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The Weather Chrissy Magee Unpredictable Not predictable with absolute certainty Can make reliable predictions With experience your predictions become more reliable Weather Change White fleecy clouds, with winds: North to northeast No rain for 48 hours Southeast to southwest Rain within 24 hours Rain or Snow Light gray to black clouds, with winds: Easterly to southerly North to northwest Heavy rain or snow Light rain West to north No rain Uncertain Sky Fish scale clouds, with winds Northeast to southwest Possible short, late afternoon rain Other winds No rain Weather Information Local television news Local newspaper Weather channel radio NOAA website (www.noaa.gov) Air Pressure Cold air sinks High-pressure Hot air rises Low-pressure Barometric (Air) Pressure Air pressure is measured in: Standard sea-level pressure Inches of mercury Millibars 29.92 inches of mercury 1,000 millibars Above 29.92 indicates high-pressure Effects of Air Pressure Cold, sinking air High-pressure Cool/cold weather Dry Stable Barometer Rising Hot, rising air Low-pressure Cloudy Stormy Unstable Barometer Falling Winds Cold air sinks and becomes heated, moving towards low-pressure areas to replace the air that is being lifted Wind Cold Front Region of warm air displaced by colder air Forces warm air to rise Potential for heavy rains or snow Line of thunderstorms Precipitation is brief Cumulonimbus and stratus clouds Cumulonimbus Clouds Stratus Clouds Warm Front Warm air replaces colder air Warm air rises above colder air Light, steady rain or snow Up to 2 day warning Precipitation over extended period of time Cirrus clouds and stratus clouds Cirrus Clouds Local Winds Caused by the terrain Use to determine campsite At night mountains cool quicker than valleys Cold winds flow to the valleys Set camp off the valley floor Standard Lapse Rate With every 1,000 foot gain in altitude: Temperature decreases 3.5 degrees Under clear and stable weather As high as 5.5 degrees in unstable To estimate temperatures Use 4 degree change Standard Lapse Rate Example Obtain temperature at starting point Trailhead at 1,000 ft and 80 degrees Calculate temperature at destination Camp at 4,500 ft and 66 degrees Varying between 69 and 61 degrees Thunderstorms & Lightning Descend as quickly as possible Take cover under lowest tree Move away from any metal objects Backpack Tent poles Trekking poles Keep spread out when in open terrain Weather Where You Are Not Flash floods in the Desert Swelling rivers in the Mountains Other Weather Considerations Research average temperatures and rainfall Contact forest service rangers Long-ranged forecasts are less reliable Prepare for the worst Remember The only time weather conditions need to be considered are: Any time you hike or backpack Questions?