Ice crystals grow at expense of water droplets and fall. Cloud Seeding • Necessary ingredient: Presence of supercooled clouds • Silver iodide (AgI) serves as ice Dry-ice-seeded racetrack pattern in supercooled cloud layer nuclei for the ice-crystal process (top temperature at 750 km & -5.6°C ) 30 min after the seeding. Utica, NY, 24 Nov 1948 (Langmuir 1961). (deposition from water droplets) to take place and initiate precipitation. • Effectiveness is uncertain. 300 km Seeding track over central China on 0735 UTC 14 Mar 2000 as observed by NOAA-14 AVHRR . Natural Cloud Seeding Cold enough for ice crystal formation Æ On the lee, precipitation may be observed in separate bands 1 SO2, CO2 Cloud seeding experiment on the global scale by human-induced Acid rain Global warming increase in aerosols (condensation Aerosols nuclei and ice nuclei). Æ Increased number of cloud droplets Æ clouds become brighter. ÆIncreased number of raindrops decrease their size Æ decreased surface rainfall and clouds persist longer. Æ Both effects help cool the earth by reflecting sun light. Aerial view shows Amazon forest burning in Mato Grosso state, Brazil, August 9, 2005. Precipitation Types Rain over Roi Et, Thailand 2 Virga Rain or snow that evaporates before reaching the surface. Cirrus as Virga Falling ice crystals that evaporate in air. 3 Mammatus Clouds Downward motion in convection caused by weight of falling precipitation and cooling by evaporating precipitation. Precipitation Types Precipitation reaching the surface can take on different forms depending on the vertical temperature profile. Sleet is solid before it reaches the ground. Freezing rain is liquid (super cooled) until it strikes. 4 Freezing Rain • Freezing rain is liquid until it strikes an object. • It coats surfaces with large quantities of ice • Trees break, power lines fall, roads are treacherous Riming (Accretion) • Small supercooled cloud drops collected by trees (or other structures) are known as rime. • Collection of supercooled rain and cloud drops poses a hazard in the form of aircraft icing and can result in ships capsizing. Mt. Washington, NH 5 Snow Dry environment Humid environment Ice pellets (Graupal, diameters < 5 mm) 6 Hail • Hail can form in clouds with – High supercooled liquid water content – Very strong updrafts • Hailstones associated with deep and intense cumulonimbus – Typically make 2-3 trips up through cloud (10-15 minutes) • Opaque and clear ice layers form – Opaque represents rapid freezing of accreted drops – Clear represents slower freezing during higher water accretion rates – Layering tells about hailstone history • Frozen precip of largest size occurs during the warmest time of the year The Coffeyville hailstone (5.5”, 1.7 lbs) Note the density layering Doppler radar measures scattering of microwave by precipitation. NEXRAD (Next Generation Radar) with wavelength of 10 cm Oklahoma 7 TRMM Satellite Eye & eyewall Global Precipitation Precipitation and Weather Maps 8 .NOW... SHOWERS OVER SOUTHWEST OAHU...WHICH DEVELOPED DUE TO A COMBINATION OF DAYTIME HEATING...LIGHT WINDS AND AN UNSTABLE ATMOSPHERE WILL CONTINUE THROUGH SUNSET. THE SHOWERS WILL DISSIPATE AS THE ISLAND COOLS OFF AT SUNSET. --NWS Unstable condition due to the moistening at the surface and cooling aloft Surface Td ~ 21 Æ 25 C, with a weak inversion yesterday Trade wind inversion is a highly stable layer (~2 km high) that caps the T Td moist surface layer (often cloudy) from the dry Hilo 9/19/06 Hilo 9/25/06atmosphere above. 2:00 am HST 2:00 pm HST Clockwise Counter-clockwise around High around low 9 Winds blow parallel to the contours of constant height: Westerlies with high to the south and low to the north The Atmosphere in Motion: Forces, and Wind Why is there wind? Why does it blow the way it is? What causes it? What are the physical rules that govern the wind? 10 Changes in temperature drive winds The gas law (eq of state): p ~ ρ x T Æ Heated air expands. 1 2 3 Forces and winds • Pressure gradients produce air movement • Multiple forces act simultaneously to cause the wind direction to differ from the direction of decreasing pressure • Newton’s laws of motion describe the relationship between forces and motion 11 Forces and Wind A force is a pushing or pulling that will result in motion if it is unopposed. Newton’s Laws of Motion Galileo 1. In the absence of forces an object at rest will remain at rest and an object in motion will remain so with the same velocity. 2. Force equals mass times acceleration (F = m·a). 3. To every action there’s an equal and opposite reaction. 12.