WEATHER REPORTS AND FORECASTS
WEATHER REPORTS
Different weather reports contain different amounts of information. The simplest and
shortest weather report contains only one piece of information, the present temperature.
This is the kind of report you often hear on the radio. More detailed weather reports contain
information about precipitation, wind speed
and direction, relative humidity, atmospheric
pressure, and so on.
A typical weather report tells you the highest
and lowest temperatures for the past day. The
day’s lowest temperature usually occurs just
after sunrise. The day’s highest temperature
is usually reached during early to midafternoon. A weather report also tells you the
present temperature. It may also give you the average temperature for the day. The
average daily temperature lies halfway between the highest temperature and the lowest
temperature.
The weather report might also tell you how many degrees the average temperature is above
or below the normal temperature for the day. The normal temperature is found by averaging
the average temperatures for the calendar day for the past 30 days.
Most weather reports give the amount of precipitation (rain or melted snow), if any, that fell
during the past day. They also tell you the totals for the current month and the current year.
Reports also indicate how much the monthly and annual precipitation totals are above or
below normal (the long-term average).
WEATHER FORECASTS
Most people are interested in what the weather will be tomorrow or in the next few days.
Predictions of the weather for up to a week in the future are called short-term forecasts.
Meteorologists also try to make long-term forecasts (called “outlooks”) of the weather for a
month, a season, or a whole year. Long-range outlooks are different from short-term
forecasts in that they specify expected departures of temperature and precipitation from
long-term averages (ex: colder or warmer than normal, wetter or drier than normal).
In earlier times, before invention of the telegraph and the telephone, weather observations
from faraway place could not be collected in one place soon after they were made. In those
times, the only way of predicting the weather was to use your local experience. Given the
weather on a particular day, what kind of weather usually follows during the next day or
two? As you can imagine, the success of such forecasting was not much better than making
a random guess.
Beginning in the 1870s, a national weather service used the telegraph to gather weather
observations from weather stations located over large areas of the country. Simultaneous
weather observations allowed meteorologists to plot weather maps and follow weather
systems as they moved from place to place, greatly improving the accuracy of weather
forecasts.
Through the 20th century, meteorologists
developed even better tools for observing and
predicting the weather. As you will learn in the
following investigations, special instruments
measure weather in the atmosphere far above
the ground. Satellites orbiting the Earth send
back images of the weather over broad areas of
the planet. In addition, computer models were
developed for weather forecasting. The
important processes operating in the Earth
system that govern weather are built into a
computer model. The model starts with the
present weather and tries to simulate how the
weather will evolve in the future. These computer models are run on supercomputers. They
can handle enormous amounts of observational data and make billions of calculations
quickly. Today’s computer models do a very good job of predicting the weather for the next
few days. You know, however, that sometimes the forecast is wrong! The science of
weather forecasting is still developing.
Weather prediction will never be perfect. One reason the absence of reliable weather
observation from large areas of the globe (especially the oceans). These observations are
needed for computer models to accurately represent the present state of the atmosphere. A
second reason is that even small changes in the weather in one place can cause much
larger changes in weather elsewhere. The effects are small at first, but they become much
greater. It’s very difficult for computers to simulate these interactions. Although forecasts will
never be perfect, they will continue to improve in the years ahead. Through research,
meteorologists learn more and more about the details of how weather in the Earth system
works.