Onions use daylight to produce a good deal of top growth before they form bulbs (and the more top growth, the bigger the bulb.) When the day reaches the right number of hours for that variety of onion, the onion will stop forming top growth, and form a bulb instead.
Intermediate-day onions will start bulbing when the day is 12 to 14 hours long.
Intermediate-day onions can be fall planted if the winter temperature goes no lower than 0 F / -17 C.
They are usually sweet onions.