"during wintertime" is correct and usable in written English.
You can use it to refer to the season of winter, usually in the context of a specific activity or event that occurs during that season. For example: "We always go skiing during wintertime."
People come down for holiday weekends, which can be very busy during wintertime as well".
A non-aligned Afghan cleric tried to get things off on the right foot by reading Koranic passages about peace: a sly attempt to silence any rabble-rousers.The talks took a pause during wintertime.
Rough weather condition with a risk of ice is a significant problem during wintertime.
The incidence of pulmonary complaints, severe tracheitis and lung function deterioration is increased during wintertime in laryngectomized individuals.
A persistent haze layer that spread over most of the northern Indian Ocean during wintertime was discovered.
Aragonite undersaturation will be reached during wintertime in parts (10%) of the Arctic when atmospheric CO2 attains 410 ppm (Steinacher et al. [2009]).
Moreover, a single peak is exhibited at certain stations during wintertime, particularly at high latitudes (Israelsson and Tammet 2001; Harrison 2004b), showing high correlations with the Carnegie curve.
The outdoor concentrations of the pollutants measured in the four cities reflect the seasonal trend, being higher everywhere during wintertime due to low atmospheric layers mixing.
In contrast, southern Atacama records (23 28°S) display only three pluvial periods which result from SE summer monsoon precipitation and outbreaks from the Westerlies during wintertime.
It doesn't need to be during wintertime: summer holidays, beach trips and short breaks can all fall foul of a post-work health slump too.
This paper examines determinants of indoor air temperatures during wintertime conditions to examine how temperature conditions vary with, for example, dwelling age or household socio-economic conditions.
