Transcript Introduction - San Jose State University

MET 10 - Lecture 4
Chapter 3: Air Temperature
Dr. Craig Clements
San Jose State University
Diurnal Temperature Variation
The daily variation in air temperature is controlled
by incoming energy (primarily from the sun) and
outgoing energy from the earth’s surface.
Where incoming energy exceeds outgoing energy
the air temperature rises. Where outgoing energy
exceeds incoming energy the air temperature falls
Incoming energy exceeds outgoing energy (orange shade), the air
temperature rises. Where outgoing energy exceeds incoming energy (blue
shade), the air temperature falls
5
MET 112 Global Climate Change
Fig. 3-11, p. 65
On a sunny, calm day, the air near the surface can be
substantially warmer than the air a meter or so above
the surface.
On a clear, calm night, the air near the surface can be much colder than
the air above. The increase in air temperature with increasing height above
the surface is called a radiation temperature inversion.
Death Valley, California
Thermal Belts: cold air in valley bottoms
Diurnal temperature distribution above the ground
Temperature sensors are attached to balloons
What controls temperature?
Latitude
Land and water distribution
Ocean currents
Elevation
Average air temperature near sea level in January (°F)
Lines of constant temperature are called isotherms
Average air temperature near sea level in July (°F)
Temperature Measurements
Temperature is measured using a few different types of
thermometers.
Typically, the daily temperatures are measured using
automated weather stations (AWS).
Sensors include:
Electrical resistance thermometers
i.e., Thermocouples (two-metals give voltage)
A portable, remote weather station
Thermometer shelter
Why do we need a
shelter?
It is impossible to
measure temperature
accurately in direct
sunlight.
What effects does this
have on the measured
temperatures?
Use of Temperature Data
Heating degree-day: is based on the assumption that
people will begin to use their furnaces when the mean daily
temperature drops below 65 °F.
This is calculated by subtracting the mean temperature for
the day from 65 °F.
Cooling degree-day: is based on the fact that people begin
to cool their indoor environment as the ambient temperature
climbs above 65 °F.
This number is used during warm weather to estimate the
energy needed to cool indoor air.
Use of Temperature Data
Growing degree-day: is an index used by farmers as a
guide to planting and determining the approximate dates
when a crop will be ready for harvesting.
This is defined as a day on which the mean daily
temperature is one degree above the base temperature-- the
minimum temperature required for growth of a crop.
For example, on a summer day in Iowa, the mean
temperature might be 80 F. On this day, sweet corn would
accumulate(80F - 50F(base temp)), or 30 growing degreedays. Theoretically, sweet corn can be harvested when it
accumulates a total of 2200 growing degree-days.
Air Temperature and Human Comfort
The human body loses heat by emitting infrared energy
Conduction and convection also cool and heat the body.
On a cold day, a thin layer of warm air molecules forms close
to the skin, protecting it from the surrounding cold air.
Once the wind starts to blow, this insulating air is swept away
from the skin and heat is rapidly removed. The faster the
wind blows, the greater the heat loss, and the colder we feel.
How cold the wind makes us feel is expressed as a
wind-chill index.
Air Temperature and Human Comfort
Air Temperature and Human Comfort
High winds, in below-freezing air, can remove heat from
exposed skin so quickly that the skin may actually freeze and
discolor. The freezing skin, called frostbite, usually occurs
on the body extremities first because they are the greatest
distance from the source of body heat.
The rapid loss of body heat may lower the body temperature
below its normal level and bring on a condition of
hypothermia!
frostbite
A thousand
degrees and
freezing to
death!