Download Proficiency Level Three - 533 St. Albert RCACS

Proficiency Level Three
Combined Aviation Assessment Study Guide
Developed by: 533 St. Albert RCACS
Last Revision: January 2014
Reference: A-CR-CCP-803/PG-001 Level Three Qualification Standard and Plan
A-CR-CCP-803/PF-001 Level Three Instructional Guides
Table of Contents
Performance Objective 331: Principles of Flight ...................................................................... 2
331.01 – Describe Aircraft Stability ........................................................................................................ 2
Performance Objective 336: Meteorology .............................................................................. 6
336.01 – Describe Properties of the Atmosphere ................................................................................ 6
336.02 – Explain the Formations of Clouds ......................................................................................... 10
336.03 – Explain the Effects of Air Pressure on Weather ................................................................... 15
336.04 – Explain the Effects of Humidity and Temperature on Weather ........................................ 20
1 Performance Objective 331
Principles of Flight: Describe Principles of Flight
331.01 Describe Aircraft Stability
Why do we need to know about aircraft stability?
Knowledge of aircraft stability is crucial in understanding how aircraft will react when flying
through turbulent weather, or perhaps when they are put through aggressive flight manoevres. It
will also allow you to understand why aircraft are designed with certain features.
What is stability?
Stability is the tendency of an aircraft in flight to remain in straight, level, upright flight and to
return to this attitude, if displaced, without corrective action by the pilot.
What are the characteristics of stability?
Static Stability
The initial tendency of an aircraft to return
to its original attitude, if displaced
Positive Stability
The aircraft is able to
return to its original
attitude without any
corrective measures
-VS-
-VS-
Dynamic Stability
The overall tendency of an airraft to return
to its original attitude
Neutral Stability
The aircraft will remain in
the new attitude of flight
after being displaced,
neither returning to its
original attitude, nor
continuing to move
away
-VS-
Negative Stability
The aircraft will continue
to move away from its
original attitude after
being displaced
What are the axes of an aircraft?
Longitudinal Axis (Axis of Roll)
Runs the length of the aircraft, from the tip of the
nose to the end of the empennage
Lateral Axis (Axis of Pitch)
Runs through the aircraft’s wings, from wing tip to
wing tip
Vertical/Normal Axis (Axis of Yaw)
Runs vertically through the aircraft from top to
bottom
All three axes intersect at an aircraft’s centre of
gravity
2 What is longitudinal stability?
• Stability around the lateral axis
• Known as pitch stability
• Achieved by designing the aircraft to be nose heavy if loaded correctly
• Influenced by the horizontal stabilizer and the centre of gravity
What are the effects of the horizontal stabilizer?
• Located at the tail end of the aircraft, and functions as a counterweight
• Forces the tail down when the nose of an aircraft is pushed up
• When the stabilizer meets the airflow at a higher angle, it produces more lift which
counters the initial disturbance
What are the effects of the centre of gravity?
• Each aircraft’s centre of gravity is naturally occuring and is dependent on its design
• When an aircraft is loaded, the centre of gravity can shift; drastic shifts in the centre of
gravity can adversely affect the stability of an aircraft
•
•
•
Too Far Forward!
Produces a “nose-down” tendency
Forces the pilot to use excessive back
pressure on the controls
Causes the aircraft to speed up and
lose altitude
•
•
•
Too Far Aft/Back!
Produces a “nose-up” tendency
Forces the pilot to use excessive
forward pressure on the controls
Causes the aircraft to slow down and
eventually stall
What is lateral stability?
• Stability around the longitudinal axis
• Known as roll stability
• Achieved by three design features built into an aircraft: dihedral, sweepback, and keel
effect
What is dihedral?
When looking at an aircraft from the front, you will notice that the wings are angled so the wing
tip is higher than the wing root – this angle is the dihedral (or in the cases of a negative angle,
the anhedral)
What are the effects of dihedral?
• Assists the aircraft in maintaining lateral stability by changing the angle that the leading
edge makes with the airflow
• Produces lift on a down-going wing when the aircraft is forced into a side-slipping motion
What is sweepback?
• A design feature providing the same effects as dihedral, where the wings sweep back
instead of protruding straight out from the fuselage
What is keel effect?
