itu840.rst

Recommendation ITU-R P.840

This Recommendation provides methods to predict the attenuation due to clouds and fog on Earth-space paths.

Introduction

For clouds or fog consisting entirely of small droplets, generally less than 0.01 cm, the Rayleigh approximation is valid for frequencies below 200 GHz and it is possible to express the attenuation in terms of the total water content per unit volume. Thus the specific attenuation within a cloud or fog can be written as:

γ_c(f, T) = M ⋅ K_l(f, T)  [dB/km]

where:

• γ_c : specific attenuation (dB/km) within the cloud;
• K_l : specific attenuation coefficient ((dB/km)/(g/m3));
• M : liquid water density in the cloud or fog (g/m3).
• f : frequency (GHz).
• T : cloud liquid water temperature (K).

At frequencies of the order of 100 GHz and above, attenuation due to fog may be significant. The liquid water density in fog is typically about 0.05 g/m3 for medium fog (visibility of the order of 300 m) and 0.5 g/m3 for thick fog (visibility of the order of 50 m).

Module description

itur.models.itu840