Temperature

Temperature scale

meteoblue expresses all temperatures in degrees Celsius (°C). Pure water freezes at 0°C and boils at 100°C at 0 meter above sea level (m asl). Conversions into other scales can be made on request.

Measurements

Official temperature measurements are usually made at a given location and altitude, at 2 meters above the ground, if measured according to WMO (World Meteorology Organisation) guidelines. Many other measurements are available, which may produce substantially different results, depending on the instrumentation (cover, ventilation, etc.), soil surface (vegetation, rock, tarmac or other), surroundings (fields, forest, cities…). These are usually not considered for standard weather services.

Simulations

Temperature forecast (simulation) is made for the temperature expected at any given location and altitude, if measured according to WMO guidelines, at 2 meters above the ground, if not otherwise indicated. Other altitudes are indicated in the legend.

Maximum and minimum temperature

Maximum and minimum temperatures indicate the highest and lowest temperature respectively that occurred in a specific recording time period (e.g. last 24h).

Temperature amplitude and change

The temperature amplitude is the difference between the minimum and maximum temperature that occurred within a specific recording time period (e.g. last 24h).

Temperature change shows how temperature changed compared to the temperature at the exact same time 24 hours earlier. Thus, positive and negative values are possible here. Both temperature amplitude and temperature change are expressed in degrees Celsius (°C) as well as in degrees Fahrenheit (°F).

Equivalent potential

The equivalent potential (Theta-e) is the temperature an air parcel would have, if all the water contained condensed and released its latent heat to the parcel, which would then be lowered adiabatically to a reference pressure level of 1000 hPa. Equivalent potential temperature is expressed in degrees Celsius (°C) as well as in degrees Fahrenheit (°F).

A moist air parcel would reach a higher equivalent potential temperature than a dry air parcel at the same height. It is the result of the exothermic process of condensation, where energy is released to the surrounding. Therefore, a larger water content means that more water is available for condensation: The result is a greater warming of the air parcel. Thus, the equivalent potential temperature gives you information about the energy content of air masses: The higher the equivalent potential, the greater the energy content.

Wet-bulb potential

The wet bulb potential (Theta-w) is the temperature an air parcel would have, if it cooled adiabatically to saturation and then were brought moist-adiabatically to the reference pressure level of 1000 hPa. Wet bulb potential temperature is expressed in degrees Celsius (°C) as well as in degrees Fahrenheit (°F).

As the equivalent potential, the wet-bulb potential temperature is useful to identify particular air masses and frontal analysis for instance. 

Notations and aggregations

Temperature data can be shown as :

  • Hourly temperatures: These are given for values measured or forecast at full hours, usually 00:00, 01:00, 02:00 etc. until 23:00. Other intervals are available on request.
  • Daily minima and maxima: Forecast minima and maxima are used from 24 hourly simulations. Sometimes, measured maxima or minima can be up to 2°C higher or lower, because temperature can still vary within hours. If the minima occurs in the evening (which rarely occurs), then that evening minimum is used for the daily minima. 
  • Daily averages: These are the averages of 24 hourly values, either from the forecast or measured data.
  • Weekly temperatures: these can be indicated as maxima or minima, sometimes as averages. Usually, calendar weeks are used, starting with week 1, 2 etc. of the year. For monthly summaries, either the weeks within a monthly or 10-day aggregations are used.
  • Monthly temperatures: these can be indicated as maxima or minima, sometimes as averages. Usually, calendar months are used.
  • Yearly temperatures: these can be indicated as maxima or minima, sometimes as averages. Usually, calendar years are used. For some applications, typical meteorological years (TMY) are used.

Interpretation

Temperatures are always subject to the actual location, and can change according to the type of measurement, location, surrounding and personal perception. For comparison purposes, the official measurements are the most suitable basis.

As for maps and Pictocasts, meteoblue uses a standard colour scale from -45 to +45°C to improve interpretation. Temperature is sometimes shown as a curve with a scale on the side of the graph, as well. Examples of temperature forecasts are shown under the point Meteograms, Pictocast and under Temperature Maps.

Felt temperature

The apparent (or "felt") temperature (°C) is the perceived temperature that people experience under forecast conditions. It is made of a heat index and a wind chill factor. The heat index (HI) calculates the temperature felt by a body when heat and humidity are combined. It rises with actual air temperature and relative humidity, which makes it increasingly difficult for the body to remove the excess heat through sweating. Exposure to direct sunlight can increase the HI by up to 10°C. When the heat index exceeds 37°C, the body will need extra help for removing heat to avoid heat stress and collapse. The wind chill factor calculates how much heat is removed from the body through air movement. It generally reduces perceived temperature compared to the actual temperature.
The meteoblue "felt" temperature is a combination of both effects. Under windy conditions, perceived temperature is lower than the actual temperature. Under moist (and mostly hot) conditions, perceived temperature is higher than the actual temperature - conditions are then described as "muggy" and occur mainly in summer and in the tropics, mostly around the Equator (e.g. in Indonesia, Kenya, Rwanda, Congo, Brazil, Peru).

Freezing level height

The freezing level height represents the zero-degree isotherm in the free atmosphere. It is the altitude where the temperature is 0°C and freezing of water begins. The height is expressed in meters (m).