• A form of built-in stability found in high-wing aircraft
• The bulk of the aircraft, which sits below the wings, acts as keel
• The fuselage acts like a pendulum and swings the aircraft back into position when the
wing is forced up by a disturbance
3 What is directional stability?
• Stability around the vertical axis
• Influenced by the vertical tail surface, also known as the fin
What are the effects of the fin?
• Weather vaning is an aircraft’s tendency of always flying head-on into the relative
airflow, and is a direct result of the fin
• If the aircraft yaws away from its course, the airflow strikes the fin from the side, forcing it
back into position
• Will only work if the side area of the aircraft is greater aft of the centre of gravity than the
area forward of the centre of gravity
4 What is the definition of aircraft stability?
Describe dynamic and static stability.
What are the factors influencing longitudinal stability?
Directional stability is stability around which axis?
5 Performance Objective 336
Meteorology: Identify Meteorological Conditions
336.01 Describe Properties of the Atmosphere
Why do we need to know about the properties of the atmosphere?
Being able to describe the properties of the atmosphere will allow you to better understand how
weather conditions are created
What is the atmosphere composed of?
The atmosphere is composed of a mixture of invisible gases, as well as trace amounts of dust
and debris in the lower levels
What types of invisible gases make up the atmosphere?
At altitudes of up to 250 000 feet above sea level, the
atmosphere is composed primarily of nitrogen (78%), oxygen
(21%), and several other gases (1%) which include argon,
carbon dioxide, hydrogen, and water vapour, among others.
Why is water vapour important?
• Only found in the lower levels of the atmosphere
• Amount is never constant
• Most important gas in terms of weather
• Can change from gas into water droplets or ice crystals
• Responsible for the formation of clouds
How is the atmosphere divided?
The atmosphere is divided into four distinct layers, and extends for many miles from the surface
of the earth. These layers are named troposphere, stratosphere, mesosphere, and thermosphere.
The exosphere is not actually a layer of the atmosphere, but rather the start of outer space
6 •
•
•
•
•
•
•
•
•
•
•
Troposphere
Lowest layer of the atmosphere
Starts at ground level and extends to
varying heights
Air pressure, density, and temperature
decrease with altitude
Temperature will drop to a low of -56°C
Most weather occurs here due to the
presence of water vapour and strong
vertical currents caused by terrestrial
radiation
Terrestrial radiation causes the
troposphere to extend to varying
altitudes – there is more radiation at the
equator than at the poles
The jet stream exists in the upper parts of
the troposphere
The top of the troposphere is known as
the tropopause, and is the boundary
between the troposphere and the
stratosphere
•
•
•
•
•
Mesosphere
Characterized by a decrease in
temperature – as low as -100°C at
275 000 feet above sea level
Meteorites usually burn up in this layer
The top of the mesopshere is known as
the mesopause, and is the boundary
between the mesosphere and the
thermosphere
•
•
•
•
•
Stratosphere
Extends 50 000 feet upwards from the
tropopause
Pressure continues to decrease
Temperature will gradually rise to 0°C
Most of the ozone layer exists here,
which prevents the more harmful solar
radiation from reaching the earth’s
surface and explains the rise in
temperature
The top of the stratosphere is known as
the stratopause, and is the boundary
between the stratosphere and the
mesosphere
Thermosphere
Highest of the four layers
Named due to its intense temperatures
First layer to be affected by solar
radiation
What little oxygen exists absorbs high
amounts of the radiation
Actual temperatures vary, but can
exceed 15 000°C
What is meant by ‘standard atmosphere’?
The decrease in temperature, pressure, and density with altitude is not consistant, but varies with
local conditions. The International Civil Aviation Organization (ICAO) has set an international
standard for the purposes of aviation, and different regions have different standards.
What are the basis of ICAO standards in North America?
• Summer and winter averages
• 40 degrees north latitude
Whare are the assumptions for standard atmosphere in North America?
• Air is a perfectly dry gas
• An average (mean) sea level pressure of 29.92 of mercury
• An average (mean) sea level temperature of 15°C
• Temperature decreases with altitude at a rade of 1.98°C per 1 000 feet
7 What are the principle properties of the atmosphere?
•
•
Mobility
Ability for air to move
from one place to
another
Explains why air masses
which form in the
arctic can affect areas
in the south
Capacity for Expansion
• Most important
property
• Air is forced to rise for
various reasons
• As air pressure
decreases, the air will
expand and cool
• The cooling may
cause condensation to
occur and clouds to
form, creating
precipitation
Capacity for Compression
• The opposite of
expansion
• When the air cools, it
becomes denser
• As the air sinks, it
decreasese in volume
and increases in
temperature
What are the factors affecting the properties of the atmosphere?
