We only have a few minutes left, so I'd like to go over a couple of points before we move on. Remember that although there are both horizontal and vertical movements of air, the term "wind" is applied only to horizontal movements, and that more air is involved in those horizontal movements than in vertical movements. And what causes these horizontal movements? Ultimately, it's solar radiation. Because the unequal heating of the earth and the atmosphere produces horizontal differences in air pressure, these differences set winds in motion. Essentially, winds are nature’s way of balancing out the uneven distribution of air pressure over the earth. Secondly let me repeat my answer to the question we had before about wind direction. Many people get confused by what they hear in weather forecasts. We talk about the wind direction in terms of where the wind’s coming from, not where it’s blowing to. There’s a good reason for this. To answer forecasters, the origin of the wind is more important than its destination. The wind’s origin helps them predict the weather. Logically in the northern hemisphere a north winds tends to bring colder weather, and a south wind warmer weather. I haven’t forgotten vertical movements of air but we don’t have time today to talk about them in depth. In our next class then, I’ll begin by discussing updraft and downdraft, and how they affect the weather. I suspect most of you can guess which of the two brings to warm weather and which brings cold.