Weather in the Sierra Nevada can be deceptively pleasant most of the time, but it can change quickly.

Snow in the spring and summer is not uncommon. A foot of snow in August can happen. Many a hunter in the fall has been snowed on. Lightning is a potential hazard on ridges, peaks and in open meadows. In summer, afternoon storms can dump heavy rain and hail.

Of particular concern is "the weather is always good in the Sierra" syndrom that affects people who have hiked and backpacked for years without encountering serious bad weather so they start leaving their storm gear at home to lighten there pack .. this is a disaster waiting to happen.

The often mild winter weather in the Sierra Nevada can be equally deceptive. Always assume the worst weather will happen and be prepared with adequate clothing, food, shelter and an emergency plan .. it can save your life.

Always check the forecast before you start your hike. Numerious sources for weather information are available on the internet. Local Ranger Stations and Visitor Centers will usually have current weather forecasts.

HEAT:
Overexertion on hot summer days can lead to heat-related injuries. Heat exhaustion will leave you feeling tired, nauseous, dizzy and headachy. Rest, rehydration, food and a good night's sleep are the treatment.

Heat stroke is a life-threatening illness. Symptoms include confusion, delirium and loss of consciousness. The skin feels hot as the body's cooling mechanism fails. THIS IS A TRUE EMERGENCY. Cooling the patient immediately is essential.

Both heat exhaustion and heat stroke can be prevented by avoiding overexertion on hot days. Pace yourself, rest often, drink plenty of water and eat regularly throughout your hike.

COLD & HYPOTHERMIA:
The lowering of your body's core temperature below normal can lead to poor judgement and confusion, loss of consciousness and death - even in summer! Early signs of hypothermia may be as mild as poor judgment, a slight sensation of chilliness, and trouble using your hands for simple tasks. Later signs can include uncontrolled shivering, unconsciousness and death.

Prevent hypothermia by having warm clothes and dressing in layers to adjust temperature as needed. Keep dry with good wind and rain gear: your body loses heat three times as fast when it's wet. Drink plenty of fluids, eat many small meals throughout the day, and avoid alcohol and caffeine.

Pay attention to your fellow hikers. Watch for symptoms of hypotheria when the weather is cold and wet.

LIGHTNING:
Pay attention to weather forecasts and avoid being caught on exposed ridges or above treeline during electrical storms. Large open meadow areas can be equally hazardous. Know an escape route in case fast-moving storms catch you off-guard.

If a storm hits, spread your group out and keep track of each other so if one person is hit, others can help.

If you don't have time to get to a lower elevation, squat down to reduce your height and minimize your contact with the earth. Take off your pack if it has a metal frame. If you have a foam pad, put it under you.

You will find more information on lightning here.

WATER CROSSINGS:
In spring and after heavy rains, streams can change from trickles to torrents, making crossings difficult. If you ford a stream, be extremely careful.

A stream that is fordable in the morning may be much deeper and unsafe to cross in the afternoon as snow melt or rain increases runoff.

Water in the Sierra Nevada is COLD. Prolonged immersion can cause hypothermia.

Use common sense and, if in doubt, don't cross.

Read more here, about making water crossings safe.