Suppose that it is 11:00 A.M. in your town, and you have been observing the
temperature for the past several hours. At 8:00
A.M. it was 20°C. Then it rose to
22°C at 9:00
A.M., 24°C at 10:00 A.M., and 26°C at 11:00 A.M. A trend forecast
would suggest that at 12:00 noon, the temperature will be 28°C.
There are limitations to this method. You can’t keep extrapolating and con-
clude that within a couple of days, the air will be hot enough to boil water! In
fact, if a cold front is approaching or you live in the tropics where rain showers
are an almost daily occurrence during the afternoon hours, the trend will soon be
reversed. By 5:00
P.M. it might be only 20°C again, after the front has passed or
the shower has cooled things down.
Trend forecasting has been applied to long-term climate prediction. The con-
troversy over global warming—Is it really happening, or not?—has been largely
resolved by observing average temperature trends since about the year 1900.
Based on this data, you’ll hear scientists warn that if current trends continue, the
entire planet will be several degrees Celsius warmer, on the average, a century
from now, as compared with conditions today. They will also report trends in
other factors such as the frequency and intensity of hurricanes, the ocean tem-
perature, the distribution of rainfall, and changes in sea level.
STEERING
Have you heard that storm systems are “blown along” by upper-atmospheric
currents, especially the jet streams? To some extent this is true, but it is an over-
simplification. The jet streams, which you learned about in the last chapter, blow
west–to–east in the upper troposphere along narrow, riverlike channels of air.
There are two or three jet streams in the northern hemisphere, and counterparts
in the southern hemisphere. The most significant of these are the mid-latitude jet
streams. One of these meanders through the temperate zones between latitudes
of approximately 30°N and 60°N. The other behaves in a similar manner
between approximately 30°S and 60°S.
Low-pressure systems in the temperate zones tend to track along the jet streams,
centered slightly poleward of them. (Hurricanes in the tropics follow atmospheric
currents, too, but do not interact with the jet streams unless or until they enter
the temperate zones.) Blizzard tracks can be predicted by watching the behavior
of the jet streams. Individual fronts, and thunderstorm cells or snow squalls
within these large cyclonic systems, however, tend to form, dissipate, and rede-
velop in a different way. Rather than following the jet stream, these smaller com-
plexes are guided by the circulation around the main cyclonic system, or “low.”
On the Eastern Seaboard and Gulf Coasts of the United States, residents hear
the term steering currents in conjunction with hurricanes. These are not jet
CHAPTER 3 Observation and Forecasting
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