Do You Know Your Weather? Also, can you read Packed with interesting facts explained well. A fascinating and informative co A fascinating and informative course!
Great course 27 Jun, Learn About Weather 25 Jun, Great cpurse 23 Jun, They come from cumulonimbus clouds, which are convective clouds. Want to keep learning? This content is taken from University of Exeter online course,. This content is taken from University of Exeter online course. See other articles from this course.
This article is from the online course:. News categories. Other top stories on FutureLearn. We explore the current business landscape in India, identity the 5 best startup opportunities and …. Find out about some of the best startup ideas for the Philippines, as well as …. We explore the challenges and opportunities of the healthcare system in India, looking at how …. Register for free to receive relevant updates on courses and news from FutureLearn.
A cap in the sounding can also enhance convection. A temperature inversion can hold heated air near the surface building convective energy which is held to the ground until the cap is broken by daytime heating… if it is sunny enough, cloud cover will spoil the event.
Busting the cap can create explosive and dangerous thunderstorms, rarely seen in the UK. Here is a summary of some other ingredients as it applies to the Spanish Plume like the one on 7 June over the UK:.
Warm air is also being moved into the country from the south by a process called advection. Charts showing the heat energy providing the potential for air to rise are called CAPE: convective available potential energy. Negative figures show the air is buoyant and ready to rise. Values of -8 or -9 are unusually low and show a very unstable airmass with the potential for plenty of lift!
In a Spanish Plume event the upper air contains enough energy and moisture to produce elevated thunderstorms even in the absence of surface heating: this means moderate thunderstorms can occur at night and with extensive cloud cover.
The morning moderate storms experienced in the SE on Saturday 7 June were all elevated thunderstorms because extensive cloud cover throughout the morning meant an absence of surface based heating to kick off more purposeful convective activity.
This is because, in summer, SE winds are often humid with a relatively high water content, advecting moving lots of moisture into the SE: high dew points illustrate this with some reaching 20c on Saturday… a muggy humid day. This moisture will be required, of course, to form clouds. To form really big clouds you need a lot of water in the atmosphere. Once water vapour starts to condense it releases latent heat and this heat gives additional lift to convection and feeds thunderstorm formation.
Interestingly and perhaps counter to what might be expected, a dry plume of air mid-way up through the atmosphere is also an important ingredient to the production of big thunderstorms. Drier air at mid-levels aloft gives the atmosphere added instability for the production of thunderstorms. The reason for this is that dry air cools more rapidly with height than moist air because rising moist air releases latent heat when clouds inevitably form and this additional heat reduces the rate of cooling of saturated air with height.
So moisture is a critical ingredient to storm formation because it controls instability and cloud formation. The skew-t charts show temperature change with altitude. They are called Skew-t because the temperature lines are skewed off the vertical slightly. Whilst they are initially odd to look at, focus on the red and blue and dashed lines: if the red and blue lines are close together it means the air is saturated cloudy.
If they are far apart then the air is dry. If the red line skews to the right then this is known as an inversion where temperatures can increase or stay the same with height. Such an inversion will prevent thermals rising and the formation of clouds from surface convection, a critical ingredient for big thunderstorms.
The other ingredient the skew-t chart shows above is plenty of wind shear with height, in this case speed shear.
Increasing wind speed with height has the effect of dragging air off the ground like a hoover. On Saturday there is a jetstream moving overhead during the day and this will cause divergence aloft and encourage more air to lift off the surface.
Strong directional wind shear, when winds turn at angles through the atmosphere and when different winds directions meet around fronts, is also a trigger for tornadoes. The UK quite regularly has tornadic conditions in lively convective thunderstorm weather but the ingredients for tornadoes are a even more fickle!
Our tornadoes are considerably less powerful than those in the USA but can be of great interest and still cause damage by uprooting trees, damaging rooves and chimneys and even tipping cars. On Saturday morning a CAP will exist in the atmosphere that will prevent air lifting far off the surface, thus preventing extensive vertical lift. The cap is known as a temperature inversion and the cap on Saturday is pretty solid, so little convection is expected early on, except possibly in unstable upper atmospheric layers which might cause elevated thunderstorms.
During the day, if there are suitable breaks in cloud cover, the sun will heat the surface and this will start to break down the cap.
A strong cap has the effect of building energy and heat below and, if the surface heats up sufficiently then the cap can be broken suddenly. This usually happens in the afternoon when a sudden explosive thunderstorm could be produced. On charts the cap is shown as convective inhibition that acts against convection but, at the same time, can be an ingredient for extreme weather. Another mechanism that is capable of lifting air rapidly is the proximity of fronts which can mechanically lift the air as different air masses converge.
Convergence of winds! Converging winds at the surface tend to slow down and pile up, like lorries slowing down uphill and causing congestion. When surface winds pile into each other converge they can only go one way… UP! Convergence often occurs at the coast where winds coming from the sea slow down due to friction over the land. This is one reason why the south coast often produces thunderstorms or enhances them as they cross the Channel so long as the Channel sea surface temperature is warm enough — another story!
The other map below shows areas where there is a sufficient combination of wind shear, heating and energy to possibly start to rotate a thunderstorm: rotating thunderstorms are called supercells and are capable of producing tornadoes.
So the action of winds converging at the surface and diverging aloft or doing the reverse, is important to thunderstorm activity. The charts below website are a cross-section through the atmosphere from S-N and W-E across the UK before and during the plume. The streamlines showing wind vectors illustrate the turbulent nature of the air during the plume.
The other main type of thunderstorm we experience is the result of air mass. In the UK we sometimes get what is known as a Spanish plume: this is a layer of warm air moving north from Spain across France.
A temperature inversion traps this warm moist air, which is continually fed by evaporation from the land below, giving very warm moist air at low levels. Hailstones are often the first form of precipitation from a cumulonimbus, usually followed by rain. At the same time as this plume of warm air works its way north, cold dry air from the Atlantic at upper levels is moving east often in the form of an upper air trough.
Over time the inversion is eroded so that the warm moist air, having lost its cap, will rise to great heights triggering large cumulonimbus clouds and thunderstorms. The atmosphere is so unstable that some of the updrafts in a big storm can reach around 60 knots although downdrafts are generally about half this speed. We now have this vast overturning cloud that will produce severe downdrafts with hailstones and strong wind — these hailstones hurt and can damage varnish and wood.
It is the downdrafts associated with the rain or hail that causes the difficulties when sailing. As the rain and hail descends into warmer air some of it evaporates, which in turn cools the air so we have a cold column of air accelerating to the surface where it spreads out as a gust front.
0コメント