Lec 16 Flashcards
(30 cards)
what is an atmosphere?
a layer of gas that surrounds a world
Atmospheric air is a mixture of gases that may consist either of individual atoms or of molecules.
–temperatures in the terrestrial atmospheres are generally low enough (even on Venus) for atoms to combine into molecules.
For example, the air we breathe consists of molecular nitrogen N2 and oxygen O2, as opposed to individual atoms (N or O).
atmospheric pressure
collisions of individual atoms or molecules in an atmosphere create pressure
-planetary atmospheres exist in a balance between the downward weight of gases and the upward push of their gas pressure
why don’t we notice atmospheric pressure?
1) the pressure pushes in ALL directions, so it pushes up and inward as well as down
2) the fluids in your body OUT with equal pressure, so theres no NET pressure trying to compress/expand your body
how does atmosphere affect planets
-atmospheres create pressure that determines whether liquid water can exist on the surface
-atmospheres absorb and scatter light, which makes daytime skies brighter and ABSORPTION prevents radiation from reaching the ground
-can create wind and weather and play a major role in long-term climate change
-interactions b/w atmospheric gases and solar wind can create protective magnetosphere around planets with stronger magnetic field
-can make planet surfaces warmer via greenhouse effect
how does the greenhouse effect warm a planet?
Greenhouse effect
Energy that warms a planet comes from sunlight (visible light)
Some of this visible light is reflected back to space, and the rest is absorbed by surface
The absorbed energy must be returned to space, but planetary surfaces are too cool to emit visible light
The greenhouse effect works by temporarily “trapping” some IR light, slowing return to space
Greenhouse effect occurs only when an atmosphere contains gases that can absorb IR light
These gases can absorb the light because their molecular structures begin rotating or vibrating when they absorb IR photon
Greenhouse gases slow the escape of IR radiation from the lower atmosphere, while their molecular motions heat surrounding air
This makes the surface and lower atmosphere warmer than they would be from sunlight alone
Note: greenhouse effect by itself does not alter a planet’s overall energy balance
without the greenhouse effect, a planet’s global average surface temp would depend on what two things:
The planet’s distance from the Sun, which determines the amount of energy received from sunlight. The closer a planet is to the Sun, the greater the intensity of the
incoming sunlight.
The planet’s overall reflectivity, which determines the relative proportions of incoming sunlight that the planet
reflects and absorbs. The higher the reflectivity, the less light absorbed and the cooler the planet.
why do atmospheric properties vary with altitude?
The greenhouse effect can warm a planet’s surface and lower atmosphere, but other processes affect the temperature at higher altitudes
-the way in which temperature varies with altitude determines what is often called the
atmospheric structure, which has 4 basic layers:
The troposphere is the lowest layer, in which temperature drops with altitude (something you’ve probably noticed if you’ve ever climbed a mountain).
The stratosphere begins where the temperature stops dropping and instead begins to rise with altitude. High in the stratosphere, the temperature falls again.
The thermosphere begins where the temperature again starts to rise at high altitude.
The exosphere is the uppermost region, in which the atmosphere gradually fades away into space.
important effects of scattering
Small amount of visible light is scattered by atmospheric molecules, and this scattering has 2 important effects:
1) Scattering makes the daytime sky bright, which is why we can’t see stars in the daytime.
Without scattering, sunlight would travel only in perfectly straight lines, which means we’d see the Sun against an otherwise black sky, just as it appears on the Moon.
Scattering also prevents shadows on Earth from being pitch black.
On the Moon, shadows receive little scattered sunlight and are extremely cold and dark.
2) Scattering explains the colors of our sky. Visible light consists of all the colors of the rainbow, but not all the colors are scattered equally.
Gas molecules scatter blue light (shorter wavelength and higher energy) so much more effectively than red light (longer wavelength and lower energy).
why does the sky appear blue?
