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Introduction
Measurements of meteorological variables with sufficient spatial coverage to represent
global observations began in the mid to late 1800s. Measurements are taken (1) a few feet (~1.6 m)
above the earth's surface over land, (2) at various elevations above the surface over land by
launches of weather balloons every 12 hours, and (3) a few feet above the water surface over oceans.
On-going measurements also are taken by satellites, but this topic is covered in
Unit 2-11 . It is important to
remember that, in our discussions about global warming, the term "surface temperatures" really means
air temperatures taken a few feet above the surface of the earth. These are the temperatures
that are normally associated with weather forecasts reported by the
media.