precipitation, and wind speed and direction, which are all directly related to the amount of
energy that is in the system and where that energy is. The ultimate source of this energy is
the sun.
Climate is the average of a region’s weather over time. The climate for a particular place
is steady, and changes only very slowly. Portland, Oregon has a mild, moist climate and
Fairbanks, Alaska has a frigid, dry one. Portland or Fairbanks may experience a warm
sunny day in February, but that doesn’t change their climate. Climate is determined by
many factors, which are related to the amount of energy that is found in that location
over time. Factors that determine the amount of energy include the angle of the sun, the
likelihood of cloud cover, the air pressure, and many others.
Humidity
Humidityis the amount of water vapor in the air in a particular spot. We usually use
the term to mean relative humidity, the percentage of water vapor a certain volume of air
is holding relative to the maximum amount it can contain. If the humidity today is 80%,
that does not mean that 80% of the molecules in the air are water vapor. It means that
the air contains 80% of the total amount of water it can hold at that temperature. If the
humidity increases to more than 100%, the excess water will condense from the air and form
precipitation.
Humidity affects weather a great deal and is important for weather forecasting. When hu-
midity is high, precipitation is more likely. The combination of high humidity and high
temperatures can threaten people’s health. People are more uncomfortable when both tem-
perature and humidity are high. As people and some other animals sweat to cool themselves
off; they lose heat as the sweat evaporates. But if the air is already saturated with water
vapor, the sweat will not evaporate and the person will not cool. They will simply be hot
and sweaty, and uncomfortable.
The National Weather Service has developed aheat index(HI). On an HI chart (Table
16.1), people can see what the temperature feels like, when the air temperature and hu-
midity are known. For example, if the temperature is 85oF, but humidity is only 40%, the
temperature feels like a pleasant 84oF. But if the temperature is 85oF, but the humidity is
90%, the air temperature feels like a very hot and sticky 101oF. This information is useful
for people who are interested in outdoor activities. High humidity cause health problems,
such as sunstroke or heatstroke, to occur more quickly.
Table 16.1: Heat Index: Temperature (F) vs. Humidity (%)90% 80% 70% 60% 50% 40%
80 oF 85 84 82 81 80 79
85 oF 101 96 92 90 86 84
90 oF 121 113 105 99 94 90