LEC.175 Atmosphere, weather, and climate Flashcards
What instrument is used to measure Temperature?
Thermometer
Why is a Stevenson screen used for measuring temperature and what are some of the features?
To limit exposure. Naturally ventilated, shielded from solar and terrestrial radiation, shielded from rain, white to reflect sunlight.
Where do temperatures get measured?
Standard screen level, grass surface, concrete surface, and at 5, 10, 20, 30, 50, 100cm depth in soil.
What is the automatic measurement instrument for Temperature?
Thermistor (and thermocouples)
How do thermocouples work?
Bimetallic strips, current flows through the strip, varies when there is a gradient in temperature across it, older variants use mechanical movement of the strip (due to thermal expansion) to move the arm
How do thermistors work?
Mixtures of metallic oxides have a resistance to the flow of electrical current that varies greatly with temperature.
Why does a Stevenson screen open on the poleward side?
So that direct sunlight does not enter during reading temperature.
Pros and cons of automatic measurements?
Pros - no person needed to take the measurement
Cons - need calibration, degrade with time (meaning they need frequent calibration)
What is the symbol and unit used for Relative Humidity?
RH, %
What is humidity?
Water vapour content of air
What is absolute humidity (pw)?
Units of concentration, partial pressure, or mass mixing ratio
What is saturation vapour pressure (ew)
The maximum amount of water vapour that air, above liquid or solid water, can contain at equilibrium. Depends very sensitively on temperature.
What is relative humidity (%RH)?
Ratio of the partial pressure to the vapour pressure of water (or ice). %RH = 100(pw/ew)
If %RH < 100% what cannot form?
Clouds, and any remnants will rapidly evaporate
What instruments can be used to measure Relative Humidity?
hair hygrometer, psychrometer
How does a psychrometer work?
Ordinary thermometer ‘dry bulb’ matched with thermometer covered in a wet muslin cloth ‘wet bulb’. The difference in temperatures is related to relative humidity because heat has been used to evaporate water form the cloth.
What is the automatic measurement instrument for Relative Humidity?
Capacitance
How does a capacitance work?
By detecting changes in the capacitance of a hygroscopic dielectric material between two electrodes, which is directly proportional to the amount of water vapor absorbed.
What is the symbol for Wind Speed?
U, V
What units are used to measure Wind Speed?
m/s, kt
What instrument is used to measure Wind Speed?
Anemometer
What is the Beaufort scale?
Subjective scale used when instruments are not present e,.g. Force 8 = gale, impedes walking, small twigs breaking off
What does conventional wind speed measure?
Horizontal component of the wind, usually 3-4 orders of magnitude bigger than the vertical component.
What instrument is used to measure Wind Direction?
Wind vane
What is the symbol and unit used for Pressure?
p, mb,hPa
What instrument is used to measure Pressure?
Barometer
How does a mercury barometer work?
Reads station pressure, which is converter to mean sea level pressure using the stations altitude. A barometer balances the weight of the atmosphere against the weight of the liquid in it which tends to be mercury.
Why is an aneroid barometer more commonly used now?
Safer and more convenient, however does require calibration.
What unit is used to measure Precipitation?
mm/day
What instrument is used to measure Precipitation?
Rain gauge
What is the automatic measurement instrument for Precipitation?
Tipping bucket
What is the measurement for Cloud coverage?
okta
What ‘instrument’ is used to measure cloud cover?
(Observer)
What is the automatic measurement instrument for Cloud coverage?
Camera
What is the measurement for Visibility?
km
What ‘instrument’ is used to measure Visibility?
(Observer)
What is the measurement for Sunshine duration?
hours
What instrument is used to measure Sunshine duration?
Campbell-Stokes
What is the automatic measurement instrument for Sunshine duration?
Radiometer
What is the measurement for Evaporation?
mm/day
What instrument is used to measure Evaporation?
Evaporating pan
What time is meteorological observation data typically collected?
0900hrs GMT
Observation requirements for data quality:
Representative, accurate, consistent
What % of the atmospheres mass is below 5km?
50%
Atmospheric composition in dry air?
78% nitrogen, 21% oxygen, ~424ppm CO2, 1% argon, tiny amounts of Ne, He, Kr, H2
Importance of the atmosphere?
Warms the surface, mixes heat vertically, mixes heat meridionally (quarter to pole), redistributes water via hydrological cycle, eternal UV shield for the biosphere
What is the atmospheric region below the homopause known as?
The turbopause - The homopause is sometimes referred to as the turbopause.
What process dominates atmospheric mixing below the homopause?
Turbulent motions
Below the homopause, the atmosphere is efficiently mixed by turbulent motions.
Above the homopause, what process becomes dominant?
Molecular diffusion
What factor influences molecular diffusion rates in the atmosphere?
Molecular mass. Molecular diffusion rates depend on the mass of the molecules.
What happens to the main components of the atmosphere above the homopause?
They become separated by mass. Above the homopause, lighter and heavier gases separate due to differences in molecular mass.
What is the significance of the Critical Level in the atmosphere?
Molecules have a significant chance of escaping to space
Which type of molecules are more likely to escape to space?
Light molecules (e.g., hydrogen)
What is the only region where the mass of a molecule matters in the atmosphere?
Above the Critical Level
What determines the composition of the atmosphere everywhere, apart from above the Critical Level?
Mixing - In regions below the Critical Level, mