UP AND DOWN THE AIR. Solution suggestions to the worksheet: Narrator: Katarína Frečková

Transcription

1 Video: DVD, 13 minutes, 2005 Addressees: pupils from 10 years of age at elementary, secondary and grammar school Keywords: windward and leeward, the High Tatras, air moisture, point of condensation, summer clouds, storm, area of high and low pressure, cold front, adiabatic warming and cooling, cloud formation, Föhn principle, Dobšinská ice cave Chapters: Windward and Leeward, Summer Clouds, Storm, Ice Cave A Film by Rainer Hahn Assistance: Jana Bryjová, Annerose Hahn, Jakub Ľudma, Rastislav Pjontek, Miroslava Suchánová, Katarína Frečková Narrator: Katarína Frečková Content: The film shows consequences of adiabatic warming and cooling of the air: emergence and dissipation of clouds; condensation niveau of the clouds on windward and leeward side of the mountains; the föhn principle and its consequences. It also illustrates the formation of summer clouds and storms, as well as shifting of cold front and formation of an ice cave. In three natural landscapes high mountains, flatland and a cave the film provides an impressive insight into enigmatic powers of the air. Up and down the air that is a constant circulation. Solution suggestions to the worksheet: 1a. emergence of water vapour droplets during cooling 1b. fog, clouds and precipitation 1c. temperature, at which water vapour condenses 1d. the side exposed to/ away from the wind 1e. warm, moist air rises on the windward side, condenses, clouds emerge, on the lee side the clouds dissipate due to adiabatic warming 2a. small, cumulus clouds 2b. the air cools down 2c. e.g. condensation trail does not dissipate 2d. cold and warm air masses collide, the cold front takes over warm front 2e. energy 3a. cold air in winter cools the cave, warm air does not get in 3b. ground glaciers and various forms of ice all year round Praxis Unterrichtsfilm Draisendorf Wiesenttal 4a. snow, rain, hoarfrost, hail, black ice, frost, etc. 4b. cloud formation on windward side, cloud dissipation on the lee side 4c. North of Slovakia, at the Polish border 1

2 Film text to PRAXIS EDUCATIONAL FILM Windward and Leeward Ahead of us: the High Tatras, the smallest high mountains in the world. The High Tatras are a famous holiday and skiing resort, as well as the location of spas for curing pulmonary diseases. The peaks are thickly veiled by the clouds. Such a common view! The airflow approaching the High Tatras blows from the north to the south. This is the Polish side. The town of Zakopane, a noted winter sports resort, is typical of almost constant stormy and rainy weather. That is the Slovak side: the city of Poprad suffused with sunshine. Between the two cities, on an area of about six kilometres (approx. four miles), sprawls the mountain range of High Tatras. Why is the weather on such a small area so unstable? That s what we d like to find out! So, let s go to the Lomnický Peak. On the funicular railway, we go over beautiful, clear, but ice cold brooks to reach the belt of dwarf mountain pine. We are approaching the peak. Thick clouds embrace us. The air is humid and it is getting colder and colder. The peak offers a breathtaking, bright view southwards. Suddenly the curtain of clouds opens. It gives us a chance to observe the formation of clouds. Warm air, rising from the valley, is humid. On its way up, it cools down and the moisture condenses into very small water droplets. The clouds arise. The point of condensation is clearly visible. Mountain peaks are heavily overcast. On the other side, we can observe quite a reverse process. The clouds sink down the peak and dissipate. This is the point of dissipation. The cool air heats up. The High Tatras a cloud machine. Summer clouds In the flatlands the air circulation is similar to that in the mountains. The sun heats the earth and the warm air rises up. The higher the air rises the lower is the temperature. The warm air cools down and the water vapour condenses. Summer clouds arise. Most of the summer clouds are visible between one and two p.m. The darker the bottom of the cloud the higher it reaches. In the afternoon, the air gets colder again and clouds disappear. On the next day, the summer clouds are formed again. However, this time they become larger and more numerous. Eventually, there comes a short, strong summer storm. Soon, everything is calm again and the air is fresh and clear. 2

