The sky can appear to be flashing without any accompanying thunder due to a phenomenon called "heat lightning." Heat lightning is actually lightning from distant thunderstorms that are too far away for the sound of thunder to reach the observer. Lightning flashes can be visible from very far distances, up to about 100 miles (160 km), but the thunder sound dissipates or gets absorbed by atmospheric conditions before it arrives. Additionally, factors like rain, wind, and terrain can further muffle or block the sound of thunder. Therefore, the flashing sky without thunder is usually just distant lightning from storms far away, where the sound doesn't carry over to where someone is observing it.
Other reasons for seeing flashes without thunder might include electrical discharges from transformers or other non-storm light sources, but the main cause in natural atmospheric conditions is distant lightning with sound that doesn't reach the observer.
In summary, the sky flashes without thunder because the lightning is too far to hear the thunder, and atmospheric conditions can prevent the sound from traveling to the observer. This is commonly known as heat lightning.
