On June 21, less than a month from now, days will start to get shorter. But have you ever noticed that the warmest days of the year don’t actually happen until later in July and August? It’s because of something called seasonal lag.
Here's how it works. Our Earth is 2/3 water. It takes time for that water to heat up.
In our demonstration, we have a hot plate, a pot of water, and a thermometer. We heat up the water and measure the temperature. Then, we take the pot off the burner, and measure the temperature again. For a brief while, the temperature of the water continues to rise even after we take the pot off the heat.
It has to do with the heat capacity of water, and this is what’s happening in our atmosphere. The warmest time of the year in the northern hemisphere happens roughly a month after the summer solstice, the longest day of the year.
This same principle applies during the coldest days of the year, which happen about a month after the winter solstice, the shortest day of the year.