Earth's Atmosphere: Unequal Heating

• The Earth's atmosphere is not heated evenly.
• Air circulation around the globe moves heated air to areas where there is less heat
• Understanding the climate of Africa, including the role that air and oceans play, is important.

Unequal Heating of the Atmosphere

• The Earth gets heated by incoming solar radiation, also known as insolation.
• The amount of heat energy in the atmosphere is determined by the amount of insolation.
• Insolation is not equal all over the Earth's surface.
• Latitude and the tilt of the Earth's axis, or seasons, determine the amount of insolation.

Latitude

• Insolation is greater at the equator (0° latitude), resulting in higher temperatures.
• Insolation decreases towards 90° North (North Pole) and 90° South (South Pole), leading to lower temperatures.
• The sun rays strike the Earth's surface at a more direct angle in lower latitudes, near the equator.
• In higher latitudes, near the poles, the sun strikes the surface at an oblique angle.
• Heat is more concentrated over a smaller area at the equator (A).
• At the poles (B), heat is less concentrated and spread over a bigger area.
• Sunlight travels a shorter distance through the atmosphere at the equator (C) compared to the poles (D).
• Less insolation is lost through reflection, scattering, and absorption at the equator.
• A smaller amount of atmosphere results in less terrestrial radiation, conduction, and convection heating at the equator.

Heat Zones of the Earth

• Heating of the atmosphere gets affected by latitude.
• The Earth gets divided into heat zones based on latitude namely; Polar, Arctic Circle, Temperate, Tropic of Cancer, Equator, Tropic of Capricorn, Antarctic Circle

Earth's Axis and Revolution

• Heating of the Earth and its atmosphere varies throughout the year.
• Summer is usually hotter than winter.
• Different parts of the world experience winter and summer at different times.
• Seasonal variations in heating get caused by Earth's axis and its revolution around the sun.

Earth's Axis

• The Earth rotates on an imaginary axis that passes through the North and South Poles.
• One rotation is completed every 24 hours.
• During the part of the 24 hours when the Earth faces the sun, it receives insolation, resulting daytime.
• The opposite side of the Earth faces away from the sun and receives no insolation, resulting nighttime.
• The length of day determines the amount of heat Earth receives.
• If the axis was vertical, every location on Earth's surface would have 12 hours of day and 12 hours of night year-round, and there would be no seasons.
• The Earth's axis is tilted at an angle of 23.5° to the vertical.
• This tilt causes unequal lengths of day and night, resulting in temperature variation.
• Longer days and more heat occur during summer, and shorter days and less heat occur during winter.
• With a vertical axis, one half of the Earth facing the sun experiences daytime and the other half experiences nighttime.
• Every location on Earth gets exposed to both day and night due to its rotation every 24 hours.
• The northern hemisphere experiences more daylight and summer when the Earth axis is tilted, and the southern hemisphere experiences winter.
• The area between the Arctic Circle and North Pole always has daylight, so the sun never sets
• The area between the Antarctic Circle and South Pole always has night, so the sun never rises

Revolution Around the Sun

• The Earth revolves around the sun, completing one revolution every 365 days/ one year.
• The path the Earth travels gets called an orbit.
• Different world parts experience different seasons at each position in the orbit.
• In the southern hemisphere, Midsummer's Day occurs on December 21, called the summer solstice with long days, short nights, and maximum insolation.
• The days become shorter between December and March, as the season shifts from summer to autumn.
• The mid-autumn Day occurs on March 21, called the autumn equinox, with equal day and night length.
• Between March and June, the season shifts to winter with even shorter days and less insolation
• Midwinter Day occurs on June 21, called the winter solstice.
• Between June and September, the season shifts from winter to spring, and Mid-spring Day occurs on September 23, called the spring equinox, with equal day and night length.
• Antarctica experiences 24 hours of daylight in summer and 24 hours of nighttime in winter.

Energy Transfer and Balance

• Unequal insolation causes the equator to be hotter than the poles.
• It also results in warmer water in the oceans at the equator and colder water near the poles.
• Air and water movements transfer heat energy and restore energy balance over the Earth's surface.

Ocean Currents

• Ocean currents can be warmer or colder than the surrounding sea.
• Warm ocean currents move from the equator towards the poles, transferring heat from tropical zones to temperate zones.
• Cold ocean currents move from polar regions towards the equator, transferring cold water from polar regions to temperate zones.
• Cold currents move from the poles to the equator, and warm currents move to the poles.
• Ocean currents move along coastlines.

Winds

• Winds get the movement of air.
• Winds transfer heat energy and warm regions from warmer/tropical zones to cooler zones
• Winds can affect temperatures over land and sea surfaces, and also partly restores energy balance over the Earth's surface.
• The tropical easterlies blow from subtropical regions towards the equator, bringing cooler temperatures.
• The westerlies bring warmer temperatures from subtropical to temperate regions and are a major cause of the West Wind Drift ocean current.
• The polar easterlies blow from polar regions towards the subpolar regions, bringing cooler temperatures.
• An infrared satellite imaging taken in July shows the warmest temperatures in red and cooler temperatures shifting from orange, yellow and green to the coldest in dark blue.

