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💨 Humidity, Wind & Evapotranspiration: Moisture and Movement in the Atmosphere

Types of humidity, atmospheric pressure, wind systems, sea and land breezes, wind effects on crops, rainfall definitions, PET, and PAR — with agricultural examples and exam mnemonics

Why Humidity and Wind Matter for Farming

Why Humidity and Wind Matter for Farming explained with a practical agricultural meteorology visual
This visual gives a practical anchor for why humidity and wind matter for farming so the concept is easier to connect with field observation and exam recall.

In the previous lesson, we explored solar radiation and temperature — the energy that drives photosynthesis and controls crop growth rates. Now we examine the moisture and movement side of the atmosphere: humidity, wind systems, and evapotranspiration — the forces that determine how much water crops actually lose and gain.

A cotton farmer in Gujarat watches humidity levels because high moisture triggers bollworm infestation. A rice farmer in coastal Andhra Pradesh benefits from humid sea breezes that keep transpiration low. A wheat farmer in Punjab knows that hot, dry winds (loo) in March can cause shrivelled grains. Humidity, wind, and evapotranspiration directly govern crop water use, pest pressure, and yield.

This lesson covers:

  1. Humidity — absolute, specific, and relative humidity with crop effects
  2. Atmospheric pressure — measurement and isobars
  3. Wind systems — prevailing winds, local breezes, and wind speed thresholds
  4. Rainfall definitions — meteorological vs crop rainy day thresholds
  5. Evapotranspiration — PET, AET, water deficit, and the moisture deficit index
  6. PAR — photosynthetically active radiation measurement

All topics are high-yield for IBPS AFO, NABARD Grade A, and FCI exams.

Humidity — Moisture in the Air

The amount of water vapour present in the atmosphere is called atmospheric moisture or humidity. There are three ways to express it:

1. Absolute Humidity

  • The actual mass of water vapour in a given volume of moist air.
  • Unit: grams of water vapour per cubic metre (g/m³).
  • Highest at the equator; minimum at the poles.
  • Daily pattern: minimum at sunrise, maximum in the afternoon (2–3 PM).
  • Diurnal variations are small in desert regions.

2. Specific Humidity

  • Weight of water vapour per unit weight of moist air.
  • Unit: grams of water vapour per kilogram of air (g/kg).

3. Relative Humidity (RH)

  • Ratio of actual water vapour to the amount needed for saturation at the same temperature and pressure.
  • Expressed as a percentage (%); no units.
  • When air temperature equals dew point, RH = 100%.

Comparing the Three Types of Humidity

Humidity comparison showing absolute, specific, and relative humidity concepts
Putting the three humidity terms side by side prevents the common exam mistake of mixing amount of vapour with saturation status.
Type Measured Per Unit Key Fact
Absolute humidity Unit volume g/m³ Highest at equator; lowest at poles
Specific humidity Unit weight g/kg Weight-based; more stable measure
Relative humidity Ratio (no physical unit) % Changes with temperature; no fixed units

IMPORTANT

Exam distinction: Absolute = per volume (g/m³). Specific = per weight (g/kg). Relative = ratio (%). Mean maximum RH occurs in the morning; mean minimum RH in the afternoon.

Effect of Relative Humidity on Crops

Condition Crops Why
Very high RH is beneficial Maize, Sorghum, Sugarcane Reduces transpiration loss; maintains turgidity
Very high RH is harmful Sunflower, Tobacco Promotes fungal diseases; reduces oil quality
Pest incidence Increases at higher RH Humid conditions favour fungal spores and insect breeding
  • Safe RH for most crops: 40–60% — this range balances transpiration control with disease avoidance.

TIP

Mnemonic — "MSS love moisture, ST hate it": Maize, Sorghum, Sugarcane benefit from high RH. Sunflower, Tobacco are harmed by it.

Atmospheric Pressure

  • Atmospheric pressure is the weight of air above a unit area.
  • Pressure at sea level: 76 cm (1013.25 mb) — the standard reference.
  • 1000 mb = 1.053 kg/cm².
  • Air presses down with a pressure of 1.034 g/cm².
  • Measured in millibars (mb); 1 mb = 100 N/m² = 1000 dynes/cm².
  • Atmospheric pressure is force per unit area.
  • Unequal heating by the sun and earth's rotation create pressure differences that drive winds.
  • Most accurate instrument: Mercurial barometer (Fortin barometer).
Atmospheric pressure and wind diagram showing isobars, high pressure, low pressure, and wind flow over a farm plain
This diagram connects the pressure chapter with the wind chapter by showing how air moves from high pressure toward low pressure across a farm landscape.

