Download Lecture 3

Meteo 3: Chapter 3
Seasons, Controllers of
Temperature and Fronts
Read Pages 77-90, 92-100
Review
 Energy transfer from the sun to the earth
 Energy transfer in the atmosphere –
radiation/conduction/convection
 What can we can apply this to?
The Reason for the Seasons
 The plane of the Earth’s orbit and the plane of the Earth’s
equator make a 23.5º angle
– Earth revolves around sun once every 365.25 days
– Orientation of Earth with respect to sun’s rays constantly changing
throughout year
 Summer Solstice (N.H.)- Sun over 23.5º N (Tropic of Cancer)around June 21- More daylight hours than any other day
 Winter Solstice (N.H.)- Sun over 23.5 º S (Tropic of Capricorn)around Dec. 21- Fewest daylight hours
 Opposite seasons in Southern Hemisphere
 On equinoxes (vernal and autumnal), sun directly over equator
Earth’s Tilt and the Seasons
For a more whizz-bang animated explanation, let’s see
the CD
Determinants of Temperature Climatology
 Latitude
 Proximity to Bodies of Water
 Elevation
Latitude
 High latitudes colder than tropics
– Solar radiation strikes more directly strikes over
tropics
– Less variation in temperature in the tropics as
compared to mid/high latitudes
– Driving force of most weather
 Warm air transported northward and cold air
transported southward, to eliminate latitudinal
temperature contrasts
Horizontal Distribution of Temperature
 What season does this show?
Proximity to Bodies of Water
 Water’s heat capacity is ~3X that of land,
so it is slow to warm and cool
 Winds also mix the topmost layer of water,
causing upwelling
 Result: Changes in air temperature are less
over water than land, because air is heated
by surface below
Temperature and Water
Locations near
water have small
seasonalitieschange in
temperature
between summer
and winter
Temperature and Water
 Moderating effect is
much greater at
locations downwind
of water, as
prevailing winds
blow air modified by
water inland
Astronomical versus Meteorological Seasons
 Time lag between astronomical and
meteorological definitions
 Meteorologists classify seasons based on average
temperature. These are as follows ( in N.H.):
–
–
–
–
“Meteorological Winter”- December - February
“Meteorological Spring”- March - May
“Meteorological Summer”- June - August
“Meteorological Fall”- September - November
More on Water’s Role
 Also explains why warmest and coldest
times of year occur about 1 month after
solstices
 Ocean currents also regulate temperature
– Driven by prevailing winds
– Try to erase temperature contrast between tropics &
poles
– Northward flowing currents transport warm water north,
southward flowing currents transport cold water south
Elephant Polo
You’re only as
good as your
elephant
Beware of
elephant polo
spies
Gulf Stream
 Gulf Stream
most important
for U.S. East
Coast weather
Flows south to
north – warm
ocean current
Ocean Currents
Elevation and Temperature
 Higher elevations are usually colder
 Lapse rate- rate of temperature decrease with
height
– Avg. environmental lapse rate in troposphere (lowest
10 km of atmosphere) = 6.5ºC/km
– Because atmosphere is heated from ground up!
Elevation on Temperature
What mountain is
this?
Why are mountains
cooler than lower
terrain during the day,
even though they’re
closer to the sun?
Controllers of Daily Temperature
 Some dependence on geographical factors, but
local weather conditions control diurnal
temperature range. First, the geographic factors:
– Proximity to water bodies: Smaller diurnal ranges near
large water bodies
– Latitude: Large range in low-mid latitudes because sun’s
angle varies greatly compared to high latitudes
– Elevation: Hills usually chillier during day, but valleys
sometimes colder than hilltops at night
– Urbanization: Cities warmer than suburbs, esp. at night
Weather’s Impact on Daily Temperatures
 Wind: Can blow in warmer or colder air masses
(temperature advection)
 Clouds: Lower daytime temperatures, but higher
nighttime temperatures
 Water Vapor: Humid areas have higher overnight
lows, dry areas rapidly cool at night
 Precipitation: Lower air temperature via
evaporational cooling
 Snow Cover: Lower temperature, esp. at night
How to measure temperature
 Thermometer: Instrument used to measure
temperature
– Housed in a white enclosed shelter with open vents on
sides to allow free flow of outside air to contact
instruments (Stevenson Screen)
– Instruments about 5 feet off ground
– Protects instruments from direct sunlight => reason why
bank thermometers are erroneously high is because
they are exposed to direct sun
– Lowest recorded temp: -129ºF (Vostok, Antarctica)
– Highest recorded temp: 136ºF (Libyan Desert)
Stevenson Screen
Air Masses
 Air mass- large volume of air with uniform
temperature and moisture properties
– Form as air acquires properties of its underlying surface
– Span several thousand kilometers in horizontal
– Described with two letter abbreviations (cP, cT, cA, mP,
mT)





c = continental
m = maritime
P = polar
T = tropical
A = arctic
Air Mass Source Regions
Fronts
 Cold and warm air masses collide, narrow
boundary between them is a front.
 Type of front is determined by movement of cold
air b/c cold air is more dense
– Cold front => cold air advances
– Warm front => cold air retreats
– Stationary front => cold air moves little or not at all
 Large temperature gradients at front (front located
on warm side of gradient) ….frontal passage
comes with wind shift & large temperature change
Fronts
Cross-sections of Fronts
Cross-sections of Fronts
Cross-sections of Fronts