You can see lightning but not hear thunder because the lightning is usually too far away. Light travels much faster than sound, so you see the flash instantly while the sound of thunder travels slower and may dissipate or be refracted away before reaching you. Thunder can often only be heard within about 15 miles (24 kilometers) of the lightning strike; beyond that distance, the sound waves weaken or are reflected by atmospheric conditions or the Earth's curvature, making thunder inaudible even if lightning is visible. This phenomenon is sometimes called "heat lightning," but it is really just distant lightning you cannot hear thunder from because of the distance and atmospheric effects. Additionally, wind conditions and temperature layers can refract or direct the sound of thunder away from the observer, so even closer lightning may produce thunder that is not heard in certain cases. The thunderstorm itself may also be too far for the sound to reach, even though the high clouds and lightning flashes are visible in the distance. Always treat visible lightning seriously, even if thunder is not heard, as the storm could be approaching. In summary:

• Light travels faster than sound, so lightning is seen before thunder is heard.
• Thunder may not be heard because it fades out over distance (beyond ~15 miles).
• Atmospheric refraction, Earth's curvature, and wind can prevent thunder sounds from reaching you.
• "Heat lightning" is really distant lightning whose thunder is inaudible.

This explanation is supported by scientific sources and meteorological studies on thunder and lightning visibility and audibility.