Tara,

Your  absolutely right questioning this thing called sun*.  I should have told said where in most parts of the world it could be used in both sun and rain....here in the Pacific Northwest where there is no sun it is used only in rain.

• ....a self-luminous heavenly body, star
    .....sunshine, the heat and light from the sun, to be exposed to the sun
   ......something likened to the sun in brightness, spendor, etc
   
  smiles