In terms of a few experimental schemes,an expression for downward atmospheric radiation form various isobaric levels under a range of cloudiness is developed in the formG_p=σT⁴[0.536+0.128ln(1+e)]The effects of cloudiness are represented mainly by the corresponding change of temperature and vapor pressure at the related levels. Verification shows that the expression has a 6.3% relative fitting error,and thereby the spatial and temporal pattern of the downward radiation is investigated at 500hPa and in the lower troposphere (500hPa to the ground) by use of the data from 104 radiosonde stations throughout the country.