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Date Created: 02/28/16
Atmospheric, Circulation, Water, and Weather Lectures 13 Of the four spheres of the Earth System, there are two that we can focus on here: o Atmosphere: Climate Greenhouse Gases Water in its various forms Weather o Hydrosphere: Aquifers Glaciers Ice Lakes Oceans Reservoirs Rivers Snow Water is Amazing: o With Earth maintaining it’s temperatures and atmospheric pressure, all three phases can exist near or on the surface o Polar (not equally shared) covalent (atoms share electrons) bonds o Opposite ends of molecule have different charges o In water, the hydrogen has a partial positive charge and oxygen has a partial negative charge Water forms Hydrogen Bonds: o Because these bonds are difficult to break water has a very high: Boiling point Melting point Viscosity o This is why liquid water is present at the Earth’s surface o Energy gets locked up in the relationship between molecules when liquid water changes to vapor o Warm air “holds” more water vapor Humidity: o Refers to the amount of water held as vapor (water gas) in the air Relative Humidity: o The ratio of the actual amount of water vapor that is in the air, relative to the maximum water vapor possible in the air at a given temperature o Therefore, relative humidity varies because of evaporation condensation, or temperature changes Saturation: o When the rate of evaporation (liquid to gas) and the rate of condensation (gas to liquid) each equilibrium o So: you feel sweatier in conditions of high relative humidity because of air is unable to accommodate the evaporation of sweat from your body Dew Point: o The temperature at which a given mass of air containing a certain amount of water becomes saturated o Aka: air is saturated when the dew point temperature and the air temperature are the same o Example The cold glass of ice water chills the air immediately around it, lowers the air temperature, increases the relative humidity to the point of saturation o Question What would happen if you put a fan nearby? Would you get as much condensation? Vapor Pressure: o Water vapor pushing against the pressure of the atmosphere o Vapor Pressure The portion of total air pressure that is comprised of water vapor molecules (mB) o Warm tropical air can hold lots of water as vapor providing the latent heat for storms o Cold polar air does not generate much precipitation, even though it is close to the dew point Specific Humidity: o The actual mass of water vapor per mass of air at any specified temperature (units of grams of water vapor per kilogram of air) o Not a relative measure only changes if we remove water vapor from that unit of air o Useful for weather predicting understanding the complex mixing of air parcels Atmospheric (in)stability: o Refers to the tendency of an air parcel to either remained in place (stability), or move vertically, by either ascending (instability) of descending o Warm air (lower density) rises o Cold air (higher density) descends o Air parcels are carrying some water vapor o Air Mass Characteristics: Air takes on characteristics of the local environment They initially take on characteristics of source environments o Moisture: m (maritime/wetter) c (continental/drier) o Temperature: A (Arctic) P (polar) T (tropical) E (equatorial) AA (Antarctic) How does air move around? o Rising, warm air is less dense, creates low pressure at the surface o Rising air expands and cools as it rises o If air cools to the point where it is saturated (100% relative humidity), it will: Release latent heat as vapor condenses to liquid Form a cloud o Continue to rise and cool, but at a slower rate How does air move around? o We now have physical rules for how air moves vertically horizontally o The evaporation and condensation of water moves a large amount of energy and matter (water) around Cyclone Low Pressure: o Converging air o Warm air rises and cools Anticyclone High Pressure: o Diverging air o Descending air warms + dries Pressure Gradient: o Air moves from high pressure to low pressure o Gravity + Pressure Gradient + Coriolis + Friction o Energy gets locked up in the relationship between molecules when liquid water changes to vapor o Warm air “holds” more water vapor How does air move around? o Rising, warm air is less dense, creates low pressure at the surface o Rising air expands and cools as it rises o If air cools to the point where it is saturated (100% relative humidity), it will: Release latent heat as vapor condenses to liquid Form a cloud Continue to rise and cool, but at a slower rate Atmospheric Lifting Mechanics: o Air contains some water as vapor o Warm air can hold more water as vapor o If you cool down air enough, the vapor will condense into liquid later o Rising air cools as it ascends o So rising air gives us clouds and rainfall o Atmospheric Lifting Mechanisms: Conduction + Convection: Parcel of air gets warmed by surface (land or water), become more buoyant (less dense) than surrounding environment, and therefore rises Convergence: Air comes together from different directions, can’t all be in the same place and can’t go down, so rises into the atmosphere Orographic Lifting: Air encounters a mountain or other topographic feature which forces it to rise. Windward side will be wet, leeward side will be dry (rainshadow) Frontal Lifting: The Meeting of Cold and Warm Air (Fronts): When cold continental air meet warm maritime air, warm air is forced upwards Cold Front: o Cold air actively jams itself underneath the lessdense warm air o Cold air forces warm, moist air to rise. Warm air cools, water condenses, forms cloud and then rains o Warm air smoothly rides up over moredense cold air Clouds: o A cloud is an aggregation, or grouping, or liquid moisture droplets and ice crystals that are suspended in air and great enough in volume and density to be seen by the human eye o The moisture droplets are very, very small a raindrop may consist of millions of these water droplets o Water droplets do not condense in each other They need condensation nuclei o Condensation Nuclei Microscopic particles that provide the surface for condensation of atmospheric water vapor to water salts, organic matter and compounds, dust soot, ash LargeScale Features: o Three Main Cells of Circulating Air: Hadley Cell Ferrel Cell Polar Cell High Pressure: o Anticyclone (anticyclonic flow spiraling down toward the surface) o Descending Air o Warming Air o Dry Air o Stable Air (No Precipitation o Divergence (Air Moving Around) Low Pressure: o Cyclone (cyclonic flow spiraling up into the atmosphere) o Ascending or Rising Air o Cooling Air o Moist Air o Unstable Air (Precipitation) o Convergence (Air Together) Seasonal Migration of these Zones: o At the surface, alternative zones of high and low pressure with surface winds moving accordingly We care about water because we need water: o Primary source of atmospheric moisture is the ocean o ~97% of water is salty ocean water o ~3% of water is fresh liquid water or ice