6.1 What causes variations in solar radiation on earth?

what causes earth’s seasons?

the tilt of Earth’s axis, NOT the elliptical orbit

plane of the ecliptic

surface connecting all points in Earth’s orbit, showing earth’s axis of rotation is not perpendicular relative to the plane of the ecliptic, rather it tilts at an angle of 23.5 degrees —> diff. hemisphere on earth are tilted more directly toward or away from the sun during earth’s yearly orbit —> seasons

**throughout its yearly cycle, earth’s axis ALWAYS points in the same direction, toward the north star (Polaris)

vernal equinox (spring)

March 21, sun is directly overhead along the equator. during this time, all places in the world experience equal lengths of night and day

summer solstice

June 21, sun reaches its most northerly point in the sky, directly overhead along the tropic of cancer

autumnal equinox

sep 23, sun is directly overhead equator

winter solstice

dec 22, sun is directly overhead along the tropic of capricorn, 23.5 deg S lat

declination

angular distance from the equatorial plane

sun’s declination

varies between 23.5 deg N, and 23.5 deg S of the equator on a yearly cycle due to Earth’s tilt -—> as a result, the region between these two latitudes (the tropics) receives much more annual radiation than polar areas

things affecting earth’s climate

• seasonal changes in angle of the sun and length of the day

• daily heating of earth influences climate in most locations except:

  • artic circle (66.5 deg N)

  • antarctic circle (66.5 deg S)

    • certain times of the year don’t experience daily cycles of daylight and darkness

is the amount of sunlight falling equal on every place on earth?

no, solar radiation is higher at lower latitudes than it is at higher latitudes

factors influencing the amount of radiation received at low and high latitudes are

• solar footprint

• atmospheric absorption

• albedo

• reflection of incoming sunlight

solar footprint

sunlight strikes a high angle of incidence in equatorial region —> solar radiation is concentrated in a small area ; closer to poles, sunlight strikes at a low angle of incidence so in high latitudes, same amount of radiation is spread over a larger area

atmospheric absorption

earth’s atmosphere absorbs some radiation, so less radiation reaches earth’s surface at high latitudes compared to low latitudes, bc sunlight must pass through more atmosphere at high latitudes

• more radiation at low lats bc less atmosphere

• less radiation at high lats bc more atmosphere

albedo

percentage of incident radiation that’s reflected back to space

• varies from material to material

  • ice can reflect as much as 90% = high albedo

• avg. albedo of earth’s surface = 30%

reflection of incoming sunlight

angle at which sunlight strikes the ocean surface determines how much is absorbed and how much is reflected

• if sun shines down on smooth sea directly overhead, only 2% of radiation is reflected

• if sun is only 5 deg above horizon, 40% is reflected back

**ocean reflects more radiation at higher lats than lower lats

is the intensity of radiation higher at high lats or low lats?

much much higher at low lats bc angle of incidence is much higher at low lats, hitting from directly overhead in many cases, therefore less is reflected. also, solar footprint: same amount of sunlight concentrated in smaller areas for low lats but larger areas for higher lats = more radiation. also, atmospheric absorption: thicker atmosphere in higher lats = less radiation passes through

oceanic heat flow

on average, heat gained and heat lost by the oceans balance each other on a global scale, since the excess heat from low lats is transferred to heat-deficient high lats by oceanic and atmospheric circulation

• the polar regions are always considerably colder than the equatorial zone, but the temp difference remains the same bc excess heat is transferred from equatorial zone to poles bc of circulation in oceans and atmosphere

along the arctic circle, how would the sun appear during the summer solstice? during the winter solstice?

(1) summer: it would rise up into the sky and then just stay overhead pretty much all day, with no “nighttime”

(2) winter: the sun would never rise and it would be dark all day