Ch 4 Quiz: Humidity & Precipitation — Dew Point, RH & Formation

Relative Humidity (RH) = (e/es) × 100% where e is actual vapour pressure and es is saturation vapour pressure at the same temperature. At the dew point temperature, RH = 100%. As temperature rises with constant moisture, RH decreases.

The dew point is the temperature at which the saturation vapour pressure equals the actual vapour pressure — i.e., the temperature at which condensation (dew) begins when air is cooled at constant pressure without adding moisture.

The dew point spread (T – Td) is the difference between the temperature and dew point. A spread close to 0 means the air is near saturation; a large spread means dry air. The spread is used to estimate cloud base height.

Saturation vapour pressure (es) increases with temperature (Clausius-Clapeyron equation). Warmer air can hold more water vapour. At 30°C, es ≈ 42 hPa; at 0°C, es ≈ 6 hPa. This is why tropical air carries far more moisture.

In warm clouds (all droplets liquid, above freezing), rain forms by the coalescence process: larger droplets fall faster and collide with smaller droplets, growing large enough to fall as rain. Predominant in tropical convective rainfall.

In mixed-phase clouds (coexistence of ice crystals and supercooled water), saturation vapour pressure is lower over ice than over liquid water. Ice crystals grow (by deposition) while water droplets evaporate. This Bergeron process produces precipitation in mid-latitude clouds.

Supercooled liquid water (SLW) droplets remain liquid below 0°C because they lack ice nuclei for freezing. When they impact an aircraft surface, they freeze instantly, causing structural icing — especially dangerous between 0°C and -20°C.

Freezing rain forms when rain falls from a warm layer above (T>0°C) through a shallow cold surface layer (T<0°C). Droplets remain liquid (supercooled) and freeze on contact with aircraft and ground surfaces, creating glaze ice — extremely hazardous.

Specific humidity (q) = mass of water vapour / total mass of moist air. Expressed in g/kg. Unlike relative humidity, it does not change with temperature (conservative property), making it useful for tracking air mass properties.

Hail forms in cumulonimbus with strong updrafts. Ice embryos are swept upward, collecting supercooled water (clear ice), then fall into warmer regions (rime layer), and may be swept up again. Multiple growth cycles create the concentric layers visible in cut hailstones.

Freezing rain and ice pellets (sleet) both require precipitation that has melted (passed through a layer above 0°C) and then entered a subfreezing layer. Freezing rain: thin cold layer, droplets don’t refreeze. Ice pellets: thicker cold layer, droplets refreeze before impact.

The warm conveyor belt (WCB) is a large-scale, ascending airstream that transports warm, moist tropical air northward ahead of and up the warm frontal surface in a mid-latitude cyclone. It is responsible for the broad precipitation area ahead of the warm front.

RH = 100% means the air is saturated (at its dew point temperature). Condensation or deposition CAN occur (fog, dew, frost), but doesn’t automatically mean precipitation. Supersaturation is needed for cloud droplets to form without CCN.

CCN are tiny particles (0.1–1 μm) on which water vapour condenses when the air reaches near-saturation. Sea salt, sulfate aerosols, dust, smoke, and industrial particles act as CCN. Their concentration affects cloud droplet size and precipitation efficiency.

Mixing ratio (w) = mass of water vapour / mass of DRY air. Similar to specific humidity but uses dry air mass as the base (vs. moist air for specific humidity). Both are conserved properties useful for tracking air mass history.

A hair hygrometer uses the property of human hair (or synthetic fibre) to change length with relative humidity — used in traditional observations. Modern electronic sensors use capacitance changes in a hygroscopic material.

Precipitable water (W) = the depth of liquid water that would result if all water vapour in a vertical column from surface to top of atmosphere were condensed and fell as rain. Typical values: ~10–15mm in temperate air, 50–60mm in deep tropical air.

The Indian Summer Monsoon draws moisture primarily from the warm Arabian Sea and Bay of Bengal, where intense surface evaporation feeds the Southwest and Northeast monsoon airflows. SSTs above 26-28°C drive large latent heat flux.

Drizzle has the smallest water droplet size (0.2–0.5mm diameter), intermediate between cloud droplets (0.5mm). Drizzle falls slowly and can cause significant icing on aircraft at subfreezing temperatures.

The LCL is where a surface air parcel, rising and cooling at the DALR, reaches its dew point (RH=100%). Above the LCL, condensation occurs and the parcel cools at the SALR. The LCL approximates the base of convective (cumulus) clouds.

