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 isthe 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.

•In the continental US the sun is never directly overhead

•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

•Cloud free summer days – the maximum temperature is as late as 3-5 pm

•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 iscolder 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?

•Duration of the night: (how long the earth is emitting energy without any incoming solar energy).

•Cloud cover (clouds absorb and radiate IR energy back to the ground)

•The moisture content of the air (similar to the clouds water vapor absorbs IR radiation very effectively)

•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) nights over land with little vegetation during the winter 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 temperaturesJanuary

July

Longitude

Latitude

Latitude

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