Daily Visual Summary Glossary
Cloudcover
Cloudcover is measured by the number of eighths of the sky that are covered by clouds. Clear (Sunny) skies contain 0/8ths cloudcover. Few clouds (Mostly Clear/Mostly Sunny) skies contain 1/8 - 2/8ths cloudcover. Scattered (Partly Cloudy) skies contain 3/8 - 4/8ths cloudcover. Broken (Mostly Cloudy) skies contain 5/8 - 7/8ths cloudcover. Overcast (Cloudy) skies contain 8/8ths cloudcover. The ceiling, or the height of a cloud base, is given (in feet) for broken or overcast skies.
Drizzle
Precipitation consisting of very small water drops that appear to float with the air currents while falling in an irregular path. Drizzle is composed of slowly falling fine water droplets.
Rain
Precipitation consisting of larger drops of water that fall in relatively straight, although not necessarily vertical, paths.
Snow
Precipitation composed of white or translucent ice crystals, chiefly in complex branched hexagonal form.
Ice Pellets (Sleet)
Small, transparent or translucent, round or irregularly shaped pellets of ice. They may be hard grains that rebound on striking a hard surface, or pellets of snow encased in ice. Sleet is a usually bouncy wintry precipitation that forms when snow encounters a warm layer in the atmosphere, melts, and then refreezes when it returns to a colder layer. It differs from hail, which is produced by updrafts in convective clouds.
Freezing Rain / Freezing Drizzle
Freezing rain / freezing drizzle is a wintry precipitation that is the result of snow melting into raindrops as they near the surface. Upon contact with a cold ground, these raindrops freeze and create an icy glaze.
Ice Crystals
A type of precipitation composed of unbranched crystals in the form of needles, columns, or plates; usually having a very slight downward motion, may fall from a cloudless sky.
Snow Grains
Precipitation of very small, white opaque grains of ice, similar in structure to snow crystals. The grains are fairly flat or elongated, with diameters generally less than 0.04 inch (1 mm.).
Snow Pellets (Graupel)
Precipitation consisting of white, opaque approximately round (sometimes conical) ice particles having a snow-like structure, and about 0.08 to 0.2 inch in diameter; crisp and easily crushed, differing in this respect from snow grains; rebound from a hard surface and often break up. They are composed of snow crystals completely coated by rime.
Thunderstorm
In general, a local storm invariably produced by a cumulonimbus cloud, and always accompanied by lightning and thunder.
Hail
A form of precipitation composed of balls or irregular lumps of ice, always produced by convective clouds which are nearly always cumulonimbus. Hail is produced by updrafts, usually in strong thunderstorms. It differs from sleet, which is the wintry precipitation caused by the freezing of raindrops as they fall.
Shower
Precipitation from a cumuliform cloud; characterized by the suddenness of beginning and ending, by the rapid change of intensity, and usually by rapid change in the appearance of the sky; showery precipitation may be in the form of rain, ice pellets, or snow. A very light and brief snow shower is commonly called a flurry, while a very light and brief rain shower is sometimes called a sprinkle.
Unknown Precipitation
The observation of Unknown Precipitation results when an automated weather station cannot discern the type of precipitation falling. It is typically the result of a wintry mix of precipitation, such as rain, snowflakes, and/or partially melted snowflakes.
Haze
A type of lithometeor composed of fine dust or salt particles dispersed through a portion of the atmosphere; particles are so small that they cannot be felt or individually seen with the naked eye, but diminish the visibility; distinguished from fog by its bluish or yellowish tinge. Haze reduces visibility.
Fog
A hydrometeor consisting of numerous minute water droplets and based at the surface; droplets are small enough to be suspended in the earth's atmosphere indefinitely. Fog results from the condensation of water vapor in the air, and is basically a cloud at ground level.
Ground Fog
A fog that conceals less than 0.6 of the sky and is not contiguous with the base of clouds. Ground fog is the result of the condensation of water vapor in air that chills while in contact with a cold ground.
Ice Fog
A type of fog composed of minute suspended particles of ice; occurs at very low temperatures and may cause halo phenomena. Ice fog occurs when water vapor in the air turns directly into tiny ice crystals.
