Download Seasons and the Diurnal Cycle

Seasons and the Diurnal Cycle
ATS 351
Lecture 3
Lecture 3
Brief discussion on last weeks Rain
Brief discussion on last weeks Rain Total rainfall Sept. 11‐12:
0.66 inches
Note the wind direction during the storm
CSU Campus
Foothills
Foothills Campus
Earth‐Sun Distance
•
This change in distance has only a slight effect on seasonal temperature – Sperihelion=1419 W m‐2 vs. Saphelion=1325 W m‐2 (ΔS=94 W m‐2) •
During the northern hemisphere winter, we are CLOSER to the sun!
– Perihelion: ~ Jan. 4th
– Aphelion: ~ July 4th
– Climate effect?
Eccentricity
• Changes the distance from g
the Earth to the sun
– Aphelion: Farthest from sun
– Perihelion: Closest to sun
• Today’s difference between aphelion and perihelion ~ 3%
•
•
Eccentricity Varies on 100,000 year timescales
The Earth’s orbit goes from being elliptical to nearly circular
– Results in about a 7% increase in received solar radiation in January than in July
– Max eccentricity would result in 20‐30% more solar energy received at aphelion
Earth’ss Axis
Earth
Axis
• Tilt: 23.5°
• Rotation is counterclockwise (when viewed from the North Pole)
• This is why the sun rises in east and sets in west
Obliquity
Obliquity → Change in lt
θ=23.5
~41,000 year cycle
 Greater the tilt, greater the difference in season

Precession of the Axis
Precession of the Axis
• ~23,000
23,000 year cycle year cycle
• Axis points to a different star
• This is not a change in Tilt!
Results in a change in
• Results in a change in Perihelion/Aphelion date
Radiation and Orbital Parameters
• Combined tilt, precession, and obliquity effects change high‐latitude insulation in summer h
hi h l i d i l i i
by as much as 30%!
• Some people have found
evidence that these cycles affect the climate
• Specifically
Specifically these these
cycles seem to be related to ice ages
Solstice and Equinox
• During the equinox, there is exactly 12 hours of day and 12 g
y
hours of night everywhere on Earth.
• During a solstice, there is either 24 hours of light or darkness North or South of 66.5° (Arctic and Antarctic Circles)
Seasons
• Notice the change in the angle of incidence with season
Arctic Circle
Arctic Ci l
Circle
Zenith Angle
Zenith Angle
0°
θ
Θ= Zenith Angle
The Solstice
The Solstice
•
When the Sun reaches its farthest north or south point and begins to p
g
move back the other way, it can be thought that the earth “stands still” as far as the north or south axial movement is concerned
– Latin: sol = sun; stice = standing
•
When the sun reaches its northernmost point at the Tropic of Cancer (23.5°N), we know this in the NH as the summer solstice
in the NH as the summer solstice – ~21st of June
•
Southernmost, Tropic of Capricorn
(23.5°S) = winter solstice – ~22nd of December
fD
b
The Equinox
The Equinox
• On 2 specific days of the year, the sun crosses the equator, p
y
y ,
q
,
and on that day, day and night are of equal length
• Latin: equi = equal; nox = night
• Equinox = equal night
• When it’s on its way north, we call it the vernal (spring) equinox
– ~20th of March
• On its way south, the fall or autumnal equinox
y
,
q
– ~23rd of September
Summer Solstice: Sun is directly above Tropic of Cancer
Equinox: Sun is directly above Equator
q
Winter Solstice: Sun is directly above Tropic of Capricorn
Something to think about:
g
• Since polar latitudes receive the longest period of sunlight during the summer, why aren’t temperatures highest there?
• Would the seasons be stronger or weaker if we i
increased the tilt of the earth?
d h il f h
h?
• What if the tilt was 90 degrees? 0 degrees?
