Road Weather Information System

Automated Weather Observation System

 

Road Weather Information System

Air Temperature
Temperature of the air measured approximately eight feet above the ground.

Dew Point
Dew Point temperature is the temperature at which the moisture in the air would start to condense in the air or form dew or frost on surfaces. The dew point is a more useful measure of the moisture present because it helps determine the temperature at which dew or frost will form on surfaces such as the road or bridges.

Relative Humidity
Measure of the amount of moisture present in the atmosphere, expressed as a percentage.

Average Wind Speed
Average wind speed within a one minute period, expressed in miles per hour.

Wind Gust
Maximum wind speed within a one minute period expressed in miles per hour.

Average Wind Direction
Average wind direction within a one minute period, expressed as the direction the wind is blowing from.

Visibility
Visibility is a measure of how far a person with normal vision would be able to see through the air, and is measured in miles. Fog, haze, blowing snow, and precipitation will reduce visibility distances by varying degrees.

Precipitation
Precipitation in the atmosphere (not the roadway surface). Expressed as "Yes" (presence of some type of precipitation), "None" (precipitation is not occurring), or an indication of the specific type of precipitation expressed in inches per hour.

Location
Physical location of the sensor.

Surface Temperature
Measurement of the temperature of the roadway or bridge surface.

Sub Surface Temperature
Temperature approximately 18" below the roadway surface.

Deep Temperature Probes
Deep Temperature Probes take the temperature of the road subsurface at varying intervals under the surface. The probes measure the temperature every three inches between the surface and 18 inches below, then every 6 inches from 18 to 72 inches below surface.

Cameras
Most RWIS have color cameras which are programmed to pan, tilt, and zoom to take several different pictures of the area around the RWIS site. The number of images that can be taken at an RWIS location differ from place to place depending on the vantage point of the camera to the road and surrounding areas, and the speed of communications available at an individual site. Sites with fast communications are able to support multiple camera shots that update frequently. Sites with slow communications can support only a few image views and may only be able to update one of its images at a time. Most RWIS are situated in rural locations with little artificial light so most nighttime images will be very dark.

Traffic Sensors
Most RWIS have traffic sensors that use radar to count cars that pass an RWIS. The sensors provide a the total number of cars that passed in a two-minute period in a given lane, and their average speed. Information from these sensors can be used to tell if and how a weather event is affecting traffic flow.
On the map you can quickly see how fast traffic is moving as indicated by the color of the arrows at a specific location.
Normal Indicates the traffic is moving at near normal speeds at this site
LessThanNormal Indicates traffic is moving at less than normal speeds at this site
MuchLowerThanNormal Indicates traffic is moving at much lower than normal speeds

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Automated Weather Observation System

Air Temperature
Air Temperature is measured in degrees Fahrenheit by a sensor located six feet off the ground.

Dew Point temperature
Dew Point temperature is the temperature at which the moisture in the air would start to condense in the air or form dew or frost on surfaces.   The amount of moisture air can hold decreases when it is cooled. As the air temperature drops there is a point at which the moisture in the air begins to condense and form fog, dew, or frost. That point is the dew point.. The dew point is a more useful measure of the moisture present because it helps determine the temperature at which dew or frost will form on surfaces such as the road or bridges.

Barometric Pressure
Barometer is the amount of downward force exerted by the weight of the air above us and is measured in inches of mercury.

Wind Speed
Wind speed is the average velocity at which the air travels over a one-minute period and is measured in nautical miles per hour (NM/H or knots). The main display is in miles per hour (mph) with the knots in parentheses.

Wind Direction
Wind Direction is the average direction from which the wind is coming and is measured in degrees from north. The main display is given by an alphabetic code denoting the direction with degrees from north in parentheses.

Wind Gusts
Wind gusts are the highest wind speed registered over the previous 10 minutes and is measured in nautical miles per hour (NM/H or knots). If no gusts are detected more than 5 NM/H faster than the average wind speed, this field is zero. The main display is in miles per hour (mph) with the knots in parentheses.

Visibility
Visibility is the distance a person can see with normal vision, and is measured in miles.

Precipitation
Precipitation is any form of water, such as rain, snow, sleet, or hail that falls to the earth's surface.

1st Cloud Layer
1st Cloud Layer is the height above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as "broken," "overcast," or "obscuration," and not classified as "thin" or "partial."

2nd Cloud Layer
2nd layer of clouds above the earth's surface.

3rd Cloud Layer
3rd layer of clouds above the earth's surface.

Radar Loop
The radar image displayed is provided by the National Weather Service and the Iowa Environmental Mesonet. You can control how quickly the radar image moves across the map by selecting a speed. You can also select Pause to stop the image. You can also control how dark the radar image is by selecting a percentage. The higher the percentage, the darker the image will display.

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