SITEMAP | HOME | CONTACT | CURRENT WEATHER | GRAPHS & MONTHLY/YEARLY HIGHS & LOWS | ARCHIVES/RAIN DATA

Glossary of Weather Data Terms

Search

Barometric Pressure

The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure. This, in turn, means that atmospheric pressure changes with altitude. For example, atmospheric pressure is greater at sea-level than on a mountaintop. To compensate for this difference in pressure at different elevations, and to facilitate comparison between locations with different altitudes, meteorologists adjust atmospheric pressure so that it reflects what the pressure would be if measured at sea-level. This adjusted pressure is known as barometric pressure.

Barometric pressure changes with local weather conditions, making barometric pressure an important and useful weather forecasting tool. High pressure zones are generally associated with fair weather, while low pressure zones are generally associated with poor weather. For forecasting purposes, the absolute barometric pressure value is generally less important than the change in barometric pressure. In general, rising pressure indicates improving weather conditions, while falling pressure indicates deteriorating weather conditions.


Source: Davis Instruments

Top


Dew Point

Dew-point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur, providing there is no change in water content. The dew-point is an important measurement used to predict the formation of dew, frost, and fog. If dew-point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dew-point is also a good indicator of the air’s actual water vapor content, unlike relative humidity, which takes the air’s temperature into account. High dew-point indicates high vapor content; low dew-point indicates low vapor content. In addition a high dew-point indicates a better chance of rain and severe thunderstorms. You can even use dew-point to predict the minimum overnight temperature. Provided no new fronts are expected overnight and the afternoon Relative Humidity 50%, the afternoon’s dew-point gives you an idea of what minimum temperature to expect overnight, since the air is not likely to get colder than the dew-point anytime during the night.

Source: Davis Instruments

Top


Growing Degree–Days

Because temperature plays an important part in the rate of development of plants and many diseases and pests (especially insects), a measurement including the accumulation of heat with passing time is necessary to predict maturation. Growing degree-days provide a measure for calculating the effect of temperature on the development of plants and pests. One growing degree-day is the amount of heat that accumulates when the temperature remains one degree above the base developmental threshold for 24 hours. One growing degree–day is also the amount of heat that accumulates when the temperature remains 24 above the base threshold for 1 hour. (What about Heating and Cooling Degree-days?) Note that there are no negative degree-days. If the temperature remains below the threshold, there is no degree-day accumulation.

Unlike strict time predictions of plant or pest development, growing degree-day predictions hold true regardless of location or temperature fluctuations. As long as you know the number of degree-days necessary for plant/pest development, you may use degree-days as an accurate predictor. For example, you may know that it takes, in general, three weeks for a specific pest to develop. What you will find, however, is that the pest may take 4 weeks to develop in cooler weather and only 2 weeks to develop in warmer weather. The time prediction can be off by up to a week by looking at time alone, while the degree–day prediction should result in far greater accuracy.

Source: Davis Instruments

Top


Heating & Cooling Degree-Days

Although degree-days are most commonly used in agriculture, they are also useful in building design and construction, and in fuel use evaluation. The construction industry uses heating degree-days to calculate the amount of heat necessary to keep a building, be it a house or a skyscraper, comfortable for occupation. Likewise, cooling degree-days are used to estimate the amount of heat that must be removed (through air-conditioning) to keep a structure comfortable. Just like growing degree-days, heating and cooling degree-days are based on departures from a base temperature. 65 F is almost always used as this base. It is assumed for heating load calculations that the occupants, lighting, equipment, appliances, cooking, bathing and other activities will raise the temperature from 65 to 68.

One heating degree–day is the amount of heat required to keep a structure at 65F when the outside temperature remains one degree below the 65F threshold for 24 hours. One heating degree–day is also the amount of heat required to keep that structure at 65F when the temperature remains 24F below that 65 threshold for 1 hour.
Likewise, one cooling degree–day is the amount of cooling required to keep a structure at 65F when the outside temperature remains one degree above the 65F threshold for 24 hours. One cooling degree–day is also the amount of cooling required to keep that structure at 65F when the temperature remains 24F above that 65 threshold for 1 hour.

