Weather Dynamics

Energy Sources Driving The Water Cycle

The Water CycleCyclic movement of water from Earth’s surface to the

atmosphere (via evaporation and transpiration) and

then back to Earth’s surface (via precipitation)

Parts of the Water Cycle

• Evaporation: Liquid water turning into water vapour (liquid to gas)

• Transpiration: Evaporation of liquid water into

water vapour from plants

• Condensation: Water vapour (gas) turning back into liquid water as it cools.

• Precipitation: Liquid water falling back to the earth’s surface in the form of rain, snow, sleet, or hail.

• Evaporation and condensation occur throughout the world at all times.

• About 23% of all solar radiation (heat energy coming from the sun) causes liquid water to evaporate or transpire into water vapour (the gas form of water)

• Most evaporation comes from oceans, but it also comes from lakes, rivers, etc.

• Nearly everywhere on earth, air contains water vapour (the gas state of water)

• The amount of water vapour varies with local weather conditions

Condensation and Precipitation

• The movement of air into cooler regions (or the lowering of air temperature) causes the water vapour to condense into water droplets, which forms clouds.

• If the air temperature is cold enough, ice crystals form instead of liquid droplets. If the liquid droplets or crystals become large and therefore heavy enough, they fall to the earth (precipitation).

The Formation of Clouds

• A cloud is a large collection of very tiny droplets of water or ice crystals. The droplets are so small and light that they can float in the air.

• When warm air rises, it expands and cools. Cool air can't hold as much water vapour as warm air, so some of the water vapour condenses on tiny pieces of dust and salt that are floating in the air, forming a tiny droplet around each dust particle.

• When billions of these droplets come together they become a visible cloud.

The Formation of Fog

• Fog can be considered a cloud at ground level.

• Like clouds, fog is made up of condensed water droplets which are the result of the air being cooled to the point where it can no longer hold all of the water vapour it contains (referred to as dew point).

• This cooling is often caused by rain cooling and moistening the air near the surface, forming fog.

• A warm moist air mass blowing over a cold surface (usually snow or ice, or over a cold ocean surface) can also cause fog to form

RECAP: Examples of Heat Storage Leading to Weather Phenomena

• Cloud formation in the atmosphere occurs because of condensation from water vapour into water droplets. This can lead to precipitation in the form of rain. When temperatures are cool enough, these water droplets freeze and precipitate as snow, sleet, or hail.

• Fog is formed by warm moist air moving over snow cover

• Fog can also be formed by cold air moving over warm lakes/the sea

The Hydrosphere as a Heat Sink

• The Hydrosphere acts as a heat sink for the earth’s water during the water cycle

• Hydrosphere: The combined mass of water found on, under, and over the surface of the earth (includes oceans, rivers, lakes, clouds, fog, etc)

• Heat sink: Stores excessive heat energy

• The high Specific Heat Capacity of water allows it to store a huge amount of heat energy from the sun. This energy stored in the oceans influences many systems on our earth.

The Atmosphere as a Heat Sink

• Atmosphere: Gaseous envelope surrounding a planet

• The atmosphere also acts like a heat sink, storing energy during the water cycle as water vapour

The Hydrosphere & Atmosphere

• Heat energy is transferred/exchanged between the hydrosphere and atmosphere.

• For example:

– Short-term fluctuations in wind or temperature (that is, weather) can directly influence the currents and temperature of the underlying ocean

– Oceanic fluctuations can magnify, diminish, or modify atmospheric fluctuations.