HEAT OF VAPORISATION
AT DEW POINT TEMP
HEAT (STORM FUEL)
Atmospheric Moisture
Evaporation & Condensation · Absolute vs. Relative Humidity · Saturation, Dew Point & Adiabatic Cooling (28 Micro-Realms)
The Thermodynamic Engine of Weather (NCERT Chapter 11)
Water vapor present in the air is known as humidity. It is governed by a strict thermodynamic balance: Evaporation turns liquid to gas by absorbing the latent heat of vaporisation, while Condensation reverses this, releasing massive latent heat that fuels storms. The air's moisture content is measured in three ways: Absolute Humidity (actual mass per volume, $g/m^3$), Specific Humidity (mass per mass, $g/kg$), and Relative Humidity (percentage of capacity, inversely tied to temperature). When air cools below its Dew Point, it reaches 100% RH (Saturation), forcing condensation onto hygroscopic nuclei to form Dew, Frost, Fog, or Clouds.
I. The Hydrological Cycle, Evaporation & Latent Heat (NCERT)
Water in the Atmosphere
- Water is present in atmosphere in 3 forms
- Gaseous (vapor), Liquid (rain), Solid (snow)
- Proportion varies from 0 to 4% by volume (NCERT)
Continuous Exchange
- Moisture is continuously exchanged between
- Atmosphere, Oceans & Continents
- Via evaporation, transpiration & precipitation
Evaporation (Liquid to Gas)
- Process by which water is transformed
- From liquid to gaseous state (NCERT)
- Heat is the main cause of evaporation
Latent Heat of Vaporisation
- The temperature at which water evaporates
- Is referred to as the latent heat of vaporisation (NCERT)
- This energy is "hidden" in the vapor molecule
Evaporation Controls
- Increases with higher Temperature
- Increases with Drier air (lower humidity)
- Increases with Wind (replaces saturated air)
Condensation (Gas to Liquid)
- The transformation of water vapour into water
- Caused by the loss of heat (cooling)
- Releases the stored Latent Heat back to air!
Sublimation (Direct Solid/Gas)
- Direct conversion from solid (ice) to gas (vapor)
- Or gas directly to solid (deposition/frost)
- Bypasses the liquid phase entirely
II. Measuring Humidity (Absolute vs. Specific vs. Relative — NCERT Trap)
Absolute Humidity ($g/m^3$)
- Actual amount of water vapour in atmosphere
- Weight of vapour per unit volume of air ($g/m^3$)
- Changes if the air volume expands/contracts (NCERT)
Specific Humidity ($g/kg$)
- Mass of water vapor per unit mass of air ($g/kg$)
- NOT affected by changes in volume/pressure
- Most reliable metric for meteorologists
Relative Humidity (%)
- Percentage of moisture present in atmosphere
- Compared to its full capacity at a given temp
- Formula: $(\text{Actual} / \text{Capacity}) \times 100$ (NCERT)
RH vs. Temperature (Inverse)
CAPACITY ↑
DROPS
- Warm air can hold MORE moisture than cold air
- If Temp INCREASES, Capacity increases $\to$ RH DROPS
- If Temp DECREASES, Capacity shrinks $\to$ RH RISES!
RH: Oceans vs. Continents
GREATEST RH
LEAST RH
- NCERT Fact: Relative Humidity is GREATEST
- Over the oceans (abundant water source)
- And LEAST over the continents
Saturation (100% RH)
- Air containing moisture to its full capacity
- At a given temperature is said to be Saturated
- It cannot hold any additional moisture (NCERT)
Dew Point Temperature
- The temperature at which a given parcel of air
- Reaches saturation is called the Dew Point (NCERT)
- Cooling below this triggers condensation!
III. Condensation Nuclei & Forms of Condensation (Dew, Frost, Fog)
Condensation Nuclei
- Condensation requires a microscopic surface
- Dust, smoke, and ocean salt act as excellent
- Hygroscopic (water-attracting) condensation nuclei
Forms of Condensation
- Classified based on temperature and location
- Ground level: Dew and Frost
- Near ground: Fog and Mist; Aloft: Clouds (NCERT)
Dew (Dew Point > $0^\circ\text{C}$)
- Moisture condenses on solid cold surfaces
- (Stones, grass blades) instead of nuclei in air
- Forms when Dew Point is ABOVE freezing point
Frost (Dew Point $\le 0^\circ\text{C}$)
- Forms on cold surfaces when condensation takes
- Place BELOW freezing point ($0^\circ\text{C}$)
- Direct deposition from vapour to minute ice crystals
Fog (Cloud at Ground Level)
- Large mass of air containing dust/smoke
- Cools below its dew point near the ground
- Essentially a cloud with its base at/near the ground
Fog vs. Mist (NCERT Contrast)
DRIER · SMOGGY
WETTER · MTNS
- Mist: Contains MORE moisture than fog. Each nucleus has a thicker water layer. Frequent in mountains.
- Fog: Drier, often prevalent where warm air meets cold currents.
Smog (Urban Industrial)
- In urban and industrial centers with high smoke
- Smoke provides abundant condensation nuclei
- Such a condition when smog and fog mix is SMOG (NCERT)
IV. Adiabatic Cooling, Lapse Rates & Air Instability (Cloud Genesis)
Adiabatic Cooling
- As a parcel of air rises, pressure drops
- The air EXPANDS, doing work, which consumes heat
- Temperature drops without transferring heat to environment
DALR (Dry Adiabatic Lapse Rate)
- Rising air that is NOT yet saturated (RH < 100%)
- Cools at a constant, fast rate: $9.8^\circ\text{C per km}$
- Continues until it reaches the Dew Point!
SALR (Saturated Adiabatic)
- Once saturated, rising air condenses water
- Condensation releases Latent Heat!
- This heat offsets cooling $\to$ SALR is SLOWER than DALR.
LCL (Condensation Level)
- Lifting Condensation Level (LCL)
- The exact altitude where rising air hits Dew Point
- Marks the flat base of all Cumulus clouds!
Latent Heat of Condensation
- The massive energy absorbed during evaporation
- Is released into the upper air when clouds form
- This powers the violent updrafts of Hurricanes!
Instability of Air
KEEPS RISING
SINKS BACK
- If rising air is warmer than surrounding environment
- It remains buoyant and continues to rise (Unstable)
- Leads to towering storm clouds
Master Moisture Matrix
- Completes Water in the Atmosphere Part 1
- Locks Humidity, Dew, Frost, Fog & Lapse Rates
- Transitions to Realm 74 (Clouds & Precipitation)
Core Distinctions
• Absolute Humidity: The actual weight of water vapour per unit volume of air, expressed in $g/m^3$. The ability of air to hold vapour depends entirely on its temperature.
• Relative Humidity: The percentage of moisture present compared to its full capacity at a given temperature. With the same amount of vapour, if temperature increases, capacity increases, and Relative Humidity DROPS. It is highest over oceans and lowest over land.
• Dew: Moisture condenses as water droplets on cold solid surfaces (blades of grass) when the dew point is ABOVE the freezing point.
• Frost: Vapour deposits directly as minute ice crystals on surfaces when condensation occurs BELOW the freezing point ($0^\circ\text{C}$).
Both are clouds with their base at/near the ground. However, Mist contains MORE moisture than fog (each nucleus contains a thicker layer of moisture). Mists are common over mountains, while Fogs are drier, common where warm air meets cold currents, and can mix with smoke to form Smog.
Moisture Dynamics Matrix
NCERT Chapter 11 Reference Guide