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1 NOTICE: The information contained within this document is copyright © U.S. Coast Guard Auxiliary Association No attempts to add, modify or delete information contained within this document shall be made. If errors are discovered, please contact the U.S. Coast Guard Auxiliary’s National Training Directorate. Slide 1:

2 WEATHER Specialty Course AUXWEAChapter 3 – Moisture, Latent Heat, Fog and Stability

3 Overview Properties of water. Composition of the atmosphereDry and with water vapor. Vertical structure of the atmosphere. Troposphere, stratosphere, mesosphere, thermosphere. Definition of lapse rate. The adiabatic process and the related lapse rates. Dry adiabatic lapse rate. Wet adiabatic lapse rate. Properties of water. Phases of water and the change among them. Latent heat of phase changes.

4 Overview (continued) How moisture in the atmosphere is reported.Absolute humidity. Relative humidity. Dew point. Fog types—formation and dissipation. Atmospheric stability. Absolute and conditional stability. Absolute instability. An Example: The Chinook or Foehn wind.

5 Composition of the AtmosphereMajor constituents. Nitrogen. Oxygen. The rest is mostly Argon, with small amounts of: Carbon dioxide. Neon. Helium. Methane. Variable amount of water vapor. ©1997, USA Today. Reprinted with permission

6 Structure of the AtmosphereThe atmosphere has four major layers and three boundaries. Primary distinguishing feature is the temperature change with height. Most of the water and most of the weather occurs in the troposphere, the lowest layer. This vertical temperature structure is from the U.S. Standard Atmosphere and is representative of mid-latitudes. Courtesy of NOAA

7 Lapse Rates The way the temperature decreases or increases with altitude in the atmosphere is called the lapse rate. The environmental lapse rate is what is seen at any given time and place and changes with altitude. Those shown on the previous slide are averages—in mid-latitudes it averages °F/1000 ft. in the lower troposphere. The dry adiabatic lapse rate is how fast unsaturated air cools as it is pushed up. Its nominal value is -5.5°F/1000 ft. The wet adiabatic lapse rate is how fast saturated air cools as it is pushed up. Its nominal value is -3.2°F/1000 ft. The reason for the differences between dry and wet rates is shown later in this chapter. The definition of adiabatic is on the next slide.

8 More About Lapse Rates The average environmental lapse rate is shown in the top figure. The air is NOT rising. Adiabatic lapse rates are named for the adiabatic process. Adiabatic processes occur without the addition or loss of any energy from/to the outside. Air rising cools adiabatically. Air sinking warms adiabatically. ©1997, USA Today. Reprinted with permission

9 The Properties of WaterThe properties of water are uniquely suited to life. It expands when frozen, making ice float. It changes phase at temperatures found on the Earth. It is an excellent solvent. Its properties are also well suited to weather processes. It releases a lot of energy when it condenses or freezes. It is an efficient absorber of infrared energy. It can transport energy from one location to another. There is an abundant supply.

10 Properties of Water (continued)The amount of water vapor in air (by weight) varies: From less than 1% (very dry air). To about 4% (very moist air). Moist air is lighter than dry air. Nitrogen (79%) has a molecular weight of 28. Oxygen (21%) has a molecular weight of 32. Argon (<1%) has a molecular weight of 14. Average: Water (H2O) has a molecular weight of 18. Every molecule of air replaced by one of water lowers the molecular weight of the air by 10.7 for every 18 grams of water.

11 Phase Changes and Latent HeatThe phases of water are shown below, along with the names of the phase-change processes. While water melts or evaporates, it stores large amounts of energy, called latent heat. = 620 calories per gram = 620 calories per gram ©2004, US Power Squadrons. Reprinted with permission

12 The Importance of Latent HeatThe latent heat tied up in water (80 cal/g), and the even larger amounts added in water vapor (540 cal/g), is carried with the water or vapor as it is transported in the atmosphere. This transport of latent heat is added to conduction, convection, and radiation as a means of energy transport. The condensation of water is a key driver of many kinds of weather phenomena, such as thunderstorms and hurricanes, because the released latent heat upon condensation provides energy to the systems.

13 Measures of Humidity The concentration of water vapor in the atmosphere is reported in three primary ways: Absolute humidity is the mass of water vapor in a given mass of air, normally given as grams per kilogram (g/Kg) of dry air. The mass the air can hold increases with increasing temperature. Relative humidity (RH) is a ratio of the mass of water vapor in the air to the maximum it can hold at that temperature (as a percent). The dew point is the temperature the air would have to be cooled to in order to reach saturation (100% RH). A fourth, seen occasionally, is the dew point depression—the temperature minus the dew point. Humidity is measured with a Psychrometer.

