Weather FAQ’s

Understanding Weather Terms

Weather FAQ’s and Terms for Weather Watchers

The Science Behind the Weather
Also known as Meteorology


Meteorological information is recordable or visible weather events which are measurable, and are explained by the weather science of meteorology. Those events are bound by the variables that exist in earth’s atmosphere. Variables that are tracked and recorded are, temperature, air pressure, water vapor, and the gradients and interactions between each variable, and how they change in time. Different scales are studied to determine how systems on local, region, and global levels impact weather and climatology. Meteorology, climatology, atmospheric physics, and atmospheric chemistry are all disciplines of the atmospheric sciences. Interactions between Earth’s atmosphere and the oceans are also a part of ocean-atmosphere studies. Meteorology has application in many diverse fields such as the military, energy production, transport, agriculture and construction.

The dictionary defines weather as the state of the atmosphere at a particular time and place with respect to heat or cold, wetness or dryness, calm or storm, and clearness or cloudiness. It involves things we can’t see like air pressure, wind, solar radiation or humidity. These elements are organized into various weather systems, such as areas of high and low pressure, thunderstorms, and tornadoes. Weather affects our everyday lives but is more important to know for some people than others, especially if you are a pilot, a construction worker, a fisherman, a farmer. The blanket of air around the earth is called the atmosphere. It is about 15 miles thick. All of our weather happens in the bottom layer of the atmosphere, which is called the troposphere, which is six to ten miles thick.

Below we will look it some of the Weather Terms and what they mean.

Air Density

In simple terms, density is the mass of anything – including air – divided by the volume it occupies.In the metric system, which scientists use, we usually measure density in terms of kilograms per cubic meter.The air’s density depends on its temperature, its pressure and how much water vapor is in the air. We’ll talk about dry air first, which means we’ll be concerned only with temperature and pressure. In addition to a basic discussion of air density, we will also describe the effects of lower air density – such as caused by going to high altitudes – on humans, how humidity affects air density – you might be surprised – and the affects of air density of aircraft, baseballs, and even racing cars. The molecules of nitrogen, oxygen and other gases that make up air are moving around at incredible speeds, colliding with each other and all other objects. The higher the temperature, the faster the molecules are moving. As the air is heated, the molecules speed up, which means they push harder against their surroundings. If the air is in a balloon, heating it will expand the balloon, cooling it will cause the balloon to shrink as the molecules slow down. If the heated air is surrounded by nothing but air, it will push the surrounding air aside. As a result, the amount of air in a particular “box” decreases when the air is heated if the air is free to escape from the box. In the free atmosphere, the air’s density decreases as the air is heated. Pressure has the opposite effect on air density. Increasing the pressure increases the density. Think of what happens when you press down the handle of a bicycle pump. The air is compressed. The density increases as pressure increases. Altitude and weather systems can change the air’s pressure. As you go higher, the air’s pressure decreases from around 1,000 millibars at sea level to 500 millibars at around 18,000 feet. At 100,000 feet above sea level the air’s pressure is only about 10 millibars. Weather systems that bring higher or lower air pressure also affect the air’s density, but not nearly as much as altitude. We see that the air’s density is lowest at a high elevation on a hot day when the atmospheric pressure is low, say in Denver when a storm is moving in on a hot day. The air’s density is highest at low elevations when the pressure is high and the temperature is low, such as on a sunny but extremely cold, winter’s day in Alaska.

Barometric Pressure

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.

Dew Point

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 thunderstorms. You can even use dew-point to predict the minimum overnight temperature. Provided no new fronts are expected overnight and the afternoon Relative Humidity ³ 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.