Isobars

Isobars are imaginary lines on a map joining places with the same atmospheric pressure. They help meteorologists identify high- and low-pressure systems that determine whether a farmer will get rain or dry weather.

Wind — Air in Motion

Wind — Air in Motion explained with a practical agricultural meteorology visual
This visual gives a practical anchor for wind — air in motion so the concept is easier to connect with field observation and exam recall.

Humidity describes the moisture content of air; wind describes its movement. Wind redistributes heat, moisture, and pollen across landscapes — and at extreme speeds, it becomes destructive. Air in horizontal motion is called wind. Vertical air movement is called an air current.

Property Detail
Main cause Variation in temperature creating pressure differences
Direction Always from higher pressure to lower pressure
Optimal crop speed 4–6 km/hr — facilitates gas exchange without causing damage
Monsoon winds Blow from cold to warm regions
  • High-speed winds are called Squalls.

Wind Terminology

Term Meaning Agricultural Context
Veering wind Turns to the right (clockwise) Indicates approaching warm front
Backing wind Turns to the left (anticlockwise) Indicates approaching cold front
Windward side Direction from which wind blows Receives orographic rainfall (Western Ghats)
Leeward side Direction towards which wind blows Rain shadow area (Deccan Plateau)

NOTE

Memory aid: Veering = Veers right (clockwise). Backing = Backs left (anticlockwise).

Wind Speed Patterns in India

Period Speed Notes
Winter months 3–6 km/hr Lowest wind speeds
Pre-monsoon (April–June) 6–10 km/hr (peak) Hot winds (loo) in north India
Strongest wind regions West Rajasthan, Gujarat, parts of Tamil Nadu Important for wind energy and windbreak planning

Prevailing Winds

Winds that blow predominantly from one direction over a particular area of the earth:

Wind Type Latitude Belt Direction (NH) Agricultural Significance
Trade winds (Tropical Easterlies) 30° towards equator From north-east Drive the SW monsoon into India
Antitrade winds (Westerlies) 30° to 60° From south-west Bring western disturbances to north India; stronger in southern hemisphere
Roaring Forties 40°–50° S Strongest westerlies No direct Indian agricultural impact
Polar Easterlies Poles towards sub-polar From poles Cold waves reaching north India
Periodic Winds Reverse direction seasonally Seasonal reversal Primary driver of the Indian monsoon

IMPORTANT

Exam summary: Trade winds: 30° → equator. Westerlies: 30° → 60°. Polar easterlies: poles → sub-polar. The equatorial low-pressure belt (5°N and S) is called the Doldrums.

Sea Breeze vs Land Breeze

These are local wind systems near coastlines, directly affecting coastal agriculture:

Sea Breeze (On-shore): During daytime, land heats faster than sea → low pressure over land → wind flows from sea to land.

Land Breeze (Off-shore): At night, land cools faster than sea → low pressure over sea → wind flows from land to sea.

Feature Sea Breeze Land Breeze
Time Day time Night time
Direction Sea → Land Land → Sea
Moisture content More moisture Less moisture
Strength Stronger Weaker
Agricultural effect Moderates coastal temperatures; benefits rice, coconut Carries farm odours seaward; minimal crop effect

TIP

Memory aid: Sea breeze = Sun time (day). Land breeze = Late night. Sea breeze is always stronger and carries more moisture.

Sea breeze and land breeze comparison over coastal farm fields showing daytime moist onshore flow and nighttime drier offshore flow
Sea breeze carries moist air from sea to land during the day, while land breeze reverses the flow at night as the land cools faster.

Mountain Breeze vs Valley Breeze

Type Description Also Called Agricultural Example
Mountain Breeze Cool, dense air flows downhill into the valley Katabatic wind Cold air pooling causes frost in valley-floor orchards at night
Valley Breeze Warm air flows uphill along heated slopes Anabatic breeze Carries moisture up slopes; benefits hill tea gardens during day
Valley breeze and mountain breeze over a farming valley showing daytime upslope warm air and nighttime downslope cold air drainage
Valley breeze lifts warm air upslope by day, whereas mountain breeze drains cold dense air downslope at night and raises frost risk on the valley floor.

