Transcript Chapter 4
Chapter 4 Moisture & Atmospheric Stability Steam Fog Hydrologic cycle 1. Water is continually cycled between the atmosphere, land & seas 2. Powered by the Sun 3. Plants absorb water through their roots, and then release it into the atmosphere - called transpiration 4. Water balance - precipitation is exactly matched by evaporation & transpiration 380,000 cubic km per year. (would cover Earth to depth of 1 m.) H2O – A unique substance • Readily converted from state to state • Solid phase (ice) is less dense than the liquid phase (water) • Water expands as it cools through 4C • Water has an unusually high specific heat • Unusual properties mostly due to water’s ability to form hydrogen bonds Water's Changes of State 1. Liquid, Solid, Gas 2. One calorie - the amount of heat required to raise the temperature of one gram of water by 1 degree C 3. Latent Heat - heat energy that is transferred to or from a body, thereby changing its state, but not its temperature. (solid to liquid, liquid to gas - ice to water, water to steam). Latent means hidden. Water's Changes of State - 2 1. Evaporation - the process of converting a liquid to a gas (vapor). Latent heat of vaporization must be supplied. Most energetic molecules escape. Liquid cools. 2. Condensation - the process in which water vapor changes to the liquid state. Latent heat of condensation is released. Water's Changes of State - 3 1. Melting - the process in which a solid changes to a liquid. Requires heat. 2. Freezing - reverse of melting - releases latent heat of fusion Water's Changes of State - 4 1. Sublimation - conversion of a solid directly to a gas 2. Deposition - reverse of sublimation. Vapor converted directly to solid. (Frost) Water in the Atmosphere 1. Humidity - describes the amount of water vapor in the air 2. Absolute humidity - mass of water vapor in a given volume of air: mass of water vapor (gm) / volume of air (cubic m) [mixed units] Water in the Atmosphere - 2 1. Mixing ratio - mass of water vapor in a unit of air, compared to the mass of the dry air mass of water vapor (gm) / mass of dry air (kg) Vapor Pressure & Saturation 1. Pressure - caused by change of momentum of molecules as they hit a surface 2. (Water) Vapor pressure - that part of the total atmospheric pressure that is due to its water vapor content Vapor Pressure & Saturation - 2 1. Saturation -- number of molecules evaporating = number condensing 2. Saturated vapor pressure - pressure when the air is saturated (cannot hold any more water) Vapor Pressure & Saturation - 3 1. For every 10C change in temperature, amount of water needed for saturation doubles 2. Rate of evaporation depends on temperature and vapor pressure in the air Relative Humidity 1. Relative humidity is the ratio of the air's actual water content to the amount of water vapor required for saturation at that temp & pressure 2. Same thing as RH = actual mixing ratio / saturation mixing ratio Relative Humidity - 2 1. Relative humidity can be changed by adding/subtracting water or by changing the temperature 2. If the water vapor content remains constant, decreasing the temperature increases the relative humidity How to Change Relative Humidity 1. Add or subtract moisture 2. Increase or decrease the temperature Dry Air at 100% Relative Humidity? • The RH is a measure of how much water the air has in it, relative to the amount that it could actually hold at that temperature. • Cold air with 100% humidity (all the water it can hold) can actually contain less water than hotter air with a lower RH. How Relative Humidity Changes 1. RH = (actual mixing ratio / saturated mixing ratio) x 100% 2. Suppose mixing ratio at 20C is 7g/kg 3. RH = (7g/kg / 14g/kg) x 100 = 50% (see Table 4-1) 4. Suppose temperature rises to 20C. 5. RH = (7g/kg / 26.5g/kg) x 100 = 26% decreases How Relative Humidity Changes - 2 1. Suppose temperature falls to 1C. 2. RH = (7 g/kg / 3.9g/kg) x 100 - increases, but is > 100%!!! 3. What actually happens is that 7 - 3.9 = 3.1 gm condenses to form droplets Humidifiers & Dehumidifiers • In winter, the air is cold and cannot hold a lot of water vapor. Water evaporates from the skin, leading to dry skin. The solution is to add moisture, with an humidifier. • In summer, the air can be very humid (high RH), preventing the body from cooling by sweating. The solution is to remove moisture from the air (dehumidifier). Natural Changes in Relative Humidity 1. Daily changes in temperature 2. Temperature changes that result as air moves horizontally from one location to another 3. Temperature changes as air moves vertically Dew Point Temperature 1. Temperature to which a parcel of air would need to be cooled to reach saturation 2. Dew point temperatures