Heating degree days are indicators of household energy consumption for space heating. It was found that for an average outdoor temperature of 65 degrees Fahrenheit, most buildings require heat to maintain a 70 degree temperature inside. Similarly, for an average outdoor temperature of 65 degrees or more, most buildings require air-conditioning to maintain a 70 degree temperature inside.

How heating and cooling degree days are computed

Take the high and low temperature for the day, and average them. If this number is greater than 65 F, then we have (Average temperature - 65) cooling degree days. If the average temperature is less than 65 degrees, then we have (65 - Average temperature) heating degree days. Running totals are kept for these units over a time period of a year so fuel distributors and power companies can assess average demands.

Source:  http://www.wunderground.com/about/faq/degreedays.asp

Dew Point

All air contains water vapor of varying quantities. The dew point indicates the amount of moisture in the air. The higher the dew point, the higher the moisture content of the air at a given temperature. Conversely, the dew point of humid air will be higher than the dew point of dry air.

Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of water vapor possible at the existing temperature and pressure.

Condensation of water vapor begins when the temperature of air is lowered to its dew point and beyond. The dew point, like other measures of humidity, can be calculated from readings taken by a hygrometer

Source: http://www.doc.mmu.ac.uk/aric/eae/Weather/Older/Dew_Point.html

DEW-POINT TEMPERATURE

A temperature less frequently reported on radio or TV is the dew-point temperature. This is the temperature that a sample of air would have if it was cooled until dew (or frost) began to form on a surface immediately adjacent. As you can see, this "temperature" is really a round-about way of saying something about the amount of moisture or humidity in the air. If the dew-point temperature (sometimes we just call it the "dew point", for short) is close to the air temperature, the relative humidity is high, and if the dew point is well below the air temperature, the relative humidity is low. If moisture condenses on a cold bottle of pop taken out of the refrigerator, the dew-point temperature of the air in the room is above the temperature of the pop bottle. This happens much more frequently in summer than winter, when there is generally more moisture in the air.

Incidentally, one of the clues a meteorologist uses for forecasting tonight's low temperature is to look at today's dew point: if no fronts are expected to come through, tonight's low temperature will not get much below today's dew point. (Unfortunately, not all forecasting is that simple.) The dew-point temperature is measured in the same enclosure as the air temperature.

Why humidity can be less than 100% when it's raining

Humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens when the air rises and cools. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.

Other Weather Definition==Definitions

Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter

Actual vapor pressure: The partial pressure exerted by the water vapor present in a parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air. Vapor pressure is also measured in millibars.

Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor to liquid water.

Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air.

Dry bulb temperature: The actual air temperature. See wet bulb temperature below.

Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent. Click here for text explaining this in more detail.

What relative humidity means

The warmer air is, the more water vapor it can "hold." Dew point is a measure of how much water vapor is actually in the air. Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it. To see how this works, let's use the chart below, which is adapted from Meteorology Today by C. Donald Ahrens, published by West Publishing.

Air temperature in degrees C   Water vapor air can hold

                               at this temperature.

30 degrees                     30 grams per cubic meter of air 

20 degrees                     17 grams per cubic meter of air 

10 degrees                      9 grams per cubic meter of air

These numbers, which apply to air at sea level pressure, are based on measurements over the years. They are basic physical facts. Source:  http://www.usatoday.com/weather/wrelhum.htm

Saturation of air: The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air.

Saturation vapor pressure: The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure is directly proportional to the temperature.

Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor)

Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the chance of ice directly into water vapor. Chemists, and sometimes meteorologists, refer to the vapor to solid phase change as "deposition."

Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity

Source: http://www.usatoday.com/weather/whumdef.htm

Cool link for conversions/calculations:  http://www.srh.noaa.gov/bmx/tables/metcal_java.html