You know the song "In, out, shake it all about, you do the Hokey Cokey, and you turn around..."? Well the dance it describes is my routine for April. When the weather allows, my tender seedlings (mostly tomatoes and chillis) go outside into the garden, and when it's too cold, too wet or too windy (which is often), they come inside again. Even if the daytime weather is quite good I still bring them indoors for the night. All this means that I have to do a lot of going to and fro with trays of seedlings.
I have an in-between arrangement too. If the light is OK, but the wind is too strong, or it's raining heavily, the seedlings go into one of the plastic mini-greenhouses:
It's a lot warmer in there; in fact I have to be careful that the greenhouses don't get too hot and scorch the delicate plants.
Anyway, the whole point of the April Hokey Cokey is to acclimatise the young plants to outdoor conditions, gradually building up their strength and resilience. More than anything else, this takes time. You can't just take an indoor-reared seedling and plant it out straight away. It needs about 10 to 14 days of "hardening-off". When they have been kept indoors, plants are often soft, so they need some toughening-up. In this respect a light breeze is beneficial. In resisting the wind, the plants effectively build up their muscles. (Yes, I know plants don't have muscles...).
One of the signs that I look at to determine whether a tomato plant has been properly hardened-off is the colour of its stem. A well hardened-off plant will have a dark coloured stem, almost purple, whereas one that has yet to acclimatise will generally still be green.
Incidentally, have you noticed that the stems of tomato plants are covered with a multitude of hairs?
The hairs serve two purposes. First, to protect them against insects - both by providing a physical barrier and also by secreting chemicals toxics to many insects. Second, to assist with protection from the weather. The hairs can reflect strong sunlight, but can also trap insulating air, thus helping to maintain a steadier temperature. Clever stuff, eh?