14 Relation of Moisture Variablesg/Kg 100% RH This is an animated slide. It initially shows the relation of saturation absolute humidity to temperature. As the slide is advanced, it shows how a parcel of air changes with cooling and relates temperature to dewpoint. 100% RH Air Parcel 30% RH Dew Point

15 Plot of Moisture Variables3-day plot of Temperature, Dew Point and Relative Humidity Temperature and Dew Point (°F) Relative Humidity (%) RH Temp This plot is an example from the author’s weather station. Note that the closer together the temperature and dew point are (the smaller the dew point depression), the higher the relative humidity. DP

16 Station Model—MoistureThis station model adds dew point, in °F. The dew point is 59°F. The dew point depression is therefore 8°F, so the relative humidity is about 75%, as estimated from the graph in slide 14. 67 965 59 -23\ As a reminder, the number in the upper left is temperature in deg F, the upper right is pressure (996.5 mb) and the number to the right is pressure tendency in tenths of a mb (pressue falling 2.3 mb in 3 hours).

17 Fog The principles in this chapter help us understand fog.Fog forms in many ways, can last for hours or days. Fog forms by either a decrease in temperature or the addition of moisture (raising the dew point). The basic types to be discussed are: Radiation fog. Temperature decrease. Advection fog. Temperature decrease. Precipitation fog. Moisture added. Steam fog (sea smoke). Moisture added. Up-slope fog. Temperature decrease. Valley fog. Similar to radiation fog.

22 Air is raised orographically, cooling to the dew point.Upslope Fog Air is raised orographically, cooling to the dew point. 1 2 1. Wind blows humid air up hills or mountains. 2. As the air rises, it cools to the dew point, fog drifts up the hill; widespread upslope fog is common on the Great Plains, where the land slopes gently upward toward the Rockies. ©1997, USA Today. Reprinted with permission

23 An especially persistent form of radiation fog.Valley Fog 1 2 3 1. In valleys, especially in the west during the winter, radiation fog can become more than 1,500 feet thick. 2. Weak, winter sun isn’t strong enough to evaporate the fog completely, but might warm the ground enough for a layer of fog up to around 500 feet above the ground to evaporate. 3. Such fogs can last for days, until a storm comes along with strong winds to push out the cold air. An especially persistent form of radiation fog. ©1997, USA Today. Reprinted with permission

24 Fog Summary Fog Type Radiation Valley Advection Upslope Precipi-tationSea Smoke Typical Occurrence Clear night with radiational cooling in low wet areas. During Winter in valleys with radiational cooling. Warm, moist air transported over colder surface Warm, moist air lifted by upslope air flow, cooled by expansion to saturation. Rain from warm air falling through colder air at surface saturates cold air at surface. Cold air blows over warmer water, taking on moisture to saturation. Winds 3-5 knots for gentle mixing 4-15 knots Sufficient for Lifting Not Important Dissipation Daytime warming of ground warms air through conduction and mixing. Advent of storm with strong winds replaces saturated air with dryer air. Wind changes direction so warm air flow is eliminated. Sometimes higher winds lift off ground. Daytime warming evaporates fog, or wind may change direction. Frontal Passage or end of precipitation. Daytime warming or wind changes direction

25 Dew and Frost Dew—Dew point >32° F Frost—Dew point <=32° F 25Whether dew or frost appears depends on the dew point of the air. Why? As the ground cools at night, it cools the air immediately above by conduction and convection. The air temperature decreases and eventually reaches the dew point. At that point, the temperature equals the dew point, so a temperature over 32° F results in dew, while a lower temperature results in frost. 25

26 Atmospheric Stability (1 of 3)Atmospheric stability can have one of three states. The first is absolute stability. An air parcel lifted to any height will return to its original position as it is heavier than the surrounding air. The key to this and the next three slides is the relationship between the existing environmental lapse rate and the adiabatic lapse rate—whether lifted air becomes colder or warmer than its surroundings. If warmer, it rises, if colder is descends. Here, as the air is pushed up, it is colder and there returns to its original height ©1997, USA Today. Reprinted with permission

27 Atmospheric Stability (2 of 3)The second type is absolute instability. An air parcel will rise on its own, since it is lighter than surrounding air, and only stop when it reaches stable air, such as the tropopause. In this case, the air is warmer than the surroundings, therefore lighter, as continues to rise. ©1997, USA Today. Reprinted with permission

28 Atmospheric Stability (3 of 3)The third type is conditional stability. Air pushed up a little is stable, but pushed up more becomes unstable. In the lower levels, the air is colder than its surroundings. Above the buoyancy level, it is warmer and so continues to rise. ©1997, USA Today. Reprinted with permission

29 How Stability Can ChangeAtmospheric stability is constantly changing. This example shows a progression during a day. The top figure shows an early-morning inversion, with stable air. In the bottom figure, the lower atmosphere has heated, becoming unstable. The same principles in the previous slides apply here. In the top graphic, the inversion makes the air stable. As the surface heats, and heats the air above by conduction and convection, the air can eventually become unstable. ©2004, US Power Squadrons. Reprinted with permission

30 How is Air “Pushed up”? In the stability discussion, we talked about air being pushed up. How does that happen? Orographic lifting, such as the wind blowing up the side of a mountain. Frontal wedging, where the cold air pushes under the warm air (cold front) or the warm air rides over the cool air (warm front). Surface convergence, where surface winds from different directions meet and the air must rise since it can’t go down. Upper air divergence, where air below rises to replace that lost. Convection, where the air is inherently unstable. We will see examples of these in future chapters, but one example is shown on the next slide.

