The importance of surface temperature for mantle convection appears with the presence of adiabatic heating and cooling
and the release and consumption of latent heat in the presence of phase transitions. For some planetary bodies these effects
cannot be neglected. The dimensionless surface temperature T0, which is the ratio between the temperature at the top of
the convective region and the temperature drop across the mantle, is close to one for Mars and Venus. For the Earth,
T0 lies between 0.2 and 0.5. The dynamical influence of T0 is especially poignant for internally heated convection with
temperature-dependent viscosity. There is a tight coupling between the magnitude of the temperature field and the viscosity
itself. We have studied temperature-dependent viscosity convection for both low-T0 (0.2) and high-T0 (1.2) situations and
with internal heating in mantle convection with two upper-mantle phase transitions. Our results show that within this range
of T0 there exist two regimes for the evolution of upwellings in the mantle. In transient situations plumeplume collisions
lead to the formation of megaplumes for high-T0 regimes but are less likely to do so for low T0. In the long-term regime,
plumes with low T0 are prone to develop from the transition zone with a supply of hot material coming from the shallow
lower mantle. In systems with high T0, however, long-lived plumes tend to have deeper mantle origins. In quasi-layered
situations high T0 may act as a positive feed-back mechanism in inducing powerful hot upwellings into the upper mantle.
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