Chapter 18 Water in the Atmosphere

#1 Water is a unique substance because it is only the substance that commonly exists in all 3 states of matter. Water is a unique substance because it is only the substance that commonly exists in all 3 states of matter. Water exists as a solid, liquid, or gas on the surface of the Earth. Water exists as a solid, liquid, or gas on the surface of the Earth.

#2 True water vapor is invisible. True water vapor is invisible. If you see fog, you are actually seeing very small droplets of liquid water. If you see fog, you are actually seeing very small droplets of liquid water. When water changes phase, changing from solid to liquid to gas, or from gas to liquid to solid, energy is either released or absorbed. When water changes phase, changing from solid to liquid to gas, or from gas to liquid to solid, energy is either released or absorbed.

#3 When ice melts, for example, we think that ice heats up. ( It does not!) When ice melts, for example, we think that ice heats up. ( It does not!) For it to heat up, that means the temperature must increase. ( It does not) For it to heat up, that means the temperature must increase. ( It does not) What in fact must happen for ice to melt is that the ice molecules must absorb enough kinetic energy to break out of the solid form and become liquid. What in fact must happen for ice to melt is that the ice molecules must absorb enough kinetic energy to break out of the solid form and become liquid.

#3 Continued So lets say that some ice did melt, now the water can increase in temperature until it starts to change from liquid to gas. So lets say that some ice did melt, now the water can increase in temperature until it starts to change from liquid to gas. This process is called evaporation, and for evaporation to occur, water must absorb huge amounts of energy. This process is called evaporation, and for evaporation to occur, water must absorb huge amounts of energy.

#4 When the reverse process occurs, and water vapor changes from vapor to liquid, huge amounts of energy are released. When the reverse process occurs, and water vapor changes from vapor to liquid, huge amounts of energy are released. This process is called condensation. This process is called condensation. Once vapor has condensed into liquid, then the liquid can cool to the freezing point. Once vapor has condensed into liquid, then the liquid can cool to the freezing point. As the liquid turns to ice (freezing) more heat is released. As the liquid turns to ice (freezing) more heat is released.

#5 Water vapor in the air is called humidity. Water vapor in the air is called humidity. The actual amount of water vapor in a given parcel of air, such as a cubic meter of air, is called specific humidity. The actual amount of water vapor in a given parcel of air, such as a cubic meter of air, is called specific humidity. But the ability of a given parcel of air to hold water depends upon it’s temperature. But the ability of a given parcel of air to hold water depends upon it’s temperature.

#5 Continued That is, the higher the temperature of air, the more water vapor it can hold. That is, the higher the temperature of air, the more water vapor it can hold. The lower the temperature of the air, the less water vapor it can hold. The lower the temperature of the air, the less water vapor it can hold.

#6 So lets say the relative humidity of the air is 50%. So lets say the relative humidity of the air is 50%. That means that the air is holding 50% of the water vapor it can hold at that temp. That means that the air is holding 50% of the water vapor it can hold at that temp. But if the specific humidity stays the same but the temperature falls, then the capacity of the air is decreased, then the relative humidity goes up. But if the specific humidity stays the same but the temperature falls, then the capacity of the air is decreased, then the relative humidity goes up. The water vapor capacity of air roughly doubles for every temperature increase of 11 C The water vapor capacity of air roughly doubles for every temperature increase of 11 C

#7 Dew point is actually a more useful measurement. Dew point is actually a more useful measurement. The definition of dew point is that it is the temperature at which the air, with a given amount of water vapor will have a relative humidity of 100%. The definition of dew point is that it is the temperature at which the air, with a given amount of water vapor will have a relative humidity of 100%. That is to say the air is saturated. That is to say the air is saturated.

#8 When water vapor condenses, it must have something to condense around. When water vapor condenses, it must have something to condense around. That could be a small particle of something in the atmosphere. That could be a small particle of something in the atmosphere. These particles are called condensation nuclei. These particles are called condensation nuclei.

#9 Dew and frost will form when water vapor comes into contact with a cold surface. Dew and frost will form when water vapor comes into contact with a cold surface. Dew forms when the temperature of the cold surface is above freezing, frost forms when the cold surface temperature is below freezing. Dew forms when the temperature of the cold surface is above freezing, frost forms when the cold surface temperature is below freezing.

#10 Fog forms when a cold surface cools the warm, moist air above it. Fog forms when a cold surface cools the warm, moist air above it. Often fog will form in low areas because cool air settles there and causes the air in the low spot to become saturated. Often fog will form in low areas because cool air settles there and causes the air in the low spot to become saturated.

#11 There are 4 basic cloud types. There are 4 basic cloud types. They are: cumulus, stratus, nimbus, and cirrus. They are: cumulus, stratus, nimbus, and cirrus. Clouds are also described by their shape. Clouds are also described by their shape.

#11 Continued Clouds that form in layers are called stratus clouds. Clouds that form in layers are called stratus clouds.

#11 Continued Puffy, cotton ball shaped clouds are called cumulus clouds. Puffy, cotton ball shaped clouds are called cumulus clouds.

#11 Continued High, thin feathery clouds are called cirrus clouds. High, thin feathery clouds are called cirrus clouds.

#11 Continued Clouds that produce rain are called nimbus clouds. Clouds that produce rain are called nimbus clouds.

#12 To make a cloud, small droplets of water form around a condensation nuclei. To make a cloud, small droplets of water form around a condensation nuclei. As the droplet collides with others, they accumulate until the droplet is too heavy to stay suspended any longer and the falls out as precipitation. As the droplet collides with others, they accumulate until the droplet is too heavy to stay suspended any longer and the falls out as precipitation.

#13 There are several different types of precipitation. There are several different types of precipitation. They include drizzle, which is composed of very fine droplets, rain, snow, sleet, freezing rain, and hail They include drizzle, which is composed of very fine droplets, rain, snow, sleet, freezing rain, and hail Hail stones are actually made up of layers of ice that form when the hail stone is taken up into a tower cumulus cloud by updrafts and then the water collects and freezes on it. Hail stones are actually made up of layers of ice that form when the hail stone is taken up into a tower cumulus cloud by updrafts and then the water collects and freezes on it.

#14 When weather systems approach mountains, the clouds are forced up to go over the mountains. When weather systems approach mountains, the clouds are forced up to go over the mountains. When this occurs, the dew points drops and the clouds will release the moisture they are carrying. When this occurs, the dew points drops and the clouds will release the moisture they are carrying.

#14 Continued The windward side of the mountains get more precipitation than the leeward side of the mountains. The windward side of the mountains get more precipitation than the leeward side of the mountains. This is why deserts often form on one side of mountain ranges while the other side is quite green and moist. This is why deserts often form on one side of mountain ranges while the other side is quite green and moist.

#15 When a drought occurs, sometimes the technique of cloud seeding is used. When a drought occurs, sometimes the technique of cloud seeding is used. In this process, super cooled pellets of dry ice might be dropped into clouds to cool them and hopefully cause precipitation. In this process, super cooled pellets of dry ice might be dropped into clouds to cool them and hopefully cause precipitation.

The End!!!! The End!!!!