First signs of Spring

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  • Diagram to illustrate the process of daytime heating.

    Satellite image and radar image from 21st February to illustrate the formation of cumulus clouds.

    Photo to illustrate cumulus clouds (Paul Knightley, MeteoGroup)

    Photo to illustrate cumulonimbus clouds (Paul Knightley, MeteoGroup)

  • First signs of Spring
    28.02.2015 14:18

     

    Over the last two weeks we have seen the first signs of spring; whether its flowers appearing in your garden, the evenings becoming lighter or feeling too hot in your winter coat, it’s definitely starting to feel more like spring. Officially tomorrow will be the first day of spring, as we move into March, and for this reason the sun has been stronger and we’ve seen crocuses come into full bloom. One way meteorologists know that spring is coming is when day time heating starts leading to the formation of cumulus clouds inland. For us, this is the first sign that the busy winter period is nearly over!

    In spring and summer, the sun is strong enough to lift the air temperature over the land on bright days. The process of the sun warming the Earth’s surface, as well as the surrounding air, leads to warm air rising due to atmospheric convection. Due to the energy used for this uplift and the expansion of air, latent heat is realised causing the air to cool and relative humidity increases. Once the relative humidity reaches 100% small cumulus clouds will begin to develop. Cumulus clouds develop at a height of roughly 1,000m (3,300ft) and can be identified by their flat bases and fluffy, cotton wool like appearance as shown in the photo. 

    Once a cumulus cloud has formed, it has a fairly short lifespan. A cumulus cloud will travel at the speed and in the direction of the winds, and as it does so will continue to collect moisture as more warm air rises as illustrated by the diagram. Eventually, the cloud will get to a point where it cannot take anymore moisture and therefore loses instability and precipitates; this is what we call a shower. These showers from cumulus clouds are often light in nature. However, in some instances, an updraft can start to develop within the cumulus cloud and it will soon turn into a cumulonimbus cloud. 

    Cumulonimbus clouds (Cb) are much larger than cumulus clouds and can often develop an anvil shape at the top. Although the base of the cloud will be at a similar level to the cumulus clouds, the top of a Cb cloud can reach 6,000m (20,000ft). Cb clouds are often associated with thunderstorms, but thunder will not always occur. The showers are often heavy and can fall as hail. There is an increased chance of Cb clouds forming along squall lines, where two winds meet, and in some instances tornadoes can be associated with them. 

    If a cumulus cloud does not pick up enough moisture to precipitate, for example a cumulus cloud forms at the end of the day when the sun is not as strong, then the cumulus cloud will simply fragment leaving clear skies. 

    Over the next month you will start to experience more days where it starts of sunny and then cloud increases as the day progresses; this is because of daytime heating and the increased formation of cumulus clouds.

    By: Sally Webb