Yelling at the top of your lungs is only going to be effective if there are people nearby to hear you. When this isn’t the case, save your strength and vocal cords.
Remember that 3 of anything is the international standard for distress. It can be 3 gunshots, 3 fires, 3 flares, 3 blows of a whistle, etc.
In a clearing or on a hilltop, make three signal fires, place them in a triangular pattern (a known signal for help). Space them about 50ft. apart so that the pattern can be distinguished from overhead.
You want the smoke from your signal fire to show up against your background area. In snowy conditions for example you will want dark smoke (black). You can make dark smoke by burning the tires on your car, your vehicles oil, even plastics may work. In a forest you would want white smoke. You can get this by burning dry leaves and or green boughs. For ample smoke, use green and wet wood on top of your burning fire.
Make ground signals. Use brightly colored objects, clothes, tarps, bags, equipment, etc. to write out SOS or HELP. Make the signal as big as possible. If you are using natural materials try and get as much color contrast as you can. Make sure the signal is set in as open an area as possible so that it has the best chance of being seen.
Try reflections. Use your watch, mirror, cell phone, anything that works to shine light off the sun on distant planes or locations. Reflect in bursts of 3.
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