What I've read is that because of its thick atmosphere there is little temperature variation on the surface:

Venus Alive! by Frederick Suppe
Chapter 4
Venus Unmasked
"In 1967 Mariner 5 and the Soviet Venera 4 gave further confirmation to a very hot surface
temperature on Venus. Eventually it would be determined that Venus is uniformly hot over its surface,
varying by only about 25°C between the nightside and the dayside. By comparison, Mercury, which is
closer to the Sun, ranges from a low of -100° K ( -173° C) to a high of about 700° K (427° C)–slightly
lower than Venus’s 725-747° K (452-474° C) average. The difference is that Mercury has insignificant
atmosphere and Venus has one 90 times more dense that the Earth’s. The Cytherian atmosphere holds in
Venus’s internal heat–the prototype greenhouse effect."
p. 12.
http://carnap.umd.edu/phil250/venus_alive_pdf/Chapter4/CH4-0.pdf

This reference does mention that earlier radar studies from the sixties did suggest large temperature changes on the surface, but later studies corrected this.

Bob Clark