A map of the relative humidity of air over the Baltic using the WRF model (11.06.2015, 12:00)

The relative humidity of air at a height of 2 m above the sea surface, obtained using the numerical weather model (representative of a 4×4 km area). The relative humidity is one of the measures of the water vapour content in the air. It is defined as the ratio, expressed as a percentage, of the instantaneous partial pressure of water vapour to the partial pressure of saturated vapour, that is, the maximum partial pressure of water vapour at a given temperature. The relative humidity is thus a measure of how far a given air mass is from saturation with water vapour. The relative humidity varies inversely with air temperature: it falls with increasing in temperature and vice versa. The water vapour content in air is one of the basic variables in numerical weather models. The SatBałtyk system uses the WRF model (Weather Research and Forecasting), one of the most commonly used mesoscale weather models worldwide (suitable for analysing weather phenomena at a scale of several tens to several hundreds of kilometres, that is, a scale corresponding to the size of the Baltic Sea). In the SatBałtyk System, the WRF model covers the whole area of the Baltic Sea and uses the results of a global weather model as a source of information about adjacent areas. The details of the WRF model and its configuration in the SatBałtyk System can be found here.

The SO SatBałtyk service provides distributions of the relative humidity of the air over the Baltic in percentages [%], 24 times a day, in the form of maps with a resolution of 4 km.

Methodology for determining the relative humidity of air

The fundamental variable in the WRF model, describing the content of water vapour in the air, is the absolute humidity, defined as the mass (in kg) of water vapour per 1 kg of air.  This variable, like all the other variables describing the state of the atmosphere, is determined on a three-dimensional grid covering the section of the atmosphere over the target area from the land/sea surface to the 50 hPa level, which lies at an altitude of approximately 20 km. The absolute humidity values at the various grid nodes are obtained as the solution of the equation that describes the changes in water vapour content in time as a result of the inflow of air with other properties, evaporation/condensation from/at the substrate and/or phase changes of the water contained in the air. For this reason, the equations describing the absolute humidity are strictly linked to those describing air temperature changes and also to those describing physical processes taking place in clouds. For the requirements of the SO SatBałtyk System, the relative humidity is determined on the basis of the absolute humidity, air temperature and atmospheric pressure. In the WRF model, the layers of the atmosphere are of different thicknesses, variable in time and space, and dependent on the land relief and the state of the atmosphere at a given place. In order to obtain the relative humidity of the air at a height of 2 m above the surface, which is a product of the SO SatBałtyk system, the model results are interpolated in the vertical.

Validation (assessment of accuracy)

The accuracy of the maps of the relative humidity of the air is assessed on the basis of an analysis of the differences between measured and modelled humidity values. The results of the model’s validation can be found here.

Link to the parameter in SatBaltic System:

Relative humidity of air (for registered users)