Nothing beats a good thunderstorm. These clouds contain a lot of water in liquid and even solid form, rain, and hail. This lesson is about the types of precipitation that can form and how they form. We will look at the phases in which water can occur, gas, liquid and solid and how water transforms from one state to another. Water is continuously moved around the earth in what is called the water cycle, shown here. Water from oceans, lakes and rivers evaporates. Then it moves to higher layers of the atmosphere where it will condense to form clouds. These clouds drift over land where precipitation may fall in the form of rain, snow or hail. This water then gathers in streams, brooks, rivers and lakes. One water will seep into the soil and ground and takes a longer route. In the end, most water will end up back into the sea. When you drink water from a spring, that water may have taken thousands of years to get there. This is also why we need to take care not to deplete groundwater as it will take a very long time to replenish. Let's have a closer look at the two main processes in the water cycle, evaporation and condensation. The confusing thing is that evaporation and condensation take place around you all the time. But you might not notice the effect. That's because there is an equilibrium between the two. In this animation, I want to show you how this works. On the left you can see a thermometer. The temperature is high. The box on the right represents one cubic meter of air. Let's suppose there is a certain amount of liquid water in the air to start with. Because there is so much liquid water and such a high temperature, the evaporation rate will be high. There is no water vapor present right now, so the condensation rate is almost zero. Now more and more water will evaporate until the evaporation rate equals the condensation rate. If there was not too much water present to start with, then there will not be any liquid water left. Suppose next the temperature drops. At a certain temperature, the condensation rate will start to exceed the evaporation and a bit of liquid water will be formed. This temperature is called the dew point, and it depends on the amount of water present in the air, as well as the pressure of the air. When the temperature drops further, liquid water is formed until evaporation and condensation rates are equal again. So in the situation we started out with, we could have added water and it would have turned into vapor up to the point where there would be a net condensation. So at the initial temperature, the cubic meter of air could have contained a certain maximum amount of water vapor. That's where the relative humidity comes in, which is the percentage of actual water vapor in the air as a percentage of the maximum water vapor at that temperature. I'll show you how that works in a moment. One last note though. I can only condense onto something, a surface or a dust particle in the air. I'll demonstrate that in the next lesson. Let's now have a look at measuring relative humidity. You can measure relative humidity with a high-grometer. Here I've got three of them. One is giving a reading of 35%, I don't trust this one at all. The other 52% and the third one 56%. You can also measure relative humidity with two thermometers. One and the ordinary thermometer, the so-called dry bulb thermometer, and in the shade this thermometer gives a temperature of 19 degrees Celsius. The other is a wet bulb thermometer. It has a little piece of wet cloth attached to the bottom. The water evaporates, which takes energy. And as a result, the thermometer will drop in temperature. But at a lower temperature, the evaporation will be less. So an equilibrium will be created that depends on the amount of water vapor in the air and the actual temperature. The wet bulb thermometer is 30 degrees Celsius. Now you can use a table like this one to determine the relative humidity. First, read off the dry bulb temperature, 19 degrees. The wet bulb temperature is 6 degrees lower than the dry bulb temperature, so you look up 6 at the top of the table. Apparently, the relative humidity is 51%, which is close to the 52 and 56% of the two high-grometers. For a certain amount of water vapor in the air, there will be a net condensation at a certain temperature. This is called the dew point. At or below the dew point, condensation wins from evaporation and water droplets will form. It is also said that the air is saturated. You can put this information in a graph, such as this one. On the horizontal axis, you can read the temperature, and on the vertical axis, the absolute humidity. That is, the amount of water in grams in 1 cubic meter of air. Now, if the temperature is above the dew point, here on the right of the graph, evaporation wins from condensation and the air is under-saturated. This is also called dry air, although there is water present in the gas state. Let's look at some numerical examples. First, let's determine the relative humidity. Suppose the temperature is 30 degrees. From the graph, it follows that the maximum absolute humidity then is 31 grams per cubic meter. That means that if the absolute humidity would be 31 grams per cubic meter or higher, then the air is saturated and there is net condensation. Now further suppose the dew point is 20 degrees Celsius. Then from the graph, you can read the absolute humidity, 17 grams per cubic meter. So the relative humidity equals 17 divided by 31 times 100 percent equals 55 percent. If the dew point would have been 25 degrees instead, then the absolute humidity would be 23 grams per cubic meter and a relative humidity 74 percent. You see, the higher the dew point, the more water vapor there is in the air and the higher the relative humidity. Important to remember, the dew point tells you what the absolute humidity is. You can also find the dew point if you know the relative humidity. Suppose the temperature is 25 degrees Celsius and the relative humidity 60 percent. Find the dew point. From the graph, it follows that the maximum absolute humidity is 23 grams per cubic meter. Remember, this is the maximum amount of water vapor in the air before net condensation occurs. Now the relative humidity is 60 percent. So the actual amount of water vapor in the air equals 60 percent from 23 grams per cubic meter equals 30.8 grams per cubic meter. From the graph, you can find the corresponding temperature, which is 17 degrees Celsius. This is the dew point. Let's now look at the four main types of precipitation. Rain forms when the air with some amount of water vapor in it is cooled below the dew point. That stays above zero degrees Celsius. There are different ways in which air can cool. For example, because it radiates and loses energy or because it is lifted. Next lesson I will explain that effect. Sleet consists of small pellets of ice. Sleet is formed when rain is partially frozen or snow melted and re-frozen. Hill forms most often in thunderstorms. Rain droplets are frozen and clumped together forming larger lumps of ice. Snow forms when water vapor directly turns into ice, forming ice crystals. Those crystals grow in a regular shape of a six-pointed star. The dew point must be below zero. Finally, there are three phenomena that have to do with dew point and condensation where the air cools. Dew forms when moist air comes into contact with a cold surface. The air cools through conduction and the dew point is reached. Water vapor will condense on the cold surface. Frost is similar to dew but now the air cools below zero degrees Celsius. Ice crystals are formed on the surfaces. Fog is like a cloud above the ground. Fog can form because land cools by sending out infrared radiation after sunset under a clear sky. When the temperature drops below the dew point calls for. You can also have warmer air containing a relatively large amount of water vapor move over cooler land. This is called advection cooling. Advection refers to the horizontal movement of air. The cooling occurs due to conduction and due to radiation. That's about it. More than enough I would say. Next lesson we will look into the formation of clouds. That of course has a lot to do with condensation as well.