You've probably heard the term "streamer" used quite a lot in Peter's forecasts recently and wondered what exactly it means. Streamers are lines of showers that form over a body of water and align themselves with the wind-flow, bringing a constant "stream" of showers over a certain area. They are also known as Sea or Lake Effect showers, as they are a common wintertime occurrence near the Great Lakes region of the USA. In Ireland they generally form over the Irish Sea and affect north and east-facing coastal regions of Leinster and eastern Ulster.
Streamers are common when we get a cold and dry easterly wind blowing off continental Europe. This airmass may originate over the snowy steppes of northwestern Russia or Scandinavia and retains its cold as it flows westward towards northwestern Europe. As it flows out over the relatively warm waters of the North Sea or Irish Sea it picks up warmth and moisture, which leads to instability in the lowest layers of the atmosphere (the Boundary Layer). The "warm" (I use the term in a relative sense) and moist air cools as it rises and will eventually condense to form clouds, rather like steam rising out of a boiling kettle.
The satellite image below shows this process nicely, with clear conditions initially to the west of Denmark, but after a certain distance, the airmass has picked up enough warmth and moisture along its track for clouds to start to form. As it continues further westwards these clouds can be seen to grow in size, leading to showers along the eastern coast of the UK (see the corresponding radar image underneath).
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| Visible satellite image from 11:45UTC, Sunday 24th March 2013, showing the formation of streamers in the easterly airflow to the west of Denmark, growing to bring showers to eastern Scotland. Also visible is the snow cover of eastern Europe. Image courtesy of Sat24.com |
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| Radar image showing the streamers along eastern Scotland and northeastern England. Image courtesy of meteoradar.co.uk |
Depending on several factors, such as the wind, temperature and moisture profiles of the bottom few kilometres of the atmosphere, these clouds can grow to form towering shower (Cumulonimbus) clouds, bringing heavy rain, sleet, hail or snow. The fact that these showers (or streamers) align with the wind direction can mean that one area gets a constant barrage of precipitation whereas another area may miss out altogether. This is what happened in December 2010, when Dublin Airport was unlucky enough to be in the wrong (or right) position to receive these streams of snow showers, leading to several days of travel chaos.
Delving a little deeper into the detail, some factors that determine if and to what extent these showers form are:
- The temperature difference between the surface and the 850 hPa level (around 1,400 m above sea level). For efficient streamer formation this difference needs to be at least 13 °C, so taking the temperature of the North Sea to be around +5 °C, we would need -8 °C at 850 hPa. Actual data shows it a little colder, at around -11 °C, so hence this condition is in place.
- Wind speed and direction. Wind should be strong enough to direct the cold airmass over the sea and its direction should not vary by more than around 60 ° throughout the lowest 3 km of the atmosphere.
- Sea fetch. This is the distance of sea surface available for modification of the airmass. Obviously, the shorter the sea fetch, the less time the airmass will have to pick up enough warmth and moisture. Longer sea fetches, such as the North Sea, allow beefy showers to develop along the east coast of the UK, but shorter fetches, such as the Irish Sea, can sometimes mean that Ireland escapes relatively unscathed. The stronger the wind, the longer the sea fetch required.
- Temperature inversions. A temperature inversion (layer of warm air overlying colder air below) can prevent the warm currents of air from rising high enough to form decent precipitation, instead leading to a shallow layer of stratus or stratocumulus and possibly just some light drizzle or snow grains (if the airmass is cold enough).
The GFS forecast temperature and moisture profile (sounding) for just off the Dublin coast today shows all of the above factors. We can see that the 850 hPa temperature is around -7 °C, so with a sea temperature of around +8 °C this should be cold enough. The wind arrows along the right hand side show a constant easterly wind of around 20 knots throughout the lowest 5 km of the atmosphere, so this too should be in favour. The sea fetch may not be long enough between the east coast and Wales, but should be long enough between the east coast and the Blackpool area.
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| GFS forecast sounding for off the Dublin coast, 12Z Sunday 24th March, 2013. Image courtesy of Meteociel.fr |
