| ||Climate, seasons and weather - Background|
Weather is the combination of different phenomena (e.g. wind, clouds, precipitation) in the lower atmosphere, the so-called 'troposphere', at a certain location and time. The weather is caused by the Sun's energy heating the Earth's surface and the overlying atmosphere. The weather depends on latitude, orography (altitude and relief energy), land-ocean distribution, natural cover and anthropo-geographical factors. The weather's physical characteristics are temperature, air pressure, and air humidity.
The Earth's movement around the Sun
The Earth moves around the Sun in an elliptical orbit. One entire circuit takes one year. The Sun stands in a focus of the ellipse, while the orbit of the Earth around the Sun lies on an imaginary plane, the 'Ecliptic'. The Earth's axis of rotation is inclined by 23.5° with respect to the Ecliptic.
During the Earth's movement around the Sun, the Earth's axis of rotation does not move. Consequently, the Northern hemisphere points towards the Sun during the summer months, whereas it points away from the Sun in winter.
These conditions are the result of the different angles at which the Sun's radiation reaches the surface of the Earth during the year. During the summer, in the Northern hemisphere the angle of incidence is high. During the winter months, the angle decreases and the Sun stands lower over the horizon. Consequently, sunrise is late and sunset is early during winter months in the Northern hemisphere. The days are short, and, because of the low insulation, cold. Simultaneously, during the Northern winter, the Sun stands high and the days are long and warm in the Southern hemisphere. During the Northern summer months it is winter in the Southern hemisphere. The seasons in the Northern and Southern hemispheres are reversed.
The point at which sunlight strikes the Earth's surface at a right angle moves between 23.5° N and 23.5° S during the year. The climatic differences in the seasons are caused by the Earth's inclination of 23.5°. The distance of the Earth from the Sun is of little importance. The Northern and Southern turning points of the Sun are called the Northern and Southern Tropics, the Tropic of Cancer and the Tropic of Capricorn, respectively.
Summer Northern hemisphere
Summer Southern hemisphere
Summer Northern hemisphere
Summer Southern hemisphere
Climate map of Himalayan region
The formation of different climate zones within the Himalayan region is a result of the strong coaction between movements in air masses and the surface structure of the Earth.
Particularly noticeable is the decrease in humidity from the South-East towards the North and West. Humidity is an elementary factor when defining any particular climate zone.
All types of humidity levels are represented within the Himalayan region. The South-East is predominantly humid and semi-humid, with six to twelve wet months. Further to the North and to the West it becomes drier, with large, semi-arid areas. The arid deserts are located to the North of the Himalayan mountain range.
Climate classes are allocated based on land-ocean distribution, orography, atmospheric and ocean streams, and so on. The Summer Monsoon has a great influence on the climate in Southern and South-Eastern Asia.
Intertropical Convergence (ITC)
The intertropical convergence zone is situated near the equator. The air mass is heated and therefore rises. Under the warm air mass a thermal low pressure is formed. The humid North-East Trade winds from the Northern hemisphere and the South-East Trade winds from the Southern hemisphere converge. As the Trade winds flow, the air mass becomes heated and rises. Because of the very high humidity of the air and the very high temperature, the rising air mass forms high cumulonimbus clouds. These clouds are much larger and more vertical than fair weather cumulus. The top of a cumulonimbus cloud can reach up to 12,000 metres. Many rain and thunder storms result from cumulonimbus cloud formations.
The location of the ITC depends on the seasons. Basically the ITC moves with the zenith of the Sun from 20° N to 20° S. Deviations can be found, caused partly by high Trade wind circulation. The actual position of the ITC defines the meteorological equator.
Trade wind circulation
Winds with an average speed of 20 km/h flow regularly to the West along the equatorial flanks of the subtropical high-pressure zone. Over the land, and at the start of their oceanic travel, they are mostly dry winds, like, for example, the Harmattan in West Africa. As they travel over the ocean, however, these winds take up large quantities of water vapour, and in mountainous barrier zones extensive clouds and precipitation are produced.
Along the equator, the North-East and South-East Trade winds converge in a trough of low pressure. Under the influence of the Sun at its zenith, and the consequent pronounced warming of the Earth's surface, the converging air masses rise and form moist cumulonimbus clouds. The rise of air masses at the equator is balanced by a fall around the Tropics, which completes the circle. See the Intertropical Convergence diagramme.
The Coriolis force is a result of the Earth's rotation. The air masses are always redirected to the right in the Northern hemisphere and to the left in the Southern hemisphere, as seen from the equator. The Coriolis force at the equator is zero, and it increases towards the poles, because the Earth's rotanional speed decreases from the equator towards the poles from 1674 km/h to 0 km/h.
The Monsoon is a wide, seasonally alternating air current with a deflection of over 120°. The best known and most important Monsoon is the South-Western Summer Monsoon in Southern Asia. From May until September this brings tremendous rainfall to the continent.
