Final answer:
The middle of the U.S. is warmer due to its continentality and lack of oceanic moderation, which is present on the coasts. The interior experiences more extremes, while coastal climates are moderated by the ocean's high heat capacity. The delayed peak heat is also due to oceans warming up more slowly than land.
Step-by-step explanation:
The average temperatures in the middle of the United States tend to be warmer than the coasts primarily due to geographical features and the influence of oceans. Mountain ranges along the eastern and western portions of North America, such as the Rockies and Appalachians, create varied climates by impacting air flow and precipitation. The vast lowlands in the center allow for a more continental climate, which experiences more extreme temperatures both in summer and winter.
Additionally, the impact of oceans on temperature moderation is significant. Oceans have a high heat capacity, which means they warm up and cool down more slowly than land. This "heat inertia" results in coastal areas experiencing more moderate temperatures, whereas areas in the interior, away from the moderating effect of the oceans, endure a broader range of high and low temperatures.
Moreover, the warmest day of the year is usually in August rather than at the summer solstice in late June because of the time it takes for the oceans to heat up. The oceans retain much of their winter coolness by the time of the solstice, and it is not until roughly a month and a half later that they reach peak warmth, subsequently contributing to the higher land temperatures observed in August.