Temperature
Changes air density
Density
Creates the vertical
movement of the air, causing
expansion and compression
(differences in pressure)
8 Pressure
Causes mobility across the
surface as the air moves
horizonally to fill gaps left by
air that has moved vertically
Which gas composes 78% of the earth’s atmosphere?
What is the lowest layer of the atmosphere?
The ozone layer can be found in which layer of the atmosphere?
What are the assumptions for standard atmosphere in North America?
How does mobility work as a property of the atmosphere?
How does capacity for expansion work as a property of the atmosphere?
9 336.02 Explain the Formation of Clouds
Why do we need to know about cloud formations?
Understanding how clouds form will enhance your knowledge in meteorology and also your
ability to predict weather
How are clouds classified?
• Formation
• Height
What are the types of cloud formations?
•
•
•
•
Cumulus
Formed by unstable air
Cottony or puffy
Seen mostly during warmer seasons
May develop into storm clouds
•
•
•
•
Stratus
Formed by stable air
Flat
Seen year-round
Associated with colder temperatures
What are categories of cloud height?
Clouds are classified into one of four categories based on their height above ground level
•
•
•
•
•
•
Low Clouds
Bases range from the surface to 6 500
feet
Composed of water droplets and
sometimes ice crystals
Use the word stratus as either a prefix
(strato-) or a suffix (-stratus)
•
•
•
High Clouds
Bases range from 16 500 feet to 45 000
feet, with an average of 25 000 feet in
the temperate regions of the earth
Composed of ice crystals
Use the word cirrus as a prefix (cirro-)
•
•
•
10 Middle Clouds
Bases range from 6 500 feet to 23 000
feet
Composed of ice crystals or water
droplets, which may be at
temperatures above 0°C
Use the word alto as a prefix (alto-)
Clouds of Vertical Development
Base may be as low as 1 500 feet and
may rise as high as the lower reaches
of the stratosphere
May appear as isolated clouds or seen
as embedded in layers of clouds
Associated with thunderstorms and
other summer phenomena
What are some common cloud types?
What is air stability?
• The normal flow of air is horizontal at the surface
• Vertical air currents develop when disturbances occur, normally caused by changes in
temperature
• If the displaced air resists the change, it is said to be stable
• If the displaced air does not resist the change, it is said to be unstable
• When air rises, it expands and cools
Stable Air
If the rising air is cooler than the air it comes
in contact with, it will sink back to its
original position
-VS-
11 Unstable Air
If the rising air is warmer than the air it
comes in contact with, it will continue to
rise
What are the effects of stable and unstable air on flight characteristics?
•
•
•
•
•
Stable Air
Poor low-level visibilty (fog may occur)
Stratus type clouds
Steady precipitation
Steady winds, which can change
greatly with height
Smooth flying conditions
•
•
•
•
•
Unstable Air
Good visibility (except in precipitation)
Cumulus type clouds
Showery precipitation
Gusty winds
Moderate to severe turbulence
What are lifting agents?
Lifting agents are conditions which provide lift to initiate rising air currents, which in turn affect
many weather conditions. The five lifting agents are described below.
•
•
•
•
•
•
Convection
Air is heated through contact with the
earth’s surface
As the sun heats the earth’s surface,
the air in contact with the surface
warms up, rises, and expands
Orographic Lift
Occurs when the sloping terrain forces
the air upwards
Process can be exaggerated if the air
mass is already unstable
Frontal Lift
Warmer air is forced upwards by
denser, colder air as two different air
masses meet
Process may be exaggerated if the
warm air mass becomes unstable
12 •
•
•
•
•
Mechanical Turbulance
Similar to orographic lift, except caused
by smaller terrain such as forests,
builidings, and large ditches
This friction can cause eddies, which
are usually confined to the first few
thousand feet of the troposphere
Process may be exaggerate if the air
mass becomes or is already unstable
Convergence
In a low pressure system, wind blows
towardsd the centre
Excess air that collects in the centre is
forced upwards to higher altitudes
What are the two ways that clouds are formed?