The South-East Trade wind air streams are deflected by the Coriolis force in a westerly direction when they cross the equator and stream towards the Indian subcontinent. This is due to the thermal differences between the surfaces of the land and sea. Land masses heat up faster than water. The air over the land rises and generates a low-pressure zone into which the air from the Equator streams. The Monsoon crosses the Indian Ocean on its way to Southern Asia, absorbing a lot of water. It starts to rise over the warm land mass. While rising, the air cools down, heavy clouds appear, and the Monsoon rain starts to fall.
The Himalayas form a natural barrier for the Monsoon, which the air streams are unable to surmount. Southern parts of the Himalayan region are subject to heavy rainfall, whereas Northern parts are extremely dry.
From December to February, the North-Eastern Winter Monsoon is predominant. This is a very cold, dry air mass. The South-Eastern part of India has the most rainfall during the months from December to February, because the Winter Monsoon crosses the Bay of Bengal.
Rainfall varies from 2000 to 4000 mm a year on the Western coast of India, to only 200 mm a year in the Tharr desert. The city of Cherrapunji in the Khasi Mountains has a rainfall of more than 10,000 mm a year, making it the rainiest city in the world!
The Normalized Difference Vegetation Index (NDVI), which is related to the proportion of photosynthetically absorbed radiation, is calculated from the visible red channel and the near infrared channel. Healthy vegetation shows a steep increase in reflection at 0.7 µm (near infrared), whereas soil, depending on its character, shows a linear increase. The more active the chlorophyll, the steeper the increase of reflection in the near infrared at 0.7 - 1 µm. This allows classification of the vitality of vegetation. Standardisation (by quotientiation) reduces topographical and atmospherical influences, and allows large areas to be observed.
The calculation for Landsat NDVI is: (channel4 - channel3) / (channel4 + channel3).
In other words,
||near infrared - red
||near infrared + red
Meteosat Second Generation artistic view
The weather satellites used for meteorological observations operate on polar or equatorial orbits. These satellites measure the reflection and radiation from the Earth's surface. The reflection and radiation (infrared) can be interpreted to obtain information on the distribution of clouds, temperature, and the amount of vater vapour in the atmosphere. Special attention is paid to the early detection of hurricanes and thunderstorms. Thanks to radiation, it is possible to measure the temperature of the layers of air and of the Earth's surface. Therefore it is also possible to determine the altitude of clouds. Even at night, cloud distribution can be observed by measuring infrared radiation.
Visible Light: The VIS-channel operates in the visible spectrum. It measures the solar radiation reflected by the atmosphere and the Earth's surface. Water and ice clouds stand out because of their conspicuously higher reflection. Admittedly, snow and ice can show a similar strong reflection. The reflection of water surfaces depends very much on the recording direction and the surface conditions.
- Meteosat First Generation
- Ground resolution
- Visible light (VIS) 2.5 km
- Infrared (IR) 5 km
- Spectral channels
- 1: 0.50 - 0.90 µm visible light
- 2: 5.70 - 7.10 µm water vapour
- 3: 10.50 - 12.50 µm thermal infrared
- Operating altitude 36 000 km
- Repeat rate 30 minutes
- Data since 1978
Water Vapour: The WV-channel operates in the vapour absorption zone (5.7 - 7.1 µm, middle infrared). Due to the strength of absorption in this wavelength range, the values recorded mainly relate to the middle and upper troposphere. In effect, atmospheric absorption of middle infrared radiation is so strong that almost no radiation is able to reach the surface of the Earth. Consequently, on the Earth´s surface there is nothing left to reflect, and it appears to be 'invisible'.
Even in areas where no clouds have formed, water vapour flow fields exist in the upper atmosphere, and these may eventually cause clouds and precipitation. The images are typically of lower resolution than IR images, but are available both day and night, which is an advantage over visible imagery. Water vapour is visible day and night, because the middle infrared exists both day and night and is not dependent on the existence of direct solar radiation. The usefulness of these images is reduced by the fact that 'low-level' water vapour content is often very important in the eventual formation of clouds and precipitation. The 'upper level' nature of the images may miss significant variations in water vapour content at lower levels.
Thermal Infrared: The IR-channel operates in a spectrum with low absorption of trace gas. It is therefore possible to measure long wave radiation from the Earth's surface and cloud surfaces. Differentiation of clouds is very good because of their lower temperature when compared to the Earth´s surface temperatures. Difficulties occur with low-lying clouds and snow and ice fields, which may show low temperatures similar to those of ice cloud surfaces.
Meteosat Second Generation
Meteosat Second Generation (MSG), also known as Meteosat 8, is now operational. It has improved technical specifications. MSG generates multi-spectrum imagery of the Earth's surface and cloud systems at double the rate (every 15 minutes instead of every half hour) of current Meteosat satellites, and over a much larger number of spectral channels (twelve compared to three for Meteosat). The geometrical resolution is also vastly improved (1 km for the high-resolution visible channel and 3 km for the others).
Eight of the channels are in the thermal infrared range, providing, among other things, permanent information on the temperature of clouds, land, and sea surfaces. Using channels that absorb ozone, water vapour and carbon dioxide, MSG also allows meteorologists to analyse the characteristics of atmospheric air masses, making it possible to reconstruct a three-dimensional view of the atmosphere. Two of the eight infrared channels are now published on the Eumetsat homepage. The current Meteosat capabilities have been retained.