Why is Phoenix hotter than Miami in the summer even though it is farther north? Why is Seattle milder than Chicago in the winter even though it is farther north? Clearly latitude is not the only factor.
— Gerry Talsky, Chicago
Many factors affect seasonal temperatures. Latitude is a major factor, though in the cases you have mentioned, it is clearly not the only influence on an area’s climate. Proximity to the Atlantic Ocean, which provides much higher amounts of humidity, explains why Miami, with an average high in the lower 90s in the summer, is cooler than Phoenix, whose average summer high is 100 to 106 with no ocean influence. Seattle has a marine West Coast climate, which is modified by air flow off the Pacific. Its temperatures run rather mild in the winter (highs in the mid-40s to low 50s; lows in the upper 30s) and cool in the summer (highs in the low and mid 70s).
Proximity to the Atlantic Ocean, which provides much higher amounts of humidity, explains why Miami is cooler than Phoenix in the summer.
Ask Tom: What affects temperatures other than an area’s latitude?