Seen from space, the layer of atmosphere surrounding the earth is very thin. In terms of the circumference of the earth, our atmospere seems to be just an inch or so thick. This atmosphere captures heat emanating from our planet to maintain reasonable temperatures. Without the atmosphere (irrespective of the need for life) the temperature on our planet's surface would be very close to that of outer space. We can use the same principle for riding in cold weather by trapping the heat that hovers just over our skin before it dissipates.