AIR DENSITY
The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0 degrees Celsius or 39.2 degrees Fahrenheit.

BAROMETRIC PRESSURE
The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth is known as atmospheric pressure. The density and weight of the atmosphere is generally related to altitude, there is less pressure on a mountain top and greater at sea level.  To compensate for this difference and enabling comparison of air mass between locations, barometers are adjusted to the equivalent of sea level pressure. This resulting pressure value is known as barometric pressure (BP). BP changes with weather conditions and is a useful forecasting tool. Generally, rising and high pressure is associated with better and fair weather; falling and low pressure is associated with poorer weather. BP measurement units are in millibars or the more commonly used term, inches or millimeters of a column of mercury (Hg).  Pressure Trends shows the direction of change (higher, lower, steady) of the barometric pressure over the last three hours.

BEAUFORT WIND SCALE
A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857, hydrographer to the British Royal Navy.It is a numerical scale ranging from 0 for calm to 12 for a hurricane. The scale is as follows:0- calm smoke rises vertically; water smooth (0–2 kph, 0–1 mph).
That is:

1- light air smoke shows wind direction; water ruffled (2–5 kph,  1–3 mph)
2- light breeze leaves rustle; wind felt on face (6–11 kph, 4–7 mph)
3- gentle breeze loose paper blows around (12–19 kph, 8–12 mph)
4- moderate breeze branches sway (20–29 kph, 13–18 mph)
5- fresh breeze small trees sway, leaves blown off (30–39 kph, 19–24 mph)
6- strong breeze,  wires whistling; wave sea spray (40–50 kph, 25–31 mph)
7- near gale large trees sway (51–61 kph, 32–38 mph)
8- gale twigs break from trees (62–74 kph, 39–46 mph)
9- strong gale branches break from trees (75–87 kph, 47–54 mph)
10- storm trees uprooted; weak buildings collapse (88–101 kph, 55–63 mph)
11- violent storm widespread damage (102–117 kph, 64–73 mph)
12- hurricane widespread structural damage above (118 kph, above 74 mph)

COMFORT INDEX
The comfort index calculates the body's reaction to heat, cold, humidity and wind chill. High temperatures and humidity stress the body's ability to cool itself, and low temperature can stress the body's ability to heat itself. Do not base important decisions on this comfort index! Everybody's reaction to heat and cold is different.
Condition Scale:

Extreme Cold Wind chill < 0 °F
Uncomfortably Cold 0 °F < Wind chill < 30 °F
Cool 30 °F < Wind Chill < 60 °F
Comfortable 60 °F < Temperature < 80 °F
Warm 80 °F < Temperature < 90 °F
Uncomfortably Hot Temperature > 90 °F and Heat Index < 100 °F
Extreme Hot Heat Index > 100 °F

CLOUD BASE
For a given cloud or cloud layer, the lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles. Virtual Weather Station estimates the cloud base on temperature and humidity readings, using the following equation: (Cloud Base (ft) = 250(Temperature - Dew Point))

DEGREE DAYS
A cooling degree day is given for each degree that the daily average temperature departs above the baseline of 75 degrees a given temperature It is used to estimate the energy requirements, and is an indication of fuel consumption for air conditioning.

A heating degree day is given for each degree that the daily average temperature is below 65 degrees a given temperature. It is used as an indication of heating fuel consumption.  That is:  Cooling Degree Day (CDD) :(T - 75) = daily CDD, where T is daily Average Temperature (F). If T is less than 65 degrees F, CDD=0 and Heating Degree Day (HDD) : (65 - T) = daily HDD, where T is daily Average Temperature (F). If T is greater than 65 degrees F, HDD=0.
Average Temperature (T) : (High Temperature + Low Temperature) / 2.

DEW POINT
Dew point (DP) is the temperature to which air must be cooled for water saturation (100% relative humidity) to occur. The DP is an measurement used to predict the formation of dew, frost, and fog. If late afternoon DP and temperature are close together and the air begins to turn colder, fog is likely during the night. It indicates the air’s actual water vapor content, unlike relative humidity, which takes the air’s temperature into account.

High DP indicates high water vapor content. Also, high DP indicates a better chance of rain and severe thunderstorms. It can predict the minimum overnight temperature, provided no fronts or other weather changes are expected overnight: the afternoon’s DP gives you an idea of what minimum temperature to expect overnight, since the air is not likely to get colder than the DP anytime during the night.

EVAPOTRANSPIRATION
Evapotranspiration or (ET) is an estimate of the amount of water vapor returned to the air in a given area by vegetation.  It combines the evaporation and the amount of water returned through transpiration (exhaling of moisture through plant skin) to arrive at a total.  Effectively, ET is the opposite measure of rainfall or negative precipitation (units of measure in inches/mm).  Air temperature, relative humidity, average wind speed, and solar radiation data is used to estimate ET.  (ET is calculated once an hour on the hour.)  Knowing ET gives an estimate of the need to replace lost water to the plants by irrigation. The Daily ET value may be used to estimate the daily replacement needs by drip irrigation. Because it is based on the amount of moisture lost – and, therefore, the need for moisture replacement -- this method is more efficient in its use of water than is a method based on fixed time intervals.