Degree-Days (Heating and Cooling)

Although degree-days are most commonly used in agriculture, they are also useful in building design and construction, and in fuel use evaluation. The construction industry uses heating degree-days to calculate the amount of heat necessary to keep a building, be it a house or a skyscraper, comfortable for occupation. Likewise, cooling degree-days are used to estimate the amount of heat that must be removed (through air-conditioning) to keep a structure comfortable. Just like growing degree-days, heating and cooling degree-days are based on departures from a base temperature. 65º F is almost always used as this base.One heating degree-day is the amount of heat required to keep a structure at 65ºF when the outside temperature remains one degree below the 65ºF threshold for 24 hours. One heating degree-day is also the amount of heat required to keep that structure at 65ºF when the temperature remains 24ºF below that 65º threshold for 1 hour.Likewise, one cooling degree-day is the amount of cooling required to keep a structure at 65ºF when the outside temperature remains one degree above the 65ºF threshold for 24 hours. One cooling degree-day is also the amount of cooling required to keep that structure at 65ºF when the temperature remains 24ºF above that 65º threshold for 1 hour.

EMC (Equilibrium Moisture Content)

The moisture content of wood below the fiber saturation point is a function of both relative humidity and temperature of surrounding air. The equilibrium moisture content (EMC) is the moisture content at which the wood is neither gaining or losing moisture; this however, is a dynamic equilibrium and changes with relative humidity and temperature. If the wood is placed in an environment at a particular temperature and relative humidity, its moisture content will generally begin to change in time, until it is finally in equilibrium with its surroundings, and the moisture content no longer changes in time. This moisture content is the EMC of the wood for that temperature and relative humidity

ET (Evapotranspiration)

Evapotranspiration is the amount of water evaporating from the soil and bodies of water in a given area combined with the water that transpires metabolically from nearby plant leaves. Evapotranspiration is an indicator of how much water a plant requires over a given period of time (day, week, month, year). Measured in the same units as rainfall (millimeters or inches), it is in effect the opposite variable. Evapotranspiration is an important measurement that growers consider in making their determination of when to irrigate crops.

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 (i.e., the air is saturated with water vapor) the apparent temperature “feels” higher than the actual air temperature, because perspiration evaporates more slowly.


Humidity itself simply refers to the amount of water vapor in the air. However, the 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 in the manual and on the screen, 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.


The Weather Station incorporates a tipping-bucket rain collector that measures 0.01″ for each tip of the bucket. The station logs rain data in inch units. Four separate variables track rain totals: “rain storm”, “daily rain”, “monthly rain”, and “yearly rain”. Rain rate calculations are based on the interval of time between each bucket tip, which is each 0.01″ rainfall increment.

Rain Rate

The rain rate is calculated by measuring the time interval between each rainfall increment. When there is rainfall within the archive period, the highest measured value is reported. When no rainfall occurs, the rain rate will slowly decay based on the elapse time since the last measured rainfall.

Solar Energy

The driving force behind all meteorological changes taking place on the earth is solar energy. Each minute, the outer portions of the earth’s atmosphere receive an average of 2 calories/sq cm. This value is known as the solar constant. The solar energy data is shown in units of a langley or Ly. One langley is equal to one gram-calorie/sq cm. A gram-calorie is the amount of heat required to raise the temperature of one gram of water one degree Celsius. Although the solar constant changes over very long periods of time, it does not vary enough to affect the general nature of the earth’s weather over short periods of time. The solar energy reaching the outer atmosphere may experience a variety of fates. Thirty percent of all solar energy is lost to space by means of scattering and by reflection off clouds and the earth’s surface. Another 19% is absorbed by gases in the atmosphere and by clouds. About a quarter of it (25%) reaches the earth’s surface directly; another quarter (26%) eventually reaches the surface after being scattered by gases in the atmosphere. An important factor in determining the fate of solar radiation is its wavelength. Shorter wavelengths tend to be absorbed by gases in the atmosphere (especially oxygen and ozone) while radiation of longer wavelengths tends to be transmitted to the earth’s surface. Solar radiation that reaches the earth’s surface is absorbed to varying degrees, depending on the kind of material on which it falls. Since darker colors and rougher surfaces absorb radiation better than lighter colors and smoother surfaces, soil tends to absorb more solar radiation than water. Solar energy that reaches the earth’s surface is re-radiated back to the atmosphere as heat, also referred to as infrared radiation. Infrared radiation consists of much longer wavelengths. This re-radiated energy is likely to be absorbed by certain gases in the atmosphere such as carbon dioxide and nitrous oxide. This absorption process, the greenhouse effect, is responsible for maintaining the planet’s annual average temperature.

