Weather For Dummies. John D. Cox

mean one thing for one person and something else for another and might even be used to express different things under different weather circumstances.

      

Technically, probability of precipitation is the chance, expressed as a percentage, that a measurable amount of rain — .01 of an inch or more — will fall someplace in the forecast area in the next 12 hours. It is a combination of two figures — the likelihood of precipitation in the area and the percentage of the area that is likely to get it. This doesn’t tell you as much as you might think. If there is a 50 percent chance that rain will develop anywhere in the forecast area, and 40 percent of the area is expected to get rain, this translates into a 20 percent chance of rain.

      A 70 percent chance of rain doesn’t tell you that there is a 70 percent chance of rain everywhere in the forecast area, for example, and it doesn’t tell you that rain will fall on 70 percent of the area. It’s easy to think that a 70 percent chance of rain means a pretty hefty rainfall is likely, and a 20 percent chance means that only a little rainfall is on the way. But that would be wrong, too. Probability of precipitation doesn’t tell you anything about how long it is going to rain or how much it is likely to rain.

      It’s easy to see why many television meteorologists put a different spin on the forecast, finding other ways to tell viewers what to expect. Other meteorologists at private forecasting firms such as AccuWeather Inc. employ a more user-friendly “public communications meteorology” that is intended to better describe how the day’s weather is likely to go than the National Weather Service’s way of saying things.

      The National Weather Service forecast might say something like this: “Partly sunny today with showers and thunderstorms likely in the afternoon. Probability of precipitation is 70 percent.”

AccuWeather’s version of such a forecast might go something like this: “Sunshine this morning, clouds will billow up during the afternoon, bringing much of the area a brief downpour between 5 and 8 p.m.”

      Often, you will hear forecasters use a variety of key words to give more meaning to the probability statistics that the computer models put out. These words can describe the likelihood of rain in other ways. They might tell you how certain the chances are of rain, for example, or they could tell you how much area the rain is likely to cover.

      A slight chance of rain, for instance, generally means that the probability of rain is 20 percent or less. Expressed another way, a slight chance of rain can be described as scattered showers. At least, that’s what the National Weather Service has in mind.

      When a forecaster says there is a 30 percent chance of showers, it is worth keeping in mind the other side of the coin: There is a 70 percent chance that it won’t rain! When a forecaster predicts scattered showers, it doesn’t mean it is going to scatter showers on you. (So count yourself lucky and give your forecaster a break!)

      Another set of four key words often is used to describe the duration of the rain that forecasters expect: brief, occasional, intermittent, frequent.

      A weather forecaster also can use key words to describe the intensity of the rain or snow to expect. Nobody does it exactly the same way, but here are how some of the key words are commonly used for describing rainfall intensities:

       Very light, when a trace, or less than .01 of an inch is expected

       Light, from .01 to .10 inch per hour

       Moderate, from .10 to .30 inch per hour

       Heavy, for anything above .30 inch per hour

      Temperature

      Daytime maximum temperatures and nighttime minimums that a forecaster expects are often expressed as ranges. If the forecast covers a wide area, the highs and lows are not going to be the same in one place as in another. Besides, few forecasters would pretend that they can predict the exact degree of high and low temperatures.

Temperatures of the air are very sensitive to local conditions, such as the presence of a tree or a parking lot. The tree will keep you cooler during the day, of course, and the parking lot is hot. But these features of the landscape may have the opposite effect after dark. You may notice that the tree may actually have a warming influence on the temperature overnight, while the exposed parking lot can be especially cold. This is known as the “heat island” effect.

      

Temperature ranges are generally forecast in units of 5 degrees Fahrenheit, and you commonly hear these key words used to describe them:

       Near, as in near 45

       Around, as in around 20 degrees

       About, as in about 75

       Lower, as in lower 60s (60–64)

       Mid, as in mid 60s (63–67)

       Upper, as in upper 60s (66–69)

      Forecasts of conditions beyond a couple of days are likely to use wider ranges of temperatures, calling for temperatures, for example, simply “in the 60s.”

      Short-term forecasts for a specific area often are expressed in specific numbers, such as 70/52/74 to indicate a high of 70 degrees, an overnight low of 52, and a following daytime high of 74.

      To avoid confusion, when temperatures get below 10 degrees or above 100, specific numbers are often used, even to express ranges. The forecast might be for a high of near 106, for example, or 102 to 108.

      Temperature’s relative humidity

      

The United States is a funny country. Everybody east of the Rocky Mountains is very familiar with the idea of humidity, the amount of water vapor in the air, and a lot of people west of the Rockies don’t know what all the fuss is about. (Chapter 12 explains these different climates.) Out west, when the temperatures get uncomfortably high, people console themselves with the idea that “It’s a dry heat.”

When it comes to the human body’s comfort zone, the temperature of the air around it is only part of the story. A summer day when the maximum temperature hits 95 degrees in Sacramento, California, for example, is a very different experience than a summer day when it reaches 95 degrees in Chicago or Pittsburgh. In Sacramento, you might hear people remarking about the “nice weather.” In Chicago or Pittsburgh, people are starting to get the look of disaster on their faces.

      The moisture of the air — the amount of water vapor it contains — is important to how you and I feel about the temperature of the air and important to how the air behaves when it comes to making weather. Moisture determines how much more water vapor it is likely to absorb through evaporation, on one hand, and how likely it is to give up water vapor through condensation, on the other. These tendencies are often measured by a property called relative humidity.

      Relative humidity is a little tricky. It is expressed as a percentage, which is easy to get, well, not quite right. When weather forecasters say the relative humidity is 40 percent, for example, they are not saying that the air contains 40 percent water, or even 40 percent water vapor. Relative humidity describes the percentage of water vapor in the air in relation to the total amount of water vapor the air can contain at that temperature. The air is 40 percent along the scale between holding absolutely no water vapor (which never happens, by the way) and holding all that it can, its point of saturation, which happens a lot.

      Water vapor is an invisible gas, but you see and feel the effects of the air’s relative humidity all the time. When you take a shower, for example, you’re adding so much water vapor to the