Solar Intensity
Solar Intensity
• When sunlight is spread over a larger area it is a less intense heat source
• “Insolation” is a term we use for solar intensity
S0 ≈ constant
Area Low Sun > Area High Sun
Bottom Line: The same amount of energy is Bottom
Line: The same amount of energy is
spread over a larger area during winter
Insolation
Daily Total Sunshine
Daily Total Sunshine
•
•
•
•
75 N in June gets more sun than the equator
North‐South temperature gradient is stronger in the winter
Very little change in the tropics
Note the larger values
Note the larger values
in the SH winter than in the NH winter. Daily Temperature: Sensible Heating
Daily Temperature: Sensible Heating
Sensible Heat (SH): Heat flux NOT associated with a phase change
‒ This heat can be sensed in the form of a temperature change
This heat can be sensed in the form of a temperature change
‒ SH is the same process of heating(cooling) the atmosphere via conduction as the surface absorbs(emits) radiation
Absorbing
Emitting
SW in Air is warmed by Air
is warmed by
surface
LW out Air is cooled by Air
is cooled by
surface
Latent vs. Sensible Heat
Latent vs. Sensible Heat
Latent Heat
Temperature ≈ Constant
Temperature Constant
Energy is used to break the liquid bonds between water molecules
water molecules
Sensible Heat
Temperature ↑
Temperature ↑
Energy excites the surface molecules, which then transfer energy to
then transfer energy to the atmosphere
Daytime Warming
Daytime Warming
• Daylight heats the atmosphere
the atmosphere from below by conduction and convection
ti
•Convection leads to vertical mixing to even out vertical g
temperature gradients
The Diurnal Cycle
The Diurnal Cycle
• Each day is like a mini seasonal cycle
• We call this the ‘diurnal cycle
cycle’
• Sun’s rays are most intense around noon
• Maximum temperatures lag the peak in insolation
because until around 3‐4 pm, there is more incoming solar radiation than there is radiative cooling
Nighttime Cooling
Nighttime Cooling
• Radiational cooling creates a g
temperature inversion at the surface
• Cold dense air sinks
Mountain/Valley Winds
• Sunlight heats mountain slopes during the day and they cool by radiation at night
g
• Air in contact with surface is heated/cooled in response
• A difference in air density is A difference in air density is
produced between air next to the mountainside and air at the same altitude away from the
same altitude away from the mountain
• Density differences produce upslope (day) or downslope
upslope (day) or downslope
(night) flow
• Daily upslope/downslope wind cycle is strongest in clear
cycle is strongest in clear summer weather when prevailing winds are light
What Controls Daily Temperatures?
What Controls Daily Temperatures?
• Temperature strongly depends on:
Temperature strongly depends on:
– Cloud cover
– Surface type
Surface type
• Albedo (desert vs. forest)
Moisture (think heat capacity)
• Moisture (think heat capacity)
• Difference between north and south facing slopes
– Wind
d
• Horizontal/vertical temperature advection
Annual Temperature Cycles
Annual Temperature Cycles
• Different
Different environments environments
effect the temperature cycle
• Some Major factors:
– Latitude
– Proximity to a body of water
P i i
b d f
• Ocean Currents
– Proximity to a mountain range
– Elevation
Global Temperatures
Global Temperatures
January
July
More variation in NH winter Why?
More variation in NH winter. Why?
Effect of Oceans/Lakes on Temperature
Heat Capacity of Silica (main constituent in soils) ≈ 700 J kg‐1
K‐11
Soils must heat from top down by conduction
Heat Capacity of H2O ≈ 4200 J kg‐1 K‐1
Water can easily mix to depth via convection and currents
• Bodies of water act as giant thermal reservoirs
• In the summer they warm slowly and keep nearby land cool
• In winter they retain the summertime heat, and warm h
h
h
d
nearby land mass
This same mechanism acts on a daily timescale …
Example:
Land/Sea Breezes
Example: Land/Sea Breezes
• “Sea Breezes” form during the day when there is solar heating
• The land is heated faster than the sea, which causes air to rise
• Air aloft flows outward from land to ocean
• Surface air responds with flow towards land at the low levels
→Cool air is transported over the land reducing its max
→Cool air is transported over the land reducing its max temperature
Example:
Land/Sea Breezes
Example: Land/Sea Breezes
• At night the land cools down faster than the adjacent water
At i ht th l d
l d
f t th th dj
t t
• Radiative cooling of the land surface leads to sinking air and offshore flow
Land/Sea Breezes
/
Ocean Effects: Currents
Ocean Effects: Currents
The impact of oceans on annual temperature cycles can be exacerbated by warm/cold currents
Example: London is warmer than Example:
London is warmer than
New York during the winter despite being at a higher latitude
London
50° N
New York
40° N
Elevation
While there are many factors that cause elevation to influence temperature, in general higher elevations tend to have colder climates
• Temperature decreases with height in the troposphere, so high elevation areas are exposed to colder air
l
d
ld
NOTE: Air is “thinner” at high elevations, ie. the air has lower density and thus less mass
and thus less mass
‒ Easier to heat or cool the air at high elevations
Mountains
Persistent Troughs
Persistent Ridges
The presence of The
presence of
mountains will lead to ridging west of the barrier, and troughing
east of the barrier
Tropical Convection
Tropical Convection
Large areas of convection (100’s to 1000’s of km2) can effect pressure patterns, and lead to a cascading effect across the globe
across the globe
NOTE: The method through hi h hi
d
i
which this and mountain effects operate is somewhat complicated:
YOU DO NOT NEED TO KNOW THIS FORMULA
Just know the effect
Just know the effect
Surface Analysis: Contouring
Surface Analysis: Contouring •
•
•
•
Isotherms: contour lines of constant temperature
p
Smooth lines; no corners
LINES DO NOT CROSS OR BRANCH
Label isotherm lines with a temperature
Hints:
1. USE PENCIL
2. Begin by finding the highest and lowest values
3. Start drawing lines on a value near the edge of the map
near the edge of the map
4. Complete the rest of the contour lines using the initial contour as a guide