are shown on weather maps Dew Point Temperature - 2 1. Example - Air at 23C contains 5 g vapor/kg of air 2. Cooling air to 5C (Table 4-1) will cause saturation & condensation 3. Thus dewpoint is 5C. Dewpoint Temps Atmospheric Hazard: Humidity and Heat Stress • • • • The body sweats in order to cool off (by the sweat evaporating). If the air is humid, the sweat can evaporate, but water from the atmosphere returns to the skin (dynamic equilibrium) Net effect is very little sweating. Physical exertion should be avoided on hot humid days Measuring Humidity 1. Instruments are called hygrometers 2. Psychrometer - wet & dry bulbs, observe temperature, look up tables 3. Hair hygrometer - not very useful 4. Electric hygrometer - electrical resistance changes as humidity changes Adiabatic Temperature Changes 1. Adiabatic - no heat lost or gained 2. Adiabatic cooling is the basis of cloud formation 3. A rising parcel of air expands because the atmospheric pressure decreases, and the parcel cools adiabatically (the expanding parcel has to do work) Adiabatic Temperature Changes- 2 1. Unsaturated air cools at 10C per km of ascent - this rate is called the dry adiabatic rate. 2. If the parcel of air rises enough, it will cool to its dewpoint, and condensation will occur. 3. The altitude at which this occurs is the lifting condensation level. Adiabatic Temperature Changes- 2 1. Above lifting condensation level, latent heat is liberated and slows the rate of cooling. Rate is called the wet adiabatic rate. 5C per km (high moisture content), 9C per km (low moisture content) Lifting Processes 1. Orographic lifting - air is forced to rise over a mountainous barrier 2. Frontal wedging - warmer, less dense, air is forced over cooler, denser air. 3. Convergence - a pileup of horizontal air flow results in upwards movement 4. Localized convective lifting - unequal surface heating heats localized pockets of air Orographic Lifting 1. Lifting causes adiabatic cooling, clouds, rain on windward side of mountains 2. Often get rain shadow deserts on leeward side of mountains. Water is extracted from the air on the windward side. Air on the leeward size is warmer than on the windward side, and therefore has a lower RH. 3. See Box 4-6 Orographic Effects • • • Causes of rain shadows. Water is extracted from the air on the windward side Air on the leeward size is warmer than on the windward side and therefore has a lower RH Rain shadow desert Frontal Wedging • Warmer, less dense, air is forced over cooler, denser air. Convergence 1. Collision of air masses 2. Obstacle slows or restricts horizontal air flow (wind) 3. Air moving from a relatively smooth surface to a rougher surface Localized Convective Lifting • On warm summer days, unequal heating of the Earth’s surface may cause pockets of air to be warmed more than the surrounding air • Warm parcels of air (thermals) rise above the lifting condensation level, and clouds form Rain Shadows & Chinooks • Orographic lifting by mountains leads to rain shadows. • Air coming down has lost a lot of its water vapor (as rain), and is heated. • RH therefore falls. • Chinooks are winds that move down the eastern slope of the Rockies - snow eater Atmospheric Stability 1. A rising air parcel cooler than surrounding air stable air - parcel tends to sink 2. A rising air parcel warmer than surrounding air - unstable air - parcel tends to keep rising 3. Absolute Stability - environmental lapse rate is less than wet adiabatic rate Atmospheric Stability 1. Absolute Instability - environmental lapse rate is greater than dry adiabatic rate 2. Conditional Stability - moist air has an environmental lapse rate between the dry and wet adiabatic rates. Air is stable with unsaturated parcel of air, but unstable with a saturated parcel of air. 3. "Conditional" because air must be forced upward before it reaches the level where it is unstable and rises on its own Stability & Daily Weather • Instability is enhanced by: 1. Intense solar heating warming the lower-most layer of the atmosphere 2. The heating of an air mass from below as it passes over a hot surface 3. General upward movement of air caused by processes such as orographic lifting, frontal wedging, convergence 4. Radiation cooling from cloud tops Stability & Daily Weather - 2 • Stability is enhanced by: 1.Radiation cooling of Earth's surface after sunset 2.The cooling of an air mass from below as it passes over a cool surface 3.General subsidence within an air column Problem 4, Part C • As the air rises, more condensation occurs because the cooling air cannot hold any more water vapor. • The dewpoint temperature is equal to the local air temperature. • So, at the top of the mountain, the dewpoint temperature is 0C.