Relative Humidity (RH) is the ratio of the water vapor content currently in the air to the maximum water vapor content the air could hold at its current temperature and pressure, expressed as a percentage. It is mathematically expressed as:
RH=
Maximum Water Vapour Capacity
Actual Water Vapour Content
​
×100%
This can be calculated using Absolute Humidity / Saturation Content, Humidity Mixing Ratio (HMR) / Saturation Mixing Ratio (SMR), or Actual Vapor Pressure / Saturation Vapor Pressure. RH tells a pilot how close the air is to saturation, not the total amount of water vapor present.
Key Data to Remember (ICAO/FAA Context):
• Definition: RH =
Maximum capacity
Actual amount
​
×100%.
• Saturation: Air is saturated when RH = 100%.
• Significance: RH is primarily dependent on moisture content and temperature. Cooling air increases RH, and warming air decreases RH, assuming constant moisture content

The Dew point (DP) is formally defined as the temperature to which a sample of moist air must be cooled at constant pressure and constant water vapor content for saturation to occur. Once the air reaches this temperature, the relative humidity (RH) is 100%. If cooling continues below the dew point, water vapor condenses into liquid droplets (dew, mist, fog, or cloud).
Key Data to Remember (ICAO/FAA Context):
• Definition: DP is the temperature required for saturation when cooling at constant pressure.
• Significance: DP is a direct indicator of the air’s actual water vapor content. High DP means high moisture content.
• Temperature-Dew Point Spread (The Spread): The difference between the air temperature and the dew point reveals the degree of saturation. When the spread is zero, saturation (RH = 100%) occurs.
• Measurement: DP is determined using a Psychrometer (Wet and Dry Bulb Hygrometer).
• Lapse Rate: The dew point has a theoretical lapse rate of approximately 0.5
∘
C per 1000 ft gain in height

Condensation is the change of state from water vapor (gas) to liquid, a process that inherently releases latent heat (Latent Heat of Condensation), warming the surrounding air. Warmer air has a greater capacity to hold water vapor when saturated compared to cold air. Consequently, condensation in saturated warm air involves copious moisture, which liberates a greater amount of latent heat.
Key Data to Remember (ICAO/FAA Context):
• Process: Condensation is exothermic (releases heat).
• Magnitude: The amount of latent heat released is directly proportional to the amount of water vapor that condenses.
• Aviation Significance: This release of heat causes the Saturated Adiabatic Lapse Rate (SALR) to be slower than the Dry Adiabatic Lapse Rate (DALR), leading to buoyancy and potentially larger cloud formation

The diurnal (daily) variation of Relative Humidity (RH) is primarily determined by the daily temperature cycle, assuming the actual water vapor content remains constant. Warm air can hold more water vapor than cold air.
1. Daytime: As solar insolation causes air temperature to increase, the air’s capacity to hold water vapor increases. Since the actual moisture content is assumed constant, the air moves farther away from saturation, causing the RH to decrease. The lowest RH typically occurs in the afternoon, around 1500 LMT, when temperature is maximum.
2. Nighttime: As terrestrial radiation causes air temperature to decrease, the air’s capacity to hold water vapor decreases. This moves the air closer to saturation, causing the RH to increase. The highest RH occurs in the early morning, approximately 30 minutes after sunrise, when temperature is minimum.
Key Data to Remember (ICAO/FAA Context):
• Relationship: RH is inversely proportional to temperature (assuming constant moisture).
• RH Max: Occurs during the coolest part of the day (near sunrise).
• RH Min: Occurs during the warmest part of the day (around 1500 LMT).
• Operational Relevance: High RH at night/early morning increases the probability of condensation forming mist, fog, or dew

Relative Humidity (RH) is defined as the ratio of the amount of water vapor actually present in the air to the maximum amount of water vapor the air could hold at that particular temperature and pressure, expressed as a percentage.
Mathematically, it is the ratio of the Absolute Humidity to the Saturation Content, or the ratio of the Humidity Mixing Ratio (HMR) to the Saturation Mixing Ratio (SMR), or the ratio of actual vapor pressure (e) to saturation vapor pressure:
RH= Maximum Water Vapour Capacity Actual Water Vapour Content ×100%
RH is “relative” because it indicates the degree of saturation (how close the air is to 100% saturation).
Key Data to Remember:
• Definition: RH= Capacity Actual content×100%.
• Saturation: 100% RH means the air is saturated.
• Change: RH changes primarily with temperature (inversely proportional, assuming constant moisture) and by changing the actual water vapor content

