1 00:00:00,000 --> 00:00:13,300 The United States is subjected to some of the world's most diversified weather conditions. 2 00:00:13,300 --> 00:00:17,740 During the course of a normal year, our country may experience plus 100 degrees temperatures, 3 00:00:17,740 --> 00:00:22,580 jungle humidity, negative 30 degrees cold, severe drought conditions, as well as being 4 00:00:22,580 --> 00:00:28,020 the world leader in tornado activity, not to mention an occasional east coast hurricane. 5 00:00:28,020 --> 00:00:32,020 As our transportation system has expanded, so too has our tendency to commute on a daily 6 00:00:32,020 --> 00:00:34,820 basis to work and recreational events. 7 00:00:34,820 --> 00:00:38,580 The potential for weather to complicate our lives is therefore also increased. 8 00:00:38,580 --> 00:00:43,300 Aviation, perhaps more than other readily available modes of transportation today, is 9 00:00:43,300 --> 00:00:45,820 strongly impacted by weather conditions. 10 00:00:45,820 --> 00:00:50,820 Did you know that 75% of all airport delays are related to weather? 11 00:00:50,820 --> 00:00:56,380 In 1996, the estimated price tag for weather-related delays, diversions, cancellations, and unexpected 12 00:00:56,380 --> 00:00:59,820 operating costs was $2.1 billion. 13 00:00:59,820 --> 00:01:05,180 The U.S. government invested approximately $830 million in aviation weather efforts during 14 00:01:05,180 --> 00:01:06,920 1997. 15 00:01:06,920 --> 00:01:11,180 As the magnitude of these numbers indicate, ensuring weather-related safety and efficiency 16 00:01:11,180 --> 00:01:17,140 of flight has economic and human life ramifications to business and traveling public communities. 17 00:01:17,140 --> 00:01:21,380 Weather has a continual impact on both the safety of aircraft in flight and the efficiency 18 00:01:21,380 --> 00:01:25,180 of operations throughout the national airspace system. 19 00:01:25,180 --> 00:01:30,300 To complicate things even more, air traffic is projected to triple over the next 20 years. 20 00:01:30,300 --> 00:01:34,220 More planes and people in the air fly longer distances more often. 21 00:01:34,220 --> 00:01:37,140 Imagine what a weather delay will mean with more air traffic. 22 00:01:37,140 --> 00:01:39,260 Talk about a traffic jam. 23 00:01:39,260 --> 00:01:42,420 How important is weather information to your daily activities? 24 00:01:42,420 --> 00:01:45,860 How well do you understand the weather system and the interaction of the various elements 25 00:01:45,860 --> 00:01:47,460 on the daily forecast? 26 00:01:47,460 --> 00:01:52,860 But most of all, how might weather in one location affect your planned travel to a different 27 00:01:52,860 --> 00:01:54,300 region? 28 00:01:54,300 --> 00:01:59,260 These meteorologists have the tools and advanced technology to explain complex weather phenomena 29 00:01:59,260 --> 00:02:03,900 in a way that's simple to understand, thanks largely to satellite information and computer-based 30 00:02:03,900 --> 00:02:04,900 modeling. 31 00:02:04,900 --> 00:02:08,800 Dennis Smith of the Weather Channel in Atlanta, Georgia, has offered to explain and help us 32 00:02:08,800 --> 00:02:11,340 understand the fundamentals of weather. 33 00:02:11,340 --> 00:02:12,340 Dennis? 34 00:02:12,340 --> 00:02:14,180 Thanks, Van. 35 00:02:14,180 --> 00:02:18,220 As many of you know, the Weather Channel provides 24-hour weather information, both from a national 36 00:02:18,220 --> 00:02:20,180 and international perspective. 37 00:02:20,180 --> 00:02:23,500 Our meteorologists constantly monitor and update weather information. 38 00:02:23,700 --> 00:02:28,420 Now, today, we're going to fly through some basic weather concepts and talk a little bit 39 00:02:28,420 --> 00:02:31,940 about winter weather, which can cause some problems for aircraft. 40 00:02:31,940 --> 00:02:39,060 To explain how our weather occurs, we must first travel out past the boundaries of our 41 00:02:39,060 --> 00:02:43,900 atmosphere, 93 million miles away. 42 00:02:43,900 --> 00:02:50,100 Our sun emits visible and invisible energy that we call solar radiation. 43 00:02:50,100 --> 00:02:54,780 Radiation from the sun travels through the Earth's atmosphere, heating the air, the land, 44 00:02:54,780 --> 00:02:56,780 and the water it contacts. 45 00:02:56,780 --> 00:03:00,780 The Earth's surface absorbs much more radiation than the atmosphere. 46 00:03:00,780 --> 00:03:06,420 This means that the sun heats the ground, and the ground heats the atmosphere. 47 00:03:06,420 --> 00:03:09,220 Not all parts of the Earth are warmed equally. 48 00:03:09,220 --> 00:03:15,220 Regions around the equator receive more concentrated solar radiation than do areas around the poles. 49 00:03:15,220 --> 00:03:19,900 The result is that the equator is warmer than the poles. 50 00:03:19,900 --> 00:03:26,700 Temperature differences also result because land and water do not absorb solar radiation equally. 51 00:03:26,700 --> 00:03:31,700 Because air moves, warm air and cold air are constantly mixing in the atmosphere. 