Download Chapter03b

Daily Temperature Variations
RECAP
•
•
•
•
Seasonal variations on the Earth result from
♦ Different amount of solar energy
received at a given location at a given
time of the year
♦ Different length of the day (the time
during which a given latitude receives
energy from the sun)
♦ The different response of the Earth
surface to the incoming solar energy
(oceans and continents)
The Earth orbits the Sun on an almost
circular orbit.
The Earth axis of rotation is tilted with
respect to the ecliptic at 23.5 degrees.
The inclination of the of the Earth axis is
the reason for the presence of seasons.
♦ The angle at which the sun rays hit
the surface of the Earth at a given
latitude changes.
♦ The length of the day changes
Parallels and Meridians
• Longitude:
describes the location of a
place on Earth east or west of a north-
•
south line called the Prime Meridian
♦ Meridians: lines of equal longitude
♦ Greenwich meridian: adopted at the
International Meridian Conference,
Washington DC, 1886. (Dom. Republic
against, France and Brazil abstained).
♦ Other prime meridians: Ferro, Paris…
Latitude: gives the location of a place on
Earth north or south of the equator
♦ Parallels: lines of equal latitude
♦ Tropic of cancer: 23° 26′ 22″ north
of the Equator
♦ Tropic of Capricorn: 23° 26′ 22″
south of the Equator
♦ Arctic circle: 66° 33′ 39″ north of
the Equator
♦ Antarctic circle: 66° 33′ 39″ south of
the Equator
The Sun’s position in the sky.
•
overhead
•
In the continental US the
sun is never directly
•
•
In the winter
♦ the sun is rising south
of east
♦ The noon sun is very low
In the summer
♦ The sun is rising north
of east
♦ The noon sun is high
The sun stays longer in
the sky in the summer
The Sun’s position in the sky
Local Seasonal Variations
• South-facing hills (in NH)
•
•
♦ Receive more sunshine (why?)
♦ Warmer
♦ Drier (moisture evaporates)
♦ Less vegetation
♦ Good for wine-making (in NY!)
North-facing hills: the opposite
♦ Good for ski runs
Applications:
♦ Home design
♦ Garden design
♦ Solar panel orientation and
roof angle
Daytime Warming
•• The
lower atmosphere (troposphere) is heated from below.
In the morning the Sun heats the ground but the air is still cool.
• The
layer closest to the ground is mostly heated by
♦ Conduction on a calm day: Only a thin layer is warmed, resulting in
a large vertical temperature difference.
♦ Convection on a windy day: Vertical mixing of the air transports
energy very efficiently, resulting in a small vertical temperature
difference
Daily Temperature Variations
• Incoming solar energy
•
•
♦ Starts at sunrise, stops at
sunset, maximum at noon
♦ Heats up the Earth’s surface
Outgoing IR radiation from Earth
♦ Rate proportional to the
temperature (SB law)
♦ Cools off the surface
The temperature is the result of
the balance between these two
♦ SE > IR rad -> T increases
♦ SE < IR rad -> T decreases
♦ SE = IR rad -> extremal (max or
min) temperatures
Min: just after sunrise
Max: in the afternoon
•
•
Local Weather Conditions and the
Maximum Daily Temperature
• is as late as 3-5 pm
•
Cloud free summer days – the maximum temperature
•
•
Cloudy, foggy, hazy days -the maximum temperature
occurs earlier in the afternoon. (Why? – more of the
incoming solar energy is reflected back to space)
Adjacent large water bodies (large lakes, oceans)-the
maximum occurs later in the day- water surfaces
respond more slowly to solar heating (large thermal
inertia)
Deserts (parking lots) – dry land responds faster to
solar heating and the maximum occurs earlier.
Nighttime Cooling
• At night there is no solar heating
• The
ground and the atmosphere emit
energy in the IR
• The air near the ground is cooled by:
♦ Conduction on calm nights: energy is
transferred from the air to the
ground. This cools only a thin layer
and results in a large T gradient
close to the ground.
♦ Convection on windy nights - vertical
mixing brings warmer air close to
the ground and transfers energy to
the ground very efficiently. The
resulting T gradient is small.
Radiation Inversion
• Radiational cooling: the process of
• cooling by radiating infrared energy.
• At night the ground cools more rapidly
•
•
•
•
•
•
than the air above -> the ground is
colder than the surface air immediately above it.
The air close to the ground is also cooled through IR radiation, but
more importantly, also through heat conduction to the ground.
The air high above the ground is cooled mostly through IR radiation
and remains warm longer.
As a result, the air close to the ground becomes colder than the air
high above the ground.
This increase of temperature with altitude which results from the
fast cooling of the ground through IR radiation is called Radiation
Inversion.
It is typically a layer which is just a few meters thick.
It occurs on calm, clear nights. The coldest temperatures occur on
“windless, starry” nights.
How cold will it get at night?
• energy without any incoming solar energy).
•
• water vapor absorbs IR radiation very effectively)
•
•
• nights over land with little vegetation during the winter
Duration of the night: (how long the earth is emitting
Cloud cover (clouds absorb and radiate IR energy back to
the ground)
The moisture content of the air (similar to the clouds
The strength of the wind (mixing of the air)
The type of the land surface (ocean, soil, desert, grass).
The coldest temperatures occur on calm, clear (starry)
just before sunrise.
•
•
•
•
Temperature Controls
Latitude : how much solar energy is received, and for
how long: both of these are controlled by the angle of
the incident solar light
Land and water distribution: the response of the
Earth to solar heating (different thermal inertia)
Ocean currents: transport of energy (advection) Gulf stream
Elevation: how high above the ground or the sea level
we are.
Average sea-level daily temperatures
Latitude
January
Latitude
July
Longitude
•
•
•
•
•
Temperature changes with altitude
Daily (diurnal) range of
temperature: the difference
between the daily max and daily
min
The largest daily range of T is
near the ground
This is not necessarily the case
for the daily minimum T.
Largest diurnal range of T:
♦ In the desert
Smallest diurnal range of T:
♦ In humid regions
♦ Near large bodies of water
Temperature averages
• Average (mean) daily temperatures
♦ (Max + Min) / 2
• Normal
temperature for a given
date: averaged over the past 30
years
• Average
(mean) monthly
temperature
• Average (mean) annual temperature
• Annual range of temperature
♦ Warmest – coldest month
Wind-Chill Index