When an unsaturated air parcel rises adiabatically (without external heat exchange), the surrounding atmospheric pressure decreases, causing the parcel to expand and cool at the Dry Adiabatic Lapse Rate (DALR).
1. Mixing Ratio (HMR) and Specific Humidity: These measurements quantify the actual mass of water vapor present relative to the mass of dry air (HMR) or total air (Specific Humidity). Since an adiabatically rising air parcel does not exchange mass with its surroundings, the actual amount of water vapor mass remains constant. Therefore, the Mixing Ratio and Specific Humidity remain constant during the ascent of unsaturated air.
2. Relative Humidity (RH): RH is the ratio of the actual water vapor content to the maximum capacity the air can hold at that temperature. Since the air temperature decreases during ascent, the air’s maximum capacity to hold water vapor decreases rapidly. Because the actual moisture content remains constant while the capacity decreases, the air moves closer to saturation, and the Relative Humidity increases. This increase continues until the RH reaches 100% at the condensation level (dew point).
3. Absolute Humidity: This measures the mass of water vapor per unit volume. As the air parcel expands during ascent, its volume increases. Since the water vapor mass is constant, the absolute humidity would decrease (although this is usually not the intended primary answer when discussing saturation criteria).

Relative Humidity (RH) is defined as the ratio of the amount of water vapor actually present in the air (actual content) to the maximum amount of water vapor the air can hold at that specific temperature and pressure (capacity), expressed as a percentage.
RH=
Maximum Water Vapour Capacity
Actual Water Vapour Content
​
×100%
If the air temperature remains constant, the maximum capacity of the air for holding water vapor remains constant. Therefore, increasing the actual water vapor content (e.g., by increasing the actual vapor pressure) directly increases the numerator of the ratio, causing the Relative Humidity to rise. This process moves the air closer to the point of saturation (100% RH).
Key Data to Remember (ICAO/FAA Context):
• Relationship: RH is directly proportional to the moisture content (assuming constant temperature).
• Mechanism: Adding water vapor causes the air to approach saturation, increasing RH.
• Controls: RH is changed by modifying either the water vapor content or the air temperature.
• Saturation: Air with 100% RH is saturated

The Dew Point (DP) is defined as the temperature to which a parcel of air must be cooled at constant barometric pressure and constant water vapor content to become saturated (Relative Humidity = 100%).
• If the air mass is saturated (e.g., in a cloud or fog), the dew point is equal to the air temperature (DP=T).
• If the air mass is unsaturated, the dew point must be lower than the air temperature (DP<T).
Therefore, the dew point can never exceed the air temperature of the air mass.
Key Data to Remember (ICAO/FAA Context):
• Relationship: DP≤T.
• Indicator: DP is a direct measure of the actual moisture content of the air.
• Saturation Spread: When T−DP=0, the air is saturated (100% RH), and condensation (cloud, fog, or dew) will begin upon further cooling

The maximum amount of water vapor air can contain, often referred to as saturation content or capacity, is primarily dependent on the air temperature. Specifically, the sources state that the higher the temperature, the more water vapor the air can hold. As temperature increases, the speed of water molecules increases, requiring a greater number of vapor molecules (and thus greater vapor pressure) to achieve saturation.
Key Data to Remember (ICAO/FAA Context):
• Capacity Control: Air temperature is the major controlling factor for the saturation capacity of air.
• Relationship: Capacity and temperature are directly related (warming air increases its capacity to hold moisture).
• Saturation Measure: This maximum capacity is quantified by the saturation content (mass per volume, g/m3) or saturation vapor pressure

Absolute humidity is defined as the actual mass (or weight) of water vapor contained within a unit volume of air. This measurement represents the density of the water vapor component in the atmosphere.
Key Data to Remember (ICAO/FAA Context):
• Definition: Actual mass of water vapor in a given volume of air.
• Units: Generally expressed in grams per cubic meter (g/m3).
• Distinction: Absolute humidity measures the actual amount of moisture present, differentiating it from Saturation Content (the maximum capacity the air can hold)

