The water cycle is driven by energy from the sun. Liquid water is evaporated and changed into a gas. In this process, energy is absorbed (endothermic). The gaseous vapour rises and circulates in the atmosphere, cools and changes back into a liquid. This process is called condensation and releases energy (exothermic). Tiny droplets of water in the atmosphere accumulate to form clouds, which can return the water to Earth as precipitation, namely rain or snow.

What affects evaporation?

The rate of evaporation depends on four main factors – water body size, heat energyatmospheric pressure and air movement:

  • Water body size is all about how much surface area a body of water has – the greater the surface area, the higher the evaporation rate.
  • Heat energy causes water molecules to vibrate faster – the more the vibration, the higher the temperature. As they vibrate, molecules near the surface of the liquid water can escape, becoming water vapour and rising up into the atmosphere. Heat energy may come directly from the sun heating the water or from ocean currents transporting heated waters.
  • Atmospheric pressure is the effect of air particles pushing down on the water. When we have a ‘low’, less pressure is pushing the water molecules down so it is easier for the molecules to lift up into the atmosphere. A ‘high’ means there is a lot more pressure pushing down on the water and fewer molecules can escape. That means there are higher rates of evaporation in periods of low pressure than in high.
  • Air movement refers to the wind, which moves across the top of the water. Strong winds like hurricanes can physically lift water droplets from the surface, this also increases evaporation because water droplets have a small surface area so will evaporate quickly.

How does precipitation occur?

Condensation is the process where water vapour returns to liquid due to cooling. As heat is lost, the water molecules slow down and condense into droplets. This process is mainly influenced by temperature but also how high the vapour has risen in the atmosphere. Rising vapour cools and condenses into droplets, becoming suspended on dust and accumulating in clouds. Clouds are not big bags that burst when they are full – they are water formations that will remain suspended in the atmosphere until the condensing droplets get too heavy and fall to earth.

What is the difference between weather and climate?

The difference between weather and climate is time. Weather refers to the conditions of the atmosphere over a short period of time (hours, days), whereas climate describes how the atmosphere ‘behaves’ over relatively long periods of time (months, years, decades and longer).

Climate is a reflection of the local, regional and global distribution of the Sun’s energy. The circulation of air masses and ocean waters plays an important role in determining global heat exchange.

What other processes affect the weather?

These processes are also important:

  • Astronomical parameters – Earth’s rotation around its axis, which affects circulation patterns in the oceans and atmosphere.
  • Earth’s eccentricity – the tilt of the Earth’s axis, which determines the intensity of the incoming solar radiation (i.e. the four seasons).
  • The distribution of continents and oceans, which determines the route of the ocean currents and heat exchange.
  • The greenhouse effect.
    Published 2 June 2009