There are times that extreme temperatures or other natural elements, such as high winds, may effect your lights. The level of the effect is greatly predicated upon the type of lights you have chosen to purchase. If you live in an area where weather can be extreme or vary drastically throughout the year, you should consider buying a set that will withstand extreme weather conditions. This should be notated on the manufacturer's packaging.
As a side note, your solar string lights will continue to charge on days where it is cloudy or not much sun. However, it will take much longer (some times the entire day) to get your lights to a full charge.