Common questions

What's the dewpoint? It is the most accurate indication of how humid the air is. Most people are familiar with relative humidity, but it doesn't take temperature into consideration - warm air can hold more humidity than cold air. Think of the air as a drinking glass that gets bigger when it gets warmer. Relative humidity measures how full the glass is, but it doesn't tell you how big the glass is. Relative humidity of 50% tells you that "the glass is half-full" but it doesn't tell you if it's a wine glass or a supersize cup from a fast-food restaurant. That's where dewpoint comes to the rescue. The dewpoint is the temperature at which the air would be completely full (at capacity) given the amount of humidity that's currently in it. The dewpoint reading fills in the blank in this statement: 'There is enough humidity to completely saturate the air if it were ___ degrees out."

When the temperature and the dewpoint are the same, it is 100% humidity (the air is full.) When that happens, there is dense fog. The temperature cannot fall lower than the dewpoint. If it tries, the air will become "over-full" of moisture and the excess will spill out (precipitate) -- we know such precipitation as rain, snow, sleet, and hail. As the air continues to cool, moisture will continue falling out of the air until the the air either stops cooling or until the amount of moisture in the air is small enough to be absorbed by the air.

What's low (air) pressure? Low pressure causes storms, and high pressure causes sunny weather. Why? When air heats up (thanks to the sun) it rises. That leaves less air on the ground--lower pressure. As more air rises up into the sky, there is less air left on the ground, and so the air pressure on the ground continues to drop. Two things happen: Nearby air rushes in to fill the void caused by the rising air--wind. The rising air cools off as it rises - let's face it, it's cold up there. When the rising air cools to the dewpoint, its moisture becomes visible as a cloud. (See dewpoint question). As it keeps cooling, the excess moisture starts to fall out of the cloud (precipitation.) All that air rushing in to fill the void also begins to spin around it like a whirlpool. Low pressure has just given birth to a storm.

What's high pressure? If you like sunny weather, high pressure is your friend. High pressure is where the air sinks, which prevents storms from forming. There is usually very little wind near a high pressure. Air sinks to the ground and flows outward several hundred miles, trying to find a void (low pressure) somewhere that needs to be filled.

What are fronts? The leading edges of air masses. They got their name from World War 2 meteorologists -- the leading edge of an army or an occupied territory was called a front. Air masses behave like armies. Cold air masses form over the cold regions of the earth, and warm air masses form over the warm places. When they start moving around, their leading edge is called a front. They battle one another, resulting in different kinds of weather along the front. Since cold air is heavy and warm air is light, cold air masses usually have no problem pushing warm air masses out of the way, and so weather caused by cold fronts usually doesn't last that long. When warm air starts making its moves, it can be a long and slow process but warm fronts do manage to win. Fronts can also be stationary, when two air masses are in a deadlock with each other. Weather from a stationary front can make many miserable days and it sometimes takes a powerful weather system to push the whole mess out of the way.

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