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Chapter 18: Water, Clouds, and Precipitation. Water in the Atmosphere. The amount of water vapor in the air can vary from 0-4% by volume depending on location. However, its importance can't be overstated.
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Water in the Atmosphere • The amount of water vapor in the air can vary from 0-4% by volume depending on location. • However, its importance can't be overstated. • Water is responsible for clouds, rainfall, snow, and for moderating Earth's temperatures.
Changes of State in Water • Earth is unique in that water can exist in three different states at normal temperatures. • When water is a solid (ice) and it melts into a liquid, it absorbs heat while it is melting. • The amount of heat that is required to change a state of water is called latent heat, because while the water is changing a state, it's temperature does not change.
Changes of State in Water • When liquid water changes into a gas by evaporation, it requires latent heat to change the liquid into a gas. • So, to clarify, if you set out a piece of ice at room temperature, the ice and the melt water that is produced by the ice would all stay at 0 Celsius until all of the ice melted. • If you boiled water in a pan, the water would not get any hotter than 100 Celsius until all of the water evaporated completely.
Other Important Changes of State in Water • If a gas changes into a liquid, this process is referred to as condensation. • If a solid turns directly into a gas without changing into a liquid first, this process is referred to as sublimation. • If a gas turns directly into a solid without changing into a liquid first, this process is referred to as deposition.
Humidity vs. Relative Humidity • The word humidity very general and can have many different meanings. • Sometimes we may say “the humidity is high”, or “it's really humid today”. • But this is a general term that has no real quantitative value associated with it. • Meteorologists use relative humidityto measure how much water vapor is in the air.
Relative Humidity • To understand relative humidity, we have to first explain the concept of percent water saturation in the air. • First of all, warm air always holds more water vapor than cold air. • If a parcel of air contains the maximum amount of water it can hold at a specific temperature, we say it is saturated.
Relative Humidity • Relative Humidity is a ratio or percentage of how much water vapor is in the air compared to how much water vapor could be in the air to make the air completely saturated. • There are basically two different ways to change the relative humidity of air. • The first way is, you could simply add or remove moisture from the air.
Relative Humidity and Dew Point • The second way you can change the relative humidity of a parcel of air is to change the temperature of the air. • If you change the temperature of air, you change the capacity of the air to hold moisture. • For example, If the temperature of air is lowered, the relative humidity increases. • Once air temperature drops to a temperature where the air becomes saturated, this temperature is referred to as the dew point temperature.
Relative Humidity and Dew Point • So for example, if a parcel of air at 20 Celsius had a relative humidity of 50%, and the air temperature dropped to 10 Celsius, the relative humidity would become 100%. • So in this example, 10 Celsius would be the dew point temperature. • Relative humidity can be measured with a hygrometer or a sling psychrometer.
Adiabatic Temperature Changes • Have you ever pumped up a bike tire and felt the pump get warmer. Or let air out of an air compressor and felt the cold air being released. • This is because the increase in pressure causes the molecules in the air to collide more frequently, creating thermal energy or heat. • As altitude increases, air pressure decreases, and temperature decreases.
Adiabatic Temperature Changes • These changes in air temperature caused by changes in altitude and pressure are called adiabatic temperature changes. • When air is unsaturated, it cools about 10 Celsius every 1km increase in elevation. • This is called the dry adiabatic rate. • When air is saturated, it cools about 5-9 Celsius, every 1km increase in elevation. • This is called the wet adiabatic rate.
The Formation of Clouds • When air rises and cools, it eventually reaches its dew point. • When air reaches its dew point, condensation occurs, and this forms clouds. • There are four different mechanisms that cause air to rise and form clouds. • They are orographic lifting, frontal wedging, convergence, and localized convective lifting.
Orographic Lifting • Orographic lifting occurs when air rises as it ascends over mountains, forming clouds.
Frontal Wedging • Frontal wedging occurs when a parcel of warm air encounters a parcel of cold air. • When this occurs, the warm air ascends over the cold air, creating a wedge-shaped front. • As the ascending warm air rises, it cools to its dew point and forms clouds.
Convergence • When air masses converge and collide, like in the case of areas of low pressure, then the resulting air masses rise. • This mechanism of rising air and cloud formation is called convergence.
Localized Convective Lifting • Because the surface of the ground is not equal, some places heat up more quickly than other places. • For example an area with bare rock is more likely to heat up than an area with vegetation. • In places that heat up more quickly, a rising pocket of hot air results causing localized convective lifting.
Stability of Air • When meteorologists refer to stability of air, what they are referring to is how rapidly air is rising or falling. • Hot air is more buoyant than cold air. This causes hot air to rise. • If air has very little difference in temperature with height, the density will be close to the same and the air will be stable.
Stability of Air • Unstable air that rises quickly, forms clouds. • Air that gets colder gradually as elevation increases is stable. • The most stable air is when air gets warmer with increasing elevation. • This is referred to as a temperature inversion.
Condensation of Water Vapor • When air becomes saturated with water vapor, condensation occurs. • When condensation occurs, there has to be some surface for the water to condense on. • In the air, the water usually condenses on small particles called condensation nuclei. • Condensation nuclei may include dust, smoke, volcanic ash, pollen, ions etc.
Types of Clouds • Cirrus – the name means “a curl of hair”. Clouds are high, white, and thin. They have a delicate wispy or feathery appearance. • Cumulus – the name means “a pile”. Clouds look like rounded cloud masses. They have flat bases with the appearance of rising domes or towers. Frequently described as having a cauliflower appearance.
Types of Clouds • Stratus – name means “a layer”. Clouds appear as sheets or layers that cover most or all of the sky. No distinct separation. • Types of clouds can also be subdivided by their elevation above the ground. • High clouds – Different types of cirrus clouds are found at these elevations. Usually associated with good weather.
Types of Clouds • Middle Clouds – Clouds that are found about 2-6 km (about 1-4 miles) above the ground are classified as middle clouds. These clouds are usually named with the prefix alto- before them. • Low Clouds – Clouds below 2 km (1 mile). • Nimbus is a Latin word that is used to describe a rainy cloud.
Types of Clouds • By joining the different names of clouds we can classify many different types of clouds. • For example, for high clouds we can have cirrus, cirrostratus, and cirrocumulus. • For middle clouds we have examples like altocumulus and altostratus. • For low clouds we have examples like stratocumulus, stratus, nimbostratus,cumulus and cumulonimbus.
Fog, Hail, and Sleet • Fog is a cloud that forms close to the ground. • One way fog can form is when a parcel of air reaches its dew point. • Another way that fog can form is when cool air moves over warmer water and evaporation from the water forms fog. • Hail and sleetform when liquid water falls from a warm pocket of air and then goes through a layer of air at freezing temperatures.
