lecture 8 earth's atmosphere

THE EARTH’S ATMOSPHERE

THE EARTH’S ATMOSPHERE The Earth's atmosphere is a thin layer of gases

that surrounds the Earth. It composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases. This thin gaseous layer insulates the Earth from extreme temperatures; it keeps heat inside the atmosphere and it also blocks the Earth from much of the Sun's incoming ultraviolet radiation.

Composition of the Atmosphere

THE EARTH’S ATMOSPHERE

The Earth's atmosphere is about 300 miles (480 km) thick, but most of the atmosphere (about 80%) is within 10 miles (16 km) of the surface of the Earth.

To compare, • Mount Everest, the highest mountain, is about 8.8 km

high. • Concorde cruises at about 16km. • The radius of the earth is about 6400 km. • Space satellites occupy orbits from 800 km to over

36,000 km above the surface.

Air PressureAt sea level, the air pressure is about

14.7 pounds per square inch. As your altitude increases (for example, if you climb a mountain), the air pressure decreases. At an altitude of 10,000 feet, the air pressure is 10 pound per square inch (and there is less oxygen to breathe).

Layers of the Atmosphere

The Troposphere

The troposphere is where all weather takes place; it is the region of rising and falling packets of air. The air pressure at the top of the troposphere is only 10% of that at sea level (0.1 atmospheres).

The Stratosphere

Above the troposphere is the stratosphere, where air flow is mostly horizontal. The stratosphere contains the thin ozone layer, which protects us from a harmful effect of the ultraviolet radiation.

The Mesosphere and Ionosphere

Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere), where many atoms are ionized (have gained or lost electrons so they have a net electrical charge). The ionosphere is very thin, but it is where aurora take place, and is also responsible for absorbing the most energetic photons from the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible.

OZONEOzone is a form of molecular oxygen (O3); it

consists of three connected oxygen atoms.

OZONE LAYERThe ozone layer is a region of the stratosphere which contains most (about 90%) of the Earth's atmospheric ozone. It is about 10-25 miles (15-40 km) above the Earth's surface. The ozone layer shields the Earth from Ultraviolet B rays that come from the Sun.

The History behind the Ozone Hole

Dramatic loss of ozone in the lower stratosphere over Antarctica was first noticed in the 1970s by a research group from the BRITISH ANTARCTIC SURVEY who were monitoring the atmosphere above Antarctica from a research station much like the picture to the right.

Another story goes that the TOMS satellite data didn't show the dramatic loss of ozone because the software processing the raw ozone data from the satellite was programmed to treat very low values of ozone as bad readings! Later analysis of the raw data when the results from the British Antarctic Survey team were published, confirmed their results and showed that the loss was rapid and large-scale; over most of the Antarctica continent.

THE OZONE HOLE Over Antarctica, stratospheric ozone has been depleted

over the last 15 years at certain times of the year. This is mainly due to the release of manmade chemicals containing chlorine such as CFC's (ChloroFluoroCarbons), but also compounds containing bromine, other related halogen compounds and also nitrogen oxides (NOx). CFC's are a common industrial product, used in refrigeration systems, air conditioners, aerosols, solvents and in the production of some types of packaging. Nitrogen oxides are a by-product of combustion processes, e.g. aircraft emissions.

What Is Being Done?

• The first global agreement to restrict CFCs came with the signing of the Montreal Protocol in 1987 ultimately aiming to reduce them by half by the year 2000.

• The main CFCs will not be produced by any of the signatories after the end of 1995, except for a limited amount for essential uses, such as for medical sprays.

• Agreement has been reached on the control of industrial production of many halocarbons until the year 2030.

THE GREENHOUSE EFFECT

THE GREENHOUSE EFFECT

Parts of our atmosphere act as an insulating blanket of just the right thickness, trapping sufficient solar energy to keep the global average temperature in a pleasant range.