TEMPERATURE

Temperature Observations

Thermometer shelter - Chapel Hill 2W

(in OWASA Compound, Jones Ferry Road, Carrboro) - 1985 view

These old views of the local observing station show a rather wide open area. Now Carrboro is much more developed and the station is surrounded by buildings. Because the energy flows in a rural area differ from those in a city (for example, a city has little evaporation to remove absorbed energy), a city is usually associated with the an urban heat island - an area of increased temperature. This might be happening to this station right now - making it difficult to determine it there really is any 'global warming' in Chapel Hill.

Interior of thermometer shelter, showing maximum and minimum thermometers (roughly horizontal) and dry and wet bulb thermometers (vertical - together measuring humidity)

Linking Energy and Temperature

Whenever net radiation is positive, heating occurs. Since it is usually positive until well after midday, temperatures continue to increase during the afternoon, even though the amount of solar radiation, which we think of a 'driving' temperature changes, starts to decrease at noon.

The same ideas as those above control the seasons - so that the warmest period of the year is well after the time when the sun is highest in the sky and solar radiation is most intense.

Temperature also change, of course, with distance as well as with time. Still the control of temperature is the energy - but on a global scale we not that there is a balance between incoming and outgoing radiant energy about about 38^oN - not very far away from us. Poleward of that, there is a net energy loss, equatorward is a net gain. Since the Poles are not continuously cool (however cold they may be), nor the Equator continually warming (however hot it may be), there has to be a movement of energy poleward from the equator.

This movement is the responsibility of the General Circulation of the Atmosphere - the major wind systems of the planet - and the companion General Circulation of the Oceans

Controls on Temperature

1. *Latitude*: energy receipt, and ultimately temperature, varies with latitude primarily because of the angle of the sun and the length of daylight, which influence the amount of incoming solar radiation. On an annual basis, temperatures tend to decrease poleward away from the equator - although this is not a smooth trend.
2. *Location: on a global scale a major controlling role is played by the distribution of land and sea. Ocean areas tend to the slow to respond to energy exchanges, and warm up and cool down very slowly in comparison to land. Consequently an inland area in the middle of a continent is likely to warm quickly in spring and have a high summer temperature whereas an oceanic area will warm slowly in spring and the summer maximum temperature will not be much above the winter water temperature. The whole contrasting effect is known as continentality.* In our area the winds commonly come from the west, bringing with them the characteristics of a continental interior. They are, however, not as extreme as the true interior, since the proximity of the ocean does modify the temperatures.
3. *Altitude*: Temperatures decrease with height. On the world map there are two major impacts: a) The major elevated plateaus - Tibet and much of Africa in particular- have lower temperatures than would be expected at their latitudes. They are big enough to be seen on a world map. b) Most mountain areas have very rapid vertical and horizontal temperature changes, which are difficult to depict. (There will be discussion later of climate regions, and the maps which go with them, but we can note here that most people just talk about 'highland' or 'mountain' climates because of this spatial diversity).
4. Local effects: There are many local modifications to the overall regional patterns of temperature - whether on a day-to-day basis, or even when averaged over the year as a whole. The valley of Airport Road, going north out of town just before the police station - is a place where cool air collects on spring of fall nights, giving very much colder conditions in teh valley bottom in the morning. THis is a *frost hollow. Akin to this, in the area around Tryon, NC are the thermal belts*, areas where the longest growing season is on the valley sides, not the tops of ridges or the bottoms of the valleys.

The global distribution of annual temperature range is the result of all of the controls on climate

The day-to-day variability of temperature in North Carolina is commonly much higher in winter (left) than in summer (right).

These diagrams include both temperature (yellow) and dew point (red) - a measure of humidity

Temperature Impacts

Some temperature impacts, including those possibly associated with global warming will be considered in later classes. Others which are temperature related - such as drought - will be considered towards the end of the course

Breugel: The Harvesters

Courtesy The Metropolitan Museum of Art, New York

A summer heat wave?

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