This relates to the ambient temperature to how fast a spa cools down. We suggest you leave the cover off to cool it down faster.
Alternatively, consider a Spa Heat Pump, as they work like a reverse cycle air conditioner. The spa heat pump takes the heat from the ambient air and transfers it into your spa through a heat exchange.
However a spa heat pump can also reverse the cycle and cools your spa in the summer months by taking heat from your spa and putting it out into the ambient air.