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Humidity

The Hydrologic Cycle

  • H2O is critical for survival on Earth and plays a key role in weather, transporting heat, and supporting all living things.
  • The Hydrologic Cycle describes where water is stored (pools) and how it moves between pools (fluxes).
  • Water exists in three main states: liquid, solid, and gas. It can be fresh, saline, or brackish depending on location and mixing.
  • Major accents from the slides:
    • Water moves at both large and small scales naturally and due to human actions.
    • Human water use alters storage locations, movement pathways, and water quality.
  • See USGS definitions for standard terms on the water cycle.

Pools and Fluxes

  • Pools are places where water is stored; fluxes are the processes that move water between pools.
  • Examples of pools and their characteristics:
    • Oceans (saltwater) – ~96% of Earth’s water is stored here and saline.
    • Saline lakes (on land)
    • Freshwater lakes, artificial reservoirs, rivers, and wetlands
    • Ice sheets and glaciers; snowpack and snowmelt
    • Soil moisture (liquid water) and permafrost (frozen water) in the soil
    • Atmospheric moisture (water vapor)
    • Groundwater in aquifers (liquid) beneath the surface
    • Interactions with human water use: domestic, industrial, municipal
  • Fluxes (ways water moves between pools):
    • Evaporation (liquid to vapor) and transpiration/evapotranspiration (plants contributing to atmospheric moisture)
    • Condensation (vapor to liquid) and precipitation (rain, snow, etc.)
    • Runoff, streamflow, infiltration, and groundwater recharge
    • Discharge to oceans, rivers, springs; groundwater discharge to surface water
  • The cycle is dynamic: water changes phase as it moves between pools; large-scale circulations mix ocean water and transport water vapor globally.
  • Human impacts on fluxes and pools:
    • Building dams to store water
    • Draining wetlands for development
    • Redirecting rivers
    • Withdrawing water from rivers, lakes, reservoirs, and aquifers for homes, irrigation, industry
    • Using surface water and groundwater for agriculture, power generation, mining, aquaculture
  • The amount of water available depends on:
    • How much water is in each pool (water quantity)
    • When and how fast water moves between pools (water timing)
    • How much water we use (water use)
    • Water quality (how clean the water is)
  • Water quality concerns: agricultural runoff (fertilizers, pesticides), urban runoff, industrial discharges, heated water from power plants, sediment and sewage in waterways; downstream impacts include harmful algal blooms, disease, and habitat harm.
  • Climate change effects on the water cycle:
    • Changes in water quality, quantity, timing, and use
    • Ocean acidification and sea level rise
    • More extreme weather events
  • By understanding these links, we can pursue sustainable water use.

The Water Cycle (Overview)

  • The water cycle maps where water sits on Earth and how it moves between pools.
  • Water can be stored as:
    • Atmosphere (water vapor)
    • Surface water on land (lakes, rivers, wetlands)
    • Groundwater below the ground (aquifers)
    • Ice and snow at high altitudes or near the poles (ice sheets, glaciers, snowpack)
    • Liquid freshwater on land (lakes, reservoirs, rivers, wetlands)
    • Saline water mainly in oceans; saline water in some lakes
  • Water can change forms (solid, liquid, gas) as it moves.
  • Large-scale circulation in oceans transports heat and water vapor around the globe.
  • The atmosphere exchanges water with the surface via evaporation, evapotranspiration, and precipitation.
  • Surface water exchange occurs via snowmelt, runoff, and streamflow.
  • Groundwater exchanges occur via infiltration and groundwater recharge; shallow groundwater may discharge to rivers, oceans, or springs.
  • Human actions alter water quantity, timing, and quality as described above.
  • Quantities, timings, uses, and quality together determine water availability for ecosystems and human needs.
  • Climate change influences the cycle through changes in water supply, timing, and quality.

Pools and Fluxes (Detailed)

  • On Earth, water can be fresh, saline, or a mix (brackish).
  • Pools mentioned in the figure/notes include:
    • Ice sheets and glaciers
    • Snowpack and snowmelt
    • Evapotranspiration (ET from land surfaces and vegetation)
    • Rivers and streams (streamflow)
    • Lakes (fresh and saline)
    • Wetlands
    • Groundwater (discharge to ocean, springs)
    • Urban, agricultural, and industrial water uses
    • Groundwater recharge and reservoir storage
    • Mixed oceanic zones (ocean deep water, ocean mixed zone)
  • General overview of motion: water moves between pools; cycles are defined by the transitions among phases and surface/groundwater interactions.
  • The USGS and their definitions provide standard terminology for explaining the cycle.

The Water Molecule

  • The water molecule is weakly polarized: one side slightly positive, the other slightly negative.
  • Polar molecules attract each other, which explains surface tension.
  • When water freezes, billions of molecules join to form a hexagonal ice crystal lattice due to hydrogen bonding.