When the Sun is overhead, this scattered blue light reaches our eyes from all
directions, so the sky appears blue
primary cause of storms
The primary cause of storms is the churning of air by convection, in which warm air rises
and cool air falls.
Recall that convection occurs only when
there is strong heating from below. In the troposphere, heating from the ground can therefore drive convection
solar wind
Important type of energy that comes from the Sun
Low density flow of subatomic charged particles.
On the Moon and Mercury, solar wind particles hit the surface, where they can blast atoms free.
On Venus and Mars, solar wind particles can strip away atmospheric gas.
In contrast, Earth’s strong magnetic field creates a magnetosphere that acts like a protective bubble surrounding our planet, deflecting most solar wind particles around it
weather vs climate
Weather refers to the ever-varying
combination of winds, clouds, temperature, and pressure that makes some days hotter or cooler, clearer or cloudier, or calmer or stormier than others.
Climate refers to the average of weather over many years.
-e.g. we say that Antarctic deserts have a cold, dry climate, even though
it might sometimes rain or snow.
atmospheric heating and circulation cells
Atmospheric heating affects global wind patterns because equatorial regions receive more heat from the Sun than polar regions
Warm equatorial air therefore rises upward and flows toward the poles, where cool air descends and flows toward the equator.
If Earth’s rotation did not influence this process, the result would be two huge circulation cells (or Hadley cells), one in each hemisphere
The circulation cells transport heat both from lower to higher altitudes and from the equator to the poles.
They therefore make Earth’s polar regions much warmer than they would be in the absence of circulation.
On Venus, the dense atmosphere allows the circulation cells to transport so much thermal energy that temperatures are nearly the same at the equator and the poles.
On Mars, the circulation cells transport very little heat because the atmosphere is so thin, so the poles remain much colder than the equator.
rotation and the coriolis effect
Planetary rotation affects global wind patterns through the Coriolis effect:
A deflection of the path of a moving object (like air or water) due to Earth’s rotation.
It doesn’t change the object’s actual speed or direction in space, but it causes a curved trajectory relative to Earth’s surface.
Equatorial regions circle around Earth’s rotation axis faster than polar regions
Air moving away from the equator therefore has “extra” speed that causes it to move ahead of Earth’s rotation to the east, while air moving toward the equator lags behind Earth’s rotation to the west.
In either case, moving air turns to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, which explains why storms circulate in opposite directions in the two hemispheres.
Uneven heating and cooling of Earth’s surface creates regions of slightly higher pressure (“H” on weather maps) or lower pressure (“L” on weather maps) than average
coriolis effect
deflection of the path of a moving object (like air or water) due to Earth’s rotation. It doesn’t change the object’s actual speed or direction in space, but it causes a curved trajectory relative to Earth’s surface.
How does the Coriolis effect shape Earth’s global wind patterns?
It splits each of the two huge circulation cells into 3 smaller circulation cells
As Earth rotates, the coriolis effect diverts the air to the right before it reaches the equator, forcing the single large circulation cell to split
The 3 resulting cells circulate the air like 3 interlocking gears
Coriolis effect on a rotating planet tends to divert air moving north or south into east-west winds.
Its strength depends on a planet’s size and rotation rate: Larger size and faster rotation both contribute to a stronger Coriolis effect.
Among the terrestrial planets, Earth is the only one with a Coriolis effect strong enough to split the two large circulation cells
clouds and precipitation
Besides being the source of precipitation, clouds can alter a planet’s energy balance.
Clouds reflect sunlight back to space,
thereby reducing the amount of sunlight that warms a planet’s surface, but they also tend to be made from greenhouse
molecules that contribute to planetary warming
why do thunderstorms occur more on summer afternoons?
Stronger convection means more clouds and precipitation.
That is why thunderstorms are common on summer afternoons, when the sunlight-warmed surface drives strong convection.
The linkage between clouds and convection also explains why Earth has lush jungles near the equator and deserts at latitudes of 20° ° –30 north or south.
