Download Temperature

Temperature
Topics
1. Energy & Temperature
2. Controls on Air
Temperature
3. Geographic Controls
a.
b.
c.
d.
e.
Latitude
Land vs. Water
Ocean Currents
Altitude and Elevation
Relative Geographic
Position
4. World
Distribution
5. Daily Cycle
6. Annual Cycle
7. Wind Chill
Readings
A&B: Ch. 3
(p. 70-86)
G109: Weather and Climate
Energy & Temperature
•
Energy balance: about how heat is transformed and
used in the Earth-Atmosphere system
•
First Law of Thermodynamics: A change in heat
content in the atmosphere can have two results:
→
→
•
Temperature distribution and changes are a result of
the processes involved in the energy balance
G109: Weather and Climate
xx: Another lecture
Energy & Temperature
•
•
Change in temperature due to an addition of heat
(energy) to an object (without expansion work)
depends on two things:
ƒ the mass of the object:
m
[kg]
ƒ the specific heat of the object: C
[J kg-1 K-1]
Specific heat: characteristic of the substance: how
much energy [J] is needed to change 1 kg of given
substance by 1 K
∆Energy = m C ∆T
G109: Weather and Climate
11: Wind Systems
Energy & Temperature
•
Start with: three objects all at the same T
•
Add energy
OR
•
Three experiments, with differing
a. Amount of energy added
b. Amount of mass (m)
c. Substance → specific heat (C)
G109: Weather and Climate
11: Wind Systems
Energy & Temperature
•
Result: SMALLER Temperature change if:
a.
b.
c.
G109: Weather and Climate
11: Wind Systems
Energy & Temperature
•
Temperature difference: property which
determines whether heat will flow in or out of
an object when it is in contact with another
object with a different temperature
•
Heat flows from:
G109: Weather and Climate
11: Wind Systems
Controls on Air Temperature
•
Main influence on air temperature (near
surface):
ƒ
• Is a response to the entire energy balance
• Heat from the surface is mixed into a layer of air
up to 1-2 km thick
• The thicker the mixing layer, the more heat
necessary for temperature change
ƒ But, QH can only heat the air to the extent that it is
available from Q*
• Q* influenced by the surface radiation balance:
ƒ Primarily by
ƒ Surface type:
G109: Weather and Climate
11: Wind Systems
Controls on Air Temperature
•
Other influences on air temperature:
ƒ Absorption / Emission of longwave radiation
• Conversion of radiation to heat (warming) or
heat to radiation (cooling)
• Usually requires very moist air (high ε)
ƒ Condensation: due to fog or cloud formation
• Releases latent heat
• Stops cooling, no direct heating
ƒ Advection: warmer / colder air arrives with wind
• Airmass change associated with front passage
• Wind from colder or warmer area
ƒ Change in altitude:
• Expansion and cooling by rising air
• Compression and warming by sinking air
G109: Weather and Climate
11: Wind Systems
Geographic Controls on Temperature
•
Five principal factors:
i.
ii.
iii.
iv.
v.
•
Latitude
Differential heating between land and water
Ocean currents
Altitude & Elevation
Relative geographic position
The result of these controls is the global
distribution of temperature, and the diurnal
and seasonal developments
G109: Weather and Climate
11: Wind Systems
Latitude
•
Influences average temperature
ƒ
ƒ
•
Influences seasonal pattern
ƒ Tropics:
• K↓ nearly constant throughout the year
• T nearly constant throughout the year
ƒ Outside tropics
• K↓ varies due to changes in solar angle
• Length of day changes
•
G109: Weather and Climate
11: Wind Systems
Differential heating between…
Land
Water
Solid → Conduction
Liquid → Convection
Opaque
Transparent
Specific Heat (3x less)
Specific Heat (3x more)
Less Evaporation
Evaporation
•
•
•
•
•
•
•
•
G109: Weather and Climate
Differential heating between…
Land
Water
→
→
→
11: Wind Systems
ƒ Daily: 10 cm
ƒ Annually: 15 m
→
ƒ Daily: 6 m
ƒ Annually: 200-600 m
→
•
E.g., Compare Coastal and Continental cities at the
same latitude
Latitude
Vancouver (Coastal)
Winnipeg (Continental)
49oN
49oN
Mean Jan. T
Mean July T
G109: Weather and Climate
11: Wind Systems
Ocean Currents
•
Impact T on coastal areas
ƒ
ƒ
Warm currents - move from equator → poles
Cold currents - move from poles → equator
G109: Weather and Climate
11: Wind Systems
Ocean Currents: Examples
•
North Atlantic Drift
ƒ Warm ocean Current
ƒ Keeps Great Britain and Western Europe much warmer
than would be expected at these latitudes
• E.g., New York (40oN) is on average 4.5oC cooler than
London, England (51oN)
ƒ Prevailing winds - westerlies
ƒ Moderating effect carried far inland
• E.g., New York (40oN) has a similar average
temperature as Berlin, Germany (52oN)
•
Warm ocean currents – most impact in
•
North Pacific Current
•
ƒ Cold ocean current
ƒ Keeps summer temperature in southern California ~6oC
cooler than southern East Coast of the U.S.
