The temperature of the atmosphere normally varies as you go higher in the atmosphere. So when you are measuring temperature with an eye to global warming, it matters a lot just where you are measuring the temperature, and how thick or thin that layer is. Thermometer-bearing balloons, called radiosondes, which beam constant readings back to earth, are one of the best ways to get a continuously varying measurement of the vertical temperature structure of the atmosphere. Surface and tropospheric temperature vary widely from place to place on the globe. The normal average temperature at the North Pole is different from that in a tropical jungle at the equator.
Temperatures differ also over land and sea.
Still other temperature differences are found according to surface altitude, land use, vegetation, ice conditions, etc. A good sampling scheme would be adjusted to represent faithfully the contribution of these many different temperature regimes to the global average. Climatologists generally feel that the current network of weather thermometers is extensive enough to provide adequate sampling for investigation of climate change. Earth s temperature and climate have varied dramatically and often on time scales ranging from days and weeks to years, decades, millennia, and longer. We can not understand Earth s normal temperature until we understand its normal temperature variability.