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Exercise 2 - Study of water vapour in the atmosphere 
Measurement of water vapour exploits the vapour absorbency zone (5.7 - 7.1 µm, middle infrared). Due to high absorption by water in this wavelength range, most of the values recorded are for the middle and upper troposphere. In fact, the atmospheric absorption of middle infrared radiation is so strong, that almost no radiation is able to reach the surface of the Earth. Consequently, there is nothing left to reflect when reaching the Earth's surface, and it appears to be 'invisible'. Even in areas where no clouds have formed, water vapour exists in the upper atmosphere and may eventually cause clouds and precipitation. Water vapour is visible day and night, because middle infrared radiation is emitted both day and night and does not depend on the existence of direct solar radiation. The usefulness of the images is decreased by the fact that 'low-level' water vapour content is often very important for the eventual formation of clouds and precipitation. The 'upper level' nature of the images may miss out on significant variations in water vapour content at lower levels. The amount of water vapour is affected by the degree of evaporation, which is in turn affected mainly by the surface temperature of the sea. The water vapour flows with the air currents towards low pressure zones (created as a result of strong incident solar radiation), then rises up and accumulates to form clouds, which eventually bring rain. Water vapour 'moves' with the Intertropical Convergence Zone. This movement is visualised in the images showing the average monthly water vapour content, evaluated by the MERIS instrument on ESA's Envisat satellite. Download the images, save them to a folder called 'Water Vapour', and use LEOWorks to produce an animation showing water vapour movement from January 2003 to December 2003. Download the water vapour images as a ZIP-file. Start the LEOWorks programme and choose Tools>Image Animation. Select the twelve images in the folder 'Water Vapour' and open them. Shift the Animation Speed to 2.9 Frames/Sec and watch the animation.
At what time of the year do we find a large amount of water vapour, and at what time do we find a small amount? Does this correspond with the Summer and Winter Monsoon periods? Where do the most intensive changes occur during the year? Examine the Himalayan mountain range region and the Tibetian Plateau, and describe the water vapour distribution from January to December Explain the differences between this area and the region lying to the South, and give the reasons for them. Pause the animation and activate the Active Slider. Shift the slider to Animation Frame 10, the image for November 2003. Explain the large amount of water vapour in the South-Eastern part of India and Sri Lanka during the period of the North-Western Winter Monsoon. Now shift the slider to Animation Frame 0 (January 2003) and explain the reason for the small amount of water vapour over India and the large amount over Northern Australia. Last update: 18 April 2013
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