Unequal Heating and Global Air Circulation

• The sun provides heat energy which does not equally spread all over the Earth's surface.
• The equator is hottest, and temperatures decrease northwards and southwards towards the poles.
• Temperature differences result in differences in atmospheric pressure.
• Atmospheric pressure is the weight of the atmosphere on the Earth's surface.
• Air movements are caused by the difference in atmospheric pressure and these air movements (vertically and horizontally) form global circulation.

World Pressure Belts

• Atmospheric pressure refers to the weight of the atmosphere on the Earth's surface.
• High Temperature = air expands and rises (creates low pressure)
• The temperature of the air influences the atmospheric pressure.
• Air moves towards low pressure areas to replace the rising air= converging air.
• Convergence is when air comes together, on the Earth's surface leading to rising air, thus creating low pressure.
• Low Temperature = air contracts and sinks (creates high pressure)
• Warm air is light, and cold air is heavy.
• Divergence occurs when descending air moves outwards from high pressure areas.
• Divergence means moving apart.

Global Air Circulation: The Tri-cellular Circulation

• The three sections of vertical air movements associated with global air circulation include; the Hadley Cell, the Ferrel Cell, and the Polar Cell.
• Hadley cells range approximately between the equator and 30°N/S.
• Ferrel cells range approximately between 30°N and 60°N, or 30°S and 60°S.
• Polar cells range approximately between 60°N and 90°N, or 60°S and 90°S.

Handley Cell

• Intense equatorial heat causes hot, moist air to rise, leading to rapid uplift and very low surface pressure, = termed the equatorial low.
• As air rises in the equatorial low, it cools, causing dew point reach and condensation to occurs in the form of giant cumulonimbus clouds.
• Thunderstorms and convection rain are common in equatorial regions.
• Air diverges polewards and drop 10,000m above the surface, =creating areas of high pressure, =subtropical highs at 30°N and 30°S
• Subtropical highs typically have warm and dry weather.

Easterly Winds

• Easterly winds get drawn into the equatorial low from the subtropical high-pressure belts (30°N and S).
• They are located with the tropics forming tropical easterly.
• In the Northern Hemisphere, the easterlies are the surface winds of the Hadley cells called north-easterlies.
• In the Southern Hemisphere, the easterlies are southeasterlies, thunderstorm activity commonly occurs.

Ferrel and Polar Cell

• The Ferrel cells are between 30°N and 60°N & 30°S and 60°S.
• Polar cells are between 60°N and 90°N & 60°S and 90°S.

Westerly Winds

• Westerly winds move from the subtropical high-pressure belts (30°N and S°) to the subpolar low-pressure belts (60°N and S).
• The westerlies are the ones at the surface winds of the Ferrel cells.
• The westerlies come from north-west in the Southern hemisphere.
• They blow from the south-west to the Northern Hemisphere.
• Polar cells are at the poles and between 60°N & S.
• Polar fronts are formed when winds meet between 50° and 60° N and S latitude,
• A front, is a zone in which contrasting air masses are at different temperatures.
• Polar fronts are the surface where warm subtropical air pushes up, which is rising above the Polar cells/colder polar winds.
• Polar easterlies are the surface winds found within the Polar cells.

Atmospheric Temperature, Pressure, and Winds

• Higher temperature causes air to heat and rise creating low pressure at the surface.
• Lower temperatures cause air to sink creating high pressure at the surface.
• Differences in atmospheric pressure generates wind and always moves from high to low pressure
• Uneven Earth heating by sun plus own rotation causes moving air-wind.
• Different forces act on moving air which causes wind in certain directions.

Pressure Gradient Force and Wind

• A pressure gradient is the varying pressure causes by difference in pressure
• Air moves toward low pressure from high along PG caused by pressure gradient force.
• When PG is steeper more pressure gradient force.
• The PG is gentle equals gentler winds = weaker
• If this was the one wind force, the diagram indicated directions which winds would cross over the Earth surface.

Coriolis Force

• Wind deflection is made once it starts blowing on PG because of Earth rotation.
• Earth rotation speed varies to 1 ,670 km/h at equator -zero at poles
• Winds blow either faster regions to slower or other way around from surfaces being rotated slower to quicker.
• Winds surface speed rotation retains + adapts to the surface for where it came wind will end behind +or surface ahead for where it blows.
• The winds get blown down or up instead + not deflected is called Coriolis force and the CF was by French scientist GC in 1835.
• In the 5th equator degree, this force does not take place and depends on how strong the wind strength is – because stronger winds stronger force. It varies the wind because from the N-S direction.

Ferrel's Law

• In 1856 (WF) William Ferrel, (American scientist) simple law Coriolis force showed + all moving air with water is deflected by Earth’s own rotational force.
• Deflected wind by Ferrel = moving matter LH + north HR

Geostrophic flow

• winds from pressure gradient force + Coriolis with Earth rotation
• The pressure gradient force causes air + from HP to LP Wind isobar parallels ( line constant for pressure/ given height).

Geostrophic flow -NH Norther Hemisphere

• First, airflow is moved form HP to LP because of the difference in PG/ Pressure gradient force.
• Immediately after airflow deflection occurs by Coriolis because of rotation form the planet’s
• When airflow from high/ pressure area = speeds increases by