Why was the heliograph especially useful to the British army in India and the U.S. Army in the Southwest?

The heliograph was particularly effective for the British army in India and the U.S. Army in the American Southwest because these regions offered the consistent and regular sunshine required for the device to function reliably.1

The instrument utilized two adjustable mirrors to reflect a beam of sunlight, which was then interrupted by a key-operated shutter to transmit messages using the dots and dashes of Morse Code. Because the heliograph relied entirely on favorable climatic conditions to maintain a steady light beam, it was not widely adopted by armies in continental Europe, where such weather consistency could not be guaranteed.1