Effect of Wind Speed on Crops

Wind speed thresholds are critical for agricultural operations and crop safety:

Wind Speed Effect Agricultural Application
> 8 km/hr Can lift sand in arid regions Sand dune movement; need for shelter belts in Rajasthan
> 10 km/hr Plant growth is inhibited Wind stress reduces leaf area and transpiration
15 km/hr Minimum for winnowing operations Traditional grain cleaning after harvest
15 km/hr Maximum for sprinkler irrigation Above this, spray drift wastes water
> 50 km/hr Crop lodging occurs Rice, wheat, sugarcane flatten; yield loss 30–50%

WARNING

Exam-favourite thresholds: 8 (sand lifting) → 10 (growth inhibition) → 15 (winnowing and sprinkler limit) → 50 (crop lodging). These numbers are very frequently tested.

Weather Phenomena by Wind Speed

Phenomenon Wind Speed Key Feature
Storm 40–120 km/hr Low-pressure centre; damages standing crops
Hurricane > 120 km/hr Severe tropical cyclone (typhoon in W. Pacific, cyclone in Indian Ocean)
Tornado Can exceed 400 km/hr Violently rotating air column from cumulonimbus cloud

Rainfall Definitions

Rainfall Definitions explained with a practical agricultural meteorology visual
This visual gives a practical anchor for rainfall definitions so the concept is easier to connect with field observation and exam recall.

Rainy Day Categories

Category Threshold Use
Meteorological rainy day Rainfall ≥ 2.5 mm per day IMD statistics
Crop rainy day Rainfall > 5 mm per day Minimum useful rain for crop growth
Effective rainfall 5 mm per day Amount actually available to crops

Mean Rainfall

  • Daily: amount collected by rain gauge in 24 hours.
  • Annual (India): mean of 35 years of data.

IMPORTANT

Meteorological rainy day = 2.5 mm. Crop rainy day = 5 mm. Do not confuse these — they are different thresholds frequently tested in exams.

Evapotranspiration (ET)

Evapotranspiration concept showing evaporation from soil and transpiration from crops
Evapotranspiration combines soil evaporation and crop transpiration, making it central to irrigation planning and drought assessment.

Humidity and wind together drive evapotranspiration — the combined water loss from soil surfaces and plant leaves. ET is the single most important parameter for irrigation scheduling and drought assessment. Evapotranspiration combines two processes: evaporation (liquid water escaping from soil/water surfaces) and transpiration (water moving through plants and released as vapour from stomata).

  • About 10% of atmospheric moisture comes from plant transpiration; 90% from ocean and lake evaporation.

PET, AET, and Water Deficit

Concept Definition Agricultural Use
PET (Potential ET) Maximum water loss if moisture is unlimited Indicates atmospheric water demand
AET (Actual ET) Water actually lost, limited by available moisture Indicates current water use by crop + soil
Water Deficit PET − AET Key indicator of drought stress and irrigation need
Property Value
Annual PET in India 1400 to 1800 mm
PET condition Occurs at potential rate only when water is non-limiting

IMPORTANT

AET ≤ PET always. When soil moisture is adequate, AET ≈ PET. As soil dries, AET drops below PET and the gap (Water Deficit = PET − AET) widens — this is a direct measure of crop drought stress.

Potential evapotranspiration and actual evapotranspiration comparison showing adequate-moisture field versus dry field with water deficit
When moisture is ample, actual evapotranspiration stays close to potential demand, but as soil dries the widening PET minus AET gap becomes water deficit.

Agricultural significance: When AET falls below PET, crops experience water stress. The difference between rainfall and PET determines whether a region is arid, semi-arid, or humid — and thus what crops can be grown.

Moisture Deficit Index (MDI): MDI = 100 (P - PET) / PET

Photosynthetically Active Radiation (PAR)

Photosynthetically Active Radiation (PAR) explained with a practical agricultural meteorology visual
This visual gives a practical anchor for photosynthetically active radiation (par) so the concept is easier to connect with field observation and exam recall.

PAR is the radiation that drives photosynthesis in plant leaves.

Property Value
Wavelength range 400–700 nm (same as visible light)
Strongest bands Red and Blue
Highest photosynthesis Red light band
Measurement above canopy Quantum sensor
Measurement below canopy Tube solarimeter

TIP

PAR = 400–700 nm = visible light range. Red light drives the most photosynthesis. Both Red and Blue bands are strongly photosynthetically active. Green light is mostly reflected (which is why leaves look green).

