Geoscience Reference
In-Depth Information
TABLE 5.2
The Heat Index (
(continued)T
)
Celsius
Fahrenheit
Impact on Humans
Caution: Fatigue is possible with continued exposure.
27°CĽ32°C
80°FĽ90°F
Extreme Caution: Possible sunstroke, heat cramps, and exhaustion.
32°CĽ41°C
90°FĽ105°F
Danger: Likely occurrence of sunstroke and heat cramps.
41°CĽ54°C
105°FĽ125°F
54°C
above 130°F
Extreme Danger: Heat stroke or sunstroke likely.
Source
: NOAA.
year. This migration of the subsolar point occurs only in the trop-
ics, but results in dramatic changes in the angle of incidence at
all latitudes, which, in turn, influences net radiation. If you need
to review this concept again, examine Figures 4.24 and 4.26. A
secondary impact of axial tilt and seasons is that length of day
can fluctuate a great deal, depending on latitude. Longer days
mean that more radiation is received, which, in turn, also influ-
ences temperature. Think of your own experience regarding the
temperatures where you live during winter and summer.
surplus (above the bold horizontal line) of net radiation during
the summer months than in the winter (Figure 5.5b). Notice also
that the number of hours of daily insolation and net radiation
also vary per day. During the summer, for example, insolation is
received for 16 h of the day, whereas during the winter it is only
8 h. This decrease occurs because Earth's orbital position has
changed. As a result of this seasonal insolation variation, the
time of a net deficit of radiation (below the bold horizontal line)
increases from about 8 h in summer to 16 h in winter.
To see how annual insolation/net radiation patterns influ-
ence temperature, study Figure 5.5c. As you can imagine, tem-
perature is related to insolation and net radiation in that maxima
and minima of each factor occur at about the same time. The
pattern is predictable from a seasonal perspective in that tem-
peratures are warmer during the summer and colder during the
winter. Examining Figures 5.5a and 5.5b shows a clear correla-
tion among temperature, insolation, and net radiation. Notice,
however, that the maximum and minimum temperatures occur
at different times than insolation/net radiation fluctuations on a
daily basis. In other words, a temporal lag occurs. With respect
to maximum temperature, this lag exists because the peak re-
lease of longwave energy by Earth occurs sometime after the
insolation peak. Similarly, the minimum temperature occurs
only after all the stored heat from the day is released.
Time of Day
As the day progresses from the morning sunrise,
the Sun arcs westward across the sky. Recall from Chapter 3
that the Sun is at its highest point, resulting in the day's most
intense radiation, at solar noon. Temperature usually follows
the Sun in that it increases as the Sun rises and decreases when
it sets. The relationship is not quite that simple, however, be-
cause a
temporal lag
occurs between the highest Sun angle and
the warmest temperature of the day. In other words, the warm-
est part of the day occurs later in the day than the peak period of
insolation and net radiation.
A good way to see how these geographical variables affect
one another is in graphical form. For example, Figure 5.5 shows
how season and time of day are interrelated with respect to tem-
perature. These diagrams show three sets of data from a typical
observing station at 40° to 45° N latitude in the interior of North
America: (a) insolation, (b) net radiation, and (c) air temperature.
Parameter values, such as the amount of net radiation, are pre-
sented on the vertical (or “y”) axis of the respective diagrams,
whereas time of day ranges across the horizontal (or “x”) axis.
Notice in Figure 5.5 that each of the measured variables at
the observation station varies both by season and time of day.
With respect to insolation (Figure 5.5a), the amount of insola-
tion is greater on the June solstice than on the December sol-
stice. This seasonal variation in insolation results in a greater
Local Factors That Influence
Air Temperature
In the previous discussion we examined the important factors
that influence air temperature on a large scale across Earth.
Besides these factors, additional, smaller-scale factors can
influence air temperature in specific places. This section of the
chapter focuses on some of these kinds of geographic patterns.
Maritime vs. Continental Locations
Although the
radiation budget is the largest influence on air and surface
temperatures, other factors yet to be discussed can play an im-
portant role. A good example of a local or regional variable
Temporal lag
The difference in time between two events, such
as when peak insolation and peak temperature occur.
