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WEATHER FORECAST Clear skies ahead! With its 72-meter wingspan and 2,400 kg weight, Solar Impulse (HB-SIB) is a light stick that has to navigate the skies while steering clear of strong atmospheric turbulence that could damage it or make it lose control. This explains why the weather and accurate forecasts are key for the success of this unprecedented challenge! This worksheet will give you the necessary information to understand major weather events, to decode the principles of forecasts, and to build the components of your weather station! Project: EPFL | dgeo | Solar Impulse Writing: Marie-Noëlle Kaempf Graphic design: Anne-Sylvie Borter, Repro – EPFL Print Center Project follow-up: Yolande Berga 1/14 A PRECEDENT... In March 1999, the team aboard the Breitling Orbiter 3 completed the first non-stop flight around the world in a balloon. This performance was not only the result of the efforts of Bertrand Piccard and Brian Jones’s team, but also of the work of meteorologists Pierre Eckert and Luc Trullemans! Indeed, a balloon without an engine has to be at the right altitude, sometimes down to the last meter, to be carried by the winds that will take it in the right direction. Now, Bertrand Piccard and his team are building on this experience to tackle Solar Impulse’s new challenge. WHAT EXACTLY IS METEOROLOGY? Meteorology is the science that studies the various parameters that influence the weather (wind, precipitation, temperature, etc.) and develops models to predict them. Since the beginning of human history, weather forecasts have had to meet specific needs. Farmers, for example, have to plant and harvest at the right time, and sailors have to avoid open ocean storms. The earliest meteorological records go back more than three millennia and were found in China. A long time elapsed before mankind was able to go beyond forecasts that were only based on the observation of local winds and clouds. Indeed, without a way to observe the weather at the scale of an entire continent, it is impossible to conclude that cloud formations move across large spatial expanses and Edmund Halley (1656 - 1742) is remembered for the astronomical calculations that allowed him to predict the return of the comet named after him. Fascinated by astronomy, he started to travel to discover the southern sky. As a result, he took a liking to the observation of the ocean and its currents. He invented the diving bell, which allows two persons to submerge over 10 meters under water and to stay in it for several hours. The diving bell’s air was renewed by lowering empty barrels that were opened under the bell. It was used, for example, in the context of uncovering works and for repairing dikes. Since he was also a captain, he led numerous scientific expeditions from which he gathered a great deal of observations. That is how he mapped ocean currents and winds. 2/14 WEATHER FORECAST to predict the weather a few days in advance. Only with the appearance of high-speed communication networks (telegraph networks during the mid-19th century) did it become possible to start making serious forecasts. The quality of the predictions improved considerably when satellites were sent to space to collect data over the entire surface of the earth and powerful computers were developed to run complex weather models. Quiz The complexity of atmospheric phenomena and the difficulty of modeling them, as well as the impossibility of knowing the many minor factors that can influence the weather, make it impossible to establish long-term forecasts. Projects like the Breitling 3 Orbiter offered an opportunity to test and improve innovative software, whose use was subsequently generalized and applied in other domains. The software that was developed to guide the balloon now helps track the movement of pollutants that escape into the atmosphere. The latest technologies are being put to use for the Solar Impulse project. It is likely that many unexpected applications will come out of this experience! What does each instrument measure? Pluviometer • • Wind speed Heliometer • • Wind direction Barometer • • Atmospheric pressure Thermometer • Anemometer • Weather vane • Hygrometer • • Precipitation • Humidity • Solar radiation • Temperature THE WATER CYCLE Our planet is surrounded by a protective layer of gas that is essential to life. The temperature and density within the layer vary with the altitude. The outer limit of the atmosphere (the exosphere), which extends to an altitude of at least 10,000 km, is not clearly