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|  | | | | | | | | Dew point | |
Background
Dew Point
Dew point is the temperature at which saturated air cools to form water vapour, with relative humidity at 100%. Should any further cooling occur, condensation would lead to the formation of clouds.
The graph shows how much water vapour the air can contain. It is clear that warm air can contain more water vapour than cooler air. The blue line shows the dew point at different temperatures.
The Foehn Wind
When air rises over a mountain, pressure decreases.
Any gas allowed to expand by lowering pressure will become cooler, whereas if it is compressed into a small volume, it becomes warmer. Such temperature changes are termed 'adiabatic', because heat is neither brought in from outside sources, nor lost to the surrounding area.
With the expansion of volume, the given energy needs to expand to cover the increased volume. This means that there will be less available energy per unit of volume than before, and the temperature will therefore decrease. On the other hand, if the volume decreases, gas is compressed, pressure is increased, more energy will be available per volume unit, and therefore the temperature will rise.
When unsaturated air rises, its temperature falls at a rate of approximately 1ºC per 100 m. The relative humidity increases, as this air can hold less and less humidity. As the air continues to rise, adiabatic cooling continues, and the air temperature gradually approaches the dew point.
When rising air is lifted to an altitude at which its dew point is reached, condensation begins to produce clouds. Most convectional clouds have a flat base marking the level at which condensation starts. Condensation releases heat. Therefore, if the air continues to rise after condensation, the temperature falls at a lower rate (approximately 0.5ºC per 100 m).
When the air begins to descend on the lee slope of a hill or a mountain, it warms rapidly and the relative humidity falls below condensation level, leading to the cloud's remaining water droplets quickly evaporating and drying the air. The temperature of the air will then increase at a rate of approximately 1ºC per 100 m.
| | | The Foehn wind |
The figure shows what happens when an air parcel with a temperature of 20ºC and a relative humidity of 57% is forced to cross a mountain range of an altitude of 2,500 m. The dew point is reached at 900 m, and clouds are formed. The air is cooled by approximately 1ºC per 100 m until the dew point is reached. Above the condensation level the air is cooled at a lapse rate of 0.5ºC per 100 m. So the temperature of the air at the top of the range will be 3ºC.
When the air starts to descend, the temperature rises at a rate of approximately 0.5ºC per 100 m. Consequently, the temperature will rise to 28ºC after the descent. The air will be very dry because warm air can hold more water vapour than colder air.
The windward slope will therefore be wet and the leeward slopes will be dry. When the air starts to descend, the temperature will rise at a rate of approximately 1ºC per 100m.
Such descending dry and warm air is called 'Foehn wind'.
| | Jet stream | |
The Jet Stream
The jet stream in the upper atmosphere determines the weather situation in Europe. When the jet stream goes from west to east, Europe will have weather dominated by cyclones with associated frontal systems.
Sometimes the jet stream takes a route from north to south. In these situations, winds from the north are perpendicular to the Alps.
The figure shows an undulating jet stream.
Satellite images
Meteosat images
Weather satellites are built to measure atmospheric conditions, whilst resource satellites are designed to map surface conditions on Earth.
While one of the main objectives of weather satellites is to secure frequent acquisitions, the resource satellites aim to achieve high spectral and spatial resolution. High resolution demands small scanning areas resulting in a more limited area coverage, which means that it takes several days to scan the entire Earth. It also means that there is a corresponding number of days during which the satellite covers the same area.
High spatial resolution, on the other hand, makes it possible to distinguish small area units, so it is possible to map in greater detail.
Because of their high spectral resolution, resource satellites can distinguish a greater amount of variation in radiation. This is exploited in area mapping as it is possible to distinguish between spectral profiles of many different surfaces.
NOAA images
The images in this case study were acquired by a NOAA satellite.
The National Oceanographic and Atmospheric Administration (NOAA) sent the first of a series of NOAA satellites into orbit in 1970. These satellites move in solar synchronous orbits about 850 km from the Earth and scan the entire globe in twenty-four hours.
As NOAA is equipped with both visible and near infrared channels, it can also be used for large-scale mapping of vegetation. The daily overpasses occur at the same local time, making it possible to put together cloudless images based upon several days' acquisitions.
The images shown here are band 4: 10.3 - 11.3 µm.
These images are thermal infrared, which means that the temperature is shown on a scale ranging from white (low temperatures) to black (high temperatures).
Surface Maps
Surface maps showing the weather situation are part of this case study. These maps show the weather in Europe. There is one map for each day showing the situation at 00.00 hours (midnight).
Image Processing Software
LEOWorks can be used to analyse satellite images. LEOWorks is a tool suitable for basic analysis of images and can be downloaded by clicking on the link on the right.
The exercise can also be undertaken by analysing the images in a different way.
A flight from Rome to Copenhagen
On a flight from Rome to Copenhagen on 16 March 2000, it was possible to document a Foehn event in the Alps like the one described above.
Departure from Rome was at 14:50, and arrival in Copenhagen was at 17:35.
The exercise contains a batch of photos and a video taken and recorded through the window of the plane as well as an interview with the co-pilot. The language is Norwegian.
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| | Foehn IntroductionExercises IntroductionPhotos from the planeMeteosat imagesNOAA imagesCloud filmSurface mapsJet StreamInterview with co-pilotSimulation of the
Foehn situationFind other examplesLinks Satellite images from SareptaMETEOSAT images from EUMETSATSurface mapsInternet Weather sourceNOAAEuropean weather chartsJet StreamForecast Charts for the Jet StreamVideos Cloud videoLakes of Albani and Nemi videoPlains of Po river valley and foothills of the Alps videoCloud cover south of Hanover, Germany videoDevelopment of weather systems animationInterview with co-pilotMathematical simulation of foehn
situation, 15-16 March 2000Eduspace - Download LEOWorks 3
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