Hi, as you are growing with HID Lamps , its quite "Normal to have high temperatures. And if temperature rises the RH falls
The only thing that really works is an humidifier. Its worth to buy one, if you think to have more grows. In the comments i leave you a VPD chart that you can look wich rh you should go in dependence to your temperature und growing stage. Your weed will grow under enarly every condition, but much better if you try to keep the VPD schedule. greetings