Geoscience Reference
In-Depth Information
In contrast to surface temperature, the term
air temperature
refers to the degree of warmth of a portion of the atmosphere.
The standard altitude at which air temperature is measured is
usually about 1.2 m (4 ft) above the ground surface. Although
air temperature usually differs from surface temperature, the
amount of heat energy in the ground influences the temperature
of the air above it. As you know, air temperature is measured
with a thermometer, which is traditionally a hollow glass tube
containing a liquid, often mercury, that expands or contracts de-
pending on the amount of energy present. More recently, digital
thermometers have become common, relying on small electri-
cal devices that sense temperature.
Three temperature scales are used around the world
(Figure 5.4). You are probably most familiar with the Fahrenheit
scale, which was named after the German physicist Gabriel
Daniel Fahrenheit, who devised the scale in the 18th century.
This scale is used by the United States National Weather
Service and American news media to report temperature. The
common reference temperatures used in the Fahrenheit scale
are the freezing and boiling points of water, defined as 32°F and
212°F, respectively.
Another temperature scale that you may be somewhat fa-
miliar with is the Celsius scale, named after Anders Celsius,
the 18th-century Swedish astronomer who devised it. This
scale is part of the International System of Measurement
(SI) because it is a decimal scale, with 0°C and 100°C be-
ing the freezing and boiling points of water, respectively. The
vast majority of the world uses the Celsius scale, including
scientists in the United States, which is why we present Cel-
sius measurements in this text first, rather than Fahrenheit.
Because most of the world uses the Celsius scale, the stated
goal of the U.S. government is to one day convert fully to this
form of measurement.
The conversions required to convert from Fahrenheit to
Celsius and from Celsius to Fahrenheit are, respectively,
Human Interactions: Calculating
the Heat Index and Wind Chill
Have you ever noticed that the air occasionally seems warmer
or colder than the temperature that is given? This kind of varia-
tion occurs when additional environmental factors, such as
wind speed and the amount of atmospheric water vapor, come
into play. The combined impact of these factors with air tem-
perature has implications for human comfort because they can
make the air feel much warmer or colder than it really is.
The measures of human comfort that we use are the
wind
chill index
in winter and the
heat index
in summer. The wind
chill index is calculated using a variety of parameters, such as
actual air temperature, wind speed, average face height, and
components of modern heat loss theory. The wind chill index
chart in Table 5.1 presents temperature data in the Fahrenheit
scale and wind speed in miles per hour. In contrast to wind chill,
the heat index measures the apparent temperature based on the
combined variables of actual air temperature and relative hu-
midity. (As we will discuss in Chapter 7, relative humidity is
the ratio of the specific amount of vapor relative to the amount
the air could hold at a given temperature.) The resulting heat
index chart, shown in Table 5.2, also uses the Fahrenheit tem-
perature scale.
Large-Scale Geographic Factors
That Influence Air Temperature
A common theme in this text is the holistic interaction of
geographical variables. These interrelationships are especially
important for understanding how temperature varies across
Earth, which incorporates some of the concepts covered in
Chapters 3 and 4. Some of these factors may be intuitive for
you at this stage, but it is nevertheless useful to review them and
their relationships. Later in this section we will look at some
factors that can influence temperature at a local scale. For now,
however, let's focus on three large-scale factors that influence
temperature across Earth, no matter the location.
F
â
9/5C
à
32°
C
â
5/9 (F
à
32°)
Thus, 1°C equals 1.8°F and 1°F equals 0.56°C.
The third temperature scale is the Kelvin scale, named
for the 19th-century British physicist William Thomson, Lord
Kelvin. This scale is used in a great deal of scientific research
because it measures absolute temperature, with absolute zero
theoretically being the point where an object has no measurable
temperature. On the Celsius scale, absolute zero is
Ź
273°C.
The Kelvin scale has no negative values as the other scales do,
but it is similar to the Celsius scale because it maintains a 100°
temperature range between the boiling and freezing points of
water. In other words, 1 K
â
1°C. Given that the Kelvin scale
is used by climatologists and meteorologists only for more ad-
vanced work, it will not be used further in this topic. For refer-
ence purposes, however, the conversions between Kelvin and
Celsius scales are
Latitude
Recall from Chapter 4 that the shape of Earth
causes rays hitting the surface to differ in the area they cover,
according to the latitude at which they strike. In other words,
differences in the angle of incidence cause the same amount of
energy to be directed at a smaller or larger area on the Earth's
surface. When a larger area is covered at a lower incidence
angle, as occurs at high latitudes, it results in less energy per
unit area on that surface compared to a smaller area struck at a
higher incidence angle, similar to what occurs at low latitudes.
This difference is most pronounced when comparing high and
low latitudes and results in distinct temperature differences
between two such regions (see Figure 4.24).
Seasons and Length of Day
As discussed in earlier chap-
ters, Earth's axial tilt causes seasonal migration of the subsolar
point because the Northern and Southern Hemispheres are tilted
C°
â
K
à
273
