Temperature
is a measure of the average speed of the random motion
of molecules that comprise a substance. Basically, it
is a measure of how hot or cold something is. In Chapter 4
we made an important distinction between temperature
and heat. Heat is the total molecular motion
associated with a substance while temperature is the
average motion of molecular that make up a substance.
There are two ways that the temperature
of the air can change. There can be a physical
exchange of heat between the air and the earth surface
in the presence of a temperature gradient. For
example, if the surface is warmer than the air above,
heat will be transferred into the air to warm it up.
This is a case of diabatic temperature change.
The temperature of the air can also change without a
physical exchange of heat. This is called an adiabatic
temperature change that occurs with rising
parcels of air. As a parcel of air rises it expands.
In the process of expansion requires the expenditure
of its internal energy and as a result the parcel
cools. We'll look at this process in more detail in
Chapter 7 "Atmospheric Moisture".
Measuring
Temperature
Surface temperature is the
temperature of the earth surface, not the temperature
of the air above it. Air temperature is
measured at a height of 4 to 6 feet above the surface.
For accurate results, instruments should be shielded
from the Sun and away from buildings and other
structures that could affect temperature measurements.
A Stevenson screen, like that shown in Figure 5.1 is
ventilated to allow air through it.
Figure 5.1 Stevenson screen for
measuring weather conditions like air temperature.
One of two scales of measurement are
common to most of us, the Fahrenheit scale in
the United States and Celsius in most other countries.
The Celsius scale is
an accepted international system of measurement
because it is based on a decimal scale having 100
units between the freezing and boiling points of
water. The Fahrenheit scale sets the freezing point of
water at 32oF and 212oF for
boiling. In much scientific research the Kelvin
scale is used. It is based on the "kelvin", a
unit increment of temperature. Unlike the Fahrenheit
and Celsius scales, the kelvin is not referred to as a
"degree". It is an absolute scale given that at
absolute zero all molecular movement ceases. It too
has 100 units between the freezing and boiling points
of water. Absolute zero is -273o on the
Celsius scale.
Figure 5.2 Temperature Scales
Temperature
Statisics
A variety of statistics are available to easily
communicate the volumes of temperature data available.
The daily
temperature range for a place is the
difference between the highest and lowest temperatures
over a 24 hour period. The average daily
temperature is the sum of highest and
lowest temperature of the day divided 2. Average
monthly temperature is the sum of the average
daily temperatures divided by the number of days in
the month. The annual
temperature range is the difference between
the highest and lowest monthly temperature for a
place. The average
annual temperature is the sum of the mean
monthly temperatures divided by 12.
Video: Taking Earth's
Temperature Courtesy NASA This short video describes how
researchers use models
to understand the Earth's changing temperature.
