Seasonal & Diurnal Temp Variations

ATS351

Lecture 3

Earth-Sun DistanceEarth-Sun Distance

Change in distance has only a minimal effect on seasonal Change in distance has only a minimal effect on seasonal temperature.temperature.

Note that during the N. hemisphere winter, we are Note that during the N. hemisphere winter, we are CLOSERCLOSER to the sun!to the sun!

EccentricityEccentricity

Changes the distance from Changes the distance from the Earth to the sunthe Earth to the sun

Aphelion: Farthest from sunAphelion: Farthest from sun Perihelion: Closest to sunPerihelion: Closest to sun Today’s difference between Today’s difference between

aphelion and perihelion ~ aphelion and perihelion ~ 3%3% Results in about a 7% Results in about a 7%

increase in received increase in received solar radiation in solar radiation in January than in JulyJanuary than in July

Max eccentricity would Max eccentricity would result in 20-30% more result in 20-30% more solar energy received at solar energy received at aphelionaphelion

• Eccentricity Varies on 100,000 year timescales

• The Earth’s orbit goes from being elliptical to nearly circular

• Period: ~365.242199 days

Earth’s AxisEarth’s Axis

Tilt: 23.5Tilt: 23.5°°

Rotation is counterclockwise Rotation is counterclockwise (when viewed from the (when viewed from the North Pole)North Pole)

This is why the sun rises in This is why the sun rises in east and sets in westeast and sets in west

This is why we have seasonsThis is why we have seasons

ObliquityObliquity

~41,000 year cycleGreater the tilt, greater the

difference in season

Precession of the AxisPrecession of the Axis

~23,000 year cycle ~23,000 year cycle Axis points to a different starAxis points to a different star This is not a change in Tilt!This is not a change in Tilt!

Radiation and Orbital Radiation and Orbital ParametersParameters

Combined tilt, precession, and obliquity effects Combined tilt, precession, and obliquity effects

change high-latitude insolation in summer change high-latitude insolation in summer

by as much as 30%!by as much as 30%!

Some Evidence that Some Evidence that

cycles affect the climatecycles affect the climate

Specifically theseSpecifically these

cycles seem to be relatedcycles seem to be related

to ice agesto ice ages

SeasonsSeasons

Notice the change in the angle of Notice the change in the angle of incidence with seasonincidence with season

Solstice and EquinoxSolstice and Equinox

During the equinox, there is exactly 12 hours of day and 12 During the equinox, there is exactly 12 hours of day and 12 hours of night everywhere on Earth.hours of night everywhere on Earth.

During a solstice, there is either 24 hours of light or darkness, During a solstice, there is either 24 hours of light or darkness, North or South of 66.5North or South of 66.5° (Arctic and Antarctic Circles)° (Arctic and Antarctic Circles)

Solstice and the Midnight SunSolstice and the Midnight Sun

24 hours of sunlight exposure during the summer 24 hours of sunlight exposure during the summer solstice in the Arctic circlesolstice in the Arctic circle

The SolsticeThe Solstice• When the Sun reaches its farthest

north or south point and begins to 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), this is known in the NH as the summer solstice – ~21st of June

• Southernmost, Tropic of Capricorn (23.5°S) = winter solstice – ~22nd of December

The EquinoxThe Equinox On 2 specific days of the year, the sun crosses the equator, and On 2 specific days of the year, the sun crosses the equator, and

on that day, day and night are of equal lengthon that day, day and night are of equal length Latin: equi = equal; nox = nightLatin: equi = equal; nox = night Equinox = equalEquinox = equal nightnight When moving north, we call it the vernal (spring) equinoxWhen moving north, we call it the vernal (spring) equinox

~20~20thth of March of March On its way south, On its way south,

the fall or autumnal equinoxthe fall or autumnal equinox ~23~23rdrd of September of September

Solar IntensitySolar Intensity When sunlight is spread over a larger area it is a less intense When sunlight is spread over a larger area it is a less intense

heat source (Intensity = Power/Area)heat source (Intensity = Power/Area) ““Insolation” is a term we use for solar intensityInsolation” is a term we use for solar intensity

InsolationInsolation

Daily Total SunshineDaily Total Sunshine 7575° N in June gets more sun than the equator° N in June gets more sun than the equator North-South temperature gradient is stronger in the winterNorth-South temperature gradient is stronger in the winter Very little change in the tropicsVery little change in the tropics

Daytime WarmingDaytime Warming

Daylight heats the atmosphere Daylight heats the atmosphere from below by conduction and from below by conduction and convectionconvection

Convection leads to vertical Convection leads to vertical mixing to even out vertical mixing to even out vertical temperature gradientstemperature gradients

Nighttime CoolingNighttime Cooling

Radiational cooling creates a Radiational cooling creates a temperature inversion at the surfacetemperature inversion at the surface

Cold dense air sinksCold dense air sinks

The Diurnal CycleThe Diurnal Cycle

Each day is like a mini Each day is like a mini seasonal cycleseasonal cycle

We call this the ‘diurnal We call this the ‘diurnal cycle’cycle’

Sun’s rays are most intense Sun’s rays are most intense around noonaround noon

Maximum temperatures lag Maximum temperatures lag the peak in insolation because the peak in insolation because until around 3-4 pm, there is until around 3-4 pm, there is more incoming solar more incoming solar radiation than there is radiation than there is radiative coolingradiative cooling

What Controls Daily What Controls Daily Temperatures?Temperatures?

Temperature strongly depends on:Temperature strongly depends on:Cloud coverCloud coverSurface typeSurface type

Albedo (desert or forest?)Albedo (desert or forest?)Moisture Moisture Difference between north and south facing slopesDifference between north and south facing slopes

WindWindHorizontal temperature advectionHorizontal temperature advection

Annual Temperature CyclesAnnual Temperature Cycles

Different environments effect Different environments effect the temperature cyclethe temperature cycle

Some Major factors:Some Major factors: LatitudeLatitude Proximity to a body of waterProximity to a body of water Proximity to a mountain rangeProximity to a mountain range ElevationElevation

Global TemperaturesGlobal Temperatures

JanuaryJanuary July July

Land/Sea BreezesLand/Sea Breezes

““Sea Breezes” form during the day when there is solar heatingSea Breezes” form during the day when there is solar heating The land is heated which causes Air to riseThe land is heated which causes Air to rise Air aloft flows outward from land to oceanAir aloft flows outward from land to ocean Surface air responds with flow towards land at the low levelsSurface air responds with flow towards land at the low levels

13 - 23

LAND BREEZE SEA BREEZE

Source: NOAA

Mountain/Valley WindsMountain/Valley Winds• Sunlight heats mountain slopes

during the day and they cool by radiation at night

• Air in contact with surface is heated/cooled in response

• A difference in air density is produced between air next to the mountainside and air at the same altitude away from the mountain

• Density difference produces upslope (day) or downslope (night) flow

• Daily upslope/downslope wind cycle is strongest in clear summer weather when prevailing winds are light

Upper Air MapsUpper Air Maps

We have looked at surface station dataWe have looked at surface station data Now recall the vertical extent of the Now recall the vertical extent of the

atmosphereatmosphere These layers showThese layers show

Complex temperature profileComplex temperature profile Decrease in pressure and densityDecrease in pressure and density

Let’s look at some examples Let’s look at some examples