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How does a Funnel Cloud form. Convection on 9th May 2020

Funnel Cloud Observed in Tipperary

One of the funnel cloud photos sent to us by Martin O’Brien

One of our Twitter followers, Martin O’Brien, tagged us in a series of 4 photos which clearly show a well-formed funnel cloud. He took these while passing through the scenic Blueway region in Carrick-on-Suir, Tipperary on Saturday afternoon 9th May, during this period of convection described below by Alan Hally.

Paul Downes, Forecast Meteorologist, Central Forecasts and Applications Office, Met Éireann, provided the following explanation.

This funnel, while not reaching the ground is a great example of the conservation of angular momentum. There are three main ingredients needed to get this.

  1. Moisture. While you might not see it until it transforms into a cloud, moisture is all around us. The more humid the air the more moisture that is being suspended in the air. Saturday was a warm humid day.
  2. Instability. Like a hot air balloon, the air inside a cloud has to be warmer than the air outside it, so when the sun heats the earth, ‘bubbles’ of warm air rises.

The air in the cools as you go up in the atmosphere, e.g. when you see ice crystals grow on the outside of an air plane window- its very cold out there! But the rate at which it cools is always changing. The air in the ‘bubble’ will cool too at a constant rate as it rises, the key here is if it cools slower than then atmosphere, we call this instability. The atmosphere is unstable and the bubble can continue to rise.

At a point called the Tropopause, approximately 10km above our heads, the atmospheric temperature begins to rise again and the air becomes stable once more. So If you have (1) moisture then as the invisible bubble rises and cools it will eventually condense into a cloud. If it is still unstable the cloud will continue to rise into a cumulonimbus  (cumulus- a towering cloud, nimbus – with rain).

So that is what starts the process. The final ingredient is:

  1. Shear. This can have several forms but for now we will just look at directional shear. If winds change direction along a front, such as a sea or lake, breeze shear can exist along the front, this can cause the air to turn. Alone it will do very little and is quite common. However if you place it under an updraft (a cloud going upwards)  it acts like an ice skater in a spin. When an ice skater spins and their arms are out they rotate slowly, if they want to spin faster they pull their arms in – this is called the conservation of momentum and it works in nature just the same. If a large eddy of rotation exists along a boundary and a shower accelerated the air upward, it will tighten the rotation below it. This causes pressure to decrease and to condense, thus forming the funnel cloud we see in the photo above. In extreme cases it can produce powerful tornadoes. However more of every ingredient would be needed- which is something we rarely ever see here- as well as a lot of help from the jet stream which didn’t really play a major role on Saturday.

 

Convection in Ireland: Thunderstorms on Saturday the 9th of May 2020
by Alan Hally, Meteorologist in the Research, Environment and Applications Division, Met Éireann

On the 9th of May, Ireland was under the influence of a high-pressure system with temperatures in the high-teens and low to mid-twenties countrywide. Dynamically, i.e. in the highest levels of the atmosphere, there was very weak forcing due to the presence of the high-pressure. Given the location of the initial thunderstorms in the south-west, orographic forcing (i.e. upwards motion of air due to mountains) was likely a very important contributory factor to these thunderstorms. The atmospheric conditions followed a conceptual model known as “convection in fair weather conditions”. Using a combination of satellite photos and vertical cross-sections of the atmosphere, we can get a very clear picture of what was occurring on Saturday the 9th of May.

In order for a surface based thunderstorm to occur, the dew-point temperature must be 12C or higher. Figure 1 below shows a satellite photo of Ireland at 1pm on Saturday the 9th of May in the infra-red along with an overlay of the 12C dew point isotherm (in purple) from an analysis provided by the European Centre for Medium Range Weather Forecasts (ECMWF) weather model. This clearly illustrates that Munster was perfectly primed for the development of strong thunderstorms.

Figures 2 (a), (b) and (c) are what we call vertical cross-sections. These figures demonstrate respectively; rising motion (known as omega), convergence/divergence (very important for illustrating areas where thunderstorms can occur) and relative humidity. These images detail the conditions of the atmosphere from the surface up to a height of approximately 12km. Image 2 (a) displays a large area of red from approximately 1000hPa to 300hPa. This represents the very strong updrafts, or upward motions of air in the atmosphere. Images 2 (b) and (c) compliment this perfectly as they show both convergence of air at the surface (in red on 2(b)) and high values of relative humidity in the upper atmosphere (green lines on image 2 (c)). The combination of all these factors; the mountains of the south-west, the high dew-point values, the strong rising motion and the moist air in the upper atmosphere led to very heavy downpours throughout the afternoon of Saturday the 9th of May. A satellite animation of the development of the storms can be seen below in what is known as the “Day Natural Colours RGB”. The presence of tall convection producing cumulonimbus clouds can clearly be seen by the blue/cyan colours in Munster and areas over the south-east.

 

Figure 1: Ireland at 1pm on Sat. 9th of May in infra-red, 12C dew-point isotherm in purple

Figure 1: Ireland at 1pm on Sat. 9th of May in infra-red, 12C dew-point isotherm in purple

Figure 2 (a): Rising motion in red and equivalent potential temperature values in solid black lines. The equivalent potential temperature values decrease from the ground to around 500hPa, a further indication of the likelihood of convection

Figure 2 (a): Rising motion in red and equivalent potential temperature values in solid black lines. The equivalent potential temperature values decrease from the ground to around 500hPa, a further indication of the likelihood of convection

 

Figure 2 (b): Convergence of air at the surface (in red) and divergence of air higher up in the atmospshere (in blue)

Figure 2 (b): Convergence of air at the surface (in red) and divergence of air higher up in the atmospshere (in blue)

Figure 2 (c): Relative humidity through a vertical layer of the atmosphere from the surface up to 200hPa. Green lines indicate values above 80% relative humidity.

Figure 2 (c): Relative humidity through a vertical layer of the atmosphere from the surface up to 200hPa. Green lines indicate values above 80% relative humidity.

 

Natural colours satellite image loop that shows the thunderstorms bubbling up from 06UTC to 18UTC in steps of 3-hours

Day Natural Colours RGB. The satellite image loop that shows the thunderstorms bubbling up from 06UTC to 18UTC in steps of 3-hours

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