A map or chart showing the principal meteorological elements at a given hour and over an extended region. Such maps usually show the height of the barometer, the temperature of the air, the relative humidity, the state of the weather, and the direction and velocity of the wind. Isobars and isotherms outline the general distribution of temperature and pressure, while shaded areas indicate the sections over which rain has just fallen. Other lines inclose areas where the temperature has fallen or risen markedly. In tabular form are shown changes of pressure and of temperature, maximum and minimum temperatures, and total rain for each weather station since the last issue, usually 12 hours.
© Webster 1913