The invention relates to a portable personal data processing device, such as a wrist-worn measuring device, a heart rate monitor, a subscriber terminal of a radio system, or a sports watch. The device includes a temperature sensor for measuring the ambient temperature and a humidity sensor for measuring the ambient humidity. Furthermore, dependence information, which defines temperature values of the dew points corresponding to the humidity values, is stored in the device. The control unit is configured to monitor the change rate of the measured temperature, to determine, on the basis of the change rate, the moment at which the temperature reaches the temperature value of the dew point corresponding to the measured humidity on the basis of the dependence information, and to provide a forecast on fog appearance in relation to the moment employing the user interface.