Heat Budget
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Daily cycles of temperature are driven by a budget, a heat budget that is. If there's a surplus it warms up, a deficit, it cools down. It's just bookkeeping. Hi I'm Dave Thurlow for the Mount Washington Observatory and this is the Weather Notebook.

The sun radiates heat to the earth, but the earth also radiates heat back out into space. A budget surplus of heat means more coming in than going out. A budget deficit shows more going out than coming in. Temperature gained from the sun is income, if you will. Heat lost from the earth is an expense.

When the sun goes down at night there's no income. However, the earth is still losing heat into space all night, an expense. This is a budget deficit, and the temperature drops.

At sunrise, the income starts flowing. The earth losses heat during the day too, but it gains more than it loses. The sun's contribution grows until there's a surplus, and the day becomes warmer.

At noon, the sun's contribution of heat starts to decrease. Yet, it remains greater than the amount of heat lost by the earth until, about four p.m. in the summer. As long as there's a surplus, the temperature will rise, even as the sun's contribution shrinks.

By late afternoon, the earth's loss is at its greatest. It's so warm it has more heat to send back into space. The warmer it becomes the more heat the earth loses. The outgoing heat is greater than the incoming and the temperature drops. The next morning, the income starts flowing again and the transactions continue. It's all bookkeeping.

The Weather Notebook is supported by the National Science Foundation. And, is underwritten by Subaru: The beauty of all-wheel drive.