Download Chapter 3 – Air Temperature

Air Temperature
Chapter 3
Overview
Daily Temperature Variations
– Daytime Warming, Nighttime cooling
– Cold Air near the surface
The Controls of Temperature
– Air Temperature data
– Daily, Monthly, and Yearly Temps
– The use of temp data
– Measuring air temp
Daily Temperature Variations
• Air temperature is a very important
weather element. Impacts us every day.
If it is warm we don’t always mind the rain,
but if its cold…..Or if it is 100°F outside…
• A sunny day has its own cycle of warming
and cooling.
• Temperature lag – Sun directly overhead
at noon, but noon is not warmest point of a
sunny day. Why?
Figure 2: 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 (orange shade), the air temperature rises. Where outgoing
energy exceeds incoming energy (blue shade), the air temperature falls.
Figure 1: On a sunny, calm day, the air near the surface can be substantially
warmer than the air a meter or so above the surface. Sunlight warms ground
and ground warms air very near the surface of the earth. [EX: Higher Field
temperatures than air temperatures. Higher track temperatures than air
temperatures.]
Nighttime Cooling
• As sun lowers, its energy is spread over a larger area,
which reduces the heat available to warm the ground.
• Sometime in the late afternoon or early evening, the
earth’s surface and air above begin to lose more energy
than they receive, they start to cool.
• Radiational Cooling – ground and air above cool by
radiating infrared energy. The ground, being a much
better radiator than air, is able to cool more quickly.
Shortly after sunset the earth’s surface is slightly cooler
than the air directly above it.
• By late night or early morning the coldest air is next to
the ground with slightly warmer air above.
Figure 3: 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
Radiation Inversion
Strong radiation inversion occurs when the
air near the ground is much colder than the
air higher up.
Ideal conditions:
• Calm conditions – no mixing
• Long nights – more time for radiational cooling
• Dry air, cloud-free – clear skies allow maximum
cooling at the surface
Figure 4: On
cold, clear nights, the settling of cold air into valleys makes them
colder than surrounding hillsides. The region along the side of the hill where
the air temperature is above freezing is known as a thermal belt
Figure 5: Orchard heaters circulate the air by setting up convection currents.
Figure 6: Wind machines mix cooler surface air with warmer air above.
Freeze
• A coating of ice protects
these almond trees from
damaging low
temperatures, as an early
spring freeze drops air
temperatures well below
freezing.
• A freeze occurs over a
widespread area when
the surface air
temperature remains
below freezing for a long
enough time to damage
certain agricultural crops
Controls of Temperature
• Latitude (colder near the poles, warmer near
equator)
• Land and water distribution (i.e. Great lakes water cools surrounding land masses in
summer)
• Ocean currents (warm/cold currents, upwelling)
• Elevation (air cools with increased elevation)
Specific Heat – the amount of heat needed to raise the
temperature of one gram of a substance by one degree
Celsius.
– Water has a higher specific heat than land
– Heats/cools more slowly than land
Isotherms
• Lines of equal temperature.
• Displayed on weather charts
Figure 8: Average air temperature near sea level in January (°F).
Isotherms – lines connecting places that have the same temperature
Figure 9: Average air temperature near sea level in July (°F).
Figure 10: The
daily range of temperature decreases as we climb away from the
earth's surface. Hence, there is less day-to-night variation in air temperature near
the top of a high-rise apartment complex than at the ground level.
Daily, Monthly and Yearly
Temperatures
• Daily Diurnal range of temperature – difference
between the daily maximum and daily minimum
• Mean daily temperature – average of the highest
and lowest temperature for a 24 hour period.
• Annual range of temperature – difference
between the average temperature of the
warmest and coldest months
• Mean annual temperature – average
temperature of a station for the entire year.
Figure 11: Temperature data for San Francisco, California
(37°N), and Richmond, Virginia (37°N) - two cities with the
same mean annual temperature.
Use of Temperature Data
• Heating degree day – means of estimating
energy needs. People begin to use furnaces
when temperature drops below 65°F. Subtract
the mean temperature for the day from 65°F. (If
mean temperature for day is 64°F, there would
be 1 heating degree day)
• Cooling degree day – subtract 65°F from mean
for day (70°- 65°F = 5 cooling degree days)
Figure 12: Mean annual total heating degree-days in thousands of °F,
where the number 4 on the map represents 4000 (base 65°F).
Figure 13: Mean annual total cooling degree-days in thousands of °F, where the number 1 on
the map represents 1000 (base 65°F).
Air Temperature and Human
Comfort
Air temperatures can feel different on different
occasions. 68°F in March can feel warm after a long
winter, but can
feel cool on a summer afternoon.
Sensible heat – Temperature we perceive
Wind Chill factor – how cold the wind makes us feel
Frostbite – freezing of the skin
Hypothermia – rapid loss of body heat that may lower the body
temperature below normal
Figure 14: A section of a maximum thermometer.
As air temp increases the mercury expand and freely moves past
The constriction up the tube until max temp is reached.
As air temp drops, the constriction prevents the mercury from flowing
Back into bulb. Must be reset by whirling.
Figure 15: A section of a minimum thermometer showing both the
current air temperature and the minimum temperature.
Instrument Shelter
• 90°F in the shade?
Figure 17: The max-min instrument shelter (middle box) and other weather
instruments that comprise the ASOS system