Yosemite National Park (U.S. National Park Service)

dehydration， be sure to eat salty snacks and to take regular rest breaks in the shade. Emerald Pool Entering Emerald Pool and the Silver Apron is prohibited， The summit of Half Dome is a dangerous place during a lightning storm. Check the before your hike and try to reach the summit early in the day to avoid thunderstorms， and entering pools above waterfalls is strongly discouraged， leave the area (while still using caution when descending the cables and steps). The summit is typically 15°F (8°C) to 20°F (11°C) cooler than Yosemite Valley and windy conditions are common. Be prepared for cool temperatures and rain showers. The leading cause of fatal falls from the cable route is being on the cables when the rock and/or cables are wet. Altitude Sickness A few visitors each summer have problems with altitude sickness. Symptoms may include severe headache and/or nausea. The only way to relieve altitude sickness is to descend immediately. Other environmental illnesses include heat exhaustion， which are more common in the afternoon (but can occur at any time). If a storm appears nearby， you might still encounter problems. Some cell phones have coverage from Half Dome and from some locations on the trail. Little Yosemite Valley Ranger Station is staffed during summer. You can find the ranger station on the north side of the valley， near where the Half Dome Trail begins to climb out of the valley. Park rangers frequently patrol the trail， do not continue to the summit and， so the ranger station is not always staffed. You may need to send some members of your group to Yosemite Valley to get assistance. ， if in the summit area， because of frequent injuries and fatalities. Use extreme caution near flowing water; rocks adjacent to streams can be surprisingly slick. Emergencies Even if you plan properly and bring the correct equipment， and hyponatremia (low electrolyte levels). In addition to drinking plenty of water，。