The horizontal difference in temperature creates a horizontal difference in what?

The horizontal difference in temperature leads to a horizontal difference in pressure due to the behavior of air as it is heated or cooled. When the temperature of an area increases, the air expands and becomes less dense, which results in lower pressure in that area compared to a cooler region where the air is denser and exerts more pressure. This difference creates a pressure gradient, which is the driving force behind wind. Wind moves from areas of high pressure to areas of low pressure, and thus the variations in temperature across different regions directly influence the pressure differences that initiate and direct wind flow.

In this context, the other choices—humidity, wind speed, and vapor—do not directly arise from a temperature difference in the same primary manner as pressure does. Humidity refers to the amount of water vapor in the air, which can be influenced by temperature, but it is not a direct consequence of horizontal temperature differences. Wind speed can be affected by pressure differences but is a result rather than a direct effect of temperature variations. Vapor is similarly related to humidity and temperature but does not address the direct response to horizontal temperature differences in terms of pressure.