Cloud base ≈ (T – Td) × 400ft = (25 – 13) × 400 = 12 × 400 = 4,800ft.

Ice nuclei (IN) are specific particles (clay minerals, bacteria, certain organic particles) that promote ice crystal formation (freezing or deposition) at temperatures below 0°C. Far less numerous than CCN — their scarcity explains why supercooled water exists.

Pre-monsoon season (April-May) in India is characterised by intense convective activity — Nor’westers (kalbaishakhi) in Bengal/northeast, heat thunderstorms over central India — producing hail, heavy showers, and severe weather.

Equivalent potential temperature (θe) accounts for both the thermal energy and the latent heat of all water vapour in an air parcel. It is conserved during reversible moist adiabatic processes and used to trace air mass origin and assess moist convective instability (CAPE).

Cumulonimbus (Cb) produces the most intense precipitation: heavy rain, hail, and sometimes tornadoes. Nimbostratus produces steady, moderate to heavy rainfall. Stratus typically produces only drizzle.

RH = (e/es) × 100%. RH = 50% means actual vapour pressure (e) = 0.5 × saturation vapour pressure (es). The air contains half the moisture it could hold at that temperature before reaching saturation.

Advection fog forms when warm, moist maritime air (from monsoon remnants or Bay of Bengal) flows over cooler land or water surfaces in winter. This is common along coastal Andhra, Tamil Nadu, and Bengal coasts in winter months.

Hygroscopic materials readily absorb water vapour from the surrounding air. Sea salt is highly hygroscopic — it can attract moisture at RH as low as 70%, forming haze particles. This is why maritime boundary layers often have reduced visibility.

Freezing rain creates a thin, transparent glaze ice layer on runways that is extremely slippery (braking action NIL). Braking action is dramatically reduced — even a slight downslope runway can render stopping impossible.

Evaporation rate is proportional to the vapour pressure deficit (es – e) — the difference between saturation and actual vapour pressure. High temperature → high es; low RH → low e. Large deficit + high wind speed maximises evaporation.

The Clausius-Clapeyron equation: des/dT = L·es/(Rv·T²) describes how saturation vapour pressure (es) increases approximately 7% per °C of temperature increase. Critical for understanding how a warmer atmosphere holds more moisture and intensifies precipitation.

A ceilometer is an automated optical instrument (typically pulsed diode laser/lidar) that measures cloud base height by timing the return of reflected light from cloud bases. It reports cloud layers in aviation weather observations (METARs).

Virga is precipitation (typically ice crystals or rain) that falls from clouds but evaporates (or sublimes) before reaching the ground. Visible as streaks hanging below clouds. Associated with dry, unstable air below cloud base and possible windshear/microbursts.

Saturation mixing ratio (ws) = 0.622 × es/(P – es), where es is saturation vapour pressure (temperature-dependent) and P is atmospheric pressure. Both temperature (main factor) and pressure affect ws.

Meghalaya (specifically Mawsynram/Cherrapunji) receives the highest average annual rainfall in India (~11,000–12,000mm/year), due to the orographic lifting of moist Bay of Bengal monsoon air against the Khasi Hills.

The Indian Summer Monsoon produces rainfall through: (1) orographic lifting against Western Ghats and Himalayas (heaviest rainfall), (2) convective activity in moist, unstable monsoon air, and (3) synoptic-scale lows and depressions in the Bay of Bengal.

Large hail requires strong updrafts (>25 m/s) to suspend hailstones long enough for them to grow. Severe thunderstorms (supercells) with 50+ kt updrafts can produce baseball-sized (>7cm) hail. Doppler radar measures updraft strength.

Approximately two-thirds (~67%) of global precipitation falls in the tropics (between ~30°N and 30°S), where the intense surface heating, warm oceans, and deep convection concentrate the atmospheric water cycle.

The wet bulb temperature (Tw) lies between the dew point (Td) and the dry bulb temperature (T): Td ≤ Tw ≤ T. When RH = 100%, T = Tw = Td. The wet bulb depression (T – Tw) is used with hygrometric tables to find humidity.

Orographic (upslope) precipitation occurs when moist air is forced upward over terrain. As it rises and cools, condensation and precipitation occur on the windward slope. The leeward side is in the ‘rain shadow’ and receives little precipitation.