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 the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly.  Heat Index is when the actual temperature is above 70°F.  Do not base important decisions on heat or wind chill index readings!

HUMIDITY
Humidity is amount of water vapor in the air. It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity, and specific humidity. Relative humidity is the usual term used in weather reporting.

Relative Humidity (RH) is a type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage of the amount of water vapor the air is capable of holding. RH changes with temperature, pressure and water vapor content. RH is an important factor in determining the amount of evaporation from plants and wet surfaces since warm air with a low relative humidity has a large capacity to receive extra water vapor.

LATITUDE
The location north or south, in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.

LONGITUDE
The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude. See Greenwich Mean Time.

MONSOON THUNDERSTORMS
The word "monsoon" comes from the Arabic "mausim" which means "a season or wind shift".  In May and June, usually, the winds are from westerly direction, so humidity is low and temperatures above 100 degrees. As the atmosphere warms, the jet stream retreats northward. this allows the winds to shift to the south and bring in  moisture from Gulf of Mexico and the Sea of Cortez. The Arizona "monsoon thunderstorms" officially begins after the third consecutive day of dew points above 55 degrees. This usually occurs around July 7 continuing for the next two months, with a minimum of 2.5" of rain or 32% of our yearly rainfall.

MOON PHASE
The moon phase is caused by sun rays reflecting off the moon's surface while it moves around the earth. The sun illuminates half of the moon at any time while the moon orbits around the earth. The variation in the angle made by the earth-moon line with respect to the earth-sun line causes changing phase of the moon. The moon completes one revolution around the earth in 27.322 days with respect to the background stars. This is called the “siderial” period of the moon. During this same time the earth moves about 27 degrees along its orbit around the sun. As a result, the moon takes about two extra days to complete the cycle with respect to the sun-earth line. This longer cycle of the moon that takes about 29.57 days is called "syndonic” period of the moon. The longer cycle is considered as “lunar month”.

RAIN RATE
The precipitation rate is calculated by measuring the time interval between each measured precipitation increment of .01".   When there is precipitation within the archive period, the highest measured value is reported. When precipitation ends, the rain rate will slowly decay based on the elapse time since the last measured precipitation. If no precipitation is measured for 15 minutes the rate is reset to zero. 

SOLAR RADIATION
Solar radiation is technically known as Global Solar Radiation, a measure of the intensity of the sun's radiation reaching a horizontal surface. This irradiation measurement includes both the direct solar energy and reflected energy from the rest of sky. The reading is expressed in Watts/ square meter (W/m²). The readings are useful as an indicator of cloud cover and  a indicator of energy input to solar collectors. For solar energy, compute the deviation from the known solar constant (1366 W/m2) as a function of the cosine of the sun's elevation above the local horizon. For an estimate of cloud cover, clear sky attenuation is applied to this value, as a cosine function of the sun's elevation.  

The most intense possible radiation is always at solar noon and tapers to zero at sunrise and sunset.

SUNRISE
The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level. See sunset for comparison of sunrise is calculated for mean sea level.

SUNSET
The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level.

TREND or RATE OF CHANGE
The derivative or change in a parameters value with respect to time. Virtual Weather Station calculates the rate of change by calculating the derivative of a parameter, and then filtering it over one hour. Thus, the rate of change equation factors all of the measurements taken in the last hour, and may not exactly match the change in one hour.

UV RADIATION
Energy from the dun reaches the earth as visible, infrared, and ultraviolet (UV) rays. It is believed that excessive exposure to UV rays can cause sunburn, skin cancer, skin aging, cataracts and immune system response suppression. The UV sensor readings does not take in account UV reflected off of snow, water, sand other surfaces which can scientifically increase UV dosages. The UV readings are only a gross indicator and do not suggest any prolong dosages are safe or healthful. Scientific evidence suggests that even low doses can be harmful.  The readings used at this site are defined in units of UV Index, which is defined by the EPA.

VAPOR PRESSURE
The pressure exerted by water vapor molecules in a given volume of air. (see Barometric Pressure)

WIND CHILL INDEX
Wind chill temperature is how cold people and animals feel when outside.  Wind chill quantifies the rate of heat loss from exposed skin caused by wind and cold.  Our bodies warm the surrounding air molecules by transferring heat from the skin. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder.

If the temperature is 0°F and the wind is blowing at 15 mph, the wind chill is -19°F. At this wind chill temperature, exposed skin can freeze in 30 minutes. The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of time for the object to cool. Wind chill is displayed when actual temperatures are below 70°F and will read the same as the actual temperature at temperatures above 50 as well as in very light wind conditions. 

WIND SPEED
The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.
(see Beaufort Wind Scale) (see Wind Chill Index)

WIND DIRECTION
The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, etc.).

WIND GUST
A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.

WIND RUN
The distance or length of flow of the air past a point during a given interval of time.