Glossary of weather terms
- Glossary of weather terms
The apparent temperature is an attempt to represent the 'feel like' temperature taking into account the temperature, relative humidity and wind speed. The formula used in an approximation from a mathematical model of heat balance of the human body outdoors in the shade and was developed by Robert G. Steadman (1994). It is the temperature, at a reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity. This is an estimate and doesn't take into account wind direction and solar radiation for example. For more information refer to here from the Bureau of Meteorology (BoM).
This is the average temperature over the day so far and uses the 'integration method'. This method calculates the average temperature from samples each minute, instead of the standard average of the minimum and maximum temperature. The 'integration method' avoids the issue when the minimum or maximum temperature occurred for a short period of time and and skewing the average. Before March 2019 the average temperature was calculated using minimum and maximum temperatures.
The beaufort scale was developed by British naval Commander Sir Francis Beaufort in 1805 to describe the effect of wind speed on the sea state before the advent of accurate wind speed sensors. Over time it has been improved and expanded also for land use and is used to describe the wind speed using the 10 minute average.
This is sometimes known as 'Growing Degree Days' and is the number of hours since 12 am on 1 April of 7 °C or less. Many fruit trees have a chilling requirement to set fruit properly and need a certain number of cold hours.
Cloud base height
An approximate calculation of the theoretical height above ground level at which Cumulus clouds might form, based on the current temperature and dew point. This is on the assumption that the difference between the ambient temperature and dew point falls by about 8 °C per 1000 metre increase in altitude. Note that this is less than the Dry Adiabatic Lapse Rate (DALR) which is the rate at which the temperature of unsaturated air changes as a parcel ascends or descends through the atmosphere. When the temperature and dew point are closer together (e.g. the air is humid) then the cloud base value will be lower.
Consecutive dry days
The number of consecutive days where the daily rainfall total is equal to 0 mm (less than the lowest measurement of 0.2 mm). If rain has fallen today this is 0 and increases by 1 for each dry day until a day with rain.
Consecutive rain days
The number of consecutive days where the daily rainfall total is equal or greater than 0.2 mm. This increases by 1 for each day with rain measured returning to 0 on the first dry day.
Cooling Degree Day
This is an integration of the number of degrees above a threshold of 24 °C with the time in hours that the temperature is above that threshold. For example if for one day in December, successive readings at one minute intervals above the threshold were 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, then it stayed 10 degrees above for 1 hour, before falling in successive minutes to 9, 8, 7, 6, 5, 4, 3, 2, and 1 degrees above the threshold, the total for that day (of 1440 minutes) would be 150 degree minutes reported as 150/1440 or 0.1 degree days.
Is a measure used in Agriculture to assess if conditions are suitable for spraying. It indicates the evaporative rate and droplet lifetime. When spraying this should ideally be between 2 and 8 and less than 10. It is calculated by subtracting the wet bulb temperature from dry bulb temperature (the ambient temperature) and also known as the wet bulb depression temperature. The BoM has more information here.
This indicates the moisture content of the air and is the temperature at which the air needs to be cooled for condensation to liquid to occur. This is calculated from the air temperature and the relative humidity. Dew point changes in respect to the moisture content of the air and not the temperature as a result of being expressed as a temperature and is useful as a measure of the moisture content of the air. Relative humidity on the other hand, a commonly used measure for humidity is temperature dependent and doesn't represent a true measure of the humidity of the air. When the dew point rises and is closer to the air temperature the air is more humid. Conversely when the dew point falls the water vapor content of the air is less. A higher dew point of over 20 °C is a good indication of moisture for the formation of thunderstorms. When the temperature and dew point are equal the air is saturated and the relative humidity is 100%.The BoM has more information here.
The formula used is from Davis Instruments:
v = RH*0.01*6.112 * exp [(17.62*T)/(T + 243.12)]
this equation will provide the vapor pressure value (in pressure units) where T is the air temperature in °C and RH is the relative humidity.
Now dewpoint, Td, can be found:
Td = (243.12*(ln v) – 440.1) / 19.43 - In v
This equation is an approximation of the Goff & Gratch equation, which is extremely complex. This equation is one recommended by the World Meteorological Organization (WMO) for saturation of air with respect to water.