Solar Radiation

Solar radiation drives atmospheric circulation. Since solar radiation represents almost all the energy available to the Earth, accounting for solar radiation and how it interacts with the atmosphere and the Earth’s surface is fundamental to understanding the Earth’s energy budget. Solar radiation reaches the Earth’s surface either by being transmitted directly through the atmosphere (“direct solar radiation”), or by being scattered or reflected to the surface (“diffuse sky radiation”). About 30 percent of solar (or shortwave) radiation is reflected back into space, while the remaining shortwave radiation at the top of the atmosphere is absorbed by the Earth’s surface and re-radiated as thermal infrared (or longwave) radiation. The intensity of solar radiation striking a horizontal surface is measured by my pyranometer sensor. The instrument consists of a sensor enclosed in a transparent hemisphere that records the total amount of shortwave incoming solar radiation. That is, pyranometers measure “global” or “total” radiation: the sum of direct solar and diffuse sky radiation.

Storm Rain

Storm rain displays the rain total of the last rain event. The Weather Station takes .02″ to begin a rain event and 24 HOURS WITHOUT RAIN to end a rain event.

THW Index (Temperature Humidity Wind)

The THW Index uses humidity, temperature and wind to calculate an apparent temperature that incorporates the cooling effects of wind on our perception of temperature.

THSW Index (Temperature/Humidity/Sun/Wind)

The THSW Index uses humidity and temperature like the Heat Index, but also includes the heating effects of sunshine and the cooling effects of wind (like Wind chill) to calculate an apparent temperature of what it “feels” like out in the sun.

UV Index

The UV index or Ultraviolet Index is an international standard measurement of the strength of the ultraviolet (UV) radiation from the sun at a particular place on a particular day. The index is a number linearly related to the intensity of UV radiation reaching the surface of the earth at a given point. It cannot be simply related to the irradiance (measured in W/m2) because the UV of concern occupies a spectrum of wavelength from 295 to 325 nm and shorter wavelengths have already been absorbed a great deal when they arrive at Earth’s surface. Skin damage, however, is related to wavelength, the shorter wavelengths being much more significant. The UV power spectrum (strictly expressed in watts per square meter per nanometre of wavelength) is therefore weighted according to a weighting curve known as the McKinlay-Diffey erythemal action spectrum, and the result integrated over the whole spectrum. This typically gives a figure of around 250 in midday sun and so is arbitrarily divided by 25 to generate a convenient index value, which becomes essentially a scale of 0 to 11+ (though ozone depletion is now resulting in values above ten).

Because the scale is linear and not logarithmic, as is often the case when measuring things such as sound level or brightness, it is reasonable to assume that one hour of exposure at index ten is approximately equivalent to two hours at index 5, although other factors like the body’s ability to repair damage over a given time period could detract from the validity of this assumption.

The UV dose is the effective UV irradiance reaching the Earth’s surface integrated over the day measured in MED. MED stands for Minimum Erythemal Dose, 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. My UV dose is measured with Environmental Canada skin type III – light brown, burns moderately, tans gradually.

Because over exposure to the sub can be hazardous to your health, please check here for more information and how to interpret the index numbers and protect yourself.

Wind Chill

Wind chill takes into account how the speed of the wind affects our perception of air temperature. Your body warms the surrounding air molecules by transferring heat from your skin. If there’s no air movement, this insulating layer of warm air molecules stays next to your body and offers some protection from cooler air molecules. Wind disperses this layer of warm air, causing the air temperature to “feel” colder. The faster the wind blows, the quicker the layer of warm air is dispersed, and the colder you feel. Above 76.7ºF (24.8ºC), wind movement has no effect on the apparent temperature.

Wind Run

Wind Run is calculated by multiplying the wind speed by the measurement period and summing over time. If the wind speed was a constant 10 miles per hour for three hours, the wind run would equal 30 miles.