Most Plains and desert dwellers have experienced heat lightning. They dismiss it as a nonthreatening phenomenon, caused by excessive heat. After all, when they see it, the skies are usually clear overhead. And, they don’t hear thunder. Even so, they’re mistaken. Part of what’s led to their belief is where heat lightning usually occurs: out where land is flat enough that you can see for miles and miles. In fact, you can see a storm when it’s so far away that you can’t hear thunder - plus, you can’t recognize the storm for what it is, especially when it’s nighttime or the clouds are close to the horizon. Since lightning is such a strong bolt of light, however, you can see it - particularly if it occurs within clouds causing a big reflection. It is important to remember that lightning CAN strike miles from the storm.
Mary Knapp, Weather Data Library