Types of rain

In Week 1 you explored how rain forms when air cools, the rate of condensation becomes faster than the rate at which water is evaporating and cloud droplets form. If these droplets get big enough they become rain – or snow, sleet or hail.

There are a number of ways in which air can cool to produce rain and depending on the processes involved, this can produce one of three categories of rainfall:

• frontal
• orographic or relief
• convective

Frontal rain

You’ve previously looked at this type of rainfall in some detail. Frontal rain is found where warm air meets cold at the cold and warm fronts in a depression (Figure 1).

Figure 1: Frontal Rain

Convective rain

Figure 2: Fair weather cumulus clouds. © RMetS

Figure 3: Cumulonimbus cloud over Kettering, UK, August 2014. © Dr Sylvia Knight

Figure 4: Cumulonimbus with Tornado, USA. © Dr. Lindsay Bennett)

Orographic or relief rain

Figure 5: Orographic cloud forming upstream of the Matterhorn. © RMetS

Figure 6: A rainfall map showing the rainfall amount (mm) annual average 1971- 2000 © Crown Copyright, Met Office

The precipitation map above (Figure 6), shows the rainfall ‘climate’ (averaged over 30 years) of the UK. With prevailing westerly winds, there is clearly more rain on the western side of the country, enhanced by the mountains of Wales and Scotland and the English Pennines.

As well as being drier on the downwind side of the mountains, it can also be warmer. Remember that water releases heat into the atmosphere as it condenses and takes it up as it evaporates. If there is less water in the air on the downwind side, then there is less to evaporate and not all the heat that was released on the upwind side will be taken up again. This is known as the Föhn Effect.

The onset of a Föhn is generally sudden. For example, the temperature may rise more than 10^°C in five minutes and the wind strength increases from almost calm to gale force just as quickly. Föhn winds occur quite often in the Alps (where the name originated) and in the Rockies (where the name chinook is used). They can also happen in eastern Scotland and over eastern parts of New Zealand’s South Island. In addition, they occur over eastern Sri Lanka during the south-west monsoon.

Where there are steep snow-covered slopes, a Föhn may cause avalanches from the sudden warming and blustery conditions. In Föhn conditions, the relative humidity may fall to less than 30%, causing vegetation and wooden buildings to dry out. This is a long-standing problem in Switzerland, where so many fires have occurred during Föhn conditions that fire-watching is obligatory when a Föhn is blowing.

However, clouds don’t just need water vapour to form – they also need cloud condensation nuclei (CCN) for the water droplets to condense on to. Clouds can’t form in clean air. This is the idea behind ‘cloud seeding’ to control when and where it rains, and some Climate Engineering proposed solutions to climate change.

You can read more about cloud seeding in this PDF on the Metlink website.

If you would like to read more about the different cloud types, you can find more information on Metlink website.