Unit 5: Precipitation • Relax • Cloud presentations • Quiz • Class Feedback • Superstorms Movie • Precipitation lecture • Class-wide exercises • Lecture • Lab on Thursday Cloud formation process • Air rises, pressure decreases, and temperature decreases • Temperature drops below the dew point • Water vapor condenses around cloud condensation nuclei • Lifting Condensation Level (LCL) Source: NOAA (https://scied.ucar.edu/imagecontent/how-clouds-form) What Makes Precipitation? Cloud droplets are formed when water vapor condenses (around CCN). What makes precipitation form? Precipitation Can Form By… 1. In warm clouds, water droplets condense - Droplets collide and coalesce into larger droplets Precipitation Can Form By… 2. In cold clouds, ice crystals form & grow - May form by deposition or by freezing supercooled water - Ice may or may not melt on way down to Earth’s surface - Named Bergeron process How Big Are…? Name Actual size range Model size Cloud condensation 0.1 µm nuclei Cloud droplets 100 µm (1-200) Drizzle 100 µm (up to 500) Raindrops 1000 µm (500-7000) Hail 5000 µm or more Why Do Clouds Float and Precipitation Falls? • Design an experiment to measure the effects of size on fall rates • Use crumpled papers of vastly different sizes • Assign roles within your group so that everyone can participate • Remember to perform repeated trials to verify findings • Pick a representative to report your findings to the class • Hypothesis • Experiment design • Results • Conclusion Types of Precipitation • Drizzle • Rain • Sleet • Freezing Rain • Graupel • Snow • Hail Rain • To fall, raindrops must be > 0.2 mm diameter • Smallest raindrops are called drizzle • True raindrops > 0.5 mm • Max size of raindrops 5-8 mm diameter Virga When Rain evaporates while falling, called virga View at EarthSky Community Photos. | The June 1, 2019 dawn sky – with a waning crescent moon and (to the moon’s left) the planet Venus – and with virga extending down from the clouds. Photo taken by Mike Lewinski. Those are the Sangre de Cristo mountains near Taos, New Mexico. Thanks, Mike! Snow • Starts in cloud as light ice crystals • Falls through freezing air gently to surface • Surface usually near or below freezing Sleet & Freezing Rain • At some point on way to ground both are rain • Sleet freezes passing through cold air • Hits ground as large, frozen ice crystal • Freezing rain hits ground as supercooled water • Freezes on contact, creating glaze Graupel Snowflakes fall through warm layer of air, collecting supercooled water droplets. Also called “soft hail” or ”snow pellets” or “popcorn snow”. Graupel from a 2013 storm in Montreal. Source: Ray Murphy, Earth Science Source: National Weather Service Picture of the Day Hail • Ice crystals held aloft by strong updrafts • Supercooled water freezes to ice crystals (graupel) • Graupel grow big enough to finally fall to surface • Hail may be 3-5 cm when it hits Earth • Rarely much bigger, largest ever grapefruit size What Causes Precipitation? • Moisture • Uplift: • Orographic Lifting • Convection (surface heating of moist air) • Frontal LiftingàCyclonic Precipitation • ConvergenceàCommon in equatorial belt • CCN • Natural—100 per cm3 over oceans and 300 per cm3 over land • Meteoric dust • Clay and silt (windblown) • Volcanic material • Sea salt • Smoke • Directly human-caused—3500 to 30,000 per cm3 in industrial areas • Combustion products including Nitrogen and Sulfur compounds • Lead to acid rain • Can outnumber natural CCN • Time to grow Source: NOAA U.S. Climate Atlas Why Are Precipitation Patterns Changing? • More Atmospheric Moisture • Warmer air can hold more moisture • More Uplift due to Convection • Urban areas • Evaporation • More directly human-caused CCN from industry • Changing temperatures • Areas around 0 degrees-C are shifting poleward, arriving earlier in fall and remaining later into spring Source: NWS https://www.weather.gov/safety /winter-lake-effect-snow Lake Effect • Long Fetch—distance wind travels over water • Longer fetch leads to more heat and moisture in air • A northwest wind travels almost 130 miles (210 km) across Lake Superior, 150 miles (242 km) across Lake Huron, but only about 30 miles (48 km) across Lake Erie. • Large Temperature Difference (Water Warmer than Land) • Mid-August to March: The average temperature of the land is colder than the average temperature of water. • Mid-November to mid-January: This time of year sees the largest temperature differences between land and water. • Those temperature differences may average: • Northern lakes: 30 degrees Fahrenheit (F) (-1 degrees Celsius). • Southern lakes: More than 15 degrees F (-9 degrees C). • Surface roughness causes convergenceàliftingàmore clouds and snow! Precipitation Writing Assignment • PRINT AND BRING TO CLASS • 300-500 words, aka one page. Edit down if your first draft is longer than this! • Pick a type of precipitation that you have a connection with—a fond or powerful memory, an association with home or family, a trip you took, or a phenomenon that interests you. This can be the same as you explored in your first essay. • Tell the story of a water molecule passing through the water cycle in the form of precipitation you picked. • Include all stages: on the surface, entering the atmosphere, uplifting, condensing, coalescing, and finally falling back to the surface Precipitation Writing Assignment • Where do you begin? In a plant? In a pond? On a parking lot? • What kind of air sucks you in? hot? Cold? Still? Windy? • What pushes you up? Buoancy? Topography? • How quickly do you rise? far up do you go? • What do you condense around? Or do you deposit? • How big does your new raindrop or ice crystal grow before starting to fall? • Do you melt or freeze? • Where do you land? Have you been transported? • CONSIDER THE GEOGRAPHY—Where is your story being told and what is likely to occur there?.