Legend. White lines & values = height of 850 hPa in geopotential decameter (gpdam) (every 2 gpdam). Black lines and colored zones & = temperature at 850 hPa in deg C (thin line every 1°C, thick line every 5°C). Some critical values of 850 hPa temperatures in maritime airflow. For Low Countries: usually  -7°C or below risk for wintry precipitation (in continental airflow wintry ppn may start from  -2°C).  For Ardennes: usually -3°C or below risk for wintry precipitation.