Singers rely on proper breathing and lots of practice to stay on key while performing a song. Professional singers understand that singing on key means learning how to read sheet music and sing with instrumental accompaniment. Singing flat is producing a note that is slightly lower than the key called for, and it is frequently caused when people sing without instruments and don't practice learning to hear musical keys and stay in pitch with them. Another common cause for flat singing is improper breathing.
Proper breathing is essential for avoiding flat singing. Singing on key starts with tone. Tone is the sound that comes out of a singer's mouth when air passes through his vibrating vocal cords. If a singer does not have enough air passing through his diaphragm, his vocal cords will not vibrate at the proper pitch which will cause a flat note to come out of his mouth. Proper breathing is controlled breathing and it is a learned art. Singers use breathing techniques such as quick breaths in and sustained long breaths out to control the amount of tension on their vocal cords so that perfectly on key notes come out. Singing on key is even more difficult in imperfect atmospheric conditions, such as humid weather.
Humid weather makes it easier to hit flat notes and harder to stay on key because hot, humid conditions cause dry mouth. A dry mouth and throat puts more tension on vocal cords, making it more difficult to control the vibration, and thus pitch and tone, of the notes created by the vocal cords. Humid air is full and wet, feeling heavier than dry air. It is more difficult to breathe in quickly and deeply, which makes it more difficult to sing on key. Dehydration caused by the heat and slower breathing can cause untrained, and even trained but unprepared, singers to falter on their notes.
The best way to avoid hitting flat notes in humid weather is to stay hydrated. Drink 10 eight-ounce glasses of water or more each day during warm, humid weather. Bring extra water with you to drink before, during and after your performance. Practice singing in humid weather so that you are prepared should you have to give an outdoor performance in it. Get plenty of rest and focus even more on breathing techniques in humid weather than on dry, cool days.