defined. At different altitudes, the atmosphere is referred to using different names. Altitude, in km Température, in Celsius degrees The lowest layer, the troposphere, is the one that we will study. Most weather events play out 100 Thermosphere in this layer, and it is also where Solar Impulse, which cannot climb beyond 10,000 m, will chart 80 its course. The stratosphere absorbs ultraviolet rays, which is why its temperature increases with increasing altitude. The ozone layer is located in the stratosphere. Weather balloons are normally used to study this environment. Auguste Piccard (1884 1962), Bertrand Piccard’s grandfather, ascended over 16,000 meters in a balloon, establishing the manned flight altitude record of the time. 40 Stratosphere 20 0 WEATHER FORECAST 3/14 Mesosphere 60 Troposphere -80°C -60°C -40°C -20°C 0 20°C In turn, the mesosphere is characterized by a decrease in temperature with increasing altitude. This is where meteors burn up. It is beyond the reach of balloons and too low for satellites, making it the least known layer. The thermosphere contains dioxygen, a molecule that absorbs ultraviolet rays with very short wavelengths. This again leads to an increase in temperature. Above it lies the exosphere, right on the limit to outer space. Particles are very rare in this layer and move freely. Some of them escape into space. An X-15 rocket plane, designed to carry out research on high speed and high altitude air travel, reached a top speed of over 7,200 km/h at a 95,900 m altitude! The sun provides energy to our planet. The energy that reaches the earth’s surface warms the oceans, the vegetation, etc. Water evaporates into the atmosphere. It condenses to form clouds and then returns to the ground as snow, rain or hail. This is the water cycle powered by the energy from the Sun. condensation solar energy water vapor precipitations evapotranspiration evaporation lake infiltrations groundwater flow surface runoff ocean CLOUD FORMATION But how do clouds form? The air contains water vapor. When the temperature of air that is carried to higher altitudes by updrafts decreases, water vapor condenses to form droplets or crystals. When sunlight hits the droplets and ice crystals that make up clouds, it is reflected in all directions. However, only the blue portion of sun’s spectrum is scattered by the air. This is why clouds are white, making them stand out nicely against a blue sky. When a cloud is thick enough, light cannot pass through it, making it look dark and ominous! 4/14 WEATHER FORECAST Do it yourself On a warm day Place a bottle of water in the fridge long enough for it to cool down. Take it out and watch it. Explain what happens. On a cold day Exhale air inside the building and then do the same outside. Explain what happens. IT’S WINDY! The atmosphere forms a protective layer of gas that helps regulate the earth’s temperature and protects the planet from solar radiation. The heat from the sun is not evenly distributed over the surface of the globe. Indeed, while one half of the earth is plunged into darkness, the other half basks in the sun. Regions close to the equator accumulate more heat than the poles because they are more exposed to the sun. Also, while the oceans act as a thermal regulator, the temperatures on the continents vary more significantly. These global temperature differences give rise to large-scale atmospheric circulations. Where air is heated, it expands and rises. This creates a depression called low-pressure zone. In contrast, where air is cooled, it contracts and descends, increasing the pressure. This is referred to as a high-pressure area (anticyclone). To locate these zones that are constantly moving, meteorologists carry out pressure measurements in multiple locations. Then, they map their measurements and connect the places with the same atmospheric pressure to obtain curves (isobars) that sometimes close. The average pressure is 1,015 hPa at sea level. This value defines the boundary between high and low-pressure areas. WEATHER FORECAST 20°O 0° 20° E Quiz In the figure above, use a L to indicate the center of lowpressure areas and an H for high-pressure areas. Indicate the direction of the wind using arrows. 