Seasonal & Daily Temperatures
The role of Earth's tilt, revolution, &
rotation in causing spatial, seasonal, &
daily temperature variations
Exam a week from this Tuesday:
Chapters 1-4
http://climate.ntsg.umt.edu/
Seasons & Sun's Distance
Figure 3.1
Earth is 5 million kilometers further from the sun in
July than in January, indicating that seasonal warmth
is controlled by more than solar proximity.
Seasons & Solar Intensity
Solar intensity, defined as the energy per area, governs
Earth's seasonal climate changes
A sunlight beam that strikes at an angle is spread across
a greater surface area, and is a less intense heat source
than a beam impinging directly.
Solstice & Equinox
• Earth's tilt of 23.5° and revolution around the sun creates
seasonal solar exposure and heating patterns
• At solstice, tilt keeps a polar region with either 24 hours of
light or darkness
• At equinox, tilt provides exactly 12 hours of night and 12 hours
of day everywhere
Midnight Sun
The region north of the Arctic Circle experiences a period
of 24 hour sunlight in summer, where the Earth's surface
does not rotate out of solar exposure
NH summer
June 21
Equinox
March 20, Sept 22
NH winter
Dec 21
http://www.geog.ucsb.edu/~joel/g110_w08/lecture_notes/sun_angle/anim_fall.gif
http://www.geog.ucsb.edu/~joel/g110_w08/lecture_notes/sun_angle/anim_sum.gif
Questions to Think About
• Since polar latitudes receive the longest
period of sunlight during summer, why aren’t
temperatures highest there?
• Why aren’t temperatures highest at the
summer solstice?
• What would happen if we changed the tilt of
the earth?
– Would we get a more/less pronounced seasonal
cycle in the NH if the tilt was increased?
– What would happen if the tilt was 90 degrees? 0
degrees?
Seasons
• Seasons are regulated by the amount of solar energy
received at Earth’s surface
• The solar energy received at the top of the
atmosphere depends on:
– angle at which sunlight strikes Earth’s surface.
– how long the sun shines per day.
• Seasons are NOT due to the elliptical nature of the
earth’s orbit.
Incoming solar radiation is not evenly distributed
across all latitudes, creating a heating imbalance.
Earth's Energy Balance
Earth's annual
energy balance
between solar
insolation and
terrestrial
infrared radiation
is achieved locally
at only two lines of
latitude
A global balance is
maintained by
transferring
excess heat from
the equatorial
region toward the
poles
Daily Temperature Variations
• Each day is like a mini seasonal cycle
– Sun rays most intense around noon
– As is the case with the seasons, the maximum
temperatures lag the peak incoming solar
radiation.
• An understanding of the diurnal cycle in
temperature requires an understanding of
the different methods of atmospheric
heating and cooling:
– Radiation
– Conduction
– Convection
What Controls Daily High
Temperatures?
• Tmax depends on
– Cloud cover
– Surface type
• Absorption characteristics
– Strong absorbers enhance surface heating
• Vegetation/moisture
– Available energy partially used to evaporate water
– Wind
• Strong mixing by wind will mix heated air near ground
to higher altitudes
Local Solar Changes
Northern
hemisphere
sunrises are in
the southeast
during winter,
but in the
northeast in
summer
Summer noon
time sun is
also higher
above the
horizon than
the winter sun
Landscape Solar Response
South facing slopes receive greater insolation, providing
energy to melt snow sooner and evaporate more soil moisture.
North and south slope terrain exposure often lead to
differences in plant types and abundance.
Atmospheric Heating by Convection
• Sunlight warms the ground
• Ground warms adjacent air by conduction
– Poor thermal conductivity of air restricts heating to a few cm
• Random motion of “hot” surface air molecules upward leads
to heat transfer (diffusion)
• Hot air forms rising air “bubbles” (thermals) leading to
convection
– Mechanical mixing due to wind enhances this mode of heat
transport
Daytime Warming
Solar radiation heats the
atmosphere from below by
soil conduction and
convection.
Winds create forced
convection … vertical mixing
that diminishes steep
temperature gradients.
Temperature Lags
Earth's surface
temperature is a
balance between
incoming solar
radiation and
outgoing terrestrial
radiation.
Peak temperature
lags after peak
insolation because
surface continues to
warm until infrared
radiation exceeds
insolation.
Nighttime Cooling
Radiational cooling creates a temperature inversion near the
surface that may be diminished by winds.
Cold air near the ground is heavy … negatively buoyant …
takes energy to “stir” this air with warmer air aloft
Cold Dense Air
Nighttime radiational cooling increases air density.
On hill slopes, denser air settles to the valley bottom, creating a
“thermal belt” of warmer air between lower and upper cooler air.
Protecting Crops from Below
Impacts of radiational cooling near the surface can be
mitigated by wind machines mixing warmer air from above.
Protecting Crops from Above
When T is below
freezing aloft,
crops are not
helped by
convection or
mixing, but by
spraying water.
Latent heat of
freezing of water
warms air,
protecting crops
Controls of Temperature
Air temperature is governed by length of day
and intensity of insolation, which are a
function of:
1) latitude
Additional controls are:
2) land and water
3) ocean currents
4) elevation
January Isotherms
Air
temperature is
warmer at the
equator than at
the poles, but
land and water,
ocean currents,
and elevation
create
additional
variations.
July Isotherms
Southern
hemisphere
has fewer
land masses
and ocean
currents
that encircle
the globe,
creating
isotherms
that are
more regular
than those in
the northern
hemisphere.
Figure 3.22
Daily Temperature Range
Surface
absorbs solar
energy and
efficiently
radiates
infrared energy,
creating a large
diurnal
temperature
range (max min) in the
lower
atmosphere.
Regional Temperatures
Regional differences in
temperature, from annual or
daily, are influenced by
geography, such as latitude,
altitude, and nearby water
or ocean currents, as well as
heat generated in urban
areas
Heating Degree Day
Temperature data are analyzed to determine when living space
will likely be heated (e.g. when below 65° F) and how much fuel
is required for that region
Cooling & Growing Degree Days
Daily temperature data are also used to determine cooling
loads for living space above 65° F, as well as growing hours for
specific crops above a base temperature.