The old folk tale that a hot summer evening can cause lighting without a thunderstorm is false. What is known as "heat lightning" is normal lightning generated by a thunderstorm that's too far away for thunder to be heard.
At night, lightning flashes can be seen at distances of up to 100 miles, but thunder usually cannot be heard more than 10 miles away. Temperatures can vary greatly within a storm and cause thunder to pass too high in the atmosphere for humans to hear it. The curve of the Earth also contributes to the inability of humans to hear a distant rumble of thunder, which can bounce off Earth's surface before reaching a human ear. Lightning without the sound of thunder can be an early signal that a thunderstorm is coming.
Lightning is caused by ice and water particles rubbing together in the lowest parts of clouds. The sound of thunder occurs when hot and cold air masses expand quickly. As recently as the mid-20th century, hundreds of Americans were killed each year by lightning strikes. As of 2014, fewer than 30 Americans die each year from lightning. The reduction is thought to be attributable to a move by many people to urban areas, which provide better protection from lightning.