1. Daily Temperature Variations

2. 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

3. 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, (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

4. 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

5. The Sun’s position in the sky

6. 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

7. 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

8. 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

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. Average sea-level daily temperatures
January
Latitude
July
Latitude
Longitude

15. 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

16. 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

17. Wind-Chill Index