Thermal radiation tends to spread out in all directions and not just up. When you are standing a few steps back from a large campfire, most of the heat you are receiving is being delivered to you via thermal radiation. Although the hot air of the campfire's flame travels mostly upwards, the thermal radiation has no problem coming out sideways and hitting you. The thermal radiation of a campfire spreads out in all directions, so that you can feel it heating you no matter where you stand (as long as you are close enough). Sunlight heating you up is another example of thermal radiation. The sunlight has no problem traveling out in all directions through space and coming down through earth's atmosphere to hit you.