When the weather is hot, your dog might not get as excited about her/his training treats as s/he usually does. Ok, some dogs get excited about the prospect of food no matter what! But not all of our dogs as excited about food in very distracting situations, and sometimes the heat is a huge distraction. The desire to stay in the shade (or air conditioning) and the lack of interest in treats can make training more difficult.
See if you can get your dog more excited about training by providing some cool refreshing treats. Some treats can be kept in the refrigerator so they are cold when given. If you're going somewhere to train, such as to class, put the treats in the freezer for a bit before you leave or put them in a cooler with an ice pack en route so they are still cold when you arrive.
Use small ice cubes as training treats. Make ice cubes from chicken or beef broth. Put treats in an ice cube tray and then fill partially with water and let freeze. Now you have ice cubes with treats stored inside!
Find other cold tasty treats you can share with your dog and s/he just might be more interested in working during these hot summer days.