Frequently we see weather terms being bandied about and have no real clue as to what they mean. Below we give explanations of some of the more common terms and hopefully they'll make sense to you.

Chandler Burning Index: The Chandler Burning Index (or CBI) uses air temperature and relative humidity to calculate a numerical index of fire danger. That number is then equated to the Fire Danger severity of either extreme, very high, high, moderate, or low. It's based solely on weather conditions.

Heating and Cooling Days: Basically, heating and cooling degree days are a measure of how much (in degrees), and for how long (in days), the outside air temperature was either above (Cooling degree day) or below (heating degree day) a certain level. They are commonly used in calculations relating to the energy consumption required to heat or cool buildings such as a home or business. This information can also be used in agriculture to determine crop planting, disease and pest management and harvesting. A base of 15 degrees C is suitable for most situations and is used at this station.

Apparent Temperatures: The apparent temperature is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity. The apparent temperature equals the actual air temperature when the dew-point temperature is 14C (57.2F). At higher dew-points, the apparent temperature exceeds the actual temperature and measures the increased physiological heat stress and discomfort associated with higher than comfortable humidities. When the dew-point is less than 14C, on the other hand, the apparent temperature is less than the actual air temperature and measures the reduced stress and increased comfort associated with lower humidities and greater evaporative skin cooling.

Apparent temperatures greater than 26.6C (80F) are generally associated with some discomfort. Values approaching or exceeding 40.5C (105F) are considered life-threatening, with severe heat exhaustion or heatstroke possible if exposure is prolonged or physical activity high. The degree of heat stress may vary with age, health, and body characteristics.

Feels Like: Often, simply learning the expected high and low temperatures isn't adequate preparation for going outdoors. Many people wait to hear the feels like temperature before they plan what to wear and where to go on any given day. The feels like temperature gives you a clue about how you will feel outdoors, rather than a simple temperature that leaves you guessing.

Measuring wind chill is one way of determining the feels like temperature. Wind chill involves measuring the temperature as it is felt on skin, as a result of air temperature and wind speed. Basically, wind chill measures the heat skin loses when the wind blows on it.

The wind chill is practically always lower than the temperature of the air. This is because wind increases the speed of moisture evaporation from your skin and serves to move heat away from your body. The exception to this rule, however, is when higher temperatures are concerned. At higher temperatures, wind chill is considered far less significant.

The heat index is another way of measuring the feels like temperature. It takes into account both temperature and humidity in determining how warm temperatures feel to your body. When a human being perspires, the water in his or her sweat evaporates. This results in the cooling of the body as heat is carried away from it. When humidity is high, the rate of evaporation and cooling is reduced, resulting in a higher heat index.

Keep in mind that the feels like temperature you hear when listening to your local weather forecast may not be enough to determine how you will feel in certain weather. For example, heat index measurements are given for shady conditions with light wind evident. In full sunshine, the feels like temperature may seem hotter than the heat index indicates. By the same token, if you are standing in a sheltered area that receives full sun, you may feel a bit warmer than the forecasted wind chill indicates.

The feels like temperature is particularly important for helping you to guard your health. When the heat index is particularly high, you may be at increased risk for heat exhaustion and heat stroke. Likewise, extreme wind chill temperatures can put you at risk for frostbite and hypothermia. As such, it is important to pay attention to the feels like temperature in addition to the actual forecasted temperature for the day.

Wind Chill: In practical weather terms, there is an actual air temperature and a "feels like" temperature. Television meteorologists often provide both numbers during especially hot summer days or cold winter days. In the summertime, the "feels like" temperature is called the heat index, but during the winter, the "feels like" temperature is often called the wind chill factor. The wind chill factor is a combination of air temperature and wind speed that affects the freezing rate of exposed skin.

A quick demonstration of the effects of wind chill can be performed at your desk right now. Simply blow a fast stream of air across your exposed forearm or hand. The area receiving the fast-moving air should feel noticeably cooler than the rest of your arm. This is the wind chill effect. When the accelerated air from your mouth moved across your exposed skin, the normal evaporation rate was temporarily raised. The heat radiating from your arm was also affected by the change in wind speed.

The actual wind chill effect works like this on a much larger scale. On a relatively warm winter's day, the air temperature might be O degreesC (32 degrees F). This would not be especially uncomfortable for the average person wearing suitable winter clothing, but what if the wind speed were raised to 40 kph (25 mph)? The wind chill factor would drop the "feels like" temperature to -7 degreesC (19 degreesF). At this temperature, exposed skin could suffer frostbite within a few hours.

