The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure. This, in turn, means that atmospheric pressure changes with altitude. For example, atmospheric pressure is greater at sea-level than on a mountaintop. To compensate for this difference in pressure at different elevations, and to facilitate comparison between locations with different altitudes, meteorologists adjust atmospheric pressure so that it reflects what the pressure would be if measured at sea-level. This adjusted pressure is known as barometric pressure.

Barometric pressure changes with local weather conditions, making barometric pressure an important and useful weather forecasting tool. High pressure zones are generally associated with fair weather, while low pressure zones are generally associated with poor weather. For forecasting purposes, the absolute barometric pressure value is generally less important than the change in barometric pressure. In general, rising pressure indicates improving weather conditions, while falling pressure indicates deteriorating weather conditions. [top]

Daily Rain is the amount of rain in millimeters accumulated since midnight of the respective day. This value is reset to zero at midnight every day.
Daily Rain is added to the values of Monthly Rain and Yearly Rain respectively. [top]

Dew point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur, providing there is no change in water content. The dew point is an important measurement used to predict the formation of dew, frost, and fog. If dew point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dew point is also a good indicator of the air's actual water vapor content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high vapor content; low dew-point indicates low vapor content. In addition a high dew point indicates a better chance of rain and severe thunder storms. One can even use dew point to predict the minimum overnight temperature. Provided no new fronts are expected overnight and the afternoon relative humidity >= 50%, the afternoon’s dew point gives you an idea of what minimum temperature to expect overnight, since the air is not likely to get colder than the dew point anytime during the night. Dew point is calculated whenever it is displayed. If one edits the temperature or humidity value, the dew point will change as well. [top]

Evapotranspiration (ET) is a measurement of the amount of water vapor returned to the air in a given area. It combines the amount of water vapor returned through evaporation (from wet surfaces) with the amount of water vapor returned through transpiration (exhaling of moisture through plant stomata) to arrive at a total. Effectively, ET is the opposite of rainfall, and it is expressed in the same units of measure (inches, millimeters).

The Vantage Pro2 uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET, which is calculated once an hour on the hour. ET requires the optional solar radiation sensor. [top]

The Heat Index uses 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 (i.e., the air is more saturated with water vapor) the apparent temperature “feels” higher than the actual air temperature, because perspiration evaporates more slowly.[top]

Humidity itself simply refers to the amount of water vapor in the air. However, the total amount of water vapor that the air can contain varies with air temperature and pressure. Relative humidity takes into account these factors and offers a humidity reading which reflects the amount of water vapor in the air as a percentage of the amount the air is capable of holding. Relative humidity, therefore, is not actually a measure of the amount of water vapor in the air, but a ratio of the air’s water vapor content to its capacity. When we use the term humidity we mean relative humidity.

It is important to realize that relative humidity changes with temperature, pressure, and water vapor content. A parcel of air with a capacity for 10 g of water vapor which contains 4 g of water vapor, the relative humidity would be 40%. Adding 2 g more water vapor (for a total of 6 g) would change the humidity to 60%. If that same parcel of air is then warmed so that it has a capacity for 20 g of water vapor, the relative humidity drops to 30% even though water vapor content does not change.

Relative humidity is an important factor in determining the amount of evaporation from plants and wet surfaces since warm air with low humidity has a large capacity to absorb extra water vapor. [top]

The humidex is a Canadian innovation, that was first used in 1965. It describes how hot, humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. Because it takes into account the two most important factors that affect summer comfort, it can be a better measure of how stifling the air feels than either temperature or humidity alone. (See also Heat Index)

An extremely high humidex reading can be defined as one that is over 40. In such conditions, all unnecessary activity should be curtailed. If the reading is in the mid to high 30s, then certain types of outdoor exercise should be toned down or modified, depending on the age and health of the individual, physical shape, the type of clothes worn and other weather conditions.

Guide to summer comfort:
Humidex less than 29: No discomfort
Humidex 30 to 39: Some discomfort
Humidex 40 to 45: Great discomfort; avoid exertion
Humidex Above 45: Dangerous; Heat stroke possible [top]

Rain Rate, given in millimeters per hour, depicts the amount of rain fall in an hour at the current force. The higher the rain rate, the harder the rainfall. [top]

What we call “current solar radiation” is technically known as Global Solar Radiation, a measure of the intensity of the sun’s radiation reaching a horizontal surface. This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky. The solar radiation reading gives a measure of the amount of solar radiation hitting the solar radiation sensor at any given time, expressed in Watts/sq. meter (W/m2). [top]

Storm Rain gives the amount of rain in millimeters since the last rain free period of 24-hours. In other words; the Storm Rain value is reset after a period of 24 hours of rain inactivity. Time of reset can be any time during the day and does not neccessarilly coincide with midnight.
Should new rainfall occur within 24 hours of the last rainfall then the new rainfall amount is added to the Storm Rain total.
This said, adding up all the daily rain values over a given rain period may not be consistent with the Storm Rain total.
Please note that the Storm Rain value will start showing after more than 0.4mm of Daily Rain only.[top]

Shows the current temperature, how warm it is, in degrees Celcius. This is a mere sensor reading and does not take into calculation the Humidity, Solar Radiation and Wind. [top]

The THW Index uses humidity and temperature like for the Heat Index, but also includes the cooling effects of wind (like wind chill) to calculate an apparent temperature of what it 'feels' like out in the open.[top]

Energy from the sun reaches the earth as visible, infrared and ultraviolet (UV) rays. Exposure to UV rays can cause numerous health problems such as sunburn, skin cancer, skin aging, cataracts and can suppress the immune system.

MED (Minimum Erythemal Dose) is defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure. In other words, exposure to 1 MED will result in a reddening of the skin. Because different skin types burn at different rates, 1 MED for persons with very dark skin is different from 1 MED for persons with very light skin.

Both the U.S. Environmental Protection Agency (EPA) and Environment Canada have developed skin type categories correlating characteristics of skin with rates of sunburn. [top]

Wind chill takes into account how the speed of the wind affects our perception of the air temperature. Our bodies warm the surrounding air molecules by transferring heat from the skin. If there’s no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from cooler air molecules.

However, wind sweeps that warm air surrounding the body away. The faster the wind blows, the faster heat is carried away and the colder you feel. Wind has a warming effect at higher temperatures. [top]