lecture #06 february 3, 2010, wednesdaycore.ecu.edu/geog/suh/courses/weather2010su/lecture06.pdf ·...
TRANSCRIPT
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