Resumen
We have used the Thermodynamic Model of the Climate to estimate the effect of variations in the low cloud cover on the surface temperature of the Earth in the Northern Hemisphere during the period 1984-1994. We assume that the variations in the low cloud cover are proportional to the variation of the cosmic ray flux measured during the same period. The results indicate that the effect in the surface temperature is more significant in the continents, where for July of 1991, we have found anomalies of the order of 0.7 ºC for the southeastern of Asia and 0.5 ºC for the northeast of México. For an increase of 0.75% in the low cloud cover, the surface temperature computed by the model in the North Hemisphere presents a decrease of ~0.11 ºC; however, for a decrease of 0.90% in the low cloud cover, the model gives an increase in the surface temperature of ~0.15 ºC, these two cases correspond to a climate sensitivity factor of 0.14 ºC/Wm-2 , which is almost half of the climate sensitivity factor for the case of forcing by duplication of atmospheric CO2. These decreases or increases in surface temperature by increases or decreases in low clouds cover are ten times greater than the overall variability of the non-forced model time series.