• Temperature drops to the saturation point of the air
• Temperature remains constant but the amount of water in the air increases
How do lifting agents relate to air stability?
• Each lifting agent either affects or is affected by air stability
• Convection, for example, is normally associated with unstable air since heat causes the
convection, and is also a source of instability in the air
• Orographic lift, for example, is usually associated with stable air, as after the air has been
forced up by the terrain it cools and becomes dense
How does air stability relate to cloud formations?
• Air stability directly affects cloud formation
• Clouds created in stable air will form as stratus-type clouds
• Clouds created in unstable air will form as cumulus-type clouds
13 What are the two main types of cloud formations?
What is unstable air?
What are the effects of unstable air on flight characteristics?
How does convection function as a lifting agent?
14 336.03 Explain the Effects of Air Pressure on Weather
Why do we need to know about the effects of air pressure on weather?
Knowing the effects of air pressure on weather will increase your understanding of how air
moves and your ability to recognize weather patterns
What is the Polar Front theory?
The Polar Front theory, created by Norwegian meteorologists, claims that the interaction
between high pressure areas over the poles and low pressure areas around the equator may
provide force to the movement of air
What is atmospheric pressure?
Atmospheric pressure is the pressure of the atmosphere at any point due to the weight of the
overlying air
How do we measure pressure?
• Pressure at the surface of the earth is normally
measured using a mercury barometer and is
expressed in mm of mercury (mm Hg) or inches of
mercury (" Hg)
• In meteorological work, it is common to use
hectopascals (hPa) to measure pressure
• 1 hPa is 1 000 dynes of force exerted on 1 cm2
• Average pressure at sea level is normally expressed as
760 mm Hg, or 1013.2 hPa
• Public weather broadcasts normally use kilopascals
(kPa), displaying the average pressure as 101.32 kPa
What are pressure systems?
•
•
•
•
Low Pressure Areas
Normally move in an easterly direction
at an average rate of 800 km per day
in the summer and 1 100 km per day in
the winter
Associated with thunderstorms and
tornadoes
Do not stay in one place for very long
Air moves around a low pressure area
in a counter-clockwise direction in the
northern hemisphere
•
•
•
15 High Pressure Areas
Winds are usually light and variable
Move very slowly, sometimes staying
stationary for days at a time
Air moves around a high pressure area
in a clockwise direction in the northern
hemisphere
How do we display pressure systems on a map?
Areas of like pressure are joined by lines called
isobars. On a weather map, isobars will look
similar to contour lines found on a
topographical map. The isobars form roughly
concentric circles, each circle being four hPa
different than the circles before and after it.
Groups of isobars will indicate areas of
relatively high pressure, or relatively low
pressure.
What are the characteristics of air masses?
Polar Regions
Cold
Dry
-VS-
Equatorial Regions
Warm
Moist
What is the polar front?
The polar front is the transition zone between the polar air and the equatorial air, and the
movement of air within the polar front is thought to be the cause of air circulation in the
troposphere
What are the characteristics of the polar front?
• Many low pressure areas due to the differences in properties of the two air masses
• Cold air moves from north-east to south-west in the northern hemisphere, while warm air
moves in the opposite direction
• Cold air bulges south and warm air bulges north, creating constant instability
• Cold air moves faster than the warm air and eventually envelopes it
What is an air mass?
An air mass is a large section of the troposphere with uniform properties of temperature and
moisture along the horizontal plane
How are the properties of air masses determined?
An air mass will take on the properties of the surface over which it has formed
How can air masses be described?
By Temperature
Arctic – Cold (formed over the Arctic)
Polar – Cool (formed over the Polar region)
Tropical – Warm (formed over the Tropical region) 16 By Moisture
Continental – Dry (formed over land)
Maritime – Moist (formed over water)
Normally these types of air masses are combined
to describe the properties of temperature and
moisture. In North America there are five air
masses:
•
•
•
•
•
Continental Arctic (cA)
Maritime Arctic (mA)
Continental Polar (cP)
Maritime Polar (mP)
Maritime Tropical (mT)
What is wind?
Wind is the horizontal movement of air within the atmosphere
What are the characteristics of wind?
• Moves parallel to the isobars of a pressure system, meaning wind direction will vary at
different locations along the pressure system because isobars are not straight lines
• Moves in different directions based on whether the pressure is a low or high system
Why is wind important?
