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Seasonal and Daily temperatures
Chapter 3
Why the Earth has seasons
• Earth revolves in elliptical path around sun
every 365 days.
• Earth rotates counterclockwise or eastward
every 24 hours.
• Earth closest to Sun (147 million km) in
January, farthest from Sun (152 million lm)
in July.
• Distance not the factor impacting seasons.
FIGURE 3.1 The elliptical path (highly exaggerated)
of the earth about the sun brings the earth slightly
closer to the sun in January than in July.
Why the Earth has seasons
• The amount of energy that reaches the Earths
surface is influenced by
– the distance from the Sun….not much variation
– the solar angle
– the length of daylight.
• When the Earth tilts toward the sun in summer
– higher solar angles
– longer daylight hours
Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that
strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a
surface than direct sun rays.
As the earth revolves about the sun, it is tilted on its axis by an angle of 231⁄2ｰ. The
earth’s axis always points to the same area in space (as viewed from a distant star).
Thus, in June, when the Northern Hemisphere is tipped toward the sun, more direct
sunlight and long hours of daylight cause warmer weather than in December, when the
Northern Hemisphere is tipped away from the sun. (Diagram is not to scale.)
The relative amount of radiant energy received at the top of the
earth’s atmosphere and at the earth’s surface on
June 21 — the summer solstice.
During the Northern Hemisphere summer, sunlight that reaches the earth’s surface in far northern latitudes has passed through a thicker layer of
absorbing, scattering, and reflecting atmosphere than sunlight that reaches the earth’s surface farther south. Sunlight is lost through both the
thickness of the pure atmosphere and by impurities in the atmosphere. As the sun’s rays become more oblique, these effects become more
pronounced.
Stepped Art
Fig. 3-8, p. 63
• Seasons in the Southern Hemisphere
– Opposite timing of Northern
Hemisphere
– Closer to sun in summer but not
significant difference from north due to:
• Greater amount of water absorbing
heat
• Shorter season
Because the earth travels more slowly when it is farther from the sun, it takes the earth a little more
than 7 days longer to travel from March 20 to September 22 than from September 22 to March 20.
Local temperature variations
• Southern exposure: warmer, drier
locations facing south. Implications for
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Vegetation
Viniculture
Ski slopes
Landscaping
Architecture
• Environmental Issues: Solar Heating
– In order to collect enough energy from solar
power to heat a house, the roof should be
perpendicular to the winter sun.
– For the mid-latitudes the roof slant should be
45°- 50°
Daily temperature variations
• Each day like a tiny season with a cycle of
heating and cooling
• Daytime heating
– Air is a poor conductor so initial heating only
effects air next to ground
– As energy builds convection begins and heats
higher portions of the atmosphere
– After the atmosphere heats from convection
high temperature occurs from 3-5PM
• lag in temperature from sun angle
On a sunny, calm day, the air near the
surface can be substantially warmer than
the air a meter or so above the surface.
Daily temperature variations
• Properties of soil affect the rate of
conduction from Earth to atmosphere
• Wind mixes energy into air column and
can force convection.
Vertical temperature profiles above an
asphalt surface for a windy and a calm
summer afternoon.
Daily temperature variations
• Nighttime cooling
– As sun lowers, the lower solar angle causes
insolation to be spread across a larger area
– Radiational cooling occurs as infrared energy is
emitted by the Earth’s surface
– Radiation inversion: air near ground much
cooler than air above
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.
Vertical temperature profiles just above the ground on a windy night and on a
calm night. Notice that the radiation inversion develops better on the calm night.
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.
Stepped Art
Stepped Art
Fig. 3-17, p. 72
• Protecting crops from cold
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Cover
Smudge pots
Fans
Sprinklers
The controls of temperature
• Latitude
– solar angle
– day length
• Land & water: specific heat differences
• Ocean currents: warm and cold currents
• Elevation
– Cooling
– increase range
• Cloud Cover
• Humidity
FIGURE 3.20 Average air temperature
near sea level in January (oF). Isothermal
map
FIGURE 3.21 Average air temperature
near sea level in July (oF).Isothermal Map
Air temperature data
• Daily(Diurnal), monthly, yearly temperature
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Range: maximum minus minimum
Mean: average of temperature observations
Maximum: highest temperature of time period
Minimum: lowest temperature of time period
 Mean(avg) daily temperature – avg of highest and
lowest T of 24 hr period
 Avg of the mean daily T for a given date for the past
30yrs = the “Normal” average
 Daily range of temperature
 difference between daily max and min T
 Mean (avg) monthly temperature
 = avg of the daily mean T’s for that month
 Annual range of temperatures
 Difference between avg. T of warmest and coldest months
 Mean (avg) annual temperature range
 = sum of 12 monthly means / 12
Air temperature data
• Special topic: What’s normal?
– Climate normal is the 30 year average for a
given temperature variable.
The high temperature measured (for 30 years) on March 15 in a city located in the
southwestern United States. The dashed line represents the normal temperature for
the 30-year period.
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.
(a) Clouds tend to keep daytime temperatures lower and nighttime temperatures higher,
producing a small daily range in temperature.
(b) In the absence of clouds, days tend to be warmer and nights cooler, producing a
larger daily range in temperature.
Monthly temperature data and annual temperature range for St. Louis,
Missouri, a city located near the middle of a continent and Ponta Delgada, a
city located in the Azores in the Atlantic Ocean.
Temperature data for San
Francisco, California (37°N),
and Richmond, Virginia (37°N)
- two cities with the same
mean annual temperature.
Air temperature data
• The use of temperature data
– Heating degree-day: people heat when
temperature below 65°F
• 65-mean daily temperature
– Cooling degree-day: people cool when
temperature above 65°F
• Mean daily temperature - 65
– Growing degree-day: temperature above of
below base temperature for specific crop
Mean annual total heating degree-days across the United States (base 65°F
Mean annual total cooling degree-days across the United States (base 65°F).
Air temperature and human comfort
• Perceived or sensible temperature
• Body heats through metabolism
• Body cools through emitting infrared
energy and evaporation of perspiration
• Wind-chill index – takes into account wind
speed at 1.5 m
Measuring air temperature
Thermometers
 liquid-in-glass
○ Alcohol - freezes at – 130 C (mercury -39C)
 maximum, minimum,
 electrical resistance
○ Thermistors
○ thermocouple
 Bimetallic
 Radiometer – infrared sensor
 Thermograph – records T
ASOS
 Automated Surface Observation Systems
• Observation: Thermometers in the shade
– Radiant energy from the Sun in direct
sunlight increases the temperature recorded
by a sensor.
– True air temperature measured in the shade.