Page 158 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

122 part I The energy–atmosphere System
Temperature readings occur daily, sometimes hourly, at more than 16 000 weather stations worldwide. Some stations also report the duration of a temperature, the rate of temperature rise or fall, and the tempera- ture variation over time throughout the day and night. In 1992, the World Meteorological Organization and other international climate organizations established the Global Climate Observing System to coordinate the reading and recording of temperature and other climate factors among countries worldwide. One goal is to estab- lish a reference network of one station per 250000 km2 across the globe. (For an overview of temperature- and climate-observing stations, go to www.wmo.int/pages/ prog/gcos/index.php?name=ObservingSystemsandData.)
NASA’s Goddard Institute for Space Studies con- stantly refines the methodology of collecting average- temperature measurements. To maintain a quality data set, scientists assess each station for the possibility of effects caused by human activities (for example, a given city location may be subject to urban heat island effects). For stations with human-altered temperature regimes, the long-term temperature trends are adjusted to match the average conditions of nearby rural sta- tions where such impacts are absent. When using ab- solute (rather than long-term average) temperatures in a data set, scientists do not make these kinds of adjustments.
Satellites do not measure air temperature in the same way as thermometers; instead they measure land-surface temperature (LST), or land “skin” temperature, which is the heating of the land surface and is often much hot- ter than air temperature. You have felt this difference when walking barefoot across hot sand or pavement— the surface under your feet is much hotter than the air around your body above. Land skin temperatures record the heating of the ground from insolation and other heat flows; LSTs tend to be highest in dry environments with clear skies and surfaces with low albedo that absorb solar radiation.
Three expressions of temperature are common: The daily mean temperature is an average of hourly readings taken over a 24-hour day, but may also be the average of the daily minimum–maximum readings. The monthly mean temperature is the total of daily mean tempera- tures for the month divided by the number of days in the month. An annual temperature range expresses the dif- ference between the lowest and highest monthly mean temperatures for a given year.
Principal Temperature Controls
Insolation is the single most important influence on temperature variations. However, several other physical controls interact with it to produce Earth’s temperature patterns. These include latitude, altitude and elevation, cloud cover, and land–water heating differences.
The effects of human activity are altering some of these natural controls on temperature. Increasing amounts of greenhouse gases and human-made aerosols affect temperatures, as discussed in previous chapters. Recent evidence suggests that soot, such as the black carbon emitted from shipping and other industrial prac- tices, may be a more significant cause of warming tem- peratures than previously thought.
Latitude
We learned in Chapter 2 that the subsolar point is the latitude where the Sun is directly overhead at noon, and that this point migrates between the Tropic of Cancer at 23.5° N and the Tropic of Capricorn at 23.5° S latitude. Between the tropics, insolation is more intense than at higher latitudes where the Sun is never directly over- head (at a 90° angle) during the year. The intensity of incoming solar radiation decreases away from the equa- tor and toward the poles. Daylength also varies with latitude during the year, influencing the duration of in- solation exposure. Variations in these two factors—Sun angle and daylength—throughout the year drive the seasonal effect of latitude on temperature.
Temperature patterns throughout the year for the five cities in Figure 5.5 demonstrate the effects of latitu- dinal position. Note the range from near-constant warm temperatures at Salvador, Brazil, near the equator, to wide-ranging seasonal temperature variation at Barrow, Alaska, at 71° N latitude. From equator to poles, Earth ranges from continually warm, to seasonally variable, to continually cold.
altitude and Elevation
From Chapter 3, remember that within the troposphere, temperatures decrease with increasing altitude above Earth’s surface. (Recall from Figure 3.3 that the normal lapse rate of temperature change with altitude is 6.4 C°· 1000 m−1.) The density of the atmosphere also diminishes
Georeport 5.2 Yukon and Saskatchewan Hold Canadian Records for Extreme Temperatures
The coldest temperature ever recorded in Canada is –63°C, occurring on February 3, 1947, at Snag, yukon, a small settle- ment located about 450 km northwest of Whitehorse at an elevation of 587 m. The hottest temperature record is shared by
two settlements in Saskatchewan for the same day. On July 5, 1937, Midale and yellow grass, Saskatchewan, southeast of regina, each recorded a high of 45°C. Both stations are just over 580 m elevation.