There may still be benefits from a cool roof even in a moderate or cool climate (especially if your roof is covered with snow during the colder months, meaning that any potential heat gain from a darker roof wouldn't materialize anyway). However, the benefits are most pronounced in a hotter climate. Here are three reasons why that is the case.
1. Sun angle
In the southern area of the United States, the sun angle is much more intense. The sun is shining more directly at you (and at your roof), meaning that it's more concentrated. A given amount of flat surface will pick up more heat from the sun's light in a given amount of time. The heat gain from the sun isn't as much of a problem at a more northern latitude, meaning that the benefits won't be as easily distinguished either.
2. Ratio of hot to cold months
Not only are the hotter months sunnier with more intense sunbeams in the South, but there are a lot more of them too. Theoretically, a light-colored, reflective roof saves energy in the summer but can also lose energy in the winter. This is the argument that some make when saying that dark-colored roofing is better in colder climates. It's really only relevant if you keep your roof snow-free all year (otherwise the snow will cancel out any warming benefit gained from the dark roof in the winter), but it does demonstrate that there are fewer opportunities in the North to benefit from heat reduction. If you live in a cold climate, heat reduction is not one of your main worries because such a large portion of the year is cold anyway.
3. Thermal shock
Thermal shock is a type of damage that can occur to dark-colored roofing material, such as asphalt shingling, when the sun heats it to an extreme temperature during the day and then the temperature drops sharply at night. Avoiding thermal shock is another significant benefit of a cool roof in a hot climate, especially in a part of the country that cools down rapidly at night (such as because of sea breezes). Because the sun angle isn't as intense in the northern climes, thermal shock is less likely to be a problem.
These three points show why hot southern climates tend to yield the most effectiveness and the highest number of benefits from changing to a cool roof. For more information, talk to a roofing company like K.P Lamarco & Associates.