MadSci Network: Earth Sciences |
Why are summer days hot when the days are getting shorter? This is an excellent question. Common sense would tell us that the hottest day of the year at a given location should be the day on which that location receives the most sunlight, or around June 21st for the northern hemisphere. We know from experience that the months of July and August are normally much warmer than the month of June – what causes this discrepancy? Both the Earth’s surface and atmosphere respond slowly to heat supplied by the sun. This is especially true for the Earth’s oceans, because water has a very high heat capacity (it takes a lot of heat to warm water a little bit). This slow response time is why costal regions on Earth stay cooler than slightly inland regions during the summer and warmer than slightly inland regions during the winter. In the summer, the water is cooler than the land (it warms up more slowly), so breezes coming off of the ocean cool costal regions. In the winter, the water is warmer than the land (cools off more slowly), so breezes off of the ocean warm coastal regions. A similar phenomenon causes the time lag between the longest day of the year (June) and the warmest day of the year (August) (and likewise the shortest day of the year (December) and the coldest month (February) (and on a shorter timescale, why the sun is highest in the sky at noon, but the warmest portion of the day is during the early afternoon). The land, ocean and atmosphere of the Earth take time to warm up in response to solar heating, thus the longest day of the year occurs earlier than the warmest day of the year.
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