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Water DangersEducator's NotesOf Tides and Time1Tides have long intrigued us. Perhaps it's the fact that they represent a predictable and wholly unstoppable force. Tides are rhythmic, predictable and periodic changes in the height of a body of water, caused by a combination of the gravitational pulls of the moon and sun, and the motion of the earth. The contribution to tidal height of the moon (lunar tide) is about twice that of the sun (solar tide). Even though the sun is 27 million times more massive than the moon, the moon is about 400 times closer to the Earth, and exerts a much stronger gravitational pull. Throughout the month, tides vary in their heights. The highest highs and lowest lows occur together during the new and full moons, when both moon and sun are pulling in the same direction. In this case, the pulls of the two bodies are added together. These extreme tides are called spring tides, which comes from the Old English word springen, meaning "to jump or move quickly". Spring tides occur when the Earth, moon and sun form a right angle. Tidal patterns (how often highs and lows occur within 24 hours) and ranges (the difference between high tide and low tide water levels) differ throughout the world. Some areas, such as much of the east and west coasts of North America, usually have two high and two low tides per 24 hours. These are semi-diurnal tides. On the other hand, the Gulf of Mexico tends to have one high and one low tide (diurnal tides) during the same period. Tidal ranges vary dramatically, depending on the shape of the water basin the tides flow through. The narrow Bay of Fundy has tides of about 50 feet. Remember, this does not mean that the water goes inshore for 50 feet. It means that the water level rises in height by that amount – 50 feet! If the land is pretty flat, the sea might flow inshore for miles before reaching the necessary elevation. Tidal ranges for most of the ocean are much smaller. On the west and east coasts of North America, tidal ranges tend to be around six to eight feet. In the Gulf of Mexico, the tides are even narrower, often only a foot or two. Tides are a major (perhaps the major) controlling force in many marine intertidal habitats, because they help dictate how long organisms are under water. In areas with wide tidal ranges, organisms must have adaptations that allow them to survive in the air. These include facing such hazards as drying out, wide temperature fluctuations, influxes of fresh water (from rain) and attacks by various terrestrial predators. Wind and Waves1Just what are waves? While waves are caused by various forces, most of the waves we see are caused by the wind. In the ocean, wind waves are generated by air molecules from the wind blowing along the sea surface and transferring energy to adjacent water molecules. As the water molecules begin to move, they start to travel in vertical circles, producing tiny wavelets. These tiny waves expose more water surface to the wind and more wind energy is transferred to the water, creating larger and larger waves. When winds slow or cease, waves continue on, though they become more rounded; these are swells. Waves come in various parts. The crest is the highest part of the wave (above the still-water level) and the trough is the lowest part. A wave's height is the distance between the crest and trough, and its length is the horizontal distance between each crest. In the open ocean, wave length averages 200 to 500 feet, but may reach 2 000 feet in extreme cases. A wave's period is the time it takes for two successive waves to pass by a particular point; wave frequency measures how many waves pass that point in a given time period. Wind period varies from a few seconds to as much as 20 seconds. How high do waves grow? Really high. The highest waves ever officially recorded were measured by the Executive Officer of the US Navy tanker Ramapo on February 7, 1933, in the North Pacific Ocean. The tanker was heading from the Phillippines to San Diego and for days a steady 66 mph wind had blown, with gusts to 80 mph. At about 3:00 am, with a bright moon illuminating the seas, the personnel on watch noted a particularly large set of waves bearing down on them. When the ship settled into the trough of one, the Executive Officer noted that the crow's nest of the main mast was level with the crest of the next wave. He calculated that the wave had to be 112 feet high. How Does the Ocean Move?1Water is in constant motion in the ocean and much of that motion occurs within currents. The term current usually refers to water flowing horizontally (parallel to the ocean's surface), but masses of water can move vertically, as well. Currents can be rapid and almost river-like (such as the Gulf Stream) or they can be slow and diffuse. What causes water to move? Ultimately, the sun does the job. Warm water expands and cold water contracts. Ocean water is warmer at the equator (the sun shines on it more) than at the poles. Equatorial water is actually about three inches thicker than polar water, because it is warmer and has expanded slightly. This global difference creates a very slight slope and warm equatorial water flows "downhill" (poleward) in response to