Soil(or compost) will change temperature more slowly than the air. Soil temperatures outside rarely get above,say 15deg C or fall below ,say 6.(Measured at root depth, would be about 10cm below surface) It may vary at about 2 degs per day except where directly heated by glass.Air temperatures vary much more quickly and have a wider range, could be plus/minus 20degs in a day.I feel your curcubits would appreciate a more stable environment within a structure. Its also a question of light intensity, day length and whether part of their day is in shadow. This year has lacked brightness on too many days, despite being generally benign.Try giving a foliar feed, comfrey tea, or seaweed together with good periods of ventilation.