This relationship is crucial for flight planning concerning moisture and potential condensation hazards.
1. Dew Point (DP) remains constant: The dew point is the temperature to which air must be cooled at constant pressure and constant water vapor content to achieve saturation. If the air mass is simply heated or cooled without adding or removing moisture, the actual mass of water vapor (humidity mixing ratio) remains unchanged. Therefore, the dew point remains constant.
2. Relative Humidity (RH) changes inversely with temperature: Relative Humidity is the ratio of the air’s actual water vapor content to its maximum capacity at that temperature. The maximum capacity of air to hold water vapor is largely dependent on temperature; warmer air holds more water vapor than cooler air.
◦ Increasing Temperature: When temperature increases, the capacity increases. Since the actual moisture (DP) is constant, the air moves farther from saturation, causing the Relative Humidity to decrease.
◦ Decreasing Temperature: When temperature decreases, the capacity decreases, causing the air to move closer to saturation, and the Relative Humidity increases.
Key Data to Remember:
• DP → Measures actual moisture content; constant unless moisture is added/removed.
• RH → Measures degree of saturation (actual content / capacity).
• T vs. RH: Inversely proportional when moisture is constant

Water vapor (H2​O) is considered the most important gas in the atmosphere from a meteorological or weather standpoint. Although it only constitutes a variable amount (up to 4% or 5% by volume), it is the sole substance in the atmosphere that naturally exists in all three states: solid (ice), liquid (water), and gas (water vapor).
The importance of water vapor stems from two primary roles:
1. Cloud and Precipitation Formation: Water vapor transforms into liquid droplets or ice crystals, forming clouds, mist, fog, and ultimately falling as precipitation, which defines much of the weather we experience.
2. Energy Transfer (Latent Heat): The change of state (e.g., condensation, freezing) involves the release or absorption of large amounts of latent heat. This latent heat is a crucial source of atmospheric energy that powers storms like thunderstorms and hurricanes and dictates the difference between the Dry Adiabatic Lapse Rate (DALR) and the Moist/Saturated Adiabatic Lapse Rate (MALR/SALR).
Key Data to Remember (ICAO/FAA Context):
• Composition: Water vapor content is highly variable, ranging from near zero up to about 4% or 5% by volume.
• Significance: It is the ingredient responsible for condensation, precipitation, and the major release of latent heat energy in the atmosphere.
• Measurement: Humidity, Dew Point, and Temperature-Dew Point Spread are primary indicators of water vapor content

The Humidity Mixing Ratio (HMR) is defined as the weight (mass) of water vapor contained in a unit mass of dry air.
When a parcel of air (that is unsaturated, or “dry”) is lifted adiabatically, it expands and cools due to decreasing ambient pressure. Although the temperature and pressure change, the actual mass of water vapor present and the mass of dry air within the parcel do not change because there is no exchange of mass with the surroundings. Consequently, the ratio of the mass of water vapor to the mass of dry air (HMR) remains constant during this ascent.
Key Data to Remember (ICAO/FAA Context):
• Definition: HMR is the ratio of the weight of water vapor contained in unit mass of dry air, typically expressed in g/kg.
• Behavior in Ascent: In unsaturated air, HMR remains constant during adiabatic ascent (cooling).
• Measurement: HMR is a measure of the air’s actual water vapor content

In aviation meteorology, air is classified rigidly based on whether it has reached maximum water vapor capacity.
• Saturated air is defined as air with 100% Relative Humidity (RH).
• Dry air (or unsaturated air) is defined as any air where the Relative Humidity (RH) is less than 100%.
Therefore, even air with a Relative Humidity of 95% is considered dry because it is not yet saturated. The source material explicitly notes that air with RH=99% is still classified as dry.
Key Data to Remember (ICAO/FAA Context):
• Saturated Condition: RH=100% (Dew Point equals Temperature).
• Dry Condition: RH<100%.
• Significance: Dry air cools at the Dry Adiabatic Lapse Rate (DALR) when lifted

The maximum amount of water vapor that a given quantity of air can contain (its saturation capacity) is primarily dependent on air temperature. Warmer air molecules move faster, requiring a greater mass of water vapor to achieve saturation. Consequently, at higher air temperatures, it takes more water vapor to saturate the air, meaning the saturation vapor pressure increases with temperature.
Key Data to Remember (ICAO/FAA Context):
• Capacity Control: Air temperature dictates the maximum water vapor capacity of the air.
• Relationship: Saturation capacity is directly proportional to temperature.
• Saturation Measure: This capacity is quantified by the Saturation Content (Absolute Humidity at saturation) or the Saturation Vapor Pressure.
• Definition of Saturated Air: Relative Humidity (RH) = 100%

