Originally Posted by Yeti

For the initial question - coldness and warmth can approached in two ways - sensible and absolute. Absolute is measured by temperature with absolute zero being the coldest temperature "possible" (when all motion inside the molecule stops). Sensible is if something feels hot or cold. In reality you're feeling a heat flux. An everyday example of this is a room at 80*F - you pick up a piece of plastic and it doesn't feel very cold yet you pick up a piece of copper and it feels cold, even though both pieces are at the exact same temperature. The reason is the heat is transfered much more quickly through the copper resulting in a higher heat flux (and thus thermal penentration depth).
As for radiation, it is a form of heat transfer like conduction that depends on a temperature difference. Infrared is just a small band of the entire electromagnetic spectrum (and the sun doesn't emmit only in the infrared, it ranges from UV to infrared with the peak in the visible).
Well, daily insolation depends on the declination angle, latitude, and day of year... and slope and azimuth angle if its a tilted surface... and clearness index for terrestrial radiation.