The exosphere, which is the outermost layer of Earth’s atmosphere, extends from about 600 kilometers (370 miles) above sea level to the edge of space. Despite being farther from Earth’s surface where temperatures can be higher, it feels very cold in the exosphere. This might seem surprising, so let’s break it down.
As altitude increases, the density of the atmosphere decreases significantly. In the exosphere, there are very few air molecules, which means there is not enough mass present to retain heat. Even though temperatures can soar to high levels in theory, there aren’t enough particles to transfer that heat effectively to anything—this is due to the low pressure prevailing in that layer.
Furthermore, temperature is a measure of how fast particles are moving. In the exosphere, individual particles can indeed move at very high speeds, but since they are so spread out, there are not enough collisions to create a feeling of warmth. Thus, while the actual temperature might be high in a specific sense, the lack of thermal energy transfer means it feels extremely cold.
In essence, it’s not just about how high you are but also about the presence of air. Since the exosphere has nearly a vacuum, you experience what feels like icy conditions, despite being far from the cool reaches of the Earth’s surface.