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You can learn how atmospheric humidity influences the weather. |
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Humidity |
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For our purposes, there are two different measures of humidity; “absolute” humidity (read from a hair hygrometer) and relative humidity (determined from a wet/dry bulb thermometer).
“Absolute” humidity is the approximate reading taken from a hair hygrometer and can effectively only be rated on a scale of about 1 to 5, with 1 being the driest and 5 being the most humid. Even so, this gives some real and useful information which can be used in creating an accurate weather forecast.
The following generalizations can be made about higher and lower absolute humidity:
· Precipitation is less likely at lower humidity because it lowers the probability that the air above will reach its saturation point, form clouds and produce rain or snow. Remember, however, that saturation point decreases with temperature, so that precipitation may be produced at a much lower humidity when it is cooler.
· High humidity indicates the reverse of the above, as might be expected.
· Higher humidity can be used as a predictor of dew or frost. If the temperature is expected to plunge into the mid to low ‘30s during a clear night, expect a heavy dew. If the temperature is expected to drop to 31° F. or lower, expect frost.
If you made a Wet & Dry Bulb Hygrometer...
Relative humidity can be determined by using the more sophisticated wet & dry bulb thermometer. Here are some details about relative humidity:
· Air that is holding as much water vapor as possible is said to be saturated. · Saturated air is said to be at 100% relative humidity.
· The amount of water vapor that the air can hold depends upon its temperature. Cold air holds less, warm air holds more.
· Therefore, the lower the temperature of the air, the lower the amount of water vapor that is necessary bring the air to saturation point, or 100% humidity.
· Warmer air can hold a lot more water than cooler air.
The rule of thumb is that raising the air temperature 18°F (10°C) doubles its moisture capacity. This means that air at 86°F (30°C) can hold eight times as much water as air at 32°F.
Knowing the relative humidity allows you to calculate the “dew point” temperature. This is the temperature to which the air must fall in order to reach saturation point with the current amount of water onboard.
U.S. weather reports traditionally include the relative humidity, telling us how much water there is in the air as a percentage the maximum possible amount. They might say, for example, that on such-and-such a date, a station reported a temperature of 96°F and a relative humidity of 46%, meaning that the air contained 46% of the moisture it could possibly hold at that temperature.
Finding the relative humidity with a wet & dry bulb hygrometer is covered in the section on making a Wet & Dry Bulb Hygrometer
The Dewpoint: If we cool air without changing its moisture content, eventually we'll reach a temperature at which the air can no longer hold the moisture it contains. Then water will have to condense out of the air, forming dew, fog or frost. The dewpoint is this critical temperature at which condensation occurs and is important to know!
For example, If the air temperature and the dew point temperature are the same at the temperature at the surface, fog will form.
Fog is typically fragile, because a breeze will mix saturated air at the surface with drier air above. If that happens, the fog will disappear because the dryer air will simply cause evaporation. Conditions of light wind to no wind is most favorable for the formation of fog. This typically occurs when an area is under high pressure with no cloud cover, and the air is able to cool to the dewpoint during the night.
The dewpoint is a measure of how moist the air mass itself is! Ordinarily the dewpoint doesn't vary much during a 24-hour period. Unlike temperature and unlike relative humidity, the dewpoint is usually the same at night as it is in the daytime.
Finding the dewpoint with a wet & dry bulb hygrometer involves learning to use a special chart called a “psychrometric chart”. A short, step-by-step course and a printable chart has been provided. Click below:
To end this brief overview of humidity, it must be said that hot air doesn't really “hold” more water than cold air, it just has more energy. When water molecules have more energy they are more likely to evaporate into a gas. When an air mass is at cooler temperatures, they have less energy and are more likely to settle down and condense into a liquid or solid.
Considerations surrounding humidity in making a short range, local weather forecast will be brought together with three other simple measurements from home weather instruments in The Weather Worksheet. |