3 Film text PRAXIS EDUCATIONAL FILM (continuation) Storm How does a storm originate? It is a lovely day. This is an area of a high air pressure with warm air. Cirrus clouds fly high above us. A condensed stripe left by an airplane does not dissipate, because the air moisture strongly increased. We can see the low air pressure area approaching. The next day begins with a nice sunny weather. When a low-pressure area with colder and moister air comes, the colder air will move over the warm air of the high-pressure area. The warm air has no more space to rise and the pressure between the two layers steadily increases. Electric discharges occur. A strong storm develops. These are the consequences of the storm: A new house was hit by a thunderbolt. Its statics was badly damaged. It must be demolished. The storm goes on. Ice Cave This is a unique ice cave in Slovakia the Dobšinská Ice Cave. One of three such caves in Europe. It is August and swarms of tourists visit this natural attraction. The huge hole is fascinating by its astonishing ice formations, stalactites, ice tunnels and ice waterfalls. It is high summer. The appearance of the cave constantly changes with the alternation of the thawing and freezing periods. Old photographs show a huge ice waterfall. It looks quite different today. When the cave was discovered, it was used as an ice rink. On some places, the ice can be up to 26.5 metres (27 yards) thick! How can the cold air and ice remain here even in summer? On the highest spot, the ice cave has got an opening a gap in the rock. Through this aperture, the cold air in winter flows into the cave as if through a funnel. Due to a special location of the cave, only the air, which is colder than the temperature inside, can get in. In winter, the cave becomes so chilled, that the glacier and other ice formations can last even through the whole year. 3

4 Supplementary material to PRAXIS EDUCATIONAL FILM Condensation Saturation of the air with water vapour is a basic condition for condensation and cloud formation. Water vapour is an invisible gas. What we can see during heating radiation from thermal power machines or during exhaling in cold air, are products containing besides the actual water vapour also products of condensation in the form of very small droplets of water. Water vapour or water droplets enter the atmosphere in the form of fog, clouds and precipitation. For condensation, the air must be cooled down to the dew point; it has to be supersaturated with water vapour. Condensation is caused by the following processes: 1. Cooling down through heat radiation: The surface of the earth loses energy, especially during cold nights, by means of heat radiation. When the radiation reaches the dew point temperature, dew arises. When the dew point temperature is reached near the ground, ground fog arises. The radiation area can also be on the obverse side of the clouds. The cloud cools down and the resulting cold air provides for condensation and increase in cloud volume. 2. Cooling down through heat exchange: Due to the approach of warm air (advection), the air above cold water, snow or cold continental regions cools down up to the dew point temperature. Fog builds up. When the cold air streams over warm water areas, such as seas or shores of the subpolar regions, a temperature exchange occurs between the water and fog due to continental winds. Such fog is often called sea fog. 3. Cooling down as a result of blending of warm and cold air: Approximation of warm and cold air masses leads to creation of an interface between air masses. In this way, the so-called front arises. On such a front, cooling of the warm air ensues, with condensation and cloud formation. 4. Cooling due to adiabatic expansion of the air (convection): During the expansion, the temperature drops by one degree per 100 metres. The cooling process up to the dew point temperature triggers condensation and cloud formation. 4

5 Supplementary material to PRAXIS EDUCATIONAL FILM (continuation) Fog Industrial areas and conurbations with heavy emissions of condensation nuclei and water vapour show high frequency of fog. Also, valleys and lowlands are more liable to the occurrence of fog. It can be often observed in coastal and boundary regions on the interface of warm and cold air. Fog is dissolved by heating or exchange of air masses. Rain and snowfall may dissipate fog, similarly as it can be dispersed by trees. Clouds Fog turns into clouds in higher layers of troposphere. They are a suspension of water droplets or ice particles. Clouds can be defined horizontally and vertically and they can move in both directions. Vertical shifts cause the existence of stratus clouds, convective processes result in cumulus clouds. Besides, there are water clouds, ice clouds and mixed clouds. Register of cloud types lists four cloud families with altogether ten cloud types. Moreover, there is a series of subtypes. Clouds cannot be unambiguously recognized from satellite pictures. In the same situation, there is a large scale of existing shapes. Large-scale convergence, convection, thermal effects of the surface of the earth and orographic influence result regionally in the cycle of seasons and, according to the properties of the present air masses, in various cloud conditions and cloud forms. Cloud Formation Saturation of the air with water vapour is the basic condition for cloud formation. Value of air saturation is dependent on temperature. The warmer the air becomes, the more water vapour it can absorb. Cooling of the air by any kind of air moisture results in such a temperature, at which the air is saturated with moisture. This value is called the dew point temperature. When air reaches this point, the moisture is condensed. This leads to cloud formation or precipitation. Thus, the air moisture is dependent on temperature. Clouds arise: A. above warm air areas: the warm air rises, cools down and condenses; B. when warm air approaches cold air areas; C. on the fronts; and D. on geographical elevations or mountain passes 5