Depending on the calculation method, both heating and cooling degree-days can accumulate in the same day. Also, note that there are no negative degree-days. If the temperature remains below the threshold, there is no degree-day accumulation.
Heating and Cooling degree-days may be calculated by either the High/Low method or the Integration method.

High / Low method:
In the high/low method, the software uses the highest temperature and the lowest temperature for a given day to calculate the average temperature for that day. The difference between the average temperature and the base threshold are assumed to be the number of degree-days accumulated on that day. For example, if the average of the highest and lowest temperatures is 24 below the base threshold, the software assumes 24 heating degree–days for the entire day.

Integration method:
In the integration method, the software calculates degree–days using the average temperature for an interval and the interval time. For example, if the average temperature during a 15 minute interval was 24 below the base threshold, the software would calculate 0.25 heating degree-days during that interval (24 * 15 minutes in interval/1440 minutes per day). The number of degree-days during each interval are added together to arrive at a degree-day total. This method calculates degree-day totals more accurately than the high/low method.

The West Juneau Weather Station uses the Integration method; except that for data prior to November 2002, the High/Low method is used.

Below are some representative heating and cooling degree-day totals from different parts of the United States.

  • Barrow, Alaska: Heating degree days 20,370; Cooling degree days 0
  • Juneau, Alaska (West Juneau): Heating Degree Days 8133; Cooling Degree Days 38 in Calendar Year 2003
  • Kansas City, Mo.: Heating degree days 5,326; Cooling degree days 1,388
  • Bismarck, N.D.: Heating degree days 8,932; Cooling degree days 499
  • Key West, Fla.: Heating degree days 68; Cooling degree days 4,820
  • Hilo, Hawaii: Heating degree days 0; Cooling degree days 3,134
  • Yuma, Ariz.: Heating degree days 983; Cooling degree days 4,244



Table data source: Williams, Jack. 1995. The USA TODAY Weather Almanac and the West Juneau Weather Station.

Source: Davis Instruments with modifications and additions by David Kent

Top


Heat Index

The Heat Index uses the temperature and the relative humidity to determine how hot the air actually "feels." When humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates rapidly to cool the body. However, when humidity is high (i.e., the air is saturated with water vapor) the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly.

Note: WeatherLink uses the Steadman (1979 & 1998) formula to calculate Heat Index, which is more accurate than the method used by the Vantage Pro console and is calculated for all temperatures.

Source: Davis Instruments

Top


High Rain Rate

The High Rain Rate is the amount of rain that would fall in one hour if the rain rate as calculated below continued for the full hour.

The rain rate is calculated by measuring the time interval between each rainfall increment. When there is rainfall within the archive period, the highest measured value is reported. When no rainfall occurs, the rain rate will slowly decay based on the elapse time since the last measured rainfall.

The West Juneau Weather Station uses a 5 minute archive period.

Source: Davis Instruments with additions and modifications by David Kent

Top


Storm Rain

Storm rain displays the rain total of the last rain event. The Davis Weather Instrument  takes two clicks (.02") to begin a rain event and 24 HOURS WITHOUT RAIN to end a rain event.

Source: Davis Instruments

Top


Wind Run

Wind run is measurement of the "amount" of wind passing the station during a given period of time, expressed in either "miles of wind" or "kilometers of wind". WeatherLink calculates wind run by multiplying the average wind speed for each archive record by the archive interval.
For Example:

Average Wind Speed = 5 mph
Archive Interval = 30 minutes (0.5 hours)
Wind Run = 5 mph x 0.5 hours = 2.5 miles of wind

The West Juneau Weather Station uses a 5 minute archive interval and the 5 minute archive records are totaled for the 24 hour period from midnight to midnight; and continuously totaled for the graphs.


Source: Davis Instruments with additions by David Kent

Top


SITEMAP | HOME | CONTACT | CURRENT WEATHER | GRAPHS & MONTHLY/YEARLY HIGHS & LOWS | ARCHIVES/RAIN DATA