Chapter 18 Evaporation, Condensation, and Precipitation

I. Evaporation and Humidity  Topic 1 States of Water –Evaporation is a phase change of water from a liquid to a gas. –Condensation is a phase change of water from gas to liquid.

 Topic 1 States of Water –When water changes phases from solid to liquid to gas, it goes through different energy states. –Between each phase change the energy gain does not affect the temperature of the substance. These points are called latent heat.

 Topic 2 Evaporation –The evaporation process involves energy being absorbed by the water. –Energy increases the movement of the water molecules. –When they absorb enough energy, they are moving fast enough to escape the surface of the water and become a gas.

 Topic 2 Evaporation –Evaporation is a cooling process.  When high energy water molecules escape water, the average energy level of the water is lowered.  Lowering the energy level cools water down.

 Topic 3 Specific Humidity and Capacity –Air holds water. The amount of water in the air is called specific humidity.  It is measured as grams of water per km of air. – Warm air can hold more water than cold air.  As the temperature increases the capacity increases. The air becomes saturated when the specific humidity equals the air’s capacity.  Water capacity doubles at every 11°C increase in temperature.

 Topic 4 Relative Humidity –Relative humidity is: specific humidity/capacity. – Answer:  If the amount of water in the air at 20C does not change when the air heats up to 30C, then the relative humidity becomes lower, because the specific humidity does not change, but the capacity increases.

 Topic 5 Finding Relative Humidity –Relative humidity can be measured using a psychrometer.  A psychrometer is a tool used to determine the relative humidity using a dry and wet thermometer.  The wet thermometer allows us to determine the evaporation/dew point of the air.  This then allows you to determine relative humidity based on capacity.

II. Forms of Condensation  Topic 6 Condensation and Dew Point –Air must cool down to condense. The point that this occurs is called the dew point.  This is the point that air is saturated. The specific humidity and capacity are equal, and relative humidity is 100%.

 Topic 7 Condensation Requires Cooling and Nuclei –In order for rain to form, water in the air needs something to condense onto, such as salt, sulfate, or nitrate particles.  These are called condensation nuclei. –If the air has no condensation nuclei, and it has cooled lower than its dew point, air can become supersaturated.

 Topic 8 Dew and Frost from Contact –Why does dew form on plants, cars, and other objects at night?  This happens because the objects cool faster than the air. When warmer air touches a cooler object, it lowers the air temperature to the dew point.  Dew forms when temperatures stay above 0°  Frost forms when temperatures drop below 0°

 Topic 9 Fogs from Radiation and Advection –Fog is literally a cloud on the ground.  When surface air cools lower than the dew point, water condenses into tiny droplets. The droplets are not heavy enough to be rain. –Fog forms in two different ways:  Radiation, in which the ground loses heat quickly, and cools the air above it past its dew point.  Advection, when warm air blows over cool ground. California fogs are formed in this way.

III. Clouds  Topic 10 The Origin of Clouds –Clouds are fogs, mist, or haze that form when air cools to its dew point and water forms.

 Topic 10 The Origin of Clouds –Stratiform clouds are formed by air moving horizontally, and they form in layers.

 Topic 10 The Origin of Clouds –Cumuliform clouds are made by air moving vertically. They are large and massive columns.

 Topic 10 The Origin of Clouds –At temperatures above 0°C, clouds are made of mostly water. – At temperatures below 0°C, clouds are made of ice crystals and supercooled water.

 Topic 11 Cloud Names and Their Meanings –There are three types of clouds  Cirrus (Latin for curl) – clouds that are thin or feathery, and very high up.

 Topic 11 Cloud Names and Their Meanings  Stratus (Latin for layer) – clouds that are layered or flat, and usually low in the sky.  Cumulus (Latin for heap) – are massive and thick, or puffy clouds.

 Topic 11 Cloud Names and Their Meanings –The three main type of clouds are given prefixes or suffixes to describe: –location, –a combination of two types of clouds, or –rain clouds. Alto means middle Nimbus or Nimbo Latin for violent rain. Cirro means high

 Topic 12 Dry- and Moist Adiabatic Lapse Rates –The shape of the cloud can tell you direction of air movement.  Such as cumulus clouds show vertical movement.

 Topic 12 Dry- and Moist Adiabatic Lapse Rates –Air inside a cloud cools more slowly than air outside a cloud.  The dry adiabatic rate (outside clouds) is 1°C per 100 meters.

 Topic 12 Dry- and Moist Adiabatic Lapse Rates  Inside the rate is.6°C per 100 meters, or 1°C per 160 meters. –The reason air cools more slowly is because water gives off heat to the air as condenses.

 Topic 13 Clouds with Vertical Development –As stated before, cumulus clouds form from rising air masses.  The air forming the cloud cools slower than the air around it, causing the cloud air to be more buoyant.

 Topic 13 Clouds with Vertical Development –Cumulonimbus clouds bring thunder, lightning, hail, and tornados. –The base of a cumulus cloud is called the condensation level.

 Topic 15 Layer Clouds – Stratus clouds are formed by air moving horizontally in two ways:  From air rising up a mountainside, or  Mixing air cools the air to its dew point.

 Topic 16 How Raindrops Form –Precipitation is the falling of any form of water from the air to the Earth. – Rain forms in two ways:  By condensation producing tiny droplets of water that collide together, and

 Topic 16 How Raindrops Form  Ice crystals that get larger through evaporation and condensation of super-cooled water.

 Topic 17 Forms of Precipitation – There are five forms of precipitation:  Drizzle, rain, snow, sleet, and hail. –Hail forms from frozen raindrops or ice crystals that get larger because high winds keep the ice ball aloft.

 Topic 18 Measuring Precipitation – Rain is measured in inches by using a rain gauge. –Snow is also measured in inches using a stick.  Snow is much less dense than rain, 10 inches of snow equals 1 inch of rain.

 Topic 19 Where does it Rain? – Rain usually occurs in places where air rises and cools, such as the windward side of a mountain, or low pressure areas.

 Topic 19 Where does it Rain? –It rarely rains on the leeward side of a mountain, because air sinks and warms.  It does not rain in areas of high pressure.  Deserts exist on the lee side of mountains.

 Topic 21 Weather Modification – Clouds can be forced to produce rain by seeding the clouds with carbon dioxide pellets, or iodide.

 Topic 22 Acid Clouds and Acid Rain –Most rain is slightly acidic.  However, some rains are very acidic.  Water nucleates around sulfates and nitrates produced from burning fossil fuels.  Sulfuric acid is a very strong acid.

 Topic 22 Acid Clouds and Acid Rain –Most rain is slightly acidic from carbon dioxide forming carbonic acid.