Polymeter with Hair Hygrometer

The polymeter is a combined hygrometer-thermometer for measuring air temperature, absolute and relative humidity of air, saturation vapour pressure, vapour pressure, dew point temperature and saturation deficit.

The polymeter has a special importance for the prediction of night frost. The prediction is based on the following physical laws:
At the night the earth radiates heat into the space strongly with clear sky. The cooling down of the earth is transmitted to the lower layers of air near the earth. But there is a limit in cooling down. With reaching the dew point temperature the water vapour contained in the air is condensating and latent heat becomes free. When the measured dew point temperature in the late evening is near 0 °C or below, then the danger of night frost is given.