Dew point depression
This expresses the dew point as the difference of the dew point temperature from the air temperature. The higher this value is the lower the water vapor content of the air and conversely the lower this is the more humid the air is.
Fire Danger Index
The Forest Fire Danger Index (FFDI) was developed in the 1960s by CSIRO scientist A.G. McArthur to measure the degree of danger of fire in Australian forests. The index combines a record of dryness, based on rainfall and evaporation, with meteorological variables for wind speed, temperature and humidity. A FFDI of between 12 and 25 on the index is considered a "high" degree of danger, while a day having a danger rating of over 50 is considered an "Severe" fire danger rating. For Forest fuels, an FDI over 75 is categorised as "Extreme" and over 100 as "Catastrophic" (In Victoria the alternate rating name of "Code Red" has been adopted). This formula is the basis of the official fire weather forecasting. Never use this fire danger data on this website for your safety.
This generally is not used in Australia and is an hot weather version of the 'feel like' apparent temperature . Compared to the apparent temperature it over reads significantly in very hot and humid conditions. Below 27 °C the heat index is equal to the air temperature. The heat index (HI) is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels. The human body normally cools itself by perspiration, or sweating, which evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which relates one temperature and humidity combination to another at a higher temperature in drier air. Further information can be found here.
Heating Degree Day
This is an integration of the number of degrees below a threshold of 18 °C with the time in hours that the temperature is below that threshold. For example if in July there were there was four hours at one degree below the threshold, two hours at five degrees below, and three hours at seven degrees below; the total for the month would be (4*1)+(2*5) +(3*7) hours reported as 35 degree days.
Mean sea level pressure
The air in the atmosphere exerts a pressure on the surface of the earth and is known as atmospheric pressure. The more air above an place, the higher the atmospheric pressure, which 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 and facilitate comparison between locations with different altitudes, atmospheric pressure is adjusted to the equivalent sea-level pressure for meteorological purposes. This adjusted pressure is known as mean sea level pressure. The station currently in use calculates the necessary correction factor to consistently translate atmospheric pressure into mean sea level atmospheric pressure based on the site elevation.
Atmospheric (or barometric) pressure also changes with local weather conditions, making barometric pressure an extremely important and useful weather forecasting tool. High pressure zones are generally (certainly not always) 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. Further information on the correction the station uses for atmospheric air pressure may be viewed here.
Rainfall event total
This is a total amount of cumulative rainfall starting with at least 0.4 mm in 3 hrs and ending with 24 hours without rainfall.
This is calculated by the weather station as is described by Davis Instruments:
Under normal conditions, rain rate data is sent with a nominal interval of 10 to 12 seconds. Every time a rain tip occurs, a new rain rate value is computed (from the timer values) and the rate timers are reset to zero. Rain rate is calculated based on the time between successive tips of the rain collector. The rain rate value is the highest rate since the last transmitted rain rate data packet. If there have been no rain tips since the last rain rate data transmission, then the rain rate based on the time since that last tip is indicated. This results in slowly decaying rate values as the rain ends, instead of showing a rain rate which abruptly drops to zero. This results in a more realistic representation of the actual rain event. If this time exceeds roughly 15 minutes, than the rain rate value is reset to zero. This period of time was chosen because 15 minutes is defined by the U.S. National Weather Service as intervening time upon which one rain "event" is considered separate from another rain "event".
This is traditionally used as a measure of humidity. It measures the ratio of the quantity of moisture in the air and the maximum capacity the air can hold at the current temperature. Or the water vapour pressure as percentage of the saturation water vapour pressure at that temperature and pressure. This is expresses as a percentage and at saturation the relative humidity will be at or close to 100 %. Relative humidity does not measure the absolute moisture content of the air as the air can hold more moisture at a higher temperature. If the actual moisture content remains unchanged the relative humidity will fall as the temperature rises. It is for this reason the relative humidity is typically highest early in the day (coolest time of the day) and lowest in the afternoon (hottest time of the day). For more information about humidity, visit the BoM here.
