Humidity: Absolute and Specific

• Moisture in the atmosphere plays a vital role in weather patterns, climate, and the distribution of water on Earth.
• It exists in various states: vapor (gas), liquid (rain, clouds), and solid (ice, snow).

Humidity

• Refers to the amount of water vapor in the air.
• Quantified through several measures, each offering unique insights.

Absolute Humidity

• Expressed as grams of water vapor per cubic meter of air.
• Defined as the mass of water vapor per unit volume of air.
• Changes as air volume expands or contracts.
• Due to its variability with air volume changes, absolute humidity is not frequently used in weather forecasting.

Specific Humidity

• Ratio of the mass of water vapor to the total mass of air, including water vapor.
• Measured as grams of water vapor per kilogram of air.
• Remains constant as long as water vapor is not added or removed from the air mass.
• Temperature or pressure variations do not affect it, making it a more useful measure than absolute humidity.

Mixing Ratio

• Mass of water vapor per unit mass of dry air.
• Measured as grams of water vapor per kilogram of dry air.
• Similar to specific humidity, it remains constant unless water vapor is added or removed.

Relative Humidity

• Most commonly used humidity measure.
• Indicates how close the air is to saturation.
• Ratio of the actual water vapor content to the maximum water vapor capacity at a given temperature.
• Expressed as a percentage using the formula: RH = (Actual water vapor content / Water vapor capacity) x 100%.
• Diurnal temperature variations lead to predictable changes.
• Highest in the morning when temperatures are lowest, and lowest in the afternoon when temperatures are highest.
• Temperature-dependent, as warm air can hold more water vapor than cold air, even if the actual moisture content remains constant.

Saturation

• Occurs when air holds the maximum possible amount of water vapor at a given temperature.
• Rate of evaporation equals the rate of condensation.
• Equilibrium is reached, and no more water can evaporate into the air.

Vapor Pressure

• Partial pressure exerted by water vapor in the atmosphere.
• Measured in Pascals (Pa) or millibars (mb).
• Saturation vapor pressure is the vapor pressure when the air is saturated.
• Warmer air has a higher saturation vapor pressure, indicating temperature dependence.

Measuring Humidity

• Hygrometers are instruments used.
• Hair hygrometer utilizes human hair, which expands and contracts with humidity changes.
• Electronic hygrometer measures changes in electrical resistance or capacitance.
• Sling psychrometer employs dry-bulb and wet-bulb thermometers to determine humidity based on evaporative cooling.

Adiabatic Processes

• Temperature changes occur due to the expansion or compression of air.
• No heat is exchanged with the surrounding environment.
• Air cools as it rises and expands.
• Air warms as it descends and compresses.
• Cooling or warming rate depends on whether the air is saturated or unsaturated.

Dry Adiabatic Lapse Rate (DALR)

• Rate at which unsaturated air cools as it rises or warms as it descends.
• Approximately 10°C per 1,000 meters (5.5°F per 1,000 feet).
• Unsaturated air lacks sufficient water vapor for condensation.

Saturated Adiabatic Lapse Rate (SALR)

• Rate at which saturated air cools as it rises.
• Typically lower than the DALR, ranging from 5°C to 9°C per 1,000 meters.
• Sufficient water vapor enables condensation.
• Latent heat released during condensation slows the rate of cooling.

Lifting Condensation Level (LCL)

• Altitude at which rising air becomes saturated and condensation begins.
• Clouds start to form.
• Determined by finding the intersection of the dry adiabatic lapse rate line and the dew point temperature line on a thermodynamic diagram.

Cloud Formation

• Air must be saturated (relative humidity reaches 100%).
• Condensation nuclei must be present.
• A cooling mechanism is needed.
• Cooling Mechanisms:
  • Orographic Lifting: Air forced over a topographic barrier.
  • Frontal Lifting (Convergence): Air masses converge, forcing air to rise.
  • Convection: Unequal heating of the surface causes air to rise in thermals.
  • Convergence: Air flows together and rises.

Condensation Nuclei

• Tiny particles suspended in the air where water vapor condenses.
• Include dust, salt, smoke, and pollutants.
• Essential for cloud formation as they provide a surface for water vapor to condense onto.

Cloud Classification

• Classified by appearance and altitude.
• Altitude:
  • High clouds form above 6,000 meters (20,000 feet).
  • Middle clouds form between 2,000 and 6,000 meters (6,500 to 20,000 feet).
  • Low clouds form below 2,000 meters (6,500 feet).
  • Vertical clouds extend through multiple altitude levels.
• Appearance:
  • Cirrus: Thin, wispy clouds made of ice crystals.
  • Cumulus: Puffy, cotton-like clouds with flat bases.
  • Stratus: Flat, layered clouds that cover the sky.
  • Nimbus: Rain-producing clouds.

Common Cloud Types

• Cirrus (Ci): High, wispy clouds made of ice crystals.
• Cirrocumulus (Cc): High, patchy clouds that look like ripples.
• Cirrostratus (Cs): High, thin sheet-like clouds that often cause halos around the sun or moon.
• Altocumulus (Ac): Middle-level, patchy clouds that may appear in rolls or layers.
• Altostratus (As): Middle-level, gray or bluish-gray sheet-like clouds that often cover the entire sky.
• Stratus (St): Low, gray, uniform sheet-like clouds that often cover the entire sky.
• Stratocumulus (Sc): Low, lumpy, layered clouds that often appear in rows or patches.
• Cumulus (Cu): Puffy, white clouds with flat bases that indicate rising air.
• Cumulonimbus (Cb): Tall, towering clouds associated with thunderstorms.
• Nimbostratus (Ns): Low, dark, gray clouds that produce continuous rain or snow.

Fog

• Considered a cloud at ground level.
• Forms when air near the ground cools to its dew point or gains moisture.

Types of Fog

• Radiation Fog: Forms on clear, calm nights when the ground cools rapidly through radiation, cooling the air above.
• Advection Fog: Forms when warm, moist air moves over a cooler surface, causing condensation.
• Upslope Fog: Forms as air rises up a slope, cooling adiabatically to saturation.
• Evaporation Fog: Forms when water evaporates into cool air, saturating it.
  • Steam fog occurs when cold air moves over warm water.
  • Frontal fog occurs when rain falls through a cold air mass.

Precipitation

• Any form of water falling from clouds to Earth's surface.
• Includes rain, snow, sleet, and hail.

Processes of Precipitation Formation

• Collision-Coalescence Process: Occurs in warm clouds (above 0°C). Cloud droplets collide and merge, becoming heavy enough to fall as rain.
• Bergeron Process (Ice-Crystal Process): Occurs in cold clouds (below 0°C). Ice crystals grow at the expense of supercooled water droplets. Ice crystals fall as snow or melt into rain.

Types of Precipitation

• Rain: Liquid precipitation with droplets larger than 0.5 mm.
• Snow: Solid precipitation of ice crystals.
• Sleet: Rain that freezes while falling through a cold air layer.
• Freezing Rain: Rain that freezes upon contact with a sub-freezing surface.
• Hail: Solid precipitation of ice balls or lumps.

Measurement of Precipitation

• Rain Gauge: Measures liquid precipitation.
• Snow Gauge: Measures snowfall.
• Weather Radar: Uses radio waves to detect precipitation intensity and location.