Weather for Gardeners - When Water Falls: Air Pressure and Fronts
I can't take the pressure!
We, along with our plants, are bottom dwellers. By this I mean we live at the bottom of the sea of air, or atmosphere, around this planet we call Earth. That sea of air is similar to the sea of water in that it has weight, and that weight is what gives rise to atmospheric pressure. Weather forecasters measure it with an instrument known as a barometer. Thus, measurement of atmospheric pressure via this instrument is referred to as barometric pressure.
Just as in the oceans, the sea of air is constantly in motion because the planet is rotating. The sun warms only a portion of the Earth at a time, meaning that some air will be rising as it warms and other air will be cooling and falling. The amount of sunlight, the density of clouds, and the rotation of the Earth all contribute to the movement of the mass of air around our planet. You've seen waves in the ocean, and between those waves, a "valley" where the water is lower even than sea level. The same occurs in the atmosphere; the peaks are called high pressure areas, or highs and the valleys, low pressure areas, or lows. These zones can be circular or elongated, and if elongated, they are called troughs (if low pressure) and ridges (if high pressure).
Right in Front of You!
Areas of air masses are not of uniform temperature and weight; colder air is denser and spreads out like freshly mixed mortar, while warmer air is less dense and rises. The rising of the warm air allows for the movement of the cooler air underneath it. If the colder air mass is growing, as they would do at night or when "flowing" due to atmospheric dynamics, the warm air will be pushed upward and clouds will form at the interface between the air masses. In the case of warm, humid air masses colliding with cold, drier ones, storm clouds will form at the interface where the humid air is cooled below the dew point.
A cold front is an atmospheric situation in which cooler air is moving into an area previously occupied by warm air, and a warm front is the opposite, where warm air advances into an area previously dominated by colder air. When viewed from the side, the advancing cold front looks like the leading edge of a blob of wet mortar, with that rounded edge advancing along the ground. The warm front looks similar, except that the "blob" is upside down with the rounded edge towards the top of the air mass. Clouds, rain or snow will form only if one of the air masses has both sufficient relative humidity and a wide enough temperature difference between it and the other air mass. This difference must be great enough for the humid air mass edge to cool below the dew point of that mass. If this temperature and humidity contrast is very high, the storms that form can be quite violent, with lightning, damaging winds and even tornadoes. Hurricanes are an extreme example of a low pressure system that has taken on a life of its own, feeding on warm humid air and growing while the upper-air currents are relatively calm.
In and Out of the Greenhouse
The means whereby air retains heat is the presence of specific molecules in a large enough quantity to absorb and emit it. The most common molecules that do this are water, carbon dioxide, nitrous oxide and methane. The action of radiation from the Sun on the Earth and all that is on it results in the emission of infrared, or longwave, radiation back into the atmosphere. When the air is rife with the right kinds of molecules, this radiation will be first absorbed, then re-emitted, by these molecules. The re-emitted energy will go either back to the surface, to other molecules, or out into space. The process whereby this happens is called the greenhouse effect, and the molecules that are capable of participating in this process are called greenhouse gases. This is because their activity produces the same effect on infrared radiation as the glass panels on a greenhouse.
Next: Greenhouse gases and the Greenhouse Effect
Photo credit: Public Domain Clip Art
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