What factors can cause long-term climate change?
Solar brightening. The Sun has grown gradually brighter with time, increasing the amount of solar energy reaching the planets.
Changes in axis tilt. The tilt of a planet’s axis may change over long periods of time.
Changes in reflectivity. An increase in a planet’s reflectivity— for example, from increased cloud cover, increased ice cover, or particles released from volcanoes—means a decrease in the amount of sunlight it absorbs, and vice versa.
Changes in greenhouse gas abundance. More greenhouse gases tend to make a planet warmer, and less make it cooler
changes in axis tilt
These small changes affect the climate by
making seasons more or less extreme.
Greater tilt means more extreme seasons, with warmer summers and colder
winters.
The extra summer warmth tends to prevent ice from building up, which reduces the planet’s reflectivity and thereby makes the whole planet warmer.
Conversely, a smaller tilt means less extreme seasons, which can allow
ice to build up and make a planet COOLER.
4 ways planets lose atmospheric gases:
Condensation. The condensation of gases that then fall as rain, hail, or snow is essentially the reverse of the release of gas by vaporization. On Mars, for example, it is cold enough for carbon dioxide to condense into dry ice (frozen carbon dioxide), especially at the poles.
Chemical reactions. Some chemical reactions incorporate gas into surface metal or rock. Rusting is a familiar example: Iron rusts when it reacts with oxygen, thereby removing oxygen from the atmosphere and incorporating it into the metal.
Solar wind stripping. For any world without a protective magnetosphere, particles from the solar wind can gradually strip away gas particles into space.
Thermal escape. If an atom or a molecule of gas in a planet’s exosphere achieves escape velocity, it will fly off into space. The relative importance of thermal escape on any world depends on its size, distance from the Sun, and atmospheric composition
More thermal escape will occur if a planet is small (low escape velocity)
3 ways terrestrial atmospheres gain gas
Terrestrial atmospheres can gain gas in 3 ways:
Outgassing: Volcanic outgassing has been the primary source of gases for the atmospheres of Venus, Earth, and Mars.
Vaporization: After outgassing creates an atmosphere, some atmospheric gases may condense to become surface liquids or ices. The subsequent vaporization of these surface liquids (evaporation) and ices (sublimation) therefore represents a secondary source of atmospheric gas.
Surface ejection: The tiny impacts of micrometeorites, solar wind particles, and high-energy solar photons can knock individual atoms or molecules free from the surface. This surface ejection process explains the small amounts of gas that surround the Moon and Mercury.
source and loss processes on the moon and mercury
The Moon and Mercury may once have had some gas released by volcanic outgassing, but they no longer have volcanic activity, and any gas released in the distant past is
long gone.
Some of the gas released long ago was probably lost through stripping by the solar wind, but it would have been lost to thermal energy
Mercury cannot hold much of an atmosphere because its small size and high
daytime temperature mean that nearly all gas particles eventually achieve escape velocity.
The Moon is cooler than Mercury, but its smaller size gives it a lower escape
velocity; as a result, gases escape about as easily on both worlds
why does earth’s climate stay stable?
Carbon dioxide cycle
The mechanism by which Earth self-regulates its temperature
Atmospheric carbon dioxide dissolves in rainwater, creating a mild acid.
The mildly acidic rainfall erodes rocks on Earth’s continents, and rivers carry the broken-down minerals to the oceans.
In the oceans, calcium from the broken-down minerals combines with dissolved carbon dioxide and falls to the ocean floor, making carbonate rocks such as limestone.
Over millions of years, the conveyor belt of plate tectonics carries the carbonate rocks to subduction zones, where they are carried downward.
As they are pushed deeper into the mantle, some of the subducted carbonate rocks melt and release their carbon dioxide, which then outgasses back into the atmosphere through volcanoes.
Acts as a long-term thermostat for Earth, because it has a built in form of self-regulating feedback that returns Earth’s temp back to normal.
