If you think it is starting to get hot, realise your cows are already very hot (and probably have been for a month). Where we prefer temperatures over 15, cows prefer temperatures under 15 degrees Celsius so their threshold of what is a comfortable temperature is a lot lower than ours. Humidity plays a big role as well as cows offload their heat mainly by evaporation, by breathing out the moisture. This is why one of the first signs of heat stress is rapid breathing. Another sign of heat stress that you can see a lot at the moment is cows mobbing, standing close together in a big group. 
 
In early summer cows can still offload some of that heat overnight but once the night temperatures are over 15 degrees for most of the hours of the night that opportunity is lost too. 
You can reduce the heat load by shade, fans and WATER. 
 
Remember that cow cools mainly by evaporation of water, which is why being able to drink as much water as needed is a crucial part of the cows cooling down process. Providing cows with ad lib water means enough volume, enough pressure but more importantly enough water access. Dominant cows can control troughs so make sure to have 2 troughs for mobs bigger than 80 cows. Ideally the races should have troughs too every 150-250 meters to make sure all cows get the opportunity to have a decent drink. Feed pads should have water troughs and also make sure you have a water trough within 50 meters of the dairy exit. 
 
Milk is 80-90% water too so making sure cows have access to enough water at all times will easily pay back for itself in the vat.