ONLINE WEATHER SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Monday, 8 February 1999

Explanation of Hourly Surface Weather Observations (In Tabular Form)

The near surface weather conditions across the country are monitored on a nearly continuous basis at several thousand locations as part of a nationwide network operated by the National Weather Service in conjunction with other governmental agencies such as the Federal Aviation Administration, the Department of Defense and various state agencies. Many of these weather observation sites are located at airports where the needs of the aviation industry requires a set of weather observations on an hourly or shorter time interval. With the modernization of the National Weather Service, many of the weather observations are made by automatic weather stations called ASOS for Automatic Surface Observation System.

Current hourly weather observations from nearly 1500 stations throughout the country can be accessed from the Online Weather Homepage under the heading State Surface Data-Text. Select the desired state using the appropriate 2 letter postal abbreviation. Once selected, a tabulation of the hourly observations from the state will appear. Remember that all times are listed in UTC or Z time. These files are updated on an hourly basis.

INFORMATION CONTENTS OF THE TABLE

• ID = Station ID
  A list of these identifiers appears in Appendix IX, D of the User Guide, arranged according to state. These three or four letter identifiers are assigned by the Federal Aviation Administration. Most of the stations in the continental U.S. will only have three letters since the leading letter "K" is assumed. Most of the stations in Hawaii and Alaska will have the letter "P" as the prefix.
• TIME = Hourly observation time in "Z" time
  Typically, the displayed weather observations are made within ten minutes before the top of the hour to permit a timely communication of the observations over the communication network in a given sequence.
• T = Air temperature in degrees Fahrenheit (F)
  The air temperature is measured by an electronic thermometer located within two meters (6 feet) of the earth's surface.
• TD = Dewpoint temperature in degrees Fahrenheit (F)
  The dewpoint temperature, a measure of the water vapor content of the air, is measured by an electronic device located near the thermometer. Typically, a larger value of the dewpoint indicates a greater amount of water vapor in the atmosphere. If the dewpoint were equal to the air temperature, the air would be saturated and fog most likely would be reported.
• RH = Relative Humidity in a percentage (%)
  The relative humidity is a percentage used to describe how close the measured water vapor in the atmosphere is to saturation. When RH = 100%, the air would be saturated with respect to water vapor, and the air temperature and the dewpoint temperature are equal.
• DIR = Wind direction in degrees measured clockwise from true north (360 degrees)
  The wind direction is measured by an instrument called a wind vane located at 10 meters (33 feet) above the earth's surface. Remembering that in accordance with meteorological tradition, a wind is named for the direction from where the wind originates, the following notation is used:
  • 090 degrees = east wind, 180 = south wind, 270 = west wind and 00 = calm
• SPD = Wind speed in knots
  The wind speed is measured by a wind sensor called an anemometer located at the same level as the wind vane. The 1-minute average is reported. By meteorological and aviation tradition, wind speed is reported as knots, from the nautical mile per hour. One knot is equivalent to 1.15 mph
• GST = Speed of the wind gust in knots
  A gust is defined as a sudden brief increase in the speed of the wind of at least 10 knots between peaks and lulls. Wind gusts are reported when the peak wind speed reaches at least 16 knots.
• SLP = Sea level corrected pressure in tenths of millibars
  The air pressure is measured by an aneroid barometric pressure sensor and corrected to sea level to remove the effects of the elevation of the station. In this report the lead "9" or "10" together with the decimal point are omitted. Thus, "962" is 996.2 mb and "013" represents 1001.3 mb.
• VIS = Horizontal visibility in statute miles
  Visibility is defined as the greatest horizontal distance that large, prominent objects can be identified with the unaided eye. The visibility is measured by a sensor that measures the extinction of a light beam.
• COV = Sky or Cloud Cover
  A three-letter code appears that uses the following convention based upon the fraction of the local sky hemisphere being covered by clouds:
  • CLR = clear, SCT = scattered; BKN = broken; OVC = overcast.
• WX = Significant Weather Phenomena
  A set of abbreviations identifies those weather phenomena that could adversely affect aircraft operations. These phenomena include all types of precipitation, severe weather events and obstructions to vision, such as fog. Many of the abbreviations are obvious, such as:
  • H = Haze; R = Rain, S = Snow.
• MAX = Maximum daily temperature in degrees Fahrenheit
  Not currently available
• MIN = Minimum daily temperature in degrees Fahrenheit
  Not currently available

GRAPHICAL DISPLAYS

The weather data appearing in the text format can be displayed graphically in the traditional surface map format. The weather data for each hour from the entire country and adjoining portions of Canada and Mexico are collected and plotted upon weather maps. A more detailed discussion of how the weather data are plotted on a surface map will be described in another Supplemental Summary that will appear this week.

Another weather data display option would entail the collection of the hourly weather data from one particular station for a 24 hour interval. Such a display is called a meteorogram. Online Weather produces meteorograms for a selected set of cities.