Why wasn't it this hot last year?

Answer 1

Weather fluctuations. Possibly there was more cloud cover last year, or there is a larger/more active sunspot this year.

(For me, however, last year was much hotter.)

The weather in the mid- and high-latitudes varies from year to year for many different reasons. For example, the east coast of the United States has been considerably warmer in the summer of 2010 because the Bermuda High (in the Atlantic) is particularly strong and is pumping a lot of hot, humid air around it and into the eastern United States. This is characteristic of a developing La Nina in the Tropical Pacific, which is indeed occurring this summer.

Things like cloud cover certainly influence the temperature, but generally not in this situation. Additionally, changes in cloud cover would only be symptomatic of something of a larger scale occurring in the atmosphere.

Sun spot activity continues to be at a nearly unprecedented minimum, and even if it had increased slightly this year the difference in solar irradiance would be masked by more influential modes of variability in the atmosphere.

Ultimately, what generally dictates what type of weather most places will experience in the summer has to do with where the air masses are coming from. If the atmospheric pattern is such that there are more frequent incursions of air from Canada, for example, then it will be cooler. In the case of the eastern United States, the air has constantly been coming from the south and has been aided by what are called higher heights, or higher thicknesses (of air), associated with this massive, persistent ridge of high pressure. This has made it extremely hot, and it is likely to continue.

Answer 2

Various factors can contribute to differences in temperature between years, such as weather patterns, climate change, and natural variability. It's possible that this year's weather conditions, like high pressure systems or heatwaves, are causing hotter temperatures compared to the previous year.