Download Q: What is Weather

Aviation Weather Notes
Q: What is Weather?
Weather is the condition of the atmosphere at a particular time and place
What is Atmosphere?
- The atmosphere is the envelope of gases that surround our planet
It is a very thin layer compared to the radius of the Earth
The atmosphere makes conditions on Earth suitable for life
What is Climate?
Climate is the long-term state of the atmosphere at a particular location
- Meteorology is the study of the Earth’s atmosphere, its motions and behavior
- Climatology is the study of the Earth’s climate, its past and future
Composition of Atmosphere
Dry air (neglecting water vapor) is composed of the following gases:
Nitrogen (N2) = 78%
Oxygen (O2) = 21%
Argon (Ar) = 1%
Trace Gases (CO2, H2, He, Ne, Kr, CH4)
Dust, bacteria, and other particles
In addition it contains water vapors.Water content (moisture) depends on TEMPERATURE !
Layers of the Atmosphere
The Earth’s atmosphere has 4 distinct layers that are identified by the way temperature changes with
ALTITUDE
These layers are:
-
Troposphere
Stratosphere
Mesosphere
Thermosphere
Troposphere is the layer closest to the Earth’s surface
Almost all the weather phenomenon occurs in troposphere
Temperature decreases with altitude increase
(Generally decreases at a rate of 4.4ºF for every 1000 ft - STANDARD LAPSE RATE)
The depth of the t. varies by the season (deeper in summer)
Stratosphere
- The next layer above the troposphere. The ozone layer is found here
Temperature increases with altitude
The boundary between the stratosphere and troposphere is called the tropopause
Mesosphere
- The mesosphere is the next layer above the stratosphere
- Temperature decreases with height. It is the coldest part of the atmosphere (Temp. nears -90o C)
Clouds are rare in the mesosphere
Thermosphere
No clouds exist in the thermosphere. Temperature increases with altitude in the thermosphere.
Contains the highest temperatures for the atmosphere
Thermosphere is divided in two sublayers:
IONOSPHERE - place where aurora borealis & australis occur
EXOSPHERE – the outermost layer of atmosphere (satellites are placed here)
Elements of weather
Weather is described by variables such as:
(1) air temperature,
(2) humidity,
(3) type and amount of cloudiness,
(4) type and amount of precipitation,
(5) air pressure, and
(6) the speed and direction of the wind.
Air Pressure & Weather
Atmospheric pressure is simply the weight of a column of air from the top to the bottom of atmosphere
The air pressure is greatest near the surface of Earth
Air pressure decreases through the layers farther out from the surface as Earth’s pull of gravity decreases.
Pressure Changes
Very important !!!!!!
Atmospheric pressure decreases as the altitude increases
- Air Pressure: force per area exerted by the mass of air above a point
- Measured in:
Inches of mercury (in. Hg)
Millibars (mb)
- Average sea-level pressure = 1013.25 mb or 29.92 in. Hg
- Air pressure is measured using a barometer
(By lucky coincidence), earth’s atmospheric pressure is approximately neat round numbers in metric
terms
14.7 pounds per square inch (1 kg/cm2)
A. P. = Pressure of a column of 10 m of water
Approximately one bar or 100 kPa (1 Pa = 1 N/m2)
Weather reports use millibars (mb)
One mb = pressure of one cm water
A pressure gradient is the amount of pressure change occurring over a given distance
Closely spaced isobars - lines on a map that connect places of equal air pressure - indicate a steep
pressure gradient and high winds
Widely spaced isobars indicate a weak pressure gradient and light winds.
Influence of temperature on weather
Temperature: measure of the motion of the molecules in a substance
Hot air: molecules moving more quickly
Cold air: molecules moving less quickly
Measured using a thermometer
Always measured in shady conditions
Units: oC, oF
Heat and Temperature
Temperature: Average energy of molecules or atoms in a material
Heat: Total energy of molecules or atoms in a material
Can have large amount of heat but low temperatures
Can have high temperatures but little heat
Heat and Temperature are of course closely related
Add heat -> molecules move faster -> temperature rises
Remove heat -> molecules move slower -> temperature falls
Q: Why is there less heating of the poles than the tropics even at the spring and autumn equinox when all
areas on Earth are experiencing the same number of hours of sunlight?
Temperature and Weather
Because SAME amount of radiant energy from the Sun strikes DIFFERENT areas on Earth
The cause of ALL weather phenomena is the UNEQUAL heating of Earth’s surface by the Sun
As a result, there is a net surplus of heat near to equatorial regions and a net loss nearer to poles
Q: Why don't the equatorial oceans boil away and the polar oceans freeze solid?
A: To compensate for heat inequities, heat is transported or circulated from a latitude to another
Q: How?
A: By oceanic currents or by convection currents in the air
In addition to modifying density of air and creating convection currents, the unequal heating of Earth
causes changes in pressure (low pressure area due to rising of hot air and high pressure areas due to
sinking of cold air)
Under NORMAL conditions, the higher the temperature, the LOWER the pressure
In the summer, the air pressure is usually lower
(As molecules move faster, they spread apart)
MANY times, this does NOT happen, due to the interaction between DIFFERENT air masses
Winds & Weather
Wind: horizontal movement of air along Earth’s surface (from a high pressure area to a low pressure area)
Vertical movements of air (updrafts and downdrafts) are called AIR CURRENTS
Q: What causes wind?
A: ALL winds are caused by differences in air pressure
Q: What causes these differences in air pressure?
A: Unequal heating of the atmosphere !
Instrument used to measure wind speed: ANEMOMETER
Two types of winds:
- Local winds - blow over short distances (e.g. on land next to a large body of water – Sea breeze /
Land breeze)
- Global winds - blow from specific directions over long distances – produced by the movement of
air between Eq. and Poles
Jet Streams
About 10 -11 km above earth’s surface are bands of high speed winds called JET STREAMS
These winds are hundreds of km wide, but only a few km deep
J.S. blow from West to East at speeds of 200 to 400 km/h
Moisture in the Atmosphere
Q: Where is the water in atm. coming from?
A: From precipitation, evaporation, transpiration, condensation, surface-water flow, and groundwater
flow
Water is always moving between the atmosphere (troposphere) and surface of Earth
Precipitation is any form of water that falls from a cloud (rain, freezing rain, sleet, hail & snow)
The process is called Water Cycle
Humidity of air
Humidity: measure of amount of water vapor in the air
Relative humidity: The relative humidity tells us how “full” the air is at the time of measurement
For example, 90% relative humidity means that at that moment the air is holding 90% of the maximum
amount of water it could hold (saturation)
Relative humidity depends on temperature
The instrument that measure relative humidity is called PSYCHROMETER
How clouds form?
- Clouds form when water vapors become liquid water or ice crystals
- The process by which vapors become liquid is called CONDENSATION
- The temperature at which condensation begins is called the DEW POINT