Download Chapter_3 - Weather Underground

Chapter 3: Air
Temperature

By the end of this chapter you should:
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Be able to describe how temperature changes both day and night
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Understand what are the controls of temperature
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Know the different ways farmers protect their
crops
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Understand wind chill and heat index
Daytime Warming
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During the day, the ____
sun heats the ground
conduction
The air near the ground is warmed by ____________
What kind of conductor is air? Poor!

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So this action happens very close to the ground
Daytime Warming
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Thermals are bubbles of air that help redistribute heat
 Are small on calm days and do not mix the air well
On windy days, thermals are more better at mixing
the air

So, on windy days the
difference in temperature
between surface and
above small

Daytime Warming
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When is the hottest part of the day?
On cloudless, summer days, maximum temperature
occurs between 3-5 p.m.
Why is the hottest part of the day after the most intense
solar time?
Daytime Warming


Temperature will
rise when
absorption
________ is
greater than
emission
________
Or when incoming
________
shortwave
radiation is greater
than outgoing
longwave
__________
radiation.
Daytime Warming

What can change
this maximum
temperature time?
Daytime Warming

Large bodies of water can help push cool air onto land
Clouds throughout the day – daytime temperatures are
usually lower…why?

High temperatures can occur at night if warm air
moves into an area

Soil conductivity can determine temperatures. Think of
sand (poor conductor) on the beach

Daytime Warming

Humidity can skew maximum temperatures
 Places with high humidity (lots of water vapor in the
air), maximum temperatures are lower than places
with low humidity
 New Orleans normal high for today is 88 degrees
(moist climate)
 Phoenix normal high for today is 101 degrees (dry
climate)
Nighttime Cooling

Why does the surface cool at night?

emission is greater than ___________
absorption
NO SUN! Or __________

Radiational cooling –
process at which the
ground and air above cool
by radiating infrared
radiation
Nighttime Cooling

The ground will lose energy much faster than the air
above it

Air radiates some energy to
ground, but it quickly gives
it away
Thus, by early morning/late
night, we end up with
cooler air at the surface
than above

Nighttime Cooling

When temperature increases with height, what is this
called?

An inversion!
Specifically, this kind of
inversion is a radiation
inversion

Cold Air Near the Surface

How do we get the strongest inversions?
Strongest inversions occur on nights that are clear, calm,
fairly dry, and long
 Strong breezes tend to mix the air
 Longer the night, the longer for radiational cooling
 Clear, dry air allows infrared radiation to escape to
space (water vapor absorbs it)
 Also, on moist nights, condensation occurs at the
ground (is the ground then warmed or cooled?…think
latent heat)
Cold Air Near the Surface


Cold (heavy or light?) air can drain downhill into valleys,
causing thermal belts
Hillsides warmer than valley floors
• Drainage winds:
cold air that
slides downhill.
• Can cause very
bad air quality
Protecting Crops From the
Cold Night Air


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Radiation
inversions and cold
air near the surface
are a problem for
farmers
One way to protect
crops is with wind
machines…how?
Helicopters are
also used
Protecting Crops From the
Cold Night Air

Yet another way is to flood the fields
Water resists changes in temperature so it cools
more slowly
 In fact, it will cool more slowly than dry soil, thus
keeping the air near the ground warmer

Protecting Crops From the
Cold Night Air


Freeze - when cold air moves into an area
and cools with height. How can the farmer
protect the crops?
Would you believe freezing the crops and
trees? How does it work?
Protecting Crops From the
Cold Night Air
Protecting Crops From the
Cold Night Air


A farmer will turn on his/her sprinkler system
to spray a fine spray of water over the crop.
The spray then freezes (a warming
process), and latent heat is released into
atmosphere
As long as the spray continues, the water
will keep freezing and the crop will be kept
at 32°F(0°C)…above the damaging point
The Controls of Temperature

Controls of temperature – main factors for
variations in temperature from one place
to another
 Latitude – lower latitudes receive more
solar radiation
 Land and water distribution – land
absorbs sun energy in very top soil,
water absorbs at a much deeper depth.
Water has a higher specific heat –
amount of heat needed to raise 1 gram
of a substance by 1 degree Celsius
The Controls of Temperature
 It
takes 5 times the amount of energy to
raise the temperature of water by 1
degree Celsius than it does soil or rock
 Ocean currents – warm water brought
up along east coast of continents and
cold water brought down along west
coast of continents
 Elevation – higher elevations usually are
cooler
The Controls of Temperature
The Controls of Temperature
Daily, Monthly and Yearly
Temperatures
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Diurnal temperature range – difference
between the daily maximum and minimum
temperatures
Clouds and humidity effects – keep solar
radiation from the ground and trap infrared
radiation from escaping
Proximity to large bodies of water additional water vapor, and specific heat of
water
Fig. 3-11, p. 65
Stepped Art
Fig. 3-11, p. 65
Daily, Monthly and Yearly
Temperatures
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
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Mean daily temperature – highest and
lowest temperature in a 24 hour period
Annual range of temperature – different
between warmest and coldest months
Mean annual temperature – average
yearly temperature
The Use of Temperature Data


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Heating degree-days – 65°F minus mean daily
temperature
Cooling degree-days – mean daily temperature
minus 65°F
Growing degree-days – mean daily temperature
minus base temperature
Air Temperature and Human
Comfort

In cold weather, a stiff breeze will blow away
your body heat from your skin, this is called wind
chill
Measuring Air Temperature

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Liquid-in-glass thermometers
Maximum and minimum thermometers
Bimetallic thermometers
Instrument shelters
• Temperatures can
also be measured
remotely using
infrared sensors
(radiometers).