Cold ocean currents – most impact in the
G109: Weather and Climate
.
11: Wind Systems
Altitude & Elevation
•
•
•
•
Altitude: height in the atmosphere above sea level
Elevation: height of land surface above sea level
Lapse rate: decrease of temperature with altitude
(height)
ƒ Average decrease of 6.5oC km-1 in the free troposphere
ƒ Cooler T are expected with height
At greater elevation, the air above the surface:
ƒ Absorbs & reflects smaller portion of K↓
• Increased K↓ receipt at the surface
•
ƒ Absorbs less L↑ and K↓
• L↓ back to the earth reduced
•
ƒ
ƒ High elevation sites are not as cold as one would expect
from the average lapse rate because of increased K↓
G109: Weather and Climate
11: Wind Systems
Relative Geographic Position
•
Examples:
ƒ Windward versus leeward side of mountains
ƒ Windward versus leeward side of lake
ƒ North slope versus south slope of hill
G109: Weather and Climate
11: Wind Systems
World Distribution of Temperature
• January:
•
•
N-S trend
ƒ Decrease in T polewards
E-W perturbations
ƒ NH winter:
ƒ SH Summer:
G109: Weather and Climate
11: Wind Systems
World Distribution of Temperature
• July:
•
•
N-S trend
ƒ Decrease in T polewards
E-W perturbations
ƒ NH Summer:
ƒ SH winter:
G109: Weather and Climate
11: Wind Systems
World Distribution of Temperature
•
Temperature Range:
Summer maximum - Winter minimum
ƒ Equator:
ƒ Continents:
G109: Weather and Climate
11: Wind Systems
Daily Cycle of Temperature
•
Daily Air Temperature and radiation
Maximum
Minimum
•
K↓
T
•
•
Night - atmosphere &
surface cool as they emit L↑
ƒ Not heated by K↓
Minimum T –
ƒ
Maximum T ƒ Before then,
→
•
LAG of the MAXIMUM
ƒ depends on local
conditions e.g. cloud,
humidity, latitude,
geographic setting
G109: Weather and Climate
11: Wind Systems
Cycles of Temperature: Daily
•
Influences on daily cycle: examples
a. Location
• Air near or above an ocean / large water mass
has small diurnal cycle compared with
continental locations
b. Clear vs. Cloudy Skies
Clear Skies
Large K↓
→ Increased surface T
→ Higher air T
L↑ lost to space, Small L↓
→ Cooler T
Larger
Cloudy Skies
Daytime
Smaller K↓
→ Reduced air T
Nighttime L↑ absorbed by clouds,
Big L↓
→ T remains warmer
T Range
Smaller
G109: Weather and Climate
11: Wind Systems
Annual Cycle of Temperature
•
Northern Hemisphere
Maximum
Minimum
K↓
T
•
•
•
Astronomical Season dates indicate the beginning of
Meteorological Seasons rather than the peak
ƒ Like daily cycle maxima are offset.
Lag effect of the Earth warming up (takes time: large
amount of mass involved)
Lag - greatest in oceanic/coastal areas
ƒ Continental areas - July maximum
ƒ Coastal areas - August/September maximum
G109: Weather and Climate
11: Wind Systems
Wind Chill
•
Wind chill – combines wind and temperature
ƒ How cold it feels
ƒ Calm conditions: a thin layer of warm air
molecules insulate the body
ƒ Windy conditions: warm air layer is blown away by
wind
G109: Weather and Climate
11: Wind Systems