From records of insolation and from observations of cloudiness and cloud density obtained at Blue Hill Observatory relations have been derived which give the amount of insolation, T, to be expected on the average with a given cloudiness, cloud density and solar air mass. These relations are of the form T=(a/m)e−bm where m= air mass (i.e. secant of zenith distance of the sun), e = base of natural logarithms, a and b are constants which depend on the cloudiness and cloud density. The effect of the cloud density on the insolation is of the same importance as cloudiness if the sky is largely covered. The following table shows the insolation for different values of cloudiness and density and for air masses 1 and 3 in cal/cm2/hr. It will be seen that, especially with larger values of the cloudiness, the effect of the density is very important. Abstract From records of insolation and from observations of cloudiness and cloud density obtained at Blue Hill Observatory relations have been derived which give the amount of insolation, T, to be expected on the average with a given cloudiness, cloud density and solar air mass. These relations are of the form T=(a/m)e−bm where m= air mass (i.e. secant of zenith distance of the sun), e = base of natural logarithms, a and b are constants which depend on the cloudiness and cloud density. The effect of the cloud density on the insolation is of the same importance as cloudiness if the sky is largely covered. The following table shows the insolation for different values of cloudiness and density and for air masses 1 and 3 in cal/cm2/hr. It will be seen that, especially with larger values of the cloudiness, the effect of the density is very important.