Drill: What is humidity?

1. Drill: What is humidity?

2. Section 1 Atmospheric Moisture Chapter 23 Objectives • Explainhow heat energy affects the changing phases of water. • Explainwhat absolute humidity and relative humidity are, and describe how they are measured. • Describewhat happens when the temperature of air decreases to the dew point or below the dew point.

3. Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water • Water in the atmosphere exists in three states, or phases. • One phase is known as a gas called water vapor. • The other two phases of water are the solid phase known as ice and the liquid phase known as water. • Water changes from one phase to another when heat energy is absorbed or released.

4. Section 1 Atmospheric Moisture Chapter 23 Latent Heat latent heatthe heat energy that is absorbed or released by a substance during a phase change • When liquid water evaporates, the water absorbs energy from the environment. • When water vapor changes back into a liquid through the process of condensation, energy is released into the surrounding air and the molecules move closer together.

5. Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water, continued Evaporation • Most water enters the atmosphere through evaporation of ocean water near the equator. • However, water vapor also enters the atmosphere by evaporation from lakes, ponds, streams, and soil. • Plants release water into the atmosphere in a process called transpiration. • Volcanoes and burning fuels also release small amounts of water vapor into the atmosphere.

6. Section 1 Atmospheric Moisture Chapter 23 Sublimation sublimation the process in which a solid changes directly into a gas (the terms is sometimes used for the reverse process) • Ice commonly changes into a liquid before changing into a gas. • When the air is dry and the temperature is below freezing, ice and snow may sublimate into water vapor.

7. Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water, continued The diagram below shows the different phases of water.

8. Section 1 Atmospheric Moisture Chapter 23 Reading Check Summarize the conditions under which sublimation commonly occurs.

9. Section 1 Atmospheric Moisture Chapter 23 Reading Check Summarize the conditions under which sublimation commonly occurs. When air is very dry and the temperature is below freezing, ice and snow change directly into water vapor by sublimation.

10. Section 1 Atmospheric Moisture Chapter 23 Humidity dew point: at constant pressure and water vapor content, the temperature at which the rate of condensation equals the rate of evaporation • Water vapor in the atmosphere is known as humidity. • Humidity is controlled by rates of condensation and evaporation. • When the rate of evaporation and the rate of condensation are in equilibrium, the air is said to be “saturated.”

11. Section 1 Atmospheric Moisture Chapter 23 Humidity, continued • The rate of evaporation is determined by the temperature of the air. • The higher the temperature is, the higher the rate of evaporation is. • The rate of condensation is determined by vapor pressure. • When vapor pressure is high, the condensation rate is high.

12. Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Absolute Humidity absolute humidity the mass of water vapor per unit volume of air that contains the water vapor, usually expressed as grams of water vapor per cubic meter of air

13. Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Absolute Humidity, continued • However, as air moves, its volume changes as a result of temperature and pressure changes. • Therefore, meteorologists prefer to describe humidity by using the mixing ratio of air. • The mixing ratio of air is the mass of water vapor in a unit of air relative to the mass of the dry air.

14. Section 1 Atmospheric Moisture Chapter 23 Humidity, continued The diagram below shows the effects of vapor pressure.

15. Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Relative Humidity relative humiditythe ratio of the amount of water vapor in the air to the amount of water vapor needed to reach saturation at a given temperature • If the temperature does not change, the relative humidity will increase if moisture enters the air. • Relative humidity can also increase if the moisture in the air remains constant but the temperature decreases.

16. Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Reaching the Dew Point • When the air is nearly saturated with a relative humidity of almost 100%, only a small temperature drop is needed for air to reach its dew point. • Air may cool to its dew point by conduction when the air is in contact with a cold surface. • The resulting form of condensation is called dew.

17. Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Reaching the Dew Point, continued • If dew point falls below the freezing temperature of water, water vapor may change directly into solid ice crystals, or frost. • Because frost forms when water vapor turns directly into ice, frost is not frozen dew. • Frozen dew is relatively uncommon. Unlike frost, frozen dew forms as clear beads of ice.

19. Section 1 Atmospheric Moisture 2-18-09 Drill: What is the difference between Relative and Absolute humidity? • Explainhow heat energy affects the changing phases of water. • Explainwhat absolute humidity and relative humidity are, and describe how they are measured. • Describewhat happens when the temperature of air decreases to the dew point or below the dew point.

20. Quiz: find the Abs Humidity and Relative humidity for the following temperatures • Temperature Celsius Saturation mass H20 sample (g/M3) -10 2.36g 2.0 12 10.66g 5.5 15 12.83 12.83 25 23.00g 20.00 Format 1) Temp: _______ Relative Hum: _______ Absolute Hum: _____ 2) ….

22. Section 1 Atmospheric Moisture Chapter 23 Measuring Relative Humidity

23. Drill: When using a psychrometer to measure the atmospheric relative humidity, describe the RH if…The readings of the thermometers are far apart.The readings of the thermometers if they are close together 2/23/09 (Objectives are on next slide)…

24. Section 2 Clouds and Fog Chapter 23 Objectives • Describe the conditions that are necessary for clouds to form. • Explain the four processes of cooling that can lead to the formation of clouds. • Identify the three types of clouds. • Describe four ways in which fog can form.