Relative Humidity (RH) measures the degree of saturation, defined as the ratio of the actual water vapor content present to the maximum capacity the air can hold at its current temperature.
When an air mass is unsaturated (RH<100%) and the actual moisture content (dew point) is held constant:
1. The maximum amount of water vapor the air can contain is dependent on temperature.
2. As air temperature increases, the capacity of the air to hold water vapor increases.
3. Since the capacity increases while the actual content remains the same, the air moves farther away from saturation, causing the Relative Humidity to decrease.
Key Data to Remember (ICAO/FAA Context):
• Relationship: RH and temperature are inversely proportional when the moisture content is constant.
• Diurnal Cycle: This principle explains the diurnal cycle of humidity: RH is lowest during the warmest part of the day (approx. 1500 LMT) and highest during the coolest part of the day (near sunrise).
• Dew Point Stability: If no moisture is added or removed, the Dew Point temperature remains constant regardless of temperature fluctuations

Relative Humidity (RH) is defined as the ratio of the amount of water vapor actually present in the air to the maximum amount of water vapor the air can hold at that specific temperature and pressure, expressed as a percentage.
Formally, it can be expressed using mixing ratios:
RH=
Saturation Mixing Ratio (SMR)
Humidity Mixing Ratio (HMR)
​
×100%
Alternatively, RH is the ratio of actual vapor pressure to saturation vapor pressure, multiplied by 100%. RH tells us how close the air is to being saturated.
⭐️ ⭐️ Key Data to Remember:
• Formula: RH=(HMR/SMR)×100%.
• Saturation: Air with 100% RH is saturated.
• Dew Point Relationship: RH increases as the air temperature approaches the dew-point temperature.

DURING THE clear night temperature will be lower, RH increases due t o cooling but dew
point does not changes.
because dew point is only affected by the change of water content.

At the same sub-freezing temperature, the saturation vapor pressure (SVP) over a liquid water surface is greater than the SVP over an ice surface.
This difference exists because water molecules escape the surface of liquid water more easily than they escape the surface of ice. Consequently, it requires a greater number of water vapor molecules (and therefore a greater pressure) to saturate the air directly above water than is required to saturate the air directly above ice at the same temperature.
Key Data to Remember:
• Relationship: SVPwater​>SVPice​ below 0∘C.
• Significance: This vapor pressure gradient is the underlying principle of the Ice-Crystal (Bergeron) process, allowing ice crystals to grow rapidly at the expense of surrounding supercooled water droplets, leading to precipitation formation in cold clouds

Saturation occurs when the air temperature (T) equals the dew point temperature (DP). The Dew Point is defined as the temperature to which the air must be cooled at constant pressure and moisture content to become saturated.
• Given T=+12∘C.
• Given DP=+5∘C.
For saturation to occur, the air temperature must be cooled exactly to the dew point temperature of +5∘C. This requires a temperature drop of 7∘C (12∘C−5∘C).
Key Data to Remember:
• Saturation: Occurs when Relative Humidity (RH) reaches 100%, which happens when T=DP.
• Moisture Indicator: Dew point is a good indicator of the air’s actual water vapor content; it remains constant unless moisture is added or removed.
• Condensation: Further cooling below the dew point causes condensation (fog, mist, or dew)

The Dew Point (DP), or Dew-Point Temperature, is the temperature to which a parcel of air must be cooled at constant barometric pressure and constant water vapor content in order for saturation to occur.
When the air temperature equals the dew point temperature, the air is saturated, and the relative humidity (RH) is 100%. Any further cooling beyond this point results in water vapor condensing to form liquid droplets (dew, fog, mist, or cloud).
Key Data to Remember:
• Definition: DP is the temperature required for an air mass to reach 100% relative humidity (saturation) upon cooling.
• Moisture Indicator: DP is a direct measure of the actual water vapor content (absolute humidity) of the air. High DP implies high moisture content.
• Temperature Relationship: The dew point can only be equal to or lower than the air temperature.
• Temperature-Dew Point Spread: The difference between air temperature (T) and DP (Td​) (the “spread”) indicates the degree of saturation. When the spread is small, RH is high; when the spread is zero (T=Td​), saturation occurs

The Dew Point Temperature (DP) is defined as the temperature to which a mass of air must be cooled, under conditions of constant barometric pressure and constant mixing ratio (water vapor content), in order for the air to become saturated (Relative Humidity = 100%).
When cooling occurs to reach saturation, water vapor begins to condense into liquid water. Since the definition stipulates constant pressure and constant moisture content, cooling the air to this specific temperature is the mechanism used to reach the dew point.
Key Data to Remember (ICAO/FAA Context):
• Definition: DP is the temperature for air to reach saturation by cooling.
• Relationship to Temperature: The dew point can only be equal to, or lower than, the current air temperature (T≥DP).
• Saturation Spread: Saturation occurs when T=DP.
• Moisture Indicator: DP is a strong indicator of the actual amount of water vapor in the air; high DP means high moisture content

