Weekend weather analysis

Libby goes through the Solent charts from the weekend, explaining the lack of moist air...

Analysis for 22nd-23rd March

The high pressure trough dominated the weekend’s weather holding off the frontal systems over the Atlantic and the shallow trough over central Spain. This provided the Solent with clear skies warm temperatures and light synoptic scale winds with an easterly element.

Saturday saw the strongest pressure of the weekend with the forecast light wind of 8 knots from the east to start the morning. The wind strength was then expected to build across the morning reaching towards 14 knots by midday, while the direction was forecast to remain in the east.

The wind strength was seen to build across the morning and into the afternoon gusting at around 18 knots. The wind direction was initially northeasterly in the early hours of the day and by 10am had begun to veer towards the east and remained around 100˚ between 1200 and 1700hours.



The wind strengthened across the afternoon until about 1530 though the forecast had predicted the wind to decrease slightly to 6 knots by 1800. The discrepancy in the synoptic forecast and the actual wind can be solved by looking at the local effects of the Solent and in this case a thermal enhancement was present.

A thermal enhancement is similar to a sea breeze cell but due to the strength and direction of the gradient wind (forecast wind 10 knots from the east) relative to the land the gradient wind was modified to be stronger and from a more onshore in direction, rather than a sea breeze building from the southwest.

The wind strength and direction both show signs of the thermal enhancement. The wind strength was between 2-5 knots stronger than the 14 knots forecast as was expected with a thermal enhancement.



The wind direction demonstrated how the wind can be veered to a more onshore direction by a thermal enhancement as long as heating of the land is occurring. As the air temperature maximum is reached around 1600 and the ground begins to cool the wind strength can be seen start its slow decrease and back to the north.

Sunday saw the effect of the thermal enhancement again, with initial10 knots of pressure from the east southeast set to decrease, the wind strength averaged around 14-16 knots and decreased after 1600hours to be less than 6 knots by 1800hours. The direction of the wind was around 120˚ as expected from the east southeast.

With a thermal enhancement, as with a sea breeze, you would expect to see some nice cumulous clouds popping up over the land and drifting out to sea. This indicates the rising air over the land and the sea breeze cell. However, the air stream that is over the country is a cool dry air having travelled for miles over Russia and Europe. The airstream therefore has not collected any moisture from the sea and with a lack of rain in the last week there is little moisture in the air to form cumulous clouds. In cases like this the presence of a thermal enhancement or sea breeze can be identified from smoke moving offshore high up or if you happen to have a balloon to let go a similar thing will be seen.


Outlook for 29th-30th
Saturday will see the return of the low pressure with wind strength around 15 knots from the west to southwest. Rain is likely on Saturday afternoon as a frontal system pushes across.

Sunday will see a ridge of high pressure extending from the Atlantic bringing in lighter warmer winds from the west southwest in region of 8-10 knots. Sunday will be a day to look out for a thermal enhancement or sea breeze effect seen this weekend with a good chance of cumulous clouds being seen over the land as the air stream will be moist.

Figures from www.chimet.co.uk showing real time wind speed and direction at Hayling Bar (east of Solent).