Air Temperature Data

The Controls of Temperature

Outline for Lesson 5

2/11/03

World Distributions of Temperature

Chapter 3 Temperature

•Daily mean•Daily temperature range •Monthly mean•Annual mean•Annual temperature range.

Temperature is one of the most basic elements of weather….

Mean temperatures are good for making comparisons; temperatures ranges give us an idea of extremes which are important to understand the weather and climate of an area.

Death Valley, California

Highest accepted temperature in the Western Hemisphere (57°C/134°F)………why?

-Very low elevation(53m below sea level)

-It is a desert-Mountains to the west shield Death Valley from the moderate, moist Pacific air.

Few clouds.

Isotherms: Lines of Equal Temperature

Given equal amounts of radiation it is the controls of temperature that determine what the temperature average andrange will be for an area.

Recall from Chapter 2 that latitude, and the corresponding sunangle, determines in general what areas will be warm (lower latitudes, i.e. the tropics) or cool (higher latitudes, i.e. the poles).However, not all lines of latitude have the same temperature profile. Therefore, latitude is not the only factor that controls temperature.

What are the other controls of temperature ?

The Controls of Temperature

• Differential heating of land and water

• Ocean Currents

• Altitude

• Geographic position

• Cloud cover and albedo

Controls of Temperature Include:

1. One reason water heats more slowly than land is that it is a mobile medium. The ability to convectively mix allows heatto be distributed through more volume than conduction.

Incoming solar radiation heats the waterand land butdifferentenergy transfer processes contribute to different rates of heatingbetween land and water. Convective heat transfer

allows for deeper mixing(more efficient)

hot

cold

Conductive heat transfer is shallow(less efficient)

water solid

Differential Heating of Land and Water

water solid

The opacity of water is less thanthat of land. Therefore, radiation is ableto penetrate deeper into water whichmore evenly distributes the radiative energy.

2. Solar radiation only reaches a few centimeters into land while, depending on the clarity, solar radiation can reach several meters into water. The result is that a greater volume of water is heated at a slower rate. The smaller volume of land however attains higher temperatures.

0

5m

dept

h

Differential Heating of Land and Water

3. Specific heat: the heat (heat energy) need to raise the temperature of one gram of a substance 1° C. The higher the heat capacity themore energy needed to raise the temperature of that substance.

Specific Heat of Various Substances

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Hg Cu Fe SiO2 CaCO3 O2 Land H2O

Substances

Sp

eci

fic

He

at

Hg

Cu

Fe

SiO2

CaCO3

O2

Land

H2O

Specific Heat: The amount of heat needed to raise the temperature of a substance 1deg C.Because the specific heat is much greater for water than for land it takes more energy to warm water than it does land.

4. Evaporation is greater from water than land.A change of phase from liquid to gas (evaporation) requires energy that does not contribute to a rise in temperature. Converselywhen gas is converted to a liquid (condensation), or liquid to solid(freezing) energy is liberated.

Graph of the temperature of a water versus heat added. The water starts out in the form of ice. As heat is added its temperature increases linearly until the ice reaches the melting point. Additional heat added does not raise the temperature of the sample but, instead, causes some ice to melt. Once all of the ice has melted, additional heat goes into raising the temperature of the water. A similar phenomenon occurs when the water vaporizes.

ice

water

steam

•Convective heat transfer in water•Opacity of water•High Specific Heat of water •Energy of Evaporation/Condensation of water

all contribute to the fact that water warms and cools more slowly than land, which can greatly effect the climate and daily weather.

How?

Water tends to moderate the temperature of a region. Other factors (i.e. latitude) being equal, a city near water will experience warmer winters and cooler summers than a city inland.

Same latitude, different proximity to water

The Southern Hemisphere is comprised of nearly 80% water, ~20 % more than the Northern Hemisphere

The difference in heating between land and water can also explain the differentlatitudinal heating between the Northernand Southern Hemisphere.

Its all a matter of scale

Variation in Mean Annual Temperature (C) Range with Latitude

0

10

20

30

40

50

0 20 40 60 80 100

Latitude (deg)

T (

C)

Northern Hemisphere

Southern Hemisphere

Because the Northern Hemisphere has more land than the Southern hemisphere it has a steeper slope than the Southern Hemisphere.

The prevailing winds drive the surface ocean currents. The general circulation of the atmosphere is in response to the unequal heating of the Earth by the Sun.

There is an energy surplus in tropics and a deficit in theMid Latitudes and Poles.

Yet the Tropics are not gettingprogressively warmer nor are the Mid Latitudes or Poles getting colder…..

Ocean Currents

OK, we know that land heats up and cools down much faster than water, and we know that the prevailing winds and ocean currents help to maintain the earth’s energy balance.

Which contributes more to the transfer of energy. Wind currents or ocean currents?

Air Currents Ocean Currents

75 % 25 %

Global Heat Transport

The exchange of energy by the winds and currents equalizes the latitudinal imbalances.

Winds can move much faster (vertically and horizontally)

Ocean Surface Currents

Mean Jan Temp (°C)~0°C ~4.8°C

Warm ocean surface currents effect the climate in middlelatitudes whereas cold ocean currents moderate tropical regions.

Monthly mean for Rio de Janeiro, Brazil and Arica, Chile. Both cities are coastal and both near sea level.

Ocean Currents can effect average temperature

Hot (28°C) Cold (9°C)

Ben Franklin’s depiction of the Gulf Stream

The Gulf Stream transports heat from the lower latitudes toward the North Pole.

How Altitude Affects Temperature

Hei

ght

(km

)

Guayaquil (alt:29m <25.5°C>) Quito (alt: 2800m <13.3°C>)

Recall that the lapse rate is –6.5°C/km.

An altitude difference of 2.8km should correspond to a temperature change of ~18°C.

2.8km x (-6.5 °C/km)= 18.2°C

Yet the difference is only 12.2°C….why?

An increase is elevation also increases the amount of incoming radiation because there is less atmosphere to reflect or absorbthe incoming radiation. As a consequence, mountain locationsexperience wide daily temperature ranges.

Geographic Position

Eureka experiences ocean breeze whereas New York Citymostly land breeze.

windward leewardMountains act like a barrier.Most of the moisture is left onthe windward side (Seattle).

Coastal Areas Mountain Barriers

Cloud cover and albedo

Cloud cover can be important because many clouds have a high albedo.

During the day clouds reflect solar radiation. Temperatures are lower than if there were clear skies.

During the evening, clouds absorb the terrestrial radiation radiation. Temperatures are higher than if there were clear skies.

The Daily cycle of temperature at Peoria, Illinois for 2 July days. On the clear day there is a wider spread in temperature than on the cloudy day.

Cloud cover and albedo

In Rangoon, the highest monthly mean temperature occurs during April and May rather than July and August (as is the case in most locations in the Northern Hemisphere).

The reason is that during the summer months the extensive cloud cover increases the albedo which reduces the incoming radiation

Competing Factors in Temperature Control

World Temperature Distributions

World mean sea level temperature in January in °C.

World mean sea level temperature in July in °C.

•isotherms trendeast to west

•decreasing temperature towardthe Poles

•“hot” and “cold” spots” migratewith the suns vertical rays

The hottest and coldest places are over land.

Warm ocean currents moving to the Poles warm theair. Equatorialbound currentsHelp cool the air.

World Temperature Distributions

World mean sea level temperature in January in °C.

World mean sea level temperature in July in °C.

Equatorial temperatures do not fluctuatesignificantly(there are no seasons).

25 °C

25 °C

-40 °C

10 °C

Middle and higher latitudes have much stronger seasonal signals.