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I THE ATMOSPHERE

2st PART

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ATMOSPHERE AND ENERGY
       The short wave radiation
The seasons
THE COMPOSITION OF THE ATMOSPHERE

THE TROPOSPHERE

THE COMPOSITION OF THE TROPOSPHERE - GENERAL CIRCULATION
- TYPES OF CLOUDS
- TYPES OF OPTICAL PHENOMENA
- THE JET-STREAM
THE STRATOSPHERE
THE COMPOSITION OF THE STATOSPHERE

- THE OZONE LAYER - THE POLAR OZONE HOLE - THE POLAR VORTEX - THE Q.B.O. (Quasi-Biennale Oscillation)

THE MESOSPHERE THE THERMOSPHERE THE IONOSPHERE
THE EXOSPHERE ATMOSPHERIC TURBULENCES
BLIZZARDS TORNADOES CYCLONES THUNDERSTORMS
HEATWAVES AND DROUGHTS

THE SEASONS

We ought to temperature differences prevailing in the atmosphere the weather it does every day. It's the same thing as saying that the distribution of energy in the atmosphere is uneven. Phenomena and factors involved in the influence of temperature are numerous and complex.

The sun is the main source of energy for Earth's atmosphere. The earth goes around the sun in 365,25 days, describing a nearly circular orbit and a roll on itself in 24 hours along an axis inclined at 23,5°. These astronomical characteristics of our planet are that we have four seasons.

During the winter solstice, December 21, the position of the Earth is that an observer
in the northern hemisphere receives less solar energy than in the summer. This is
because the sun's rays reach the observer at an angle by traversing a greater
distance in the atmosphere during the summer. More rays travel a great
distance in the atmosphere more energy of these rays decreases.

In summer the Earth is farther from the Sun than in winter, and then traverses its orbit more slowly. As the seasons are defined astronomically when the Earth reaches the longitude of 0°, 90°, 180° and 270°, it does not flow the same time between the beginning of each season, because the Earth isn't going to same speed in its orbit by distance, which depends on the date. Fall and spring have substantially the same time because the Earth travels in its orbit segments equivalent to these moments as "sides", so to say.

SEASONS
(NORTH HEMISPHERE)
CURRENT
LENGHT
Spring
92 days 19 hours
Summer
93 days 23 hours
Autumn
89 days 13 hours
Winter
89 days 0 hours

Seasons Length in the Northern Hemisphere.
For the southern hemisphere everything is the opposite.

 

A video explaining the cause of the seasons

Seasons dates

There are several types of revolutions :

- The revolution tropic
The tropic revolution is the time between two passages of the Earth to the point of spring equinox. It is a little shorter than the sidereal period because the precession of the equinoxes is in the retrograde direction (50,2877 "per year). This movement is called precession of the equinoxes due to the movement of the axis of rotation Earth describes a cone in the retrograde direction in about 25,858 years. For the Earth, the tropical year is 365,2422 days (or 365 days 5h 48m 46s). the astronomical year is the time of the year tropic.

- The sidereal revolution
The sidereal revolution is the period of revolution, is the time between two passages of the Earth facing the Sun used as a fixed reference. This period of 365,2563 days (365 days and 6 hours and 9 minutes and 9,50 seconds) exceeds the Tropic revolution.

Our calendar (Gregorian calendar) is the one closest to the Earth Tropic revolution length. There are 365 days, 0,2422 days per year of delay. That's why every 4 years it is added in the year one day in February (leap year of 366 days).
One day is greater than 0,2422. X.4 .=. 0,98688. The Gregorian calendar has an average value of 365,25 days which is too large compared to the tropical year (365,2422). This creates a lag of 7,50 days for 1000 years. So to be as close to reality it doesn't add a leap year every four years in those the multiples of 100 without to 400. Thus 1600 and 2000 are leap years, but not the 1700, 1800, 1900 and 2100.

The evolution of saisons lenght

The orbit of the Earth-Moon barycentre rotates in its plane in the forward direction at about 12" per year (one revolution in about 100.000 years). On the contrary, the precession of the equinoxes rotates in the opposite direction (backward motion) at a rate of 50,2877" per year (one revolution in about 25.868 years). Whole of these two movements allow to calculate the period of the passage of the perihelion of the Earth by the direction of the vernal equinox, which is about 21.000 years and called climatic precession. So every 10.500 years (half period of the climatic precession) the aphelion moves from summer to winter. Although this isn't the distance from the Earth to the Sun which is the most important factor for the type of seasons, when the Earth is at aphelion in winter, they are more severe. For more information see the Glaciations.

 

As you can see, the seasons don't depend on the distance between the Sun and Earth.
In the northern hemisphere, winter doesn't occur when the Sun is closest to the Earth.

 

The effects of season


Annual cycle of air temperature at the surface (°C),
obtained by a control simulation of CGCM1

These results were obtained in a control experiment involving several centuries, and to simulate the natural variability of the coupled system. The model reproduces the observed annual temperature cycle. Note that the amplitude of annual cycle is much higher over the poles and extra-tropical land areas than over the oceans. Note also differences in the shape of the annual cycle, which reflects the different thermal properties of land areas, ice and water on the surface of the Earth. For more information, see our page on the First Generation Coupled Model (CGCM1).
Click here to see the animation larger

As can be seen in the animation above, the atmosphere was "one step behind" compared to winter solstice and summer, so we find the hottest days of the year between 21 July and 10 August and the coldest between Jan. 21 and Feb. 10. The atmosphere in fact stores the heat and cold depending on the season with one month apart.

We find the same thing every day the sun reaches its zenith at 24:00 (GMT) but the temperature reaches its maximum around 14/15 hours UT, for the same reasons.

 


The changes in vegetation according to the season
Click here to see the animation larger

 

The average monthly rainfall from 1961 to 1990
Click here to see the animation larger


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