• Major factor in flight planning and flight characteristics
• Awareness of wind speed and direction is crucial in all parts of a flight, and especially
during landing
What is a pressure gradient?
• Rate of change of pressure over a give
distance measured at right angles to
the isobars
• When isobars are close together, the
rate of change will steep and the wind
speed will be strong
• When isobars are far apart, the rate of
change will be shallow and the wind
speed will be weak
What causes land and sea breezes?
Land and see breezes are caused by the differences in temperature over land and water
17 What is a sea breeze?
• Occurs during the day when the
land heatsup more rapidly than the
water
• Creates a lower pressure area over
the land
• Pressure gradient caused by this
change is usually steep enough to
create a wind from the water
What is a land breeze?
• Occurs at night when the land
cools down faster than the water
• Creates a higher pressure area
over the land
• Pressure gradient moves the air
from the land to the water
What is diurnal variation?
Surface winds are generally stronger during the day than at night. This is due to the heating
processes, which occur during the day, creating vertical currents and pressure gradients. At
night, when the heating processes cease, the vertical currents diminish and the pressure
gradients become shallower.
What is Coriolis Force?
As air moves from a high pressure system to a low pressure system, the air will not flow directly
from one to the other. The rotation of the earth causes a deflection to the right (in the northern
hemisphere). This force is known as the Coriolis Force. Coriolis Force also explains why air moves
clockwise around a high, and counter-clockwise around a low pressure system.
What is veering and backing?
• A change in wind direction clockwise relative to the cardinal points of a compass is
known as veering
• A change in wind direction counter-clockwise relative to the cardinal points of a
compass is known as backing
• Occur with changes in altitude; increases will cause veering and an increase in wind
speed, while decreases will cause backing and a decrease in wind speed
• Changes are due to increases in friction with the surface of the earth at lower altitudes
and decreases in friction at higher altitudes
18 What are areas of equal atmospheric pressure joined together by?
What is a low pressure area?
What is wind?
19 336.04 Explain the Effects of Humidity and Temperature on Weather
Why do we need to know the effects of humidity and temperature on weather?
Knowing the effects of humidity and temperature on weather will allow you to make informed
decisions about activities in the field and in aviation… like whether or not you need an umbrella!
What is humidity?
• A representation of the moisture or water vapour present in an air mass
What are some characteristics of water vapour?
• Makes up only a small percentage of the overall atmosphere
• Only gas which can change into a solid or a liquid in ordinary atmospheric conditions
• Causes most weather to develop
How does moisture enter an air mass?
• Originates from a body of water, such as a pond or an ocean, over which the air mass
forms or passes
• The size of the body of water will determine how much water is available for the air mass
to collect
• The rate of evaporation will determine how much water is collected by the air mass
In what forms can water exist in the atmosphere?
Invisible
Gaseous
Visible
Liquid – Water Droplets
Solid – Ice Crystals
What is condensation?
• Process by which a gas changes into a liquid by becoming denser
• Usually created by a cooling process
What is sublimation?
• Process by which a gas changes into a solid without first becoming a liquid
• Usually occurs by freezing
• Occurs whenever snow, ice or hail fall from the sky
• Usuualy occurs in the winter or during exceptional summer storms
What is dew point?
• The temperature to which unsaturated air must be cooled, at a constant pressure, to
become saturated
• Temperature and dew point are responsible for the creation of clouds and precipitiation
• If the difference between temperature and dew point is smaill, the air is considered to be
nearly saturated and a small drop in temperature will see the formation of clouds or
precipitation
What is relative humidity?
• The ratio of the actual amount of water present in the air compared to the amount of
water which the same volume of air would hold if it were saturated
• Temperature and pressure must remain the same, otherwise relative humidity will change
• Saturated air will have a relative humidity of 100%
• Perfectly dry air will have a relative humidity of 0%
20 What is temperature?
• A representation of the amount of heat in a given object
• Measured using a thermometer
• In aviation weather reports, normally expressed in degrees Celcius (°C)
What is the source of heat in the atmosphere?
• The sun is responsible for heating the earth and the atmosphere
• Solar radiation is absorbed by the earth’s surface
• The earth radiates heat into the troposphere through terrestrial radiation
• This means the atmosphere is heated from below, not above
What is diurnal variation?