25. Chapter 23 Cloud Formation cloud a collection of small water droplets or ice crystals suspended in the air, which forms when the air is cooled and condensation occurs • For water vapor to condense and form a cloud, a solid surface on which condensation can take place must be available. • In addition, for clouds to form, the rate of evaporation must initially be in equilibrium with the rate of condensation.

26. Chapter 23 Cloud Formation, continued condensation nucleus a solid particle in the atmosphere that provides the surface on which water vapor condenses • Although the lowest layer of the atmosphere, the troposphere, does not contain any large solid surfaces, it contains millions of suspended particles of ice, salt, dust, and other materials. • Because the particles are so small—less than 0.001 mm in diameter—they remain suspended in the atmosphere for a long time.

27. Section 2 Clouds and Fog Chapter 23 Cloud Formation, continued The diagram below shows the molecular formation of a water droplet.

28. Chapter 23 Adiabatic Cooling adiabatic cooling the process by which the temperature of an air mass decreases as the air mass rises and expands • As a mass of air rises, the surrounding atmospheric pressure decreases. • Thus, fewer collisions between the molecules happen. • The resulting decrease in the amount of energy that transfers between molecules decreases the temperature of the air.

29. Section 2 Clouds and Fog Chapter 23 Adiabatic Cooling, continued Adiabatic Lapse Rate • The rate at which the temperature of a parcel of air changes as the air rises or sinksis called the adiabatic lapse rate. • The adiabatic lapse rate of clear air is about –1°C per 100 m that the air rises. • The slower rate of cooling of moist air results from the release of latent heat as the water condenses.

30. Chapter 23 Adiabatic Cooling, continued Condensation Level • When air cools to a temperature that is below the dew point, net condensation causes clouds to form. • The altitude at which this net condensation begins is called the condensation level. • The condensation level is marked by the base of the clouds. • Further condensation allows clouds to rise and expand above the condensation level.

31. Section 2 Clouds and Fog Chapter 23 Reading Check What is the source of heat that warms the air and leads to cloud formation? The source of heat that warms the air and leads to cloud formation is solar energy that is reradiated as heat by Earth’s surface. As the process continues, latent heat released by the condensation may allow the clouds to expand beyond the condensation level.

32. Drill: What is the adiabatic process? • Explain the four processes of cooling that can lead to the formation of clouds. • Identify the three types of clouds. • Describe four ways in which fog can form.

33. Section 2 Clouds and Fog Chapter 23 Mixing • Some clouds form when one body of moist air mixes with another body of moist air that has a different temperature. • The combination of the two bodies of air causes the temperature of the air to change. • This temperature change may cool the combined air to below its dew point, which results in cloud formation.

35. Chapter 23 Lifting • Air can be forced upward when a moving mass of air meets sloped terrains, such as a mountains range. • As the rising air expands and cools, clouds form. • The large cloud formations associated with storm systems also form by lifting. • These clouds form when a mass of cold, dense air enters an area and pushes a less dense mass of warmer air upward.

37. Chapter 23 Advective Cooling advective cooling the process by which the temperature of an air mass decreases as the air mass moves over a cold surface • As air moves over a surface that is colder than air is, the cold surface absorbs heat from the air and the air cools. • If the air cools below its dew point, clouds form.

39. Section 2 Clouds and Fog Chapter 23 Classification of Clouds • Clouds are classified by their shape and their altitude. • The three basic cloud forms are stratus clouds, cumulus clouds, and cirrus clouds. • There are also three altitude groups: low clouds (0-2,000 m), middle clouds (2,000 to 6,000 m), and high clouds (above 6,000 m).

41. Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Stratus Clouds stratus cloud a gray cloud that has a flat uniform base and that commonly forms at very low altitudes • Stratus means “sheet-like” or “layered.” • Stratus clouds form where a layer of warm, moist air lies above a layer of cool air. • Stratus clouds cover large areas of sky and often block out the sun.

43. Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Stratus Clouds, continued • The two variations of stratus clouds are known as nimbostratus and altostratus. • Unlike other stratus clouds, the dark nimbostratus clouds can cause heavy precipitation. • Altostratus clouds form at middle altitudes. They are generally thinner than the low stratus clouds and produce very little precipitation.

44. Nimbostratus Altostratus

45. Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Cumulus Clouds cumulus cloud a low-level, billowy cloud that commonly has a top that resembles cotton balls and has a dark bottom • Cumulus means “piled” or “heaped.” • These clouds form when warm, moist air rises and cools. As the cooling reaches its dew point, the clouds form. • The flat base that is characteristic of most cumulus clouds represents the condensation level.

47. Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Cumulus Clouds, continued • High, dark storm clouds known as cumulonimbus clouds, or thunderheads, are often accompanied by rain, lightning, and thunder. • If the base of cumulus clouds begins at middle altitudes, the clouds are called altocumulus clouds. • Low clouds that are acombination of stratus and cumulus clouds are called stratocumulus clouds.

48. Cumulonimbus

49. Altocumulous

50. Stratocumulous