Phase Changes (Water Phase Transitions)

  • Sublimation: ice → vapor
  • Deposition: vapor → ice
  • Evaporation: liquid → vapor
  • Condensation: vapor → liquid
  • Melting: ice → liquid
  • Freezing: liquid → ice
  • Each phase change involves energy release or absorption (latent heat):
    • Evaporation/Boiling: energy absorbed
    • Condensation/Freezing: energy released
    • Melting: energy absorbed
    • Sublimation: energy absorbed

States and Transitions (Condensed View)

  • Gas (water vapor)
  • Liquid water
  • Ice
  • Water can transition among these states via the phase changes listed above, driven by temperature, pressure, and ambient energy exchange.

Evaporation and Condensation (Kinetics and Net Effect)

  • Molecules at any temperature above 0 K are in perpetual motion.
  • Temperature measures the average kinetic energy of molecules.
  • At the water surface, some fast-moving molecules escape into the air as water vapor; others in the air collide with the surface and stick, condensing back into liquid.
  • Water molecules continually evaporate and condense; what matters is the net balance between the two processes.

Saturation and Saturation Balance

  • Saturation occurs when the rate of evaporation equals the rate of condensation.
  • If more molecules leave water than enter, evaporation dominates and the air becomes unsaturated.
  • If more molecules enter than leave, condensation dominates and the air becomes supersaturated (in the strict sense, the air cannot hold more vapor without condensation).
  • When the rates balance, the air is saturated.

Humidity Variables (Overview)

  • Several metrics quantify atmospheric moisture:
    1) Absolute humidity
    2) Specific humidity
    3) Vapor pressure
    4) Relative humidity
    5) Mixing ratio
    6) Wet-bulb temperature
    7) Dew-point temperature
  • Each variable has its own definition, interpretation, and use in weather and climate analysis.
  • Note: These variables are interrelated but respond differently to changes in temperature and moisture content.

Absolute Humidity (AH)

  • Definition: AH = m{v} / V, where m{v} is the mass of water vapor and V is the volume of air.
  • Units:
    • grams of water per cubic meter: AH = rac{m_{v}}{V} \ [\text{g m}^{-3}]
  • Conceptual view: AH is like a density of water vapor in air.
  • Question posed: Is absolute humidity a good variable for measuring moisture in air? (The slide prompts evaluation; in practice AH can vary with air volume, so it may not always be the best standalone indicator for moisture content in dynamically moving air.)

Specific Humidity (q)

  • Definition: q = \frac{m{v}}{m{total}}
  • Example: If a parcel contains 1 g of water vapor and the total parcel mass is 1 kg, then q = 1 g / 1000 g = 0.001 (dimensionless, often in kg/kg).
  • Key property: Specific humidity does not change with volume expansion or compression of the air parcel; it changes only when water vapor is added or removed from the parcel.

Mixing Ratio (w)

  • Definition: w = \frac{m{v}}{m{dry}}
  • Example: If a parcel has 1 g of water vapor and 1.0 kg of dry air (i.e., excluding water vapor) then w = 1 g / 1.0 kg = 0.001 kg/kg.
  • Difference from specific humidity:
    • Specific humidity uses total mass in the denominator: q = \frac{m{v}}{m{total}}
    • Mixing ratio uses only the mass of dry air in the denominator: w = \frac{m{v}}{m{dry}}
  • Both q and w are commonly used in meteorology and do not change with volume expansion without adding or removing water vapor.

Representing and Calculating q and w

  • Notation:
    • m = mass, r = density, d = dry air, v = vapor
  • These variables (q and w) are standard for lab exercises and calculations; they provide volume- and mass-based moisture representations that remain stable under compression/expansion (for q) or depend only on the dry-air component (for w).

Vapor Pressure

  • Definition: In a parcel with water vapor, the total pressure is the sum of partial pressures of all gases; the partial pressure from water vapor is the vapor pressure p_v.
  • Vapor pressure is related to the number of water vapor molecules: more molecules yield higher vapor pressure.
  • Conceptual idea: p_v represents the contribution of water vapor to the total air pressure.

Saturation Vapor Pressure

  • When an air parcel is saturated, pv equals the saturation vapor pressure p{vs}(T).
  • For unsaturated air, saturation can be reached by either lowering temperature or increasing water vapor content.
  • Saturation vapor pressure increases with temperature (an important relationship in atmospheric moisture and cloud formation).
  • Important figure: the temperature dependence of p_{vs}(T) governs how easily air becomes saturated as it cools or warms.

Relative Humidity (RH)

  • Definition: RH = (actual water vapor content) / (capacity for saturation at the same temperature and pressure).
  • Formula forms:
    • In terms of vapor pressures: RH = \frac{pv}{p{vs}(T)}
    • In terms of mixing ratios or other moisture content measures: RH can be computed from q, w, or other formulations (depending on available data).
  • Practical usage: RH is a widely used measure of atmospheric moisture because it expresses how close the air is to saturation at the current temperature.
  • The slide notes that RH can be computed using either vapor pressure or mixing ratio when data are available.

