diametlogoWhen water changes state, heat is either absorbed or released.

Melting (ice to liquid water) and evaporating (liquid water to gas) are processes that require heat to be added.

Freezing (liquid water to ice) and condensing (gas to liquid water) are processes that actually release heat back into the atmosphere.

Latent Heat

Latent heat is the term used to describe the quantity of heat either absorbed or released by a substance as it changes state.

When a substance is in the process of changing its state, its temperature actually remains constant.

This can be demonstrated by placing some crushed ice in a beaker with a thermometer and taking temperature readings as the ice melts. The temperature gradually rises from below freezing until it reaches 0oC. The temperature then remains constant until all the ice has melted. The temperature then starts to rise again. The energy supplied during the time when the temperature was constant is called latent heat.

Latent heat is sometimes called the 'hidden' or 'stored' heat used to change the state of a substance without a change in temperature.

The heat energy is actually being used to overcome the forces of attraction, which hold the water particles together in the solid state (ice), so they can become free to roll on top of each other and become the liquid state (water).

The same theory applies when water boils at 100 °C. The temperature does not increase as the water (liquid) starts to evaporate and become water vapour (a gas). The heat energy is being used to separate the water particles and allow the formation of water vapour.

Carry out the 'Melting Ice Experiment'

The Release of Latent Heat

Condensation and Freezing are two processes that do not require the addition of heat energy. In fact the opposite is true. When air is cooled and water vapour condenses to form water liquid, energy is released as water particles 'give up' some of their energy in order to slow down and move closer together again. The same is true when liquid water freezes to become ice. This energy is also called latent heat.

Carry out an experiment to demonstrate the release of latent heat document  The Release of Latent Heat (75 kB)

The Role of Latent Heat in the Atmosphere

When air at the surface of the Earth is warmed, it rises. This is because warm air is 'less dense' than cold air.

When air is warm, its particles have more energy and move further apart, but in cold air the particles remain closer together. If we look at a cold parcel of air and a warm parcel of air of the same size, the warm parcel has fewer particles inside. This makes it 'lighter' than the cold air parcel and it will float upwards.

As the warm air rises, it starts to cool. Air will cool by 1 °C every 100 metres, as long as condensation is not taking place. As the air cools, the particles will slow down. Water vapour particles within the air will become closer together until they start rolling over each other. The water vapour is condensing into liquid water droplets which can form clouds.

When the water vapour particles condense to make clouds, latent heat is also being released into the atmosphere and this reduces the amount of cooling from 1 °C per 100 metres to around 0.5 °C per 100 metres. This release of latent heat also warms the air surrounding the droplet. This warmer air will want to rise, cool and possibly condense once again. This process can increase the height of the cloud until the warm air reaches an altitude where it has cooled to the same temperature as its surroundings.

Diamet storm

Large storms can develop in the atmosphere when a layer of cold air becomes positioned above a layer of warm moist air. These are ideal conditions for warm air to rise, condense and release significant amounts of latent heat.

Test your knowledge of latent heat by completing the  document  The Latent Heat Quiz (65.5 kB)

Latent Heat and the Diamet Project

The DIAMET project aims to improve our understanding of the role latent heat plays in storm development. This knowledge can then be used by the Met Office to make improvements to its computer models for forecasting the weather.

Tracks SatelliteThe DIAMET scientists made detailed measurements from a specially-instrumented research aircraft operated by NERC (The Natural Environment research Council) and the Met Office.

This aircraft, a BAe 146, can fly into the storms, making detailed measurements of the temperature, humidity and wind distribution as well as the cloud particles. Together with ground-based radar and satellite measurements this provides a powerful insight into exactly what is happening inside the storms.

Results should lead to an improvement in the way the model represents detailed processes like, the freezing of cloud droplets, convection (warm air rising) that starts well above the Earth's surface, and the input of heat and moisture from the ocean.