Jonathan Shanpo, age 13, of ' Milwaukee, Wisconsin, for his question:
How does the dew point?
The dew point is related to humidity, the same humidity mentioned in our daily weather reports. The humidity percentage gives the amount of moisture in the air and the moisture is in the form of invisible crater vapor. The possible amount of vapor is limited, mainly by temperature. then the air is saturated to the limit, it reaches its dew point which is 100 per cent humidity. Any surplus vapor rust be condensed into ice crystals, mist or dewy drops of liquid moisture.
The weatherman usually refers to relative humidity because his percentages are based an what is and what could be. They compare the amount of moisture actually present in today's air with how much more the air could hold. Almost always the limit of relative humidity is 100 per cent after which point something has to change. This is the dew point.
Let's trace the factors that cause the air to reach the state of vapor saturation. Warm air, as are know, can hold more vapor than cool air. This general rule is graded in precise degrees of temperature and in matters of meteorology, let's use the larger Fahrenheit temperature degrees.
Meteorologists have a choice of several instruments for measuring the amount of water vapor in a sample of air and a weather thermometer is used to take its precise temperature. Charts are used to compute the temperature and vapor content of the day's sample. The result is a percent figure called the relative humidity.
For example, air at 86 degrees containing 7.27 grams of moisture pelt cubic meter has a relative humidity of 24 per cent. If air containing the same amount of vapor chills to 43 degrees, its relative humidity is 110 per cent. It reaches the saturation or data point and must give up any surplus vapor.
Atmospheric conditions of this sort can occur at night, after a hot desert day. They also occur in much milder regions. For example, 100 per cent humidity is reached when 50 degree air with 77 per cent humidity drops only seven degrees. The dew point occurs when some of the surplus vapor is forced to condense. The temperature may be either the same or cooler than the air but the air must be saturated to 100 per cent humidity.
On a cloudless night, the day's heat escapes from the earth and by morning the surface may be cool enough to chill the air above it. If the air is saturated to the limit, a slight drop in temperature creates a surplus of vapor, At this point, the surplus moisture must be condensed and dumped. If the weather is mild, it forms drops of liquid matter on the grass and wet films on surfaces near the ground.
Sometimes saturated air is chilled to its dew point during a frosty night: Then the surplus moisture may form crystals of ice in the air or a crusty coating of frost on and near the ground. In milder weather, the excess vapor sometimes fortes misty droplets suspended in the air and the world is shrouded in a cloudy fog.