Lecture #06

February 3, 2010, Wednesday

Influences on Temperature

(1) Latitude (discussed in last lecture)

(2) Altitude & elevation

(3) Differential heating of land and water

(4) Atmospheric circulation

(5) Ocean currents

(6) Cloud cover

(7) Local effects

Temperature indices (human comfort)

Temperature measurement

Influences of

Temperature

Altitude & Elevation

Point A and Point B are

above the same

elevation, but point B is at

a higher altitude. Point A

is closer to the surface

and heated more during

the day and cooled more

at night by the warmer

(day) and cooler (night)

surface through

conduction and

convection. Thus, point A

experiences greater

diurnal temperature range

than point B.

Clear Night

Clear Day

Influences of

Temperature

Altitude & Elevation

Point C and Point B are

at the same altitude but

point C is above a surface

location with greater

elevation and thus closer

to the surface, and thus is

also heated more than

point B during the day

and cooled more than

point B at night.

Clear Night

Clear Day

Altitude: height above mean

sea level of a point in the air.

Elevation: height above mean

sea level of ground surface.

Influences of

Temperature

Altitude & Elevation

Point C and Point A are

located the same distance

above local surfaces but

the local surface at point C

has a higher elevation.

There is much less

atmosphere above point C

than above point A to

prevent loss of longwave

radiation emitted by the

surface (less greenhouse

effect). Evaporative

cooling is also greater at

point C due to lower

pressure. Point C is

warmed less in the day

and cooled more rapidly at

night than point A.

Clear Night

Clear Day

Tem

pera

ture

Dec July Nov.

Same latitude

Influences of Temperature

Note the differences in the range

and time lag of annual temperature

over ocean and over land

Differential heating of land and water

Influences of Temperature

Differential heating of land and water

Water surface has a higher albedo than most land surfaces.

Continentality

Defined as the exacerbation of seasonal temperature extremes experienced by continental interiors.

World record high = 57oC (137oF) at Azizia, Libya, 1913

World record low = 89 oC ( 129 oF) Antarctica, 1960

Maritime locations have moderate seasonal temperature extremes due to presence of nearby water bodies that change temperature very slowly.

Influences of Temperature

Differential heating of land and water

Atmospheric

Circulation

Latitudinal

temperature and

pressure differences

cause large-scale

horizontal energy

transport through

atmospheric circulation.

Also influences

latitudinal moisture

regimes and cloud

cover which then also

impact temperature.

Influences of

Temperature

Ocean Circulation

Western ocean

basins have warm

ocean currents

while eastern

ocean basins

maintain cold

currents.

Coastal air

temperatures are

affected

accordingly.

Influences of

Temperature

Clear day: larger temperature range

Daytime: high albedo of clouds reflects

more insolation, and reduces daytime

maximum temperature.

At night: clouds reduce the loss of

longwave radiation emitted by the

surface and thus increases nighttime

minimum temperature.

Influences of TemperatureCloud Cover

Influences of

Temperature

Local Effects

Aspect

The direction a

slope faces.

Equatorward

facing slopes

heat more

quickly than

poleward slopes

N

Vegetation reduces

insolation reaching the

surface during the day

and traps the

longwave radiation

emitted by the ground

surface at night, which

leads to cooler

daytime and warmer

nighttime

temperatures.

Influences of

Temperature

Local Effects

Vegetation

Temperature Indices

Temperature and human comfort

Temperature can feel hotter or colder depending on wind speed (wind chill temperature index) or humidity (heat index).

Extreme heat or cold are the two most deadly events in US.

Stronger wind leads to more loss of heat, higher humidity limits cooling through evaporation.

Degree days

Generally, a measure of the departure of the mean daily temperature from a given standard (threshold).

One degree-day for each degree (oC or oF) of departure from (above or below) the standard.

Degree-days are accumulated over a “season” or at any point of a year during which the total can be used as an index of temperature effect on some quantity, such as plant growth, energy demand and consumption.

Temperature Indices

Degree days

First used in connection with plant growth.

Heating degree-days: determines the energy required to bring interior temperature to 21 oC (70 oF) when it is below 18 oC (65oF), an example of lower limit.

Cooling degree-days: determines the energy required to cool interior temperature to 21 oC (70 oF) when it is above 24 oC (75oF), an example of upper limit.

Growing degree-days = daily mean temperature – base crop temperature (which varies with crop type). When accumulated during the growing season, the accumulated value can be used to estimate approximate dates for crop harvesting.

Old formula (1945): Twc = 0.0817(3.71V0.5 + 5.81 – 0.25V)(T-91.4)+91.4

New

formula:

Heat Index

Heating Degree-days

Cooling Degree-days

Measuring Temperature

Liquid-in-glass thermometers: based on substances expand and contract as temperature changes

Thermograph: two thin metal strips with different expansion properties bonded together which curl with change in temperature

Mechanic Thermometers

Electrical Thermometers

Thermistor: electrical resistor, its resistance changes with temperature

Fast response, used in radiosondes

Note the radiation shield

Measuring Temperature

Vented weather shelters are

necessary to accurately gauge air

temperatures by shading from

direct sun light and rain.

Painted white to increase albedo

and reduce absorption of solar

radiation and heating

Must be 5 ft from a vegetated

(grass) surface to reduce laminar

layer (close to the surface) bias.

An open door opens towards the

pole (to avoid direct sunlight

when open to make readings).

Measuring TemperatureInstrument Shelter