Sublimation is the process where water changes state directly from a solid (ice) to a gas (water vapor), bypassing the liquid state. The reverse process, the change from water vapor directly to ice (gas to solid), is also referred to as sublimation, or specifically as deposition.
Key Data to Remember (ICAO/FAA Context):
• Definition: Direct change between the solid (ice) state and the gaseous (water vapor) state.
• Heat Exchange: When water vapor changes to ice (deposition), latent heat is released. When ice turns to water vapor (sublimation), latent heat is absorbed.
• Operational Relevance: Deposition is the process by which hoarfrost forms on an aircraft or the ground when the air temperature and dew point (or frost point) are below freezing

When a mass of air descends, it is compressed by the increasing ambient pressure, leading to adiabatic warming.
1. Moisture Content: The actual amount of water vapor in the air (measured by dew point or humidity mixing ratio) remains constant during adiabatic processes since no moisture is added or removed.
2. Capacity: The air’s capacity to hold water vapor increases rapidly as temperature increases.
3. Relative Humidity (RH): Since RH is the ratio of the constant actual moisture content to the increasing capacity, the relative humidity decreases as the air warms.
The air moves farther away from saturation, resulting in a lower relative humidity percentage.

Relative Humidity (RH) is the ratio of the actual amount of water vapor in the air to the maximum amount the air could hold at that temperature (capacity), expressed as a percentage.
RH=Maximum Water Vapour CapacityActual Water Vapour Content​×100%
A change in Relative Humidity can be brought about in two primary ways: changing the air’s water vapor content or changing the air temperature.
If the air temperature remains constant, the air’s maximum capacity to hold moisture remains fixed. Therefore, if water vapor is added to the air, the actual water vapor content (the numerator) increases, causing the Relative Humidity to rise. Adding water vapor moves the air closer to saturation (100% RH).
Key Data to Remember:
• RH Control: RH is directly proportional to the moisture content when temperature is constant.
• Operational Context: Adding moisture (e.g., evaporation over water or falling precipitation) increases RH, raising the potential for saturation, fog, or condensation

Absolute Humidity is the precise term used to define the mass (or weight) of water vapor contained within a unit volume of air. It represents the density of the water vapor in the air.
Key Data to Remember:
• Definition: Actual mass of water vapor / unit volume of air.
• Units: Generally expressed in grams per cubic meter (g/m3).
• Contrast:
◦ Relative Humidity (RH) is a ratio, expressing how close the air is to saturation (actual content/capacity, as a percentage).
◦ Specific Humidity and Humidity Mixing Ratio are mass-per-mass ratios (g/kg)

The change of state from a liquid (water) to a gas (water vapor) is called evaporation.
This process requires energy to allow water molecules to escape the liquid surface. This energy, known as latent heat, is absorbed during the change from liquid to vapor. Because heat energy is removed from the surrounding environment during this phase change, evaporation is categorized as a cooling process.
Key Data to Remember:
• Process: Liquid → Gas is Evaporation.
• Heat Transaction: Latent heat is absorbed.
• Effect: Evaporation is a cooling process.
• Reverse: The reverse (Gas → Liquid) is condensation, where latent heat is released, causing warming

The Dew Point (DP) is defined as the temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur.
• Equal Temperatures: When air is fully saturated (Relative Humidity = 100% ), the air temperature and the dew point temperature are the same (T=DP). This condition occurs in phenomena like fog or cloud.
• Lower Temperature: If the air is unsaturated (Relative Humidity <100%), the dew point must be lower than the air temperature, indicating how much cooling is required to reach saturation.
Therefore, the dew point temperature can be equal to, but never higher than, the air temperature.
Key Data to Remember (ICAO/FAA Context):
• Saturation State: T=DP implies 100% Relative Humidity.
• Definition: DP is the temperature necessary for saturation via cooling.
• Moisture Content: DP is a measure of the actual water vapor content present in the air