52 00:03:31,700 --> 00:03:36,380 This mixing not only evens out the global temperature contrast, but results in the various 53 00:03:36,380 --> 00:03:40,900 weather conditions we see every day. 54 00:03:40,900 --> 00:03:45,740 If solar radiation penetrates all layers of the atmosphere, then why does nearly all weather 55 00:03:45,740 --> 00:03:49,220 occur in the lowest layer, the troposphere? 56 00:03:49,220 --> 00:03:53,300 The troposphere is warmest near the ground and cooler the higher you go. 57 00:03:53,300 --> 00:03:57,900 This temperature pattern is favorable for the development of vertical air currents. 58 00:03:57,900 --> 00:04:03,020 Relatively warm air tends to rise, and relatively cool air tends to sink. 59 00:04:03,020 --> 00:04:07,220 As warm air rises in the atmosphere, it expands and cools. 60 00:04:07,220 --> 00:04:12,180 As air cools, clouds can form, and precipitation can fall. 61 00:04:12,180 --> 00:04:17,860 Warm air rising causes less pressure to be exerted by the atmosphere, lower air pressure. 62 00:04:17,860 --> 00:04:22,820 Cool air falling causes more pressure to be exerted, high air pressure. 63 00:04:22,820 --> 00:04:28,660 As warm and cool air flow from one region to another, pressure changes, and so can the weather. 64 00:04:28,660 --> 00:04:33,540 When air pressure falls quickly, it usually means that stormy weather is approaching. 65 00:04:33,540 --> 00:04:35,980 That's the result of a low-pressure system. 66 00:04:35,980 --> 00:04:39,940 When air pressure rises, fair weather typically results. 67 00:04:39,940 --> 00:04:42,940 That's the result of a high-pressure system. 68 00:04:42,940 --> 00:04:45,900 Wind is another byproduct of changing air pressure. 69 00:04:45,900 --> 00:04:49,420 Winds flow because of the pressure differences in the atmosphere. 70 00:04:49,420 --> 00:04:53,820 Air moves from the areas of high pressure to areas of low pressure. 71 00:04:53,820 --> 00:05:00,020 Wind spiral inward toward low pressure, causing a piling up of air, forcing air to rise and 72 00:05:00,020 --> 00:05:04,340 cool, forming clouds and eventually precipitation. 73 00:05:04,340 --> 00:05:09,820 Around high pressure, winds spiral outward, promoting sinking air and fair weather. 74 00:05:09,820 --> 00:05:14,460 Okay, now, while you're soaking up that information, let's think of something a little bit more 75 00:05:14,460 --> 00:05:17,940 fun, snow days. 76 00:05:17,940 --> 00:05:20,980 When you think of winter, is this the picture you see? 77 00:05:20,980 --> 00:05:24,180 Snow, snow, and more snow. 78 00:05:24,180 --> 00:05:26,900 But there are other types of winter precipitation. 79 00:05:26,900 --> 00:05:31,260 The type of precipitation that falls during winter depends on how warm air moves over 80 00:05:31,260 --> 00:05:33,500 a layer of below-freezing air. 81 00:05:33,500 --> 00:05:36,100 A good place to start is with sleet. 82 00:05:36,100 --> 00:05:39,380 Sleet is frozen precipitation falling as ice pellets. 83 00:05:39,380 --> 00:05:44,380 These ice pellets form when snowflakes pass through a thin layer of warm air and melt. 84 00:05:44,380 --> 00:05:48,460 They refreeze into ice pellets as they fall through another layer of colder air close 85 00:05:48,460 --> 00:05:50,700 to the ground. 86 00:05:50,700 --> 00:05:54,820 Freezing rain is made of water droplets that fall to the Earth's surface and freeze upon 87 00:05:54,820 --> 00:05:58,500 contact with the ground or objects near the ground. 88 00:05:58,500 --> 00:06:02,860 For freezing rain to develop, cold air close to the ground needs to be shallow. 89 00:06:02,860 --> 00:06:07,660 The rain doesn't have time to freeze into ice pellets, but upon contact with frozen 90 00:06:07,660 --> 00:06:11,540 objects, it turns into ice. 91 00:06:11,580 --> 00:06:15,780 Snow is frozen precipitation in the form of a six-sided ice crystal. 92 00:06:15,780 --> 00:06:20,500 Snow is produced in clouds where water vapor changes directly into ice crystals that remain 93 00:06:20,500 --> 00:06:22,940 frozen as they fall to Earth. 94 00:06:22,940 --> 00:06:28,300 Snow will fall when temperatures remain below freezing from the clouds to the ground, or 95 00:06:28,300 --> 00:06:33,100 when a very shallow layer of above-freezing air is present near the ground. 96 00:06:33,100 --> 00:06:36,820 Winter weather can be a lot of fun to play in, but it can cause some problems for us 97 00:06:36,820 --> 00:06:43,020 when we try to get out and travel, either by foot, by car, and especially by air. 98 00:06:43,020 --> 00:06:48,140 Now that you have a better understanding of weather, here's a question for you. 99 00:06:48,140 --> 00:06:52,020 Weather reports of snow are typically based on visibility. 100 00:06:52,020 --> 00:06:56,240 Snowfall is considered heavy when an observer cannot see very far through the flakes. 101 00:06:56,240 --> 00:07:00,300 But visibility is not the critical element for the meteorologist interested in aircraft 102 00:07:00,300 --> 00:07:01,300 operations. 103 00:07:01,300 --> 00:07:05,820 What do you suppose is the main issue that is considered in looking at snowfall from 104 00:07:05,820 --> 00:07:08,020 the perspective of aviation? 105 00:07:08,020 --> 00:07:08,740 Back to you, Van.