A variable microwave noise temperature standard having an output from below 40 to above 370 K is described. This temperature standard is unique in that a closed cycle low temperature cooler used in conjunction with a proportional control heater network provides continuously variable low temperatures which are independent of the boiling temperatures of cryogenic liquids and may be readily changed and known with an accuracy of better than 1.0 K. A variable temperature output is obtained without the use of a microwave attenuator thus eliminating the inherent errors associated with the attenuator. The output noise temperature is completely calculable using measured parameters (i.e., thermal temperatures, waveguide insertion loss, and impedance mismatch factors) and as a result may qualify as a primary noise standard.