5/14 Differences in atmospheric pressure from one place to another set the air masses into motion, giving rise to high-altitude winds. These winds do not blow in straight lines because of the rotation of the earth. Instead, they spiral. In the northern hemisphere, the air blows clockwise around an anticyclone and counter-clockwise around a low-pressure area. The greater the pressure difference between low and high-pressure areas, the stronger the wind. In the southern hemisphere, the winds blow in the opposite direction. Within a high-pressure area, the pressure of the descending air increases, ensuring fair, clear and dry weather. In high-pressure areas, the winds are light, even at the center. high pressure (anticyclone) low pressure Even though anticyclones move and vary in intensity, there are areas where they are often located. In Europe, the “Azores high” is an anticyclone that is located near the Portuguese Azores Islands during the winter and spring. It is called the “Bermuda high” in North America because it usually settles there during the summer and autumn. This high-pressure system warms Europe, carrying dry air from America and blocking the passage of weather disturbances. However, the Caribbean and Central America sometimes face rainy weather or hurricanes because they receive humid air from the tropics. Can the mission begin? The green light depends in part on weather forecasts. How can weather changes be predicted with enough precision to ensure the success and safety of a flight? This is the responsibility of Luke Trullmans and his team. PORTRAIT LUC TRULLMANS, METEOROLOGIST What is Luke passionate about? Certainly the adrenaline rush during the preparation of a flight, but also the lifelong learning that comes with his profession, as there is always room to improve weather forecasting tools. He is Solar Impulse’s meteorologist and is seconded from the Royal Meteorological Institute of Belgium. He earned a master’s degree in physics at the University of Brussels (ULB), and then carried out an internship in meteorology. A few years later he participated in the creation of a research group focused on pollution in the Royal Meteorological Institute. His goal: to study the dispersion of pollutants in order to warn populations about them. After this, Luke and his team created a program to predict the trajectory of pollution in the troposphere. This same program helped Luc guide Bertrand Piccard and Brian Jones during their circumnavigation of the world in a balloon in 1999 – and today it is at the service of Solar Impulse. 6/14 WEATHER FORECAST LOCAL WINDS So far we have only referred to large-scale air currents. Locally, winds take specific directions following the features of the landscape. Surface winds develop according to the topography, rock walls, and bodies of water. In the mountains, along sunny slopes, the air close the ground warms up as the ground is heated by the sun. This causes it to expand. Because this air is warmer than the air higher up in the mountains, it rises, creating an updraft. This phenomenon is more pronounced when it happens high in a valley, near cliffs that are more exposed to the sun. It then leads to the formation of a wind that flows up the valley (valley breeze) that starts at sunrise. VALLEY BREEZE upward breeze longitudinal section MONTAIN BREEZE cross section downward breeze longitudinal section cross section In the evening, the cooling is first felt at high altitudes. The air contracts and descends, creating a breeze that flows towards the plain (mountain breeze). In narrow valleys, these winds can be quite violent (50 km/h). Paragliders and other engineless “sky wanderers” take advantage of these phenomena to gain altitude. Breezes do not only blow in the mountains. At the seaside, sea and land breezes shape the days (see Exercise 4). We can observe a similar phenomenon in large cities in the form of urban breezes. During the day, cities accumulate heat. Air that is in contact with the city’s concrete surfaces heats up much more easily than air above rural areas with more vegetation. Heat absorbed by the stone and the concrete during the day is released again at night. Also, many buildings are heated during the winter. Together, these effects can lead to temperature differences of up to 10 °C at night in cities like Paris or London. This can give rise to a gentle breeze towards the city that rarely exceeds 5 km/h. WEATHER FORECAST 7/14 IT’S RAINING, IT’S POURING... The confluence of air masses that differ in temperature and humidity creates weather disturbances. Because they differ in density, these air masses do not mix. Instead, they push and raise each other. The boundary between such air masses is called a front. Different configurations can be observed. When a cold air mass arrives in a region where the air is milder, it is referred to as a cold front. Because it is denser, it will push the warmer air away, quickly