Sager station forecast
This is a forecast generated from the weather station data. It is based on the The Sager Weathercaster which is more accurate than the Zambretti station forecast. Whilst it not of the accuracy of official forecasts it is quite accurate nevertheless and is provided for general interest. Please don't complain when it comes up with the wrong forecast. It uses the temperature, change in wind direction in the last 6 hours, atmospheric pressure and trend and present weather reported by Brisbane Airport (METAR station YBBN) based on Buys-Ballot laws. For more information visit this site.
This is generated by the weather station itself and is not of the accuracy of official forecasts. The forecast is calculated from atmospheric pressure and trend, wind speed and direction, rainfall, temperature, relative humidity, latitude, longitude and time in the year. Sky conditions, changes in precipitation, temperature and wind speed or direction are reported. It shows the forecast for up to the next 48 hours and updates once an hour on the hour.
Temperature is a measure of the heat energy that characterises the mean random motion of molecules in a physical body (kinetic energy). All matter is made of particles which are in constant motion which mean they have kinetic energy. The kinetic energy is higher when the particles are moving faster. It is a mean measure as the particles are constantly moving but not always in the same directions and speed. This results in different amounts of kinetic energy of each particle. Measuring the temperature is the best approximation of the kinetic energy of the particles. Therefore the higher the temperature the faster on average the particles of the physical body are moving, irrespective of the number of particles involved.
Wet bulb temperature
This is a calculated measure of humidity expressed as a temperature. It is the temperature measured by a traditional liquid in glass thermometer covered in a thin muslin cap that is kept constantly wet with distilled water. The evaporation of water from the thermometer has a cooling effect, and so records a lower temperature compared to the normal dry bulb thermometer. The evaporation rate depends on the humidity of the air; the higher the humidity the slower the evaporation. The difference between this and a dry bulb thermometer indicates the moisture content of the air which is known as the wet bulb depression.
Is a measure of the 'feel like' temperature from the effect of the wind on exposed skin. This is dependent on the air temperature and wind speed. The lowest wind chill are reported with low temperatures and high wind speeds. This is often called the wind chill factor and is never higher than the air temperature. This wind chill factor is also factored into the apparent temperature. When the wind speed is less than 4.8 km/hr and the temperature above 10 °C the wind chill temperature will be the same as the air temperature. The formula used is the North Amercian wind chill.
Wind direction is defined as the direction the wind is blowing from referenced from true north, and not magnetic north.
The wind gust is defined by the WMO as the highest mean wind speed measured over a 3 second period. Many factors can affect the measured wind gust including the anemometer type, height and exposure and sampling interval and method. This is the sudden increase of wind over a short period of time, usually for only a few seconds.
Is a measure of the mean wind speed expressed as the distance traveled over the day. A mean wind speed of 10 kph for a whole hour would produce a wind run of 10 kph in 1 hour and 240 kph in 24 hours. This is a good measure of the quantity of wind received over the day and indicate how windy the day is.
Wind is the constant movement of air over the surface of the Earth as a result of the complex multi-dimensional heat exchange processes that distributes the heat around the planet. The wind speed is the 10 minute average of the constant gusts and lulls of the wind speed as well as equally rapid changes of direction which is a result of turbulent effects caused by the boundary layer of the atmosphere in contact with the surface of the Earth. Wind speed at this site is measured as high as possible, at 10 metres above ground is the standard, as the wind speed is lowest and most variable close to the Earth surface. The higher obstacles are the greatest the short variance of the wind speed which we feel as gustiness. It is for this reason that winds are stronger and steadier at sea compared to built up urban environment with high and variable obstacles producing variable wind speeds that are different to the natural undisturbed flow of the wind. A squall is a sudden and sustained increase of the mean wind speed over several minutes before returning to previous mean wind speed.
Zambretti station forecast
This forecast is generated from the weather station data. It is based on the Zambretti Forecaster which is not that accurate. It is therefore provided for only general interest. Please don't complain when comes up with the wrong forecast. It uses the air pressure and trend, wind direction and trend based on Buys-Ballot laws and forecasts sky condition and precipitation. For more information visit this site.