gravity. However, this movement is only the beginning. Surface water also is propelled by winds. Winds move water through friction between moving air molecules and water molecules. As the surface water molecules begin to move, they pull with them some of the molecules below, which triggers the current. Water also moves vertically, as was earlier stated. Winds can drive surface water away from the coast and deep water can move upward (upwelling). However, water also moves downward. Ocean water sinks when it is saltier or colder than surrounding water. A prime example of this takes place in Antarctica, where Antarctic Bottom Water is formed. This is the most dense water in the ocean, and it is created in the water when sea ice forms. This . This only takes up about 15% of the ocean's salt, and the rest forms an extremely cold brine. This sinks to the bottom and spreads northward from Antarctica. In the Pacific Ocean, this water actually may reach the Aleutian Islands, a trip that takes about 1 600 years. Ocean currents have a profound effect on the weather. Mark Twain exemplifies this fact in his apocryphal remark, "the coldest winter I ever spent was a summer in San Francisco". Summer months there are cool, windy and foggy. On the other hand, Washington, D.C. – at about the same latitude but on the Atlantic Ocean – has hot and muggy summers. The reason for this difference is that San Francisco sits on the edge of the cold California Current. Winds approaching the California coast lose heat to this cold water and chill San Francisco. Washington, D.C. is hit by winds that have flowed over the warm Gulf Stream, picking up heat and moisture as they pass over that current.
ActivitiesChanging TidesGrades 5, 6 – Science PurposeThis demonstration shows the gravitational force of the moon and sun can displace water toward these sources of gravity. Materials
ProcedureFill the balloon with water, let out all excess air and tie the top with a know. Tape the paper circle to the centre of the balloon. Notice as you hold one hand at the bottom of the balloon and the other at the knot that the water is evenly distributed around the paper circle (form the balloon into the shape of a ball). Remove the hand supporting the bottom of the balloon and notice how elongated the balloon becomes. The gravity of earth pulls the balloon and water down. Likewise, the moon's and sun's gravitational pulls cause the ocean tides to rise and fall. A Current AffairGrades 4, 5, 6 – Science PurposeStudents will create their own water currents, using differences in water salinity and water temperature. Deep currents are generated by differences in salinity and temperature between two bodies of water. Salinity Currents - salt water is more dense than fresh water and sinks below it2. In the first experiment, the blue salt water on top will soon replace the clear fresh water below. Similarly, in the second half of the exercise, the clear tap water above will remain on top of the blue salt water. Temperature Currents - like salt water, cold water is more dense than hot water2. When place on top it will sink down and displace the hotter water. Hot water will sit on top of cold water. However as the temperatures equalize, they will begin to mix. Materials
ProcedureStudents will fill one bottle with clear tap water and the other with tap water salt and food colouring. They will predict what will happen to the coloured water before doing the experiment, then will observe and record the direction of the actual water flow. Students should record their predictions before beginning the experiment. Salinity Currents
Temperature Currents
Tornado in a BottleGrades 4, 5, 6 – Science PurposeIn this experiment, water forms a spiralling, funnel-shaped vortex as it drains from a 2 litre pop bottle. A simple connector device allows the water to drain into a second bottle. The whole assembly can then be inverted and the process repeated. This experiment works well in activity centres. Vortices occur in nature in many forms: tornadoes, whirlpools, weather systems, galaxies, etc. The essence of a vortex is that objects are drawn together toward the centre, then miss! Spiral waves form in the water surface of the vortex. These waves appear to move in slow motion as they travel upward through the downward flowing water. Violent weather is an unwelcome and often unexpected fact of life in most regions of the world. Coastal regions experience periodic rain accompanied by high winds (in excess of 117 km/h) that are called "hurricanes". Hurricanes are produced in tropical regions, and, when fully developed, are the most destructive of all storms. Meteorologists give human names to hurricanes in order to keep track of concurrent storms. The most violent, and perhaps the most spectacular of all storms is the tornado. The tornado is similar to the hurricane, but is much smaller – usually only a few hundred metres across. In a tornado, the air moves around the central core very fast – sometimes at speeds of 600 km/h. These fast winds can uproot trees, demolish houses, and even fling automobiles hundreds of metres. Materials
