This page is a work in progress and is not complete. More info will be added as time goes along.
What is UTC Time?
How does NEXRAD Radar work?
What are the different types of radar images?
What is dew point temperature?
Occasionally I see the term "convection". What is that?
What does "ASOS" stand for?
What does "APRS" stand for?
What is "Wetbulb temperature"?
What is UTC Time?
Weather observations around the world (including radar observations) are always taken with respect to a standard time. By convention, the world's weather communities use a twenty four hour clock, similar to "military" time based on the 0° longitude meridian, also known as the Greenwich meridian.
To obtain your local time here in the United States, you need to subtract a certain number of hours from UTC depending on how many time zones you are away from Greenwich (England). The table (below) shows the standard difference from UTC time to local time.
The switch to daylight saving time does not affect UTC. It refers to time on the zero or Greenwich meridian, which is not adjusted to reflect changes either to or from Daylight Saving Time.
However, you need to know what happens during daylight saving time in the United States. In short, the local time is advanced one hour during daylight saving time. As an example, the Eastern Time zone difference from UTC is -4 hours during daylight saving time rather than -5 hours as it is during standard time.
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How does NEXRAD radar work?
NEXRAD (Next Generation Radar) obtains weather information (precipitation and wind) based upon returned energy. The radar emits a burst of energy (green). If the energy strikes an object (rain drop, bug, bird, etc), the energy is scattered in all directions (blue). A small fraction of that scattered energy is directed back toward the radar.
This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned pulse, the time it took to travel to the object and back, and the phase shift of the pulse. This process of emitting a signal, listening for any returned signal, then emitting the next signal, takes place very fast, up to around 1300 times each second.
There are six different types of images currently available for the RIDGE radar display: Base Reflectivity, Composite Reflectivity, Base Velocity, Storm Relative Motion, One-hour Precipitation and Storm Total Precipitation.
Base Reflectivity
This is a display of echo intensity (reflectivity) measured in dBZ (decibels of Z, where Z represents the energy reflected back to the radar). "Reflectivity" is the amount of transmitted power returned to the radar receiver. Base Reflectivity images are available at several different elevation angles (tilts) of the antenna and are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries and determine hail potential.
The base reflectivity image is from the lowest "tilt" angle (0.5°). This means the radar's antenna is tilted 0.5° above the horizon.
Composite Reflectivity
This display is of maximum echo intensity (reflectivity) from any elevation angle at every range from the radar. This product is used to reveal the highest reflectivity in all echoes. When compared with Base Reflectivity, the Composite Reflectivity can reveal important storm structure features and intensity trends of storms.
Base Velocity
This display of radial velocity represents the overall wind field. Green colors indicate wind moving toward the radar with red colors indicating wind moving away from the radar. The maximum range of this product is 124 nm (about 143 miles) from the radar location.
Storm Relative Motion
This display is of radial velocity of the wind relative to the storm's motion. The result is a picture of the wind as if the storms were stationary. This often unmasks storms that rotate (supercells) which can be a precursor to the formation of tornadoes. Green colors indicate wind moving toward the radar with red colors indicating wind moving away from the radar. The maximum range of this product is 124 nm (about 143 miles) from the radar location.
One-hour Precipitation
This is an image of estimated one-hour precipitation accumulation. This product is used to assess rainfall intensities for flash flood warnings, urban flood statements and special weather statements. The maximum range of this product is 124 nm (about 143 miles) from the radar location. This image will not display accumulated precipitation more distant than 124 nm, even though precipitation may be occurring at greater distances. To determine accumulated precipitation at greater distances you should link to an adjacent radar.
Storm Total Precipitation
This image is of estimated accumulated rainfall, continuously updated, since the last one-hour break in precipitation. This image is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event.
The maximum range of this product is 124 nm (about 143 miles) from the radar location. This product will not display accumulated precipitation more distant than 124 nm, even though precipitation may be occurring at greater distances. To determine accumulated precipitation at greater distances link to an adjacent radar.
The dewpoint temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature. If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%. If there is then further cooling of the air, more water vapor must condense out as even more dew, fog, or cloud, so that the dew point temperature then falls along with the air temperature. While relative humidity is (as its name suggests) a relative measure of how humid the air is, the dewpoint temperature is an absolute measure of how much water vapor is in the air. In very warm, humid conditions, the dewpoint temperature often reaches 75 to 77 degrees F, and sometimes exceeds 80 degrees. No matter how hot the temperature gets, a dewpoint temperature of (say) 75 deg. F always represents the same amount of water vapor in the air. During the summer, the dewpoint temperature -- not the relative humidity -- is usually a better measure of how humid it feels outside. It is also a good measure of how much "fuel" is available to showers and thunderstorms, with a higher dewpoint representing more water vapor available for conversion to rain.
Generally, transport of heat and moisture by the movement of a fluid.
In meteorology, the term is used specifically to describe vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cumulonimbus clouds, towering cumulus clouds, and AltoCumulus clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.
The acronym ASOS means Automatic Surface Observing Station. Used for automatic METAR report generation. Has a ceilometer to measure cloud height and type automatically as well as other current weather data.
APRS is the acronym for Automatic Position Reporting System. An amateur radio system originally developed to maintain an ongoing record of the location of a mobile radio transmitter. It has been expanded to allow reporting of weather information within the position report. Originally restricted to reporting via amateur radio, it is now possible to send and recieve these reports via the Internet. The Internet APRS feeds data into the USA Hurricane warning center and other NOAA systems.
Wet Bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one's skin is wet and is exposed to moving air. Unlike dry bulb temperature, wet bulb temperature is an indication of the amount of moisture in the air.