If the air temperature continued to drop and the wind speed remained constant, the wind chill factor could become cold enough to cause immediate frostbite to exposed flesh. This is why cold weather experts always recommend covering the face, head, neck, hands and ears if you have to venture outside during a winter storm event. The wind chill factor primarily affects human flesh, not most metal or machinery. The actual air temperature will cause a car's transmission fluid to freeze, for example, not the wind chill factor.

Different agencies responsible for official weather information use different formulas to calculate the wind chill factor. The easiest way for average people to calculate wind chill factor on their own is to access an online conversion table provided by official weather services. Air temperature and wind speed can be entered into dialog boxes, and the approximate wind chill factor can be calculated within seconds. There are also charts available that show the wind chill factor at different temperatures, combined with the estimated exposure times before the risk of frostbite appears.

Heat Index: Those who live in hot, humid climates have all heard the lament, "It?s not the heat, it?s the humidity!" on a particularly hot, sticky day. A heat index takes the actual air temperature and factors in the relative humidity to arrive at the temperature that the human body feels. Because humidity has a real effect on the human body's ability to cool itself effectively, measuring the temperature a body feels under the influence of high heat and humidity is important to keep people safe from possible heat disorders.

When a body cools itself, perspiration forms on the skin and the air evaporates it. When humidity is high, since the air is already so saturated with water, the perspiration is not evaporated as quickly, or not at all. Therefore, when it is not only hot, but also humid, the temperature feels hotter because the body cannot cool itself effectively.

The heat index is calculated in the shade, with a light wind factored in. Direct sun exposure can raise the heat index by as much as 8 degrees Celsius (about 15 degrees Fahrenheit). Relative humidity is the ratio of the amount of water vapor in the air to the greatest amount of vapor possible. The formula is very complicated, especially for non-mathematicians, so numerous heat index charts are available on the Internet. In most areas hot and humid enough to require a heat index, weather forecasters usually give the actual temperature along with the heat index temperature.

For example, the heat index temperature for a 32 degrees C (90 degrees F) day with a relative humidity of 90% would be 49 degrees C (121 degrees F). At 80% humidity, a temperature of 35 degrees C) (95 degrees F) would feel like 56 degrees C (133 degrees F). The highest recorded heat index reading was on 8 July 2003 in Dhahran, Saudi Arabia. The relative humidity was 67% and the temperature was 42 degrees C (108 degrees F), making it feel like 80 degrees C (176 degrees F). When one compares a desert city with a relatively high temperature to a southern city near the ocean with a lower temperature, it may feel hotter in the southern city because the relative humidity pushes the heat index up.

The heat index, which is called the humidex in Canada, is a useful tool in predicting heat disorders. By calculating the heat index, health officials can issue helpful warnings to citizens about potential dangers from the heat. Temperatures from 32 degrees to 41 degrees C (90 degrees to 105 degrees F) can contribute to sunstroke and heat exhaustion. From 41 degrees to 54 degrees C (105 degrees to 130 degrees F), one can add heat stroke to the list of possible heat complications. During extended periods of exposure to temperatures over 54 degrees C (130 degrees F), heat stroke is nearly guaranteed.

Temperature/Humidity/Sun/Wind (THSW) Index: 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.

Humidity: Humidity itself simply refers to the amount of water vapour in the air. However, the amount of water vapour 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 vapour 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 vapour in the air, but a ratio of the air's water vapour 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 vapour content. A parcel of air with a capacity for 10g of water vapour which contains 4g of water vapour, the relative humidity would be 40%. Adding 2g more water vapour (for a total of 6g) would change the humidity to 60%. If that same parcel of air is then warmed so that it has a capacity for 20g of water vapour, the relative humidity drops to 30% even though water vapour 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 vapour.

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 vapour 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 vapour content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high water vapour content; low dew point indicates low water vapour content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. You can also 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 cannot get colder than the dew point anytime.

Solar Radiation: 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/square meter.

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

Weather station software uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET.

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 mountain top. To compensate for this difference and facilitate comparison between locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent sea-level pressure. This adjusted pressure is known as barometric pressure. Barometric pressure also changes with local weather conditions, making barometric pressure an extremely 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, however, 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. How to convert sea level to station barometric pressure and visa-versa Atmospheric pressure changes by approximately 1hPa (1mbar) for every 10 meters of altitude change (decrease for increase in altitude and visa-versa). So for example, if the barometric pressure at sea level was 1000hPa, then at an altitude of 10 meters, the pressure would be 999hPa.