6 Supplementary material to PRAXIS EDUCATIONAL FILM (continuation) Föhn principle as a thermodynamic phenomenon is apparent on the wind and lee side of a geographical elevation. Föhn principle is an atmospheric process, taking place in an air mass when overcoming an orographic obstacle. In the föhn principle, the air pressure and temperature decrease with height, while the proportion of pressure and temperature remains constant up to the point of condensation. Cooling of the air results. Such a change of state without an increase or decrease in heat is called adiabatic. The air can only absorb such an amount of moisture, which corresponds to the temperature. During an upward motion of an air volume the temperature increases by 1 per 100 metres. The moisture (the share of water vapour in air volume) increases up to the dew point. The air is 100 % saturated. If the air rises further, water vapour condenses. As the hilltops or mountain passes are located in quite high altitudes, drizzle or rain often occurs during condensation and therefore the descending air on the lee side displays slight air moisture. Now, the decrease in temperature per 100 metres is even more minute. On the other side of the obstacle, the air mass is heating up under rising pressure and the remaining clouds reach the point of dissolution. Now the air can absorb more moisture and thus evaporate condensation droplets. The air becomes warmer. By different altitudes of the windward and leeward sides are the temperatures adequately different. Thus, the mountain functions as a sieve for the moisture and also heats the air. The air comes on the lee side warmer, dryer, with a high saturation deficit and leads to high evaporation in the landing area. Several days of clear weather follow, with unusually far range of vision. The result of adiabatic heating and cooling are the origin and dissolution of clouds. Condensation and dissipation niveau of the clouds is changed according to the volume of moisture, temperature and elevation. 6

7 Supplementary material to PRAXIS EDUCATIONAL FILM (continuation) Precipitation Condensation yields water droplets. In order to release precipitation, bigger water drops have to emerge first. However, it was discovered, that ice crystals arise in the clouds to fulfil certain conditions. During precipitation, the ice-crystals fall through warmer layers and reach the earth as drops. Otherwise, it would not be possible, that ice-crystals in winter arise from water droplets in the bottom layers; thus, these must have emerged first. Arguments supporting this theory are not yet sufficient, though. There are several forms of precipitation, e.g. rain, snow, hoarfrost, hail, black ice, frost, etc. A specific form of precipitation is storm. Basic conditions for a storm are large scale condensation of water vapour and convective changes of state. During the discharge of condensation heat, high temperatures and moist air prefer strong vertical currents. Therefore, the storm clouds are mostly of high towering shapes. A storm normally consists of several layers of clouds, which collide with each other. By means of the following condensation between cold and warm air, or through changes conditioned by temperature and height, great amount of heat and other energy is released into the atmosphere. These are discharged as lightning and thunder. Approximately storms and about 8 million cases of lightning are discharged daily all across the world. Most of the lightning flashes discharge in the upward direction, only a few are headed towards the surface of the earth. A storm of average strength has the power of several hundred megawatts, which equals the capacity of a small nuclear power station. Classification of storms distinguishes three types of storms: 1. Air mass storms are heat storms and emerge due to overheating of the layers close to the ground, when the air is highly moistened. This is typical of tropes and summer storms; 2. front storms occur mostly along cold fronts, when warm air moves over the cold air. Such storms can take a long time and they occur most frequently in Europe/central Europe, where cold continental polar air overtakes the warm air of the Golf stream. This can also happen conversely, when cold air takes over warm air, which cannot rise any higher to cool down. Resulting pressure between the air masses is discharged; 3. Orographic storms occur when advective air glides over mountains, with unstable humidity and swells into great heights. These stormsx are often long lasting. 7

8 WORKSHEET to PRAXIS EDUCATIONAL FILM 2a. Can you describe summer clouds? 2b. Why do they dissipate in the afternoon? 2c. What are the signals forecasting a rainy weather? 2d. What fronts collide during a storm? What happens then? 2e. What is discharged during thunder and lightning? 1a. How would you explain the notion of condensation? 1b. In what forms does water vapour occur in the atmosphere? 1c. What does dew point temperature mean? 1d. Can you explain the terms windward and leeward? 1e. Can you explain the emergence and dissipation of clouds on the example of föhn principle? 3a. Why does the cold air and ice hold in the ice cave? 3b. What are the consequences of strong cooling of the cave in winter? 4a. What forms of precipitation can you name? 4b. Why can we call the High Tatras a cloud machine? 4c. Describe the location of the High Tatras with the help of a map. 8