Following the visualization of estimated average IQ by state that I was able to create with the neat site Many Eyes, Fat Knowledge astutely observed that the farther north a state is located, the higher its IQ tends to be.
That is clearly the global trend. At the national level, I found distance from the equator and IQ correlate at a statistically significant .67 using data from Vanhanen and Lynn's IQ and the Wealth of Nations. The relationship is more rigorous even than the contemporary relationship between purchasing power parity and IQ (as an aside, life expectancy shows the single strongest correlation with IQ at .85, followed by infant mortality at an inverse .84, and then--without Mike Judge's endorsement--an inverse relationship with fecundity at .81).
The trend also holds at the state level within the US. Using the latitude of each state's most populous city and comparing it with estimated IQ yields a correlation of .70, made considerably stronger (.81) if only the lower forty-eight are considered. As the relationship is at the global level, much of this is attributable to race, with blacks concentrated in the Deep South and Hispanics in the Southwest.
More interestingly, the relationship between latitude and IQ firmly exists in the US when only whites or only blacks are considered. And the correlations are almost identical: .52 for whites (click here for visual representation of estimated white IQ by state) and .51 for blacks (here for visual).
There's plenty of room for speculation as to why the farther north you go, the smarter the population tends to become.
Regarding whites, the English Puritan families of the northeast that had become established and affluent stayed put while those with less to leave had more to gain by heading out onto the frontier. Whites working in the South as indentured servants made up a large portion of the labor supply early in the 1600s until well into the 18th Century. This was the ticket to the New World for many Europeans with few prospects at home, and sometimes even presented convicts a way to avoid prison time. Give consideration also to how the gullible types, hoping to strike it rich, flocked to California in the 1840s and 1850s with dreams that, for the most part, didn't quite 'pan' out. A rebirth of the California Dream pulled destitute menials from the Heartland to the Southwest during the Great Depression (a la The Grapes of Wrath).
As the more southerly states are more ethnically diverse, it's reasonable to assume that a greater number of whites in these states have some non-European ancestry than in the whitebread upper Midwest and Northeast, especially from Amerindians in the Southwest. Notice also the Scottish/Irish Appalachian streak of banjo-playin' toothless inbreds like those put on display in movie version of Deliverance (I can say that. It's self-deprecation, as my lineage traces back to western Virginia).
Regarding blacks, those in southern states tend to, with exceptions, have less European ancestry than those in the north. Pulling off successful escapes from the antebellum South could have selected for intelligence among those who were able to do so as well.
Contemporarily, for both whites and blacks, warmer climates produce a greater demand for outdoorsy labor (landscapping, lawn mowing, construction, tourism, etc), which tend to have low skill-level requirements. That rougher climates selected for attributes to attenuate the physical threats posed by severe cold, while also selecting for greater intelligence to deal with more scarcity, may mean that, on average, smarter people are less bothered by chilly weather. Personal experience suggests it to me.
The relationship doesn't hold with any rigor for Hispanics in the US (although it trends in the same direction, it's not statistically significant). This illustrates the difficulty in grouping all people of Latin American ancestry under the "Hispanic" umbrella. The highly professional Cuban population in southern Florida provides the best example. Puerto Ricans, concentrated in the Northeast, have a very different profile than the mostly white Cubans of Miami.