Summary Table

Summary Table explained with a practical agricultural meteorology visual
This visual gives a practical anchor for summary table so the concept is easier to connect with field observation and exam recall.
Topic Key Fact Exam Value
Absolute humidity g/m³; highest at equator; minimum at sunrise Unit question
Specific humidity g/kg (per unit weight) Unit question
Relative humidity % (no units); ratio-based Definition question
RH = 100% When air temperature = dew point Condition question
High RH beneficial Maize, Sorghum, Sugarcane Crop matching
High RH harmful Sunflower, Tobacco Crop matching
RH suitable range 40–60% for most crops Range question
Atmospheric pressure at sea level 76 cm (1013.25 mb); 1000 mb = 1.053 kg/cm² Standard question
Atmospheric pressure unit Millibar (mb) = 100 N/m² Unit question
Isobar Lines of equal atmospheric pressure Definition question
Optimal crop wind speed 4–6 km/hr Threshold question
Squalls High-speed winds Definition question
Wind direction High pressure → Low pressure Direction question
Veering wind Clockwise (right) Direction question
Trade winds 30° → Equator; NE in NH Belt question
Sea breeze Day time; sea → land; stronger Timing question
Mountain breeze Katabatic; downhill at night Name question
Wind > 10 km/hr Growth inhibited Threshold question
Wind > 50 km/hr Crop lodging Threshold question
Winnowing wind speed 15 km/hr Threshold question
Meteorological rainy day ≥ 2.5 mm Threshold question
Crop rainy day > 5 mm Threshold question
Periodic Winds Seasonal-reversing; drive Indian monsoon Wind type question
Storm wind speed 40–120 km/hr Threshold question
Hurricane wind speed > 120 km/hr Threshold question
Tornado wind speed Can exceed 400 km/hr (from cumulonimbus) Threshold question
AET Actual ET limited by available moisture Definition question
Water Deficit PET − AET (drought stress indicator) Formula question
Atmospheric moisture source 10% transpiration, 90% ocean/lake evaporation Ratio question
PET in India 1400–1800 mm annually Range question
PAR range 400–700 nm Range question
Doldrums Equatorial low pressure belt (5°N and S) Name question

Summary Cheat Sheet

Summary Cheat Sheet explained with a practical agricultural meteorology visual
This visual gives a practical anchor for summary cheat sheet so the concept is easier to connect with field observation and exam recall.
Concept / Topic Key Details
Absolute humidity g/m³; highest at equator; minimum at sunrise
Specific humidity g/kg (per unit weight of air)
Relative humidity % (no units); RH = 100% when air temp = dew point
High RH beneficial Maize, Sorghum, Sugarcane
High RH harmful Sunflower, Tobacco
RH suitable range 40–60% for most crops
Atmospheric pressure at sea level 76 cm (1013.25 mb); 1000 mb = 1.053 kg/cm²
Atmospheric pressure unit Millibar (mb) = 100 N/m²
Isobar Lines of equal atmospheric pressure
Optimal crop wind speed 4–6 km/hr (gas exchange)
Squalls High-speed winds
Wind direction High pressure → Low pressure
Veering wind Clockwise (right)
Trade winds 30° → Equator; NE in Northern Hemisphere
Sea breeze Daytime; sea → land; stronger
Mountain breeze Katabatic; downhill at night
Wind > 10 km/hr Growth inhibited
Wind > 50 km/hr Crop lodging
Winnowing wind speed 15 km/hr
Meteorological rainy day ≥ 2.5 mm
Crop rainy day > 5 mm
Periodic Winds Seasonal-reversing; drive Indian monsoon
Storm 40–120 km/hr
Hurricane > 120 km/hr
Tornado Can exceed 400 km/hr (from cumulonimbus)
AET Actual ET, limited by available moisture
Water Deficit PET − AET (drought stress indicator)
Atmospheric moisture 10% transpiration, 90% ocean/lake evaporation
PET in India 1400-1800 mm annually
PAR range 400-700 nm (visible light)
Strongest PAR bands Red and Blue; highest photosynthesis = Red
Doldrums Equatorial low pressure belt (5°N and S)

TIP

Next: Lesson 06 covers clouds and cloud seeding — the 10 cloud types in 4 families, cloud cover categories, and artificial rainmaking techniques for drought mitigation.

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