Lecture 14 Outline of Last Lecture I. Weather of the dayII. How do clouds form?III. AerosolsIV. Dry HazeV. Wet HazeVI. More on how do clouds forma. Curvature effectb. Swollen aerosolsc. Super saturationVII. What exactly is a cloud?VIII. What is rain?a. Why do large droplets fall faster than small droplets?b. What is rain continued….Outline of Current Lecture II. RemindersIII. Weather of the dayIV. The issue with cloud droplets becoming rain dropletsV. How does rain form?VI. Ice FormationVII. Ice nucleiVIII. Ice NucleationIX. Process by which Ice growsCurrent LectureReminders:Test is a week from today. Look at homework problems, read through the lectures. The test will cover material from the text we have read and may not have discussed in class. Monday we will have a review discussion and go over the material. Test will only go up through chapter 18 and this recent set of chapters stability and thunderstorms only make it to there for the test. But there are homework questions due next Friday. Weather of the day: Atm Ocn 100 1st Edition250 mb map. Colors are wind speed. Yellows and reds are stronger wind speeds. If we look ahead to Saturday morning we can see that over the U.S. there will be a jet off the East Coast. But the most important to us is the purple area off the West Coast. It represents 160-knot winds. This is coming into the West Coast on Saturday morning. If we keep going ahead in time we can see the jet coming in on the West Coast and it is moving towards us. We can see that going ahead in time there will be a pretty sharp trough and ridge area near us, butthis is further to the East then it was on Monday when we looked at it. If we go ahead further to next Wednesday trough to ridge we may expect a strong storm to develop in the East Coast. At this point the storm would be off the East Coast not near us like we thought on Monday. If we look at surface map we can verify that. Storm maybe near us 992 but it is nowhere near as strong as we thought it would be on Monday. The storm will be over the East Coast instead of near us. There are strong storms coming to the West Coast. On the Gulf of Alaska 955 happens every year. Storms on the East Coast, they can become very strong. They can produce pressures as low as 955 like hurricanes. But inland this is less likely to occur because of the thermal contrast and the moisture involved is nowhere near as great. Allows water to store heat from over the year, water is a great energy source. Contrast between water and land can make for very great storms. This is what we are seeing right now.There is a tropical cyclone that is headed towards Japan right now. Japan relies on tropical cyclones for annual precipitation. But they do not like super typhoons. These are a little counterproductive because they do a lot ofdestruction if they hit. Tops of this storm are in the low -80s which is very cold. Coldest temperatures on the planet! There is a whole tropical anomaly with a jet stream over it heading towards the United States. It will mean thatthere will be a major energy pulse that will come off the super typhoon and head towards us next week. We may see interesting weather towards the end of next week because of this! Back to lecture:Review from last time: Cloud droplets are tiny compared to rain droplets. Rain droplets are not perfect spheres; they tend to be oblate spheroids. Small droplets are not distorted because they have such low terminal velocity that they don’t have much air relative to them. And being so small surface tension of the water overcomes anything so the cloud droplets look like perfect sphere. Rain droplets look like lifesavers without a hole. Smaller rain droplets even have the oblate spheroids look.By comparing vertical polarized radar to horizontal polarized radar you can see how big droplets are and if rain is being formed or not. What is the issue with cloud droplets becoming rain dropletsThe major issue: there are just too many cloud droplets and not enough condensed water to make them all large enough to fall Less is more: How do we get less droplets so the sizes are larger? How does rain form?Warm Rain – collision-coalescence leading to spectral broadening (larger sizes) – Issue here is that for cloud droplets to collide, they must move (i.e. fall) relative to each other but they tend to be all the same size falling with the same miniscule terminal velocityCold Rain – Rain forming from melting snow, the snow hydrometeors are fewer and therefore grow large enough to fall– The issue here is that ice crystals are much more difficult to form at ice saturation than are cloud dropletsFurther notes: There is mostly one size for cloud droplets. We need droplets to run into each other and collide to make larger droplets through collision process. But theyneed to move relative to each other for this to happen. Issue is about how can we get collision if they are all the same size. Another issue we will not get into is that it is hard to form ice! Ice is picky it doesn’t grow very easily; it isn’t always obvious how we are going to form ice. How does rain form?• Warm Rain (only in maritime, tropical clouds)– Collision/Coalescence process• Larger droplets fall faster than smaller droplets• Larger droplets “sweep up” smaller droplets through these collisions• Average size of rain droplets is capped at about 0.6 mm because very large droplets breakup forming more smaller droplets– Warm rain clouds may have no ice– Since ice formation can produce lightning, warm cumulus have no lightning• Cold Rain (most common mechanism for rain drop formation)– Ice forms first on aerosol through nucleation– Bergeron-Findeisen Growth Mechanism• Ice saturation is a lower vapor pressure than water saturation• Ice is supersaturated when water is saturated at 100% RH• Ice grows rapidly in supersaturated environment driving vapor pressure below 100% RH, causing cloud droplets to evaporate– Many less ice particles than cloud drops• Cloud droplet concentration ~ 100/cm**3 or 100,000 per liter• Ice crystal concentration ~ 1-10 /liter• Ice particles wind up growing to be much larger than cloud droplet because there are fewer of them to ultimate absorb the same available condensed (or frozen) water.• Ice then falls (big enough ice) down to elevations where the temperature is above freezing and melts to become rain– If this rain is still above the cloud base, it will continue to