Water and weather
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.
Water and weather
In this video, Dave Campbell and Keith Hunter discuss what weather and climate are, and how the water cycle is key to the weather. Weather describes the condition of the air masses overlying land and sea. Climate refers to a pattern of weather over a period of time (such as a season). Weather is the ‘here and now’, while climate is an accumulation of many days’ or even years’ worth of information about the weather.
Weather and climate are complex processes with many influences: solar radiation, the tilt of the Earth’s axis, landscape (including buildings and cities), seasons, time of day and human activities. Approximately half of the incoming solar radiation is used in evaporating water to the atmosphere. This water then becomes our weather, which can then recharge water resources of the rest of the planet through precipitation
Points of interest: Find out what latent heat is and why it is important. What is the source of heat energy that drives the water cycle?
What affects evaporation?
The rate of evaporation depends on four main factors – water body size, heat energy, atmospheric 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.
Evaporation and transpiration
Dave Campbell explains that evaporation occurs when water changes from a liquid state to a gaseous state. It can happen anywhere there is water – in the soil, lakes, oceans and plants. When it occurs in plants, water is lost through microscopic pores in the plant’s leaves (stomata). This process is called transpiration. Transpiration differs from evaporation not only because it occurs in plants, but also because the plants have some control over how much water they lose. Plants can actively open and close their stomata, limiting how much water the plant will lose.
Points of interest: Think about what affects the rate of transpiration.
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.
Measuring evaporation
In this video, University of Waikato Earth and Ocean Sciences master’s student Tehani Kuske explains how scientists use the eddy covariance instrument.
Precipitation and evaporation are two important processes in the water cycle. It is relatively easy to measure rainfall, but how can scientists measure water evaporating into the atmosphere? To do this, they use the eddy covariance instrument, which measures wind movement and water vapour in the air.
Scientists also measure the soil and air temperature, wind speed and incoming solar radiation.
Point of interest
A wind eddy is a current of air, moving contrary to the direction of the main current, typically in a circular motion.
Learn more about eddy covariance systems in the article Measuring gases using eddy covariance.
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).
New Zealand weather map
This map of New Zealand shows areas of high and low atmospheric pressure. We can use maps such as this to determine what the weather will be like, with highs indicating fine weather and lows meaning rain.
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.
Space and time scales
In this video, Dr Dave Campbell talks about 4 different time and space scales that are commonly used by meteorologists to describe weather phenomena – global scale, synoptic scale, mesoscale and microscale.
The global scale consists of the largest weather elements and patterns that cover tens of thousands of kilometres and affect large parts of the world. The global scale includes the general circulation features, such as the trade winds and jet streams, and is also used to describe weather patterns in regions of the atmosphere such as the tropics, the mid-latitudes, the polar regions and the ozone layer.
The synoptic scale covers weather elements such as high and low pressure systems, air masses and frontal boundaries, features that can be found on standard weather maps. The synoptic scale ranges from tens to thousands of kilometres in breadth, the area of one continent, and may extend from the surface to the lower stratosphere
The mesoscale describes scales from a few kilometres to tens of kilometres and spans between a few minutes to a day. It includes storms as well as sea-land breezes. Weather maps and forecasts are on the mesoscale.
The microscale includes very small atmospheric processes, usually less than a few kilometres in size – this is the scale in which we live.
Related content
Weather – literacy learning links contains a selection of weather-related articles from the Connected and School Journal series, along with supporting Hub resources.