The change of state directly from water vapor (gas) to ice (solid), bypassing the liquid phase, is called sublimation or deposition.
During this process, energy must be removed from the water vapor molecules. This energy is known as latent heat. When water vapor changes to ice, latent heat is released to the surrounding atmosphere. This release of latent heat warms the environment and is important in phenomena like frost formation.
Key Data to Remember (ICAO/FAA Context):
• Process: Gas → Solid (Deposition/Sublimation).
• Heat Transaction: Latent heat is released.
• Atmospheric Effect: Warming of the air

In an unsaturated atmosphere, water evaporates from the muslin wick surrounding the wet bulb. Evaporation is the change of state from liquid to gas. This phase change requires energy, known as latent heat of vaporization, to be absorbed by the water molecules.
This required heat energy is drawn from the immediate surroundings, primarily the glass bulb of the thermometer (the heat source). The removal (release) of sensible heat from the bulb causes its temperature to drop until the rate of cooling balances the heat input from the surrounding air, reaching the lower wet-bulb temperature.
Key Data to Remember (ICAO/FAA Context):
• Evaporation: A cooling process; latent heat is absorbed.
• Wet Bulb Depression: The temperature drop is proportional to the rate of evaporation.
• Saturation: If the air were saturated (Relative Humidity = 100%), no net evaporation would occur, and the wet bulb temperature would equal the dry bulb temperature

The depression of the wet-bulb temperature below the ambient (dry-bulb) temperature is a direct consequence of evaporative cooling.
1. Evaporation: The wet-bulb thermometer has a muslin wick saturated with water. If the air is unsaturated, water from this wick evaporates into the surrounding air.
2. Latent Heat Absorption: The process of evaporation (liquid changing to gas) requires energy, specifically the latent heat of vaporization. This latent heat is drawn from the immediate surroundings, which includes the thermometer bulb.
3. Cooling Effect: The removal (absorption) of this heat from the bulb causes the sensible temperature reading of the wet bulb to drop below the ambient air temperature.
Key Data to Remember:
• Wet Bulb Depression: The difference between the dry-bulb temperature and the wet-bulb temperature.
• Mechanism: Evaporation is a cooling process because it absorbs latent heat from the thermometer bulb.
• Saturation State: If the air is saturated (Relative Humidity = 100%), evaporation ceases, and the dry-bulb temperature equals the wet-bulb temperature (which also equals the dew point)

The Dew Point (DP) is formally defined as the temperature to which a sample of air must be cooled, while keeping the pressure and moisture content constant, in order to reach saturation.
Once the air reaches the dew point, the relative humidity (RH) is 100%. Further cooling below the dew point causes water vapor to condense into liquid water (or sublimate into ice if below freezing), forming clouds, fog, or dew.
Key Data to Remember:
• Indicator of Moisture: The dew point is a direct measure of the actual water vapor content in the air.
• Saturation: When the air temperature and dew point are equal, the air is saturated (RH = 100%).
• Lapse Rate: As air rises, the dew point typically decreases by about 0.5
∘
C per 1000 ft gain in height

The correct statement defines Absolute Humidity (AH), which is a specific measure of atmospheric moisture content.
Absolute Humidity (AH) is formally defined as the actual mass of water vapor present in a given volume of air. It is typically expressed in units of grams per cubic meter (g/m
3
).
Key Data to Remember:
• Definition: AH represents the density of water vapor in the air.
• Contrast with Dew Point (DP): While AH measures the physical mass concentration, DP is the temperature representation of the air’s actual water vapor content. High AH means a high DP.
• Incorrect Options Review (Pilot Context):
◦ Wet Bulb Temperature: The wet bulb temperature equals the dew point temperature only when the air is saturated (RH 100%).
◦ Diurnal Variation of DP: The dew point is primarily dependent on the actual moisture content, which generally varies only slightly during the day, making the statement that its diurnal variation is greatest (presumably compared to temperature or RH) incorrect.
◦ Falling Temperature: If air temperature falls, the mass of water vapor remains constant unless cooling results in condensation, in which case water vapor is converted to liquid/ice, causing the mass (and therefore AH) to decrease

The Wet Bulb Temperature is the lowest temperature air can reach by evaporation. When the Relative Humidity (RH) reaches 100%, the air is saturated, meaning it holds the maximum possible amount of water vapor at that pressure and temperature. Under saturation conditions, no net evaporation occurs from the wet bulb thermometer’s wick, so no latent heat is absorbed, and the temperature does not drop. Therefore, the Dry Bulb (air) temperature and the Wet Bulb temperature are the same. Since the air is saturated, this common temperature is, by definition, the Dew Point temperature.
Key Data to Remember:
• Saturation State: RH =100% means the air is saturated.
• WBT Indication: At saturation, Dry Bulb Temperature = Wet Bulb Temperature = Dew Point Temperature.
• Pilot Relevance: This condition (small temperature-dew point spread) indicates maximum moisture content and is critical for anticipating condensation phenomena like fog or cloud formation.

