Transcript 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