• Causes the heating and cooling of the atmosphere
• During the day, the solar radiation exceeds the terrestrial radiation and the surface of the
earth becomes warmer
• At night, there is no solar radiation and the terrestrial radiation cause the surface of the
earth to cool
What is seasonal variation?
• The axis around which the earth rotates is tilted
• This causes different amounts of solar radiation to strike the surface of the earth at
different times of the year
• In the northern hemisphere, this results in the summer months being warmer, while the
winter months are colder
What are heating processes?
Since air is a poor conductor of heat, there are four processes which assist in getting warm air
into the higher levels of the atmosphere
•
•
•
•
•
•
Convection
Air over a warm surface becomes
buoyant and rises
Cooler air moves into the vacant
location
Vertical current of air distributes the
heat to higher levels
•
•
Turbulence
Created as a result of friction with the
surface of the earth
Causes a mixing process
Moves the heated air to other areas of
the atmosphere
•
•
•
•
21 Advection
Cool air moves horizontally over a
warm surface
Air gets heated from below
Compression
Air masses are forced down
Air pressure increases, compressing the
air mass
Compression forces the particles
together, causing heat
Also known as subsidence
What are cooling processes?
• Since the atmosphere is heated from below, temperature usually decreases as altitude
increases
• The rate of temperature change is known as a lapse rate
• Lapse rate is only a guideline as there is a variation in air masses and the three main
cooling processes
Radiation Cooling
Earth’s temperature
decreases at night
with terrestrial radiation
• Air in contact with the
ground cools
•
•
Advection Cooling
Air from a warm region
moves over a cold
region and cools
Adiabatic Process
As air warms it rises
As air rises it expands
and cools
• In a rising air current,
temperature
decreases at a rate
independent of the
surrounding air
•
•
What are the effects of temperature on relative humidity?
• Temperature will affect the relative humidity of an air mass by changing the volume of
the air mass
• Increasing the temperature of an air mass will cause it to expand, which will increase its
volume
• Increasing the volume decreases the relative humidity, since the air mass will now have a
higher capacity for water
• Decreasing the temperature of an air mass will cause it contract, which will decrease its
volume
• Decreasing the volume increases the relative humidity, since the air mass will have less
capacity for water
How does temperature and humidity affect weather?
•
•
•
Temperature & Dew Point
Temperature changes during the
heating and cooling processes
As the temperature nears the dew
point, the air becomes more saturated
The increase in relative humidity allows
clouds to form
•
•
•
Relative Humidity
As relative humidity increases, the
weight of the air mass also increases
When the dew point is reached, the air
becomes saturated and clouds form
Once an air mass reaches 100% relative
humidity, any additional water or drop
in temperature will cause precipitation
What is precipitation?
• May be solid or liquid, depending on the temperature of the air mass
• Snow will occur when the air has a temperature below freezing
• Rain will occur when the air has a temperature above freezing
• Temperatures in an air mass change with altitude
• Water may freeze at higher levels of an air mass, causing snow or hail in summer months
22 What kinds of precipitation are there?
The World Meteorological Organization (WMO) has listed seven main categories of precipitation,
depending on temperature and cloud type
•
•
•
•
•
•
•
•
Drizzle
Small water droplets
which appear to float
In near freezing
temperatures, water
droplets may freeze on
contact with objects
(freezing drizzle)
•
•
Rain
Large water droplets
Freezing rain occurs
when liquid water
droplets make contact
with objects in near
freezing conditions
Hail
Formed in clouds which have strong
vertical currents (like thunderstorms)
Is a water droplet which has been
prevented from leaving the cloud until
reaches a certain mass
Stronger vertical currents create larger
hail stones
•
•
•
Snow
Result of sublimation
Flakes are a collection of ice crystals
Usually in the shape of a hexagon or
star
•
•
•
•
23 •
•
Ice Pellets
Raindrops which are
frozen before contact
Generally rebound
after hitting the ground
Snow Pellets
Formed in clouds over shallow water
regions
Water droplets do not form the hard
shells of hail
Pellets fall as soft snow pellets instead
Ice Prisms
Created in stable air masses at very low
temperatures
Tiny ice crystals in the form of needles
Can form with or without clouds
Often confused with ice fog
What is relative humidity?
What is dewpoint?
24