Over a period of 24 hours temeratures change quite a bit.
In the open air at sea the feel of temperature is dependent on the wind.
A canvas over the main deck is sometimes necessary to provide some cool shade. 
Half way the night you will surely need more than just a t-shirt to stay warm.
During the two weeks of our trip it is obvious that our internal thermostats have been adjusted.
And they will have to be adjusted even more because we go south.
From the way people dress it is clear that our 
individual thermostats are all programmed differently.