Understanding Weather: Precipitation (1992)

[Music] Wesley calling Wesley. The Weather Bureau of Investigation is calling Wesley. Are you there, Wesley? This is Wesley, sir. Are you ready for another day of training? Yes, sir. You're doing well, Wesley. Soon you'll be a real agent like all of us at the WBI, not just an agent in training. What is my assignment today, sir? Precipitation. [Music] [Music] Wesley, come in, please. Wesley. Yes, sir. Well, Wesley, I'm glad to see that you have precipitation on your mind. Thank you, sir. I'm thinking very hard about the rain that's falling out of the clouds. But rain is not the only form of precipitation, you remember. If you'd look at the viewing screen on your weather watch. Oh, you didn't have to remind me, sir. I know that snow and sleet are forms of precipitation, and so is hail. But what about the precipitation that's falling on you now? The rain? Well, it's stopping. But do you know where it came from? Clouds. But do you know how it got into the clouds? No, sir. But I'll see if I can figure that out the next time there's any precipitation. I'll watch for it day and night. This is Wesley calling the WBI. Our office is closed. Please leave a message. The sun is setting, sir. I'm going to spend the whole night outdoors. Just reporting that the moon is rising, sir. Nothing else to report. Good morning, sir. The sun is rising. Just reporting that there's nothing to report. Oh, wait. There is something. Dew. I see it on a plant. And I see some on a bicycle seat. And there's dew on the windshield of a car. De seems to be all over. Yes, it's off and on surfaces early in the morning. Oh. Oh, you're there, sir. And u where will it go? Well, could you give me a hint just to get me started? Sir, keep your eye on some surface where there's dew. Wesley, there does seem to be something happening to the dew. Oh, I think I know what's happening. It's evaporating. Is that right, sir? That's right, Wesley. The water is changing from a liquid into a water vapor, which is an invisible gas. Yes, Wesley, water vapor is part of the air. It's there, but you can't see it. Well, if dew goes into the air when it changes from liquid water into water vapor, then maybe dew comes out of the air when water vapor in the air changes to liquid water. But I still need to know why it comes out of the air, don't I? Yes, you do. Try to find out how the temperature of the air changed between last night and this morning. Sir, this is Wesley calling in. I'm in a weather bureau reporting station. They keep track of the temperature 24 hours a day here. Yes, I know. And a very helpful person here drew a chart of the change in temperature for me. It started getting cooler a few hours before sunset and it kept getting cooler all night. So things out in the air were getting cooler too. Sometime during the night the dew must have formed on these things. Then after the temperature started going up the dew must have started disappearing. Yes. You see water comes out of the air and forms on the surfaces when the air temperature goes down and cools them. And water goes back into the air when the temperature of the air and things in it goes up. Very good, Wesley. You've discovered something that's very important in understanding precipitation. Now, go over to the lake. We've arranged for a boat and a guide to take you out on the water. Now, we were talking about water vapor. It's going into the air all the time from water that's all over the world, from lakes and rivers, but mainly from the oceans. You can't see that happen, but you can see what happens when, look at your weather watch. When wet clothes dry on a line and wet cars get dry after they've been washed. These are examples of water going into the air. But when the air temperature drops enough and surfaces get cold enough, water comes out of the air faster than it goes in. That's why dew forms. I know dew isn't precipitation, but it doesn't look like there's going to be any precipitation soon because I don't see any or many clouds. But I can wait if my guide has the time. Wesley. Yes, sir. See any clouds yet? Actually, I can't see much at all, sir. There's a fog out here. Now, Wesley, you do remember, don't you, that a fog is a cloud? Yes, I knew that. But now you have to think about what both fogs and clouds are. What's that? Well, is the guide wearing glasses? Yes, sir. Look at them closely and tell me what you see. I see water on them. Tiny drops. The glasses are wet, but the lake is calm, so it isn't spray. The water must have come from the fog. But it isn't raining in the fog, so the water must be the fog. It looks like fog or a cloud must be made of drops of water. Very tiny drops. That's right. And they weigh so little that they can float in the air. We call them droplets. But where do the droplets come from? From the same place that dew comes from. From water vapor in the air. That's right. But the air in the fog doesn't feel cooler than the air out of the fog was. Well, all clouds don't form because of cooling air. Sometimes they form when air at one temperature mixes with air that's at a different temperature and which has a different amount of water vapor in it. Fog on a lake is just one example of that sort of cloud making. Look at your weather watch, please. Breathing out on a cold day forms clouds in that way. Clouds from a hot bowl of soup or a boiling tea kettle form in the same way. They're all called mixing clouds. Fogs, like other mixing clouds, don't usually result in precipitation. Later in the morning, Wesley, as the sun warms the air, the fog will slowly disappear. You see, but if the air is warmer, why are there clouds up there? Now, a good question, Wesley. I'd like you to go to the airport now. I'll call in a weather plane and we'll fly you up to where the clouds are and maybe you can find out for yourself why clouds are up there. Bring a warm jacket with you. Hi Wesley. Climb in. Wesley calling Wesley. Yes, sir. I'm in the weather plane, sir. Anything to report yet? Well, we're heading for some clouds. We'll have to climb a bit. The plane's engine is really working hard. Hey, there's a glider. It's climbing pretty fast, and it doesn't even have an engine. And can you explain that? Well, I guess it's being pushed upward by something. Something like Well, the only thing I could imagine pushing up on the glider is air. And that's just exactly what's happening, Wesley. You see, the sun heats the ground, and the ground heats the air over it. But some parts of the ground get warmer than others. So, the air over those parts rises faster than the air around it that isn't as warm. Air that's warm is always rising through cooler air. Thank you for suggesting that I take along a jacket, sir. It's getting cooler as we go higher. I'll turn on the cabin heater. Wesley, it's getting cooler, Wesley. Because as air warmed by the ground rises, it expands. Expanding makes it get cooler. And if it gets cool enough, what's going to happen, Wesley? Well, there'll be tiny drops of water. If there are surfaces up here for the droplets to come out on, and there are, such as salt particles and dust, then the droplets should form and stay together and make clouds. So, clouds are forming up here because the air is cooling off. That's right. Cooling air is the usual reason why clouds form. Wow, look at those clouds. It looks like they're still growing. Well, if there's enough water vapor in the rising and cooling air, clouds get bigger and bigger. Look at your weather watch. Clouds grow as more and more droplets of water come out of the air. But the clouds might not be made of drops of water. If the air temperature is low enough, like below freezing, right? Then when water vapor comes out of the air, it can form tiny ice crystals. Now, these clouds of water droplets or ice crystals can float in the air because they weigh so little. I suppose as more and more of the cloud droplets form, they could bump together and get bigger and heavier, too, right? And what happens when they get too heavy to float? Precipitation. Good. And how would you explain snow? Ice crystals grow too heavy to float and fall out of the clouds. Very good, Wesley. But it's pretty warm today. If any snowflakes fell out of those clouds, they'd melt. We'd have rain. That's quite right, Wesley. But remember, all rain isn't melted snow. And it sounds like we're about to have some rain. Better tell the pilot to land. It's umbrella up time, Wesley. Umbrella already up, sir. What do you know about precipitation now, Wesley? Well, it comes from clouds. And clouds come from water vapor in the air. When the water droplets or ice crystals that form the clouds get too heavy, we have precipitation. Very good, Wesley. But that's not all I know, sir. I also know enough to come in out of the rain. [Music] [Music]