31 Chinook Winds Chinook is a warm wind on the lee side of mountains.Air is lifted orographically. It cools at the dry adiabatic lapse rate. When saturated, it cools at the wet adiabatic lapse rate. On the lee side, it warms at the dry adiabatic lapse rate. Other places have other names (Foehn in Europe). Since the wet adiabatic lapse rate is less than the dry adiabatic lapse rate, the air cools more slowly once it reaches saturation. On the lee side is warms at the higher dry adiabatic lapse rate. ©2004, US Power Squadrons. Reprinted with permission

32 Summary (1 of 3) The Atmosphere is primarily Nitrogen and Oxygen.With small amounts of Argon and other gases. A variable concentration of water vapor (1 to 4 percent). Temperature profile varies with altitude in four layers: Decreasing in the troposphere (top is tropopause). Increasing in the stratosphere (top is stratopause). Decreasing in the mesosphere (top is mesopause). Increasing in the thermosphere (blends into space). Temperature change with height is called the lapse rate. Environmental lapse rate: What is currently in the atmosphere. Adiabatic lapse rate: How air cools when ascending or warms when descending.

33 Summary (2 of 3) Water is a very important substance in the atmosphere. It changes state at normal temperatures. It carries very large amounts of latent heat. It is an efficient absorber of infrared energy. Phases of water are solid (ice), liquid, and gas (vapor). Ice <-> Water: Melting or freezing. Latent heat 80 cal/g. Water <-> Vapor: Evaporation or condensation cal/g. Ice <-> Vapor: Sublimation or Deposition cal/g. Latent heat: Energy needed to melt ice or evaporate water. Release of this latent heat by condensation is a major driver of atmospheric processes. Note that = 620.

34 Summary (3 of 3) Humidity is a measure of water vapor in the atmosphere. Absolute humidity is concentration in g of water per kg of dry air. Saturation absolute humidity is maximum air can hold at a temperature. Relative humidity is the ratio of absolute to saturation absolute humidity. Dew point is the temperature air must cool to for saturation to occur. Stability of the atmosphere is its tendency to rise. Absolute stability means air, if lifted, will return to the original height. Conditional stability means air lifted far enough will continue to rise. Absolute instability means air will rise on its own. Air can be lifted four ways. Orographic lifting is wind blowing up a mountain. Frontal wedging is cold air pushing up warm air. Surface convergence forces air between to rise. Convection.

35 Chapter 3 Questions QUESTION ANSWERIn the lower atmosphere, the temperature generally ___________ with altitude. The troposphere is where most of the weather occurs because: The two most abundant gasses in the atmosphere are: A temperature increase with altitude is called what? A warm, dry wind on the east side of the Rockies is called what? Decreases. It contains most of the moisture. Nitrogen and Oxygen. Slide 35: Some Chapter Questions (1 of 2) ANIMATIONS Five questions are displayed. [CLICK] to display each answer in sequence. Font change used to simulate human handwriting. After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer). * * * * * * * * * * (end comments). An inversion. A Chinook wind.

36 Chapter 3 Questions QUESTION ANSWERWhen water vapor condenses heat energy is: The energy that is released when water freezes or vapor condenses is called: Saturation can be accomplished by what two methods? The temperature at which water vapor begins to condense is called: When the relative humidity of air is 100%, it is said to be: Released into the atmosphere. Latent heat. Lowering the temperature or adding moisture. Slide 36: Some Chapter Questions (1 of 2) ANIMATIONS Five questions are displayed. [CLICK] to display each answer in sequence. Font change used to simulate human handwriting. After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer). * * * * * * * * * * (end comments). The dew point. Saturated.

37 Chapter 3 Questions QUESTION ANSWERA change in the temperature or density of air without the addition or release of heat energy is called: The term describing the tendency of lifted air to return to its former level is: The term describing the tendency of air to rise on its own is: The term describing the tendency of air to continue to rise after being lifted to some higher level is: Fog that forms at night under clear skies with little wind is called: An adiabatic process. Absolute Stability. Absolute Instability. Slide 37: Some Chapter Questions (1 of 2) ANIMATIONS Five questions are displayed. [CLICK] to display each answer in sequence. Font change used to simulate human handwriting. After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer). * * * * * * * * * * (end comments). Conditional stability. Radiation fog.

38 End of Chapter 3 Are there any questions?The next three chapters make up Part II, Sensible Weather (what we see and feel). Chapter 4 covers Air Masses, Fronts, and Cyclones

NOTICE: The information contained within this document is copyright © U.S. Coast Guard Auxiliary Association No attempts to add, modify or delete information.

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