PROB30 (or PROB40) in a TAF indicates the probability (30% or 40%) of a significant weather change occurring during the forecast period. Below 30% probability, forecasters do not mention it; 50% or more becomes a TEMPO or BECMG.

Ice pellets (sleet) form when rain or melted snowflakes pass through a deep-enough subfreezing layer to completely refreeze before reaching the surface. Freezing rain passes through a shallower cold layer and arrives as supercooled liquid water.

At -40°C, homogeneous nucleation occurs — supercooled water freezes spontaneously without any ice nuclei. Below -40°C, essentially no liquid water exists in clouds (all ice), making structural icing very rare below this temperature.

DGCA regulations require pilots to report en-route weather hazards including icing, severe turbulence, wind shear, and volcanic ash via PIREPs (Pilot Reports) or ASREPs (Air Safety Reports) to assist ATC and other pilots.

The psychrometer (wet-dry bulb thermometer) measurement uses the psychrometric equation: e = ew – A·P·(T – Tw), where A is the psychrometric coefficient, P is pressure, T is dry bulb, and Tw is wet bulb temperature. Used to calculate actual vapour pressure.

The rain shadow effect: moist air rising over the windward side of mountains precipitates most of its moisture. The descending air on the leeward side warms dry-adiabatically, becoming warm and dry. Example: Deccan plateau in the rain shadow of Western Ghats.

SLD (drizzle droplets 0.05–0.5mm or freezing rain droplets) travel farther from the stagnation point before freezing, depositing ice beyond the normally protected areas (leading edge de-icing boots, heated areas), threatening control effectiveness.

Bay of Bengal Monsoon Depressions (cyclonic circulations in the Bay during monsoon season) draw in large amounts of moisture and produce widespread heavy rainfall over central, eastern, and northeastern India, often causing significant flooding.

Precipitable water of 50mm indicates an atmosphere saturated with moisture from surface to tropopause. Combined with instability and lifting mechanisms (fronts, orography, surface heating), such values support extreme rainfall events (100-300mm/day).

Evapotranspiration (ET) = evaporation from bare soil and open water surfaces + transpiration from vegetation (water released through leaf stomata). ET is the primary pathway for returning precipitation to the atmosphere over land.

Freezing drizzle is most commonly produced by low stratus and stratocumulus clouds with temperatures slightly below 0°C. These warm supercooled-water clouds release small droplets that freeze on impact — can create thin but very slippery ice deposits.

A dryline is a boundary separating moist (higher dew point) air from dry (lower dew point) air. In India, a dryline (moisture boundary) can develop in the pre-monsoon season between dry northwesterly winds and advancing moist monsoon air, triggering severe convection.

Frost (fine ice crystals) on wing surfaces creates surface roughness that disrupts the smooth laminar boundary layer. This increases drag and reduces lift, potentially causing loss of control on takeoff. Frost must be completely removed before flight.

The Clausius-Clapeyron relationship shows that saturation vapour pressure (and thus the maximum moisture capacity of air) approximately doubles for every 10°C increase in temperature. This means warm tropical air can hold vastly more moisture than cold polar air.

In METAR, FG = Fog (visibility reduced to below 1,000m by water droplets in suspension). FZFG = Freezing Fog (temperature at or below 0°C). BR = Mist (visibility 1,000–5,000m). HZ = Haze (visibility reduced by dry particles).

The Indian Monsoon is driven by the thermal low over the heated Asian continent (summer) vs. the relatively cool Indian Ocean. The large pressure gradient drives the southwesterly monsoon flow. The ITCZ migration north is also critical.

Convective precipitation: intense, short-duration, associated with thunderstorms and strong vertical motion. Stratiform precipitation: steady, widespread, longer-lasting, from nimbostratus or altostratus. Most precipitation events contain elements of both.

Ice Crystal Icing (ICI) is an airframe and engine hazard in regions of high ice crystal content (HICCs), particularly in deep convective systems and tropical cirrus. Ice crystals can melt on warm internal engine surfaces, then refreeze, blocking fuel nozzles or damaging compressor blades.

An automatic tipping bucket rain gauge measures precipitation by counting the number of times a small calibrated bucket (typically 0.2mm or 0.5mm) tips as it fills with rain. Each tip is recorded, accumulating rainfall data.

Drizzle is defined as very small water droplets (0.2–0.5mm diameter) that fall slowly (terminal velocity 0.3–1 m/s). They originate from low stratus/fog and can cause icing and reduced visibility without appearing heavy.