Dew Point Temperature (Td)

  • Definition: Temperature to which air must be cooled (at a given moisture content) to reach saturation.
  • When air is cooled to Td, dew forms.
  • Td is a practical proxy for the amount of water vapor in the air; higher Td indicates more moisture.
  • Dew point depression: DD = T - T_d
    • The difference between actual temperature T and dew point Td.
    • A smaller DD indicates higher humidity (air is closer to saturation).

Two Ways to Change Relative Humidity

  • Change water vapor content (with temperature held constant): increasing vapor raises RH; decreasing lowers RH.
  • Change air temperature (with water vapor held constant): increasing temperature lowers RH; decreasing temperature raises RH.
  • Note: It is possible to have RH exceed 100% in supersaturated conditions, but under normal atmospheric conditions, condensation acts to prevent sustained RH > 100%.

Diurnal Relative Humidity Variation

  • In the absence of weather systems, the amount of water vapor is relatively stable day-to-day.
  • Diurnal RH changes are primarily driven by daytime temperature increases and nighttime cooling:
    • Higher temperatures reduce RH (assuming constant water vapor)
    • Lower temperatures increase RH
    • This inverse relationship means RH can swing widely daily even if moisture amount remains similar.

Kansas City Example (Illustrative Diurnal RH)

  • Dew points around 60°F for about 8 days.
  • Temperature swung from 60°F to 90°F.
  • Relative Humidity varied inversely with temperature from about 30% to 90%.
  • This example illustrates how RH tracks the balance between moisture content and temperature.

Polar Air vs Desert Air (RH and Moisture Content)

  • Polar air can have 100% RH even though moisture content is low, because the air temperature is very low and the air is near saturation for that temperature.
  • Desert air can have high temperatures but low moisture content, leading to relatively low RH (e.g., RH around 21%) even though the air is hot.
  • Example figures: Polar air temperature and dew point both around -2°C with RH = 100%; Desert air temperature around 35°C with dew point around 10°C giving RH ≈ 21%.

Relative Humidity Inside vs Outside (Illustrative)

  • Inside example: T = 20°C (68°F), Td = -15°C (5°F), RH = 8% (outside air conditions described separately: Td = -15°C, Td = -15°C, RH = 100%).
  • This illustrates how interior air can be drier than the exterior air despite similar temperatures, depending on moisture content and temperature differences.

Wet-Bulb Temperature (Tw)

  • Definition: The lowest temperature that can be reached by evaporating water into the air.
  • Relationship: Td ≤ Tw ≤ T, i.e., the wet-bulb temperature lies between the dew point and the actual air temperature.
  • Examples:
    • If T = 90°F and RH = 90%, Tw is high.
    • If T = 90°F and RH = 10%, Tw is low.
  • Practical implication: A lower Tw generally means it feels more comfortable when air is hot because evaporation can cool the body more effectively.

Measuring Humidity – Sling Psychrometer

  • The Sling Psychrometer is a common instrument used to estimate humidity by measuring:
    • Dry-bulb temperature (actual air temperature)
    • Wet-bulb temperature (temperature with evaporative cooling from a wick soaked in water)
  • From these two measurements, one can infer dew point and/or RH via established psychrometric relationships.
  • A lab demonstration typically shows how Tw and T relate to RH.

Mean Dew Point Temperature Maps (January and July)

  • Mean dew point maps illustrate how moisture content varies geographically across seasons.
  • Features to note:
    • In January, certain regions show hot and humid conditions with higher dew points, while others are cooler with lower dew points.
    • In July, higher dew point values are common in more humid regions (e.g., near the Gulf Coast and southeastern U.S.) and vary by latitude and elevation.
    • Examples referenced include cities like Peoria, IL; Sacramento, CA; New York City; Seattle; Denver; New Orleans; Miami, etc., illustrating spatial patterns of dew point and hence humidity.
  • The maps underscore that high temperature alone does not determine humidity; dew point provides a more direct measure of atmospheric moisture.

Summary and Real-World Relevance

  • Understanding humidity and moisture variables is essential for predicting weather, weather-related hazards, and comfort indices.
  • Moisture variables interact with temperature to control cloud formation, precipitation, and storm intensity.
  • Humidity and phase-change processes influence energy budgets: evaporation absorbs latent heat, condensation releases latent heat, influencing atmospheric stability and convection.
  • Knowledge of RH, dew point, and wet-bulb temperature informs forecasts of fog, dew, frost, and heat stress risk.
  • The water cycle links ecosystems, agriculture, industry, and urban infrastructure to climate and water resource planning.

Key Formulas to Memorize

  • Absolute Humidity: AH = \frac{m_{v}}{V} \ [\text{g m}^{-3}]
  • Specific Humidity: q = \frac{m{v}}{m{total}}
  • Mixing Ratio: w = \frac{m{v}}{m{dry}}
  • Relative Humidity (via vapor pressure): RH = \frac{pv}{p{vs}(T)}
  • Saturation Vapor Pressure: p_{vs}(T) \text{ increases with } T
  • Dew Point Depression: DD = T - T_d
  • Wet-Bulb Constraints: Td \leq Tw \leq T