The difference between the air temperature (T) and the dew point temperature (T
d
​
), often referred to as the temperature-dew point spread, is inversely related to the Relative Humidity (RH).
• When T and T
d
​
are far apart (greater difference), the Relative Humidity is low. Air with low RH (less than 100%) is classified as “dry air”.
• When T and T
d
​
are close together or equal, the Relative Humidity is ** high** or 100% (saturated/moist air).
Therefore, a large temperature-dew point spread indicates that the air is far from saturation, defining it as relatively dry.
Key Data to Remember (ICAO/FAA Context):
• Low RH → Large Spread
• High RH → Small Spread
• The spread is crucial for predicting condensation phenomena (e.g., fog or dew), which occur when cooling causes the spread to approach zero (T approaches T
d
​
).

Relative Humidity (RH) is defined as the ratio, expressed as a percentage, of the amount of water vapor actually present in the air compared to the maximum amount of water vapor the air can hold at that particular temperature and pressure. It tells an observer how close the air is to reaching saturation.
Key Data to Remember (ICAO/FAA Context):
• Saturation: When Relative Humidity is 100%, the air is saturated.
• Formula: RH=(Actual Vapour Pressure/Saturation Vapour Pressure)×100%.
• Operational Definition: For meteorological purposes, air is considered “dry air” if the RH is less than 100% (e.g., 99% RH is still technically defined as dry air).

Relative Humidity (RH) is defined as the ratio of the actual water vapor content in the air to the maximum amount of water vapor the air can hold at that specific temperature and pressure. This relationship is often expressed as:
RH=
Saturation Vapor Pressure
Actual Vapor Pressure
​
×100%
The maximum amount of water vapor air can hold (represented by the Saturation Vapor Pressure) is directly dependent on temperature; warmer air can hold more water vapor than cooler air.
If the air is cooled while maintaining the water vapor content (Actual Vapor Pressure) constant:
1. The air’s capacity for water vapor (Saturation Vapor Pressure) decreases.
2. Since the numerator (Actual Vapor Pressure) remains constant and the denominator (Saturation Vapor Pressure) decreases, the resulting ratio (Relative Humidity) must increase.
Cooling the air to the point where the RH reaches 100% is the definition of reaching the Dew Point.
Key Data to Remember (Operational Context):
• Inverse Relationship: If the air’s moisture content (dew point) remains unchanged, air cooling leads directly to increased Relative Humidity.
• Diurnal Variation: Due to cooling overnight, the highest Relative Humidity usually occurs in the early morning, coinciding with the minimum air temperature (assuming constant absolute humidity).
• Saturation Limit: Relative humidity reaches 100% when the air temperature equals the dew point temperature.

When air is saturated, the air temperature (T) equals the dew point temperature (T
d
​
). The difference between these two values is known as the spread. When T equals T
d
​
, the spread is zero, which is the least possible value. This condition signifies that the Relative Humidity (RH) is 100%.
Key Data to Remember
• Condition: Saturated air / Relative Humidity (RH) = 100%.
• Relationship: Air Temperature (T) = Dew Point Temperature (T
d
​
).
• Spread: The difference (T−T
d
​
) is zero.

The change of state from water vapour (gas) to liquid water droplets is defined as condensation. Since the liquid state possesses a lower energy level than the gaseous state, energy—referred to as latent heat—must be released into the surrounding environment when this transition occurs. This release of latent heat warms the atmosphere and is a fundamental process in cloud formation and atmospheric stability, as it slows the rate of adiabatic cooling in saturated air (Saturated Adiabatic Lapse Rate).
Key Data to Remember:
• Process: Condensation.
• Heat Exchange: Latent heat is released.
• Significance: Condensation releases heat (latent heat of condensation) that adds energy to the troposphere, driving convection and reducing the cooling rate of rising air.

