—the rule of thumb is: if the pressure is on the rise, the greater the likelihood of fair weather...if the barometer is falling, the more likelihood of a storm.
This happens for the following reason: air flows outward from a high pressure area. With air moving away from this region, air must sink from above to replace it. This sinking motion leads to generally fair skies and no precipitation near the high.
Winds spiral clockwise around a high pressure center in the northern hemisphere (diagram left).
Temperatures are dependent upon the location relative to the east or west side of a high.
Northerly winds associated with an approaching high are likely to result in colder temperatures while southerly winds found on the backside of a high, or once a high has passed through, typically result in a warming trend.
A low pressure area, on the other hand, is produced when a large, warm air mass moves rapidly upward and cools, lowering the pressure and drawing more air up from the surface to cool in its turn. In the cooler temperatures, water vapor condenses out of the atmosphere, producing clouds and precipitation.
(See: Making a Cloud in a Jar).
Winds spiral counter-clockwise around a low pressure center in the northern hemisphere (diagram right).
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High pressure areas remain at higher pressure because air is being replaced at the top faster than it is spiraling out the bottom. Conversely, low pressure areas stay at lower pressure because air is being compressed at the top (through cooling) faster than it is spiraling in at the bottom.
Air always flows from a high pressure area to a low. But, because of the Earth’s spin (Coriolis force), the winds do not blow in a straight line, but curve in a manner similar to the stylized, clock spring “S” in our logo (top, left of page).
The lowest pressure associated with a low, is located at its leading edge in the direction of its travel. This why, if the low is fast moving, the brunt of a storm follows quickly after a rapid fall in barometric pressure, and begins rising again soon after the worst has past.
In the same way, the highest pressure associated with a high is also in the leading edge. This leading edge tends to kick up a dust cloud as it travels, and provides the basis for the “Red Sky” poem. If the thin dust cloud is backlit by the sun in the morning, that means that the high has passed, and in the cyclic nature of things, another low can soon be expected.
If the dust cloud is lighted by the setting sun, it means that the high is approaching and fair weather is coming soon.
If the fall in barometric pressure is gradual but persistent, that information combined with other observations can be used to detect an approaching, slow moving low.
Remember that high and low pressure areas have (for our purposes) to do only with whether or not the weather can be expected to be clear, partially clear or cloudy. It is only when combined with other data that predictions can be made about the likelihood of precipitation and its quantity. This keeps things dreadfully simple; up=good-down=bad, with good and bad understood to be relative terms.
Time is also a determiner in our use of barometric readings.
· A rapidly falling barometer means a rapidly approaching low.
· A slowly falling barometer indicates a more slowly approaching low.
· A rapidly rising barometer indicates rapid clearing.
· A gradually rising barometer means slower improvement in cloud & wind conditions.
· A steady higher or lower pressure over several days indicates that a pressure area has stalled overhead.
Try to envision what happens in a high or low pressure area (rising or falling air, the effects of heating and cooling, winds spiraling in or out, etc.); and do the water vapor experiments Making a Cloud in a Jar and Making It Rain in a Jar.
The information contained in this section will be brought together with the other three weather instrument measurements in the section on the Forecast Worksheet. The movement of high and low pressure systems is discussed in Wind.