forcing it upwards. This rise causes the temperature of the warm air mass to drop until it can no longer hold water vapor it contains. The water vapor condenses, forming nimbostratus or cumulonimbus clouds, and the weather becomes rainy or very stormy. Consequently, the arrival of a cold front marks a quick change of weather. After its passage, the weather remains unstable. When a mass of warm air arrives to an area where the air is colder, we speak of a warm front. This less dense warm air will gradually rise above the cold air layer, forcing it upwards. This happens progressively. An observer on the ground will first see the formation of cirrus clouds, then medium altitude clouds, before finally nimbostratus clouds are formed due to the condensation caused by the cooling of the air. Then it rains. This change of weather is slow and may take a day. COLD FRONT warm air raised and then pushed by the cold front Altocumulus Cumulonimbus cumulus stratocumulus rear sky cold air WARM FRONT Cirrostratus warm air Cirrus Altostratus Nimbostratus cold air pushed by the warm air rear sky Sometimes, when fronts are not very pronounced, they are called weak fronts. We might observe a change in the direction of the wind, but no significant change of the weather. Stationary fronts can have the characteristics of cold or warm fronts but only move slightly. In Europe, we often experience stationary fronts with polar air masses. This can cause major regional contrasts with heavy snowfall in one place and warmer and drier weather somewhere else. During the passage of a depression, a warm front precedes a cold front. Because the cold front moves faster than the warm front, it catches up with it and forms an occluded front or an occlusion. In this case, the warm air mass is trapped between two cold air masses. Cirrus Cirrostratus Altocumulus Cumulus Cirrus Cumulonimbus Stratocumulus 8/14 Cumulonimbus Altostratus Nimbostratus Stratocumulus WEATHER FORECAST HOW TO READ A WEATHER MAP There are several indications on a synoptic weather chart. You can read not only the isobars representing the pressure measurements, but also the position of the high-pressure centers (H) and low-pressure centers (L). There are also lines that represent the various types of fronts. Cold front Cold fronts are indicated by a line with triangles and warm fronts with semicircles. Stationary fronts are represented by a line with both types of marks distributed on both sides of the line. Occlusions have alternating semicircles and triangles on the same side, towards the direction of the front. Warm front Occlusion Stationary front direction from which the wind blows indicator of the wind speed The direction and strength of the winds are also represented. The shaft indicates the direction from which the wind is coming. West wind indicator of cloudiness The symbol indicates the wind speed depending on how many feathers it contains. The wind speed is given in knots. 1 knot corresponds to 1.852 km/h. 5 knots 10 knots 15 knots Nord-East wind 20 knots 50 knots 55 knots The small circle indicates whether the sky is clear or not. Here are some of the symbols: Clear sky Mostly clear < 10% cloudy WEATHER FORECAST Partly cloudy 20 to 30% cloudy 9/14 Partly cloudy 50% cloudy Very cloudy 60% cloudy Overcast sky Quiz What can you say about the weather in a place where this symbol is represented? Tuesday 12.11.2013 Here is a synoptic map of Europe on which you can find different weather symbols. source : MétéoSuisse 10/14 WEATHER FORECAST ALL THIS IN NUMBERS… Exercise 1 Here is a world map with isobars. Indicate the areas with high (H) and low pressure (L). Also draw the direction of the wind. Exercise 2 Precipitation is usually measured in mm. This measurement represents the thickness of the water layer that would have accumulated if it had not infiltrated into the ground. The record for the most abundant precipitation in 10 minutes in Switzerland was in Locarno, where 33.6 mm fell on 29 August 2003*. a) How much rainwater would we drain if the gutter of a building measuring 25 m by 8 m and with a height of 10 m were blocked during these 10 minutes? b) The most abundant rain in one day was recorded in Camedo in 1983: 414 l/m2. By how much would Camedo’s record have been broken if Locarno’s 2003 storm had continued with the same intensity all day long? Exercise 3 Students built a rain gauge using a graduated cylinder with a capacity of 500 ml fitted with a funnel with a diameter of 18 centimeters. They want to measure daily precipitation and chart it in mm and l/m2 to compare