3 Recording the weather: filling in the middle
There is a sophisticated network of surface observation stations that can catalogue what occurs at ground level, at least over land, and an array of geostationary satellites that look in from above. Between them, these systems furnish a valuable range of data on the current state of the weather and the way it is changing, but quite a lot is missing.
Winds above ground level and the nature of air masses are key to the development of local weather. Ground-based and satellite observations of clouds can be used to infer the wind directions and nature of the air masses - interpreting sequences of images in this way is an essential skill for a meteorologist. However, even for the skilled, there are blind spots: little can be inferred about winds where there are no clouds tracing out the airflow; there are few surface observations made over the oceans, where weather systems may be developing; and geostationary satellite data are not useful for monitoring weather at high latitudes (although satellites in polar orbit are more useful in these regions).
There are two more ways to gather data from the bulk of the atmosphere. The first is to get a set of instruments up there and make measurements; the second is to probe it from the ground.