Clouds affect surface temperature in three ways:

• By reflecting sunlight (albedo), lowering temperature by increasing albedo
• By absorbing surface infrared (IR) radiation and re-emitting a good portion downward, acting as a blanket, raising temperature.

The figure below depicts the radiative effects of high and low clouds.  (The width of the yellow (sunlight) and red (emitted infrared) arrows is proportional to the intensity of the radiation.)

Historical data shows that clouds have the net effect of lowering surface temperature:

The curve fit parameter of the figure above (R squared) is not very good, at 28%.  The figure below is similar, but for low cloud cover only, R squared being 32%.  A perfect fit would have R squared of 100%

These figures are taken from climate4you.com, an excellent source in plain language on the science of climate change.  See below.