How is upper wind defined in the context of meteorology?

Upper wind in meteorology is defined as the sum of lower wind and thermal wind. This definition recognizes the complexities of atmospheric dynamics, particularly how winds at different altitudes are influenced by various factors such as temperature gradients.

The lower wind refers to the winds found closer to the Earth's surface, which are significantly affected by surface friction and local weather phenomena. In contrast, the thermal wind is an idealized representation that arises from temperature differences in the atmosphere and helps to understand how wind speed and direction change with height.

When considering upper-level winds, such as those found in the free atmosphere, these winds are not solely products of surface conditions but rather result from the interplay of both the lower wind and thermal wind. This interaction is important for understanding weather patterns, including the development and movement of weather systems.

In this context, upper winds can also play a role in the behavior of the jet stream, which is relevant to option C, but the key aspect is how they are derived from the combination of lower level winds and thermal dynamics, making option B the most accurate in defining upper wind.