ProcedureFill one of the 2 litre bottles about two-thirds full of water. For effect, you can add a little food colouring and/or the bits of plastic confetti. Screw the bottles onto both ends of the plastic connector, making sure that you do not screw the bottles together too tightly. Place the two bottles on a table with the filled bottle on top. Watch the water slowly drip down into the lower bottle as air simultaneously bubbles up into the top bottle. The flow of water may come to a complete stop. With the filled bottle on top, rapidly rotate the bottles in a circle a few times. Place the assembly on a table. Observe the formation of a funnel-shaped vortex as the bottle drains. Ask the students to notice the shape of the vortex. Also, ask the students to pay attention tot he flow of the water as it empties into the lower bottle. Background NotesWhen the water is not rotating, surface tension creates a skin-like layer on top of the water and across the small hole in the centre of the connector (see the section "Water Science" for further discussion of surface tension). If the top bottle is almost full, the water can push out a bulge in this surface to form a bulbous drop, which then drips into the lower bottle. As water drops into the lower bottle, the pressure in the lower bottle builds until air bubbles are forced into the upper bottle. The pressure that the water exerts on the surface in the connector decreases as the water level in the upper bottle drops. When the water level and pressure drop low enough, the water surface can hold back the water and stop the flow completely. If you spin the bottles around a few times, the water in the upper bottle starts rotating. As the water drains into the lower bottle, a vortex forms. The water is pulled down and forced toward the drain-hole in the centre by gravity. If we ignore the small friction forces, the angular momentum of the water stays the same as it moves inward. This means that the speed of the water around the centre increases as it approaches the centre of the bottle. This is the same principle at work when ice skaters increase the speed of their spins by pulling in their arms. To make the water move in a circle, forces called centripetal forces must act on the water. These "centre pulling" forces are provided by a combination of air pressure, water pressure, and gravity. You can tell where the centripetal forces are greater by looking at the slope of the water. Where the water is steeper, such as at the bottom of the vortex, the centripetal force on the water is greater. Water moving with higher speeds and in smaller radius curves requires larger forces. The water at the bottom of the vortex is doing just this, and so the wall of the vortex is steepest at the bottom. Think about race cars: race tracks have steeper banks on high-speed, sharp corners to hold the cars in their circular paths around the track. The hole in the vortex allows air from the lower bottle to flow easily into the upper bottle. This enables the upper bottle to drain smoothly and completely. Some Snowy FactsGrades 4, 5, 6 – Science PurposeShades of White Snow Temperatures Snow Density Some areas of the world stay very warm all year round and do not get any snow. In other areas of the world, winter is a time of year when temperatures dip below freezing and snow is common. Some areas of the world have opposite seasons to our own. As a Technology/Language Arts activity, students could communicate with students in other areas of the world via Internet school links. Snow Melt Technically speaking, the term "snowflake" is defined as a cluster of snow crystals that have stuck together as they fall to the ground. The world's biggest snowflake was 38 cm in diameter. A large snowflake can fall at a speed of 5 km/h. Different kinds of snow crystals result from certain combinations of conditions, particularly temperature and moisture level, in the clouds and near the Earth's surface. There is an international system for grouping snow crystals into ten general categories. The system is based on the structure of the crystals. The international system applies to falling snow. Snow crystals change when they reach the ground and lose their original identity. As a snow crystal melts, its parts blend into a spherical shape and it ends up as a drop of water. Have you ever noticed that snow sometimes squeaks when you walk on it? When the temperature is well below freezing and you walk on snow, the ice crystals rub and grind against one another. This produces the squeaking noise. If the temperature is just a little below freezing, the pressure of your foot melts some of the snow. This creates a thin layer of water underneath your foot, which lubricates the snow crystals so that they don't make any noise. Weather A major factor in climate is "latitude", the distance of a place from the equator. More of the sun's energy reaches the area close to the equator and so it is hottest there. Another important factor in climate is ocean currents. Newfoundland is at about the same latitude as Britain, but has a cooler climate, because of the cold Labrador Current. Yet another factor in climate is height above sea level. "Rain before seven, Fine after eleven"... this saying indicates a truth about rain at any time, not just before seven. Belts of rain brought by fronts tend to last less than six hours. Notes:
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