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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