The standard instrument category used for measuring the water vapor content of the air is the hygrometer.
• Hygrometer: This is the general term for an instrument designed to measure the air’s water vapor content. Examples include the hair hygrometer, electrical hygrometer, and psychrometer.
• Wet Bulb Thermometer: This is only a component of the Psychrometer (Wet and Dry Bulb Hygrometer). The wet-bulb temperature measures the lowest temperature attainable by evaporation; this measurement, combined with the dry-bulb temperature (air temperature), is used to compute relative humidity and dew point via tables.
Key Data to Remember (Humidity Measurement)
• Primary Instruments: Atmospheric humidity is measured using a Dry Bulb and Wet Bulb Hygrometer, or Psychrometer, or an electrical hygrometer.
• Measurement Principle: The psychrometer uses the wet-bulb depression (the difference between dry and wet bulb temperatures) to determine relative humidity (RH) and dew point (DP).
• Saturation Check: If the air is saturated (100% RH), the dry bulb, wet bulb, and dew point temperatures will all be the same.

The Troposphere is the atmospheric layer where the vast majority of atmospheric humidity (water vapor) is concentrated.
• Location and Content: The troposphere extends from the surface up to the tropopause (an average height of 11 km). It holds virtually all the water vapor in the atmosphere.
• Weather Significance: Because water vapor is essential for cloud formation and precipitation, the troposphere is the layer where almost all weather phenomena occur.
Key Data to Remember (Atmospheric Layers)
• Troposphere: Contains virtually all water vapor, generally characterized by decreasing temperature with altitude.
• Stratosphere: Located above the tropopause, characterized by low moisture content and absence of clouds (except for rare types like nacreous clouds).
• Tropopause: The boundary between the troposphere and the stratosphere. It is a thin layer where the temperature lapse rate changes abruptly.

On a rainy day, the required runway length (for both takeoff and landing) is generally more compared to a dry, sunny day, primarily due to factors related to air density and runway surface contamination.
1. Effect on Takeoff Distance (Density/Performance): Rain is often associated with high atmospheric humidity. Water vapor is less dense than dry air. This reduced air density (high density altitude) negatively impacts aircraft performance by reducing engine thrust and aerodynamic lift. When density is low, the aircraft requires a longer takeoff run to achieve the necessary True Airspeed (TAS) for liftoff.
2. Effect on Landing Distance (Runway Contamination): Rain leads to a wet runway surface. Standing water or heavy rain affects braking action and may result in aquaplaning. Reduced braking efficiency significantly increases the distance required for the landing roll.
Key Data to Remember (Performance/Runway Length)
• Low Density: Reduces lift and engine performance (thrust).
• Wet Runway: Reduces friction and braking efficiency.
• Result: Low density air combined with a wet runway surface dictates a larger required runway length compared to dry conditions.

The wet-bulb temperature (T
W
​
) is defined as the lowest temperature to which air can be cooled by the evaporation of water. This measurement is obtained using a wet-bulb thermometer, which is part of a Psychrometer (or Wet and Dry Bulb Hygrometer). Evaporation from the wet muslin cloth surrounding the bulb draws latent heat from the thermometer, causing its temperature to decrease below the ambient (dry bulb) temperature.
Key Data to Remember (Wet-Bulb Temperature)
• Definition: T
W
​
is the lowest temperature attainable by evaporative cooling.
• Saturation Condition: If the air is saturated (100% Relative Humidity), no evaporation occurs, and the wet bulb temperature, dry bulb temperature, and dew point temperature are all equal.
• T
W
​
vs. Dew Point (T
D
​
): The wet-bulb temperature is generally not the same as the dew point temperature, except at saturation. T
W
​
is used, along with the dry bulb temperature, to find T
D
​
and Relative Humidity using meteorological tables.
• Human Comfort: The wet-bulb temperature is a measure of how cool human skin can become, relating directly to the efficiency of evaporative cooling (perspiration), which affects sensible temperature and the Heat Index (HI).

The definition provided—”It is the lowest temperature which air would attain by evaporating water into it to saturate it”—is the precise definition of the wet-bulb temperature (TW).

• Wet-Bulb Temperature: This measurement is obtained using a wet-bulb thermometer, which has a muslin cloth soaked in distilled water covering the bulb. As water evaporates from the cloth, it absorbs latent heat from the surrounding air and the thermometer bulb, causing the temperature reading to drop. The lowest temperature achieved through this evaporative cooling process is the wet-bulb temperature.
• Saturation: The process of evaporative cooling ceases when the air around the bulb reaches saturation. Thus, the wet-bulb temperature represents the minimum temperature achievable by evaporation.
• Hygrometry Context: The wet-bulb temperature is crucial because the difference between it and the dry-bulb temperature (the ambient air temperature) is used in a Psychrometer to calculate the dew point, relative humidity, and humidity mixing ratio.