11/25/2014 1 Earth and Space Science - Weather Systems Water Water • Water is all around us. • Water may be found in many forms ,such as: snow, ice, lakes, rivers, • within the ground as ground water, • or in the air as water vapour. • The majority of water on earth is found within the oceans. • All the water found on earth, whether it be fresh or salt, makes up what is referred to as the hydrosphere. The Water Cycle • Solar radiation strikes earth, the energy will be transferred to land surfaces and into bodies of water. • This transfer and ultimate absorption of energy by water, will cause the surface liquid to evaporate. • Evaporated water from lakes, rivers, streams, the ground, or mostly from oceans, will then move into the atmosphere and take the form of water vapour. • The amount of water vapour in the atmosphere will vary from one location to another and will depend upon temperature and the amount of surface water present. • As temperatures increase, the rate of evaporation increases. Likewise, as temperatures decrease, the evaporation rate decreases. • As the water vapour moves up higher into the atmosphere or into cooler areas over the earth surface, the decrease in air temperature, will cause the water to cool and condense. be discussed later in this activity. • As the water vapour cools, it will condense into water droplets that will then be visible in the form of clouds. • Continued cooling and collecting of the water will eventually form larger droplets that will increase in mass, until they are so heavy that they will then fall back down to earth. • Whether the water droplets return to earth in the form of snow, sleet, rain, hail, or fog, will depend on what the specific local weather conditions are at the time.
Transcript
11/25/2014
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Earth and Space Science - Weather Systems
Water
Water • Water is all around us.
• Water may be found in many forms ,such as: snow, ice, lakes, rivers,
• within the ground as ground water,
• or in the air as water vapour.
• The majority of water on earth is found within the oceans.
• All the water found on earth, whether it be fresh or salt, makes up what is referred to as the hydrosphere.
The Water Cycle • Solar radiation strikes earth, the energy
will be transferred to land surfaces and into bodies of water.
• This transfer and ultimate absorption of energy by water, will cause the surface liquid to evaporate.
• Evaporated water from lakes, rivers, streams, the ground, or mostly from oceans, will then move into the atmosphere and take the form of water vapour.
• The amount of water vapour in the atmosphere will vary from one location to another and will depend upon temperature and the amount of surface water present.
• As temperatures increase, the rate of evaporation increases. Likewise, as temperatures decrease, the evaporation rate decreases.
• As the water vapour moves up higher into the atmosphere or into cooler areas over the earth surface, the decrease in air temperature, will cause the water to cool and condense. be
discussed later in this activity.
• As the water vapour cools, it will condense into water droplets that will then be visible in the form of clouds.
• Continued cooling and collecting of the water will eventually form larger droplets that will increase in mass, until they are so heavy that they will then fall back down to earth.
• Whether the water droplets return to earth in the form of snow, sleet, rain, hail, or fog, will depend on what the specific local weather conditions are at the time.
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• The water that returns back to earth’s surface may be absorbed by plants or into the soil.
• If the water is not absorbed, it will then run off into streams, rivers, lakes and eventually into oceans.
• This cyclical pattern of water evaporating, condensing, and then evaporating again,
WATER VAPOUR • As liquid water evaporates, it turns into a
vapour.
• This change of state may be observed when a kettle of water is being heated. As the water is heated, it evaporates to form water vapour. As the water vapour rises and moves further away from the kettle and heat source, it begins to cool and condense.
• The particles then begin to coalesce (collect and grow in size) as they come in contact with one another. It is this collection of water particles that can be seen as steam above the
kettle or as clouds in the sky.
CLOUD FORMATION • Three things must be present or exist
within the air:
– water vapour
– appropriate air temperature
– particles for droplet formation.
• The warmer the air the more water vapour that can be held.
• The amount of water in the air or humidity will
vary from one location to another and will depend upon temperature and the amount of surface water present.
• In the tropics, the air is very humid and generally saturated or close to the saturation point. This means that the air is holding the maximum amount of water vapour possible at that temperature.
• When air becomes saturated, water droplets are able to form more easily when the dew point is reached.
• Dew point is the temperature at which dew forms (water vapour turns into liquid water). Water droplets will form when air reaches the saturation temperature and the relative humidity is 100%.
• In order for water droplets to form there must be a drop in temperature.
• The final criteria needed in order for clouds to form, is the presence of cloud condensation nuclei (CNNs) or cloud seeds.
• These tiny particles provide a surface where condensation may take place. CNNs are tiny liquid or solid particles that may include: sea salt, dust, pollen, or smoke.
TYPES OF CLOUDS • Clouds come in many different shapes and sizes.
• There are two basic cloud shapes: cumulus and stratus.
• Cumulus clouds take on the giant cotton ball shape with rounded and lobed edges that often
grow upwards. These clouds indicate a change in weather.
• Stratus clouds appear as a stack of flattened pancakes with broad flat layers that spread out across the sky. These clouds are generally present during times of stable weather.
• Clouds are further classified based upon the height in which they originate.
• Mid-height clouds are given the prefix alto and higher altitude clouds the prefix cirrus.
• The following diagram shows the classification of cloud types based on height.
• Ci = Cirrus Cs = Cirrostratus Cc = Cirrocumulus As = Altostratus Ac = Altocumulus
• Ns = Nimbostratus Cb = Cumulonimbus Cu = Cumulus Sc = Stratocumulus St = Stratus
• Clouds are formed when water vapour is cooled and condenses to form water droplets.
• How do we know when these water droplets will fall to earth?
• Under what conditions will these droplets present themselves as some form of precipitation?
• With respect to the formation of precipitation, size does matter!
• In order for water droplets to fall to earth as some form of precipitation, either rain, snow, sleet, hail, or fog, they must be large enough.
• Water droplets must grow in size before they have a mass great enough to allow gravity to cause them to fall to earth.
• The water droplet (or in the case of northern climates like Canada, ice crystal) starts off quite small.
• As the air moves the droplet around (warming and rising, cooling and falling), it will bump into other droplets and begin to collect and grow together.
• This coalescing process causes the droplet to grow in size to a few millimetres.
• Once this size is reached, the droplet is now heavy enough to fall to the ground.
RAIN, SNOW, SLEET, FOG, OR HAIL?
• The form that precipitation takes is determined by the air temperature above the ground surface.
• Early in the fall you have probably seen the first flakes of snow appear.
• The air temperature high above determines what form precipitation will take when it begins to fall and the air temperature at or near earth’s surface, determines what form the precipitation will be when it reaches the ground.
• In the case of the disappearing snow, air high aloft is below the freezing point forming crystals which initially fall as snow.
• As the snow encounters warmer air near ground level, the snow turns into rain or if the flakes are small enough, they just seems to disappear in a mist.