Blowing Snow
A type of hydrometeor composed of snow picked up from the surface by the wind and carried to a height of six feet or more. If blowing and/or falling snow reduces visibility to <= 1/4 mile and winds are > 35 mph for three consecutive hours, a formal blizzard is occurring.
Time
Time for the Daily Visual Summary Page is expressed in local 24-hour time (00:00 - 23:59). It has been converted from Greenwich Mean Time (UTC / Zulu) time to be more familiar to the user. Daily maximum and minimum temperatures are taken midnight to midnight.
Weather
Weather conditions include precipitation and obscurations to visibility. When there are no such "weather" conditions reported, conditions are reported according to cloudcover. Conditions are measured today mainly by automated weather stations. For this web site, conditions have been converted from weather code into icons and plain English descriptions. These icons are not actual pictures from the specific date; they are meant to graphically represent the conditions at the time, and are displayed either in day or night form according to the sunrise / sunset times of the day. To see what possible weather conditions are displayed by the page, visit the Icons Gallery.
Temperature
Temperature is the measure of the average molecular kinetic energy of the ambient air. Temperatures on this site are expressed in degrees Fahrenheit. Temperature is measured by a thermometer.
Heating Degree Days
A heating degree day (HDD) is a relative measure used by the energy industry to compute roughly how much energy is required to heat a home on a given day. It is equal to 65 degrees Fahrenheit minus the day's average temperature (thus, the colder the day, the more energy required to heat a home).
Cooling Degree Days
A cooling degree day (CDD) is a relative measure used by the energy industry to compute roughly how much energy is required to cool a home on a given day. It is equal to the day's average temperature minus 65 degrees Fahrenheit (thus, the hotter the day, the more energy required to cool a home).
Dew Point
Dew point is the temperature to which the air must be cooled in order to be saturated with moisture. Dew point temperatures on this site are expressed in degrees Fahrenheit. High dew point values (60s or 70s) are considered by most to be uncomfortably humid. Dew point is measured by a psychrometer.
Relative Humidity
Relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor needed to saturate the air. Humidity is measured by a hygrometer.
Pressure
Air pressure is the weight of the atmosphere above a given area of the earth's surface. Pressures are expressed in inches of mercury (in Hg or ") or millibars (mb). Pressures have been adjusted to sea level pressure to account for differences in elevation among sites. The average sea level pressure is equal to 1 atmosphere, or 1013 mb, or 29.92 in Hg. Typically, low pressures are areas of storminess, while high pressures are areas of fair weather. Pressure is measured by a barometer.
Precipitation
Precipitation is the amount of water that has fallen from the sky, recorded to the nearest hundredth of an inch. Any snow or ice has been melted to obtain the liquid equivalent.
Wind Direction
Wind direction is typically expressed in degrees, with 0 degrees being at north, and the number of degrees increasing clockwise (90 degrees - east, 180 degrees - south, 270 degrees - west). For easier comprehension, wind direction has been displayed as a compass graphic, with directions being expressed to the nearest 1/16th of the circle (N, NNE, NE, ENE, E, ESE, SE, SSE, S, SSW, SW, WSW, W, WNW, NW, WNW).
Wind Speed
Wind speed is taken as a two minute average. On this page, it is expressed in miles per hour. A calm wind has a speed of 3 mph or less. Hurricane force winds are winds 74 mph or greater, while tropical storm force winds are 39 mph or greater. Wind speed is sometimes expressed in knots, or nautical miles per hour. A knot is equal to 1.15 statute miles. Wind speed is measured by an anemometer.
Visibility
Visibility is the furthest distance at the horizon that one can see at a given time. It is recorded in statute miles.
Wind Chill
The wind chill scale attempts to quantify what the air feels like to human flesh based on the combined effects of cold (air temperature) and wind speed. The scale was originally crafted from Antarctic experiments in the 1940s, and was revised in 2001.
Definitions in italics are taken from Aviation Weather and Weather Services by Irvin N. Gleim, which references the FAA glossary of weather terms. Definitions and comments in regular text are composed by the web author. Additional understanding of phenomenon was gained from A World of Weather by Lee Grenci and Jon Nese.