We analyze tomographic models of the S-wave velocity in the mantle. The depth variation of laterally heterogeneous structure is parametrized in terms of the skewness, which yields information on mantle convection. Recent models show negative skewness in the lower half of the mantle. Assuming that the velocity anomaly is related to the temperature anomaly, this suggests that there is significant heat flow from the core. ¿ 1999 American Geophysical Union