their measurements to the statistics provided by the weather agency. Determine the calculations needed. Is the device adapted to record rainfall records? * www.meteosuisse.admin.ch/web/fr/climat/climat_en_suisse/en_suisse.html WEATHER FORECAST 11/14 Exercise 4 People who live on coastlines are subject to the tides but also to the breeze. When in the course of a day does the breeze blow towards the sea? Complement your response with a diagram to explain this phenomenon. Thursday 21.06.2012 source : MétéoSuisse Exercise 5 In Switzerland and in neighboring France, the “bise” is a wind that blows over the plateau from the north to the northeast (that is: it blows from that direction). It is a cold, dry wind that usually occurs following weather disturbances and announces the return of good weather. Which of the three synoptic maps represents the aforementioned bise? A Friday 08.12.2006 Monday 25.11.2013 source : MétéoSuisse B source : MétéoSuisse C Exercice 6 The following table contains monthly rainfall records in a number of cities in different continents. The measurements are provided in mm. These are average values collected over a period of thirty years. Create a graph containing all these records. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. TOTAL Geneva 82 76 67 65 71 81 73 79 80 81 86 81 922 Quebec 95 81 85 79 98 110 118 121 117 95 112 104 1,215 Cambera 51 55 53 49 42 37 41 58 61 71 65 52 635 Tombouctou 0 0 0 0 2 45 51 35 6 0 0 0 139 Calcutta 12 34 41 68 139 277 389 343 280 144 41 17 1,785 Sao Paolo 237 219 168 109 76 62 40 51 78 122 143 187 1,492 12/14 WEATHER FORECAST TECHNOLOGY: BUILD YOUR OWN WEATHER STATION! Meteorologists conduct regular surveys of the atmospheric conditions not only to look at the weather objectively and to avoid resting on misguiding memories, but also to predict the weather. Here are three basic types of measurement devices that you can build with basic material. THE WEATHER VANE Materials required • A pole or tool handle • Washers (or beads) • A stick • A compass • Nails • A blade (knife or cutter) • A plastic sheet Procedure Cut a slit in each end of the stick. Cut the point and tail of the arrow out of the plastic sheet and insert them into the slits. You can use a nail to fix them. Determine the center of gravity of the arrow and make a hole in it. Attach the arrow to the top of the pole using a long nail, after sliding washers between the stick and the pole to reduce friction (you can also use beads, if you find beads with large enough holes). Place the weather vane outside. Use the compass to mark the cardinal points on the base or on the floor. THE RAIN GAUGE Materials required • A funnel • A measuring cylinder • Cellar tape Procedure Attach the funnel to the graduated cylinder with the tape. You only need to calculate the surface on which the water collected by the funnel falls and work out a correspondence between the cylinder’s graduation and the number of mm of rainfall. Take a look at exercise 3 if you need help. Make sure that the mouth of the funnel mouth is horizontal. You can make many other devices. You can find many online. Thermometers or barometers are often in science classes. WEATHER FORECAST 13/14 THE ANEMOMETER Materials required • A bamboo pole • Two light sticks • Staples or nails • A long wood screw (considerably longer than two times the thickness of the stick) • Two hollow spheres (e.g. tennis balls) • A small metal tube whose inner diameter is slightly larger than the screw’s diameter (a ballpoint pen body will do if you do not find anything better) Procedure Use the screw to attach the two sticks to each other perpendicularly. They must cross at the center. Cut each ball into two hemispheres. Mark one of them to make it easily distinguishable from the others. Stable a hemisphere to each end of the sticks. They should be positioned so that the fixing screw is exactly at the center of gravity of the cross. They should be on the same side of the sticks as the tip of the screw. Pierce the bamboo pole to just the right depth to support the screw at the bottom of the hole. If done well, the anemometer will be able to rotate freely in the wind. You might have to enlarge the hole in the bamboo to slide in the tube. File the screw so that its tip does not crack the wood while moving and to reduce friction to a minimum. To calibrate the device, you will either need a vehicle with a speedometer or a second calibrated anemometer. Make a table representing the number of spins (over 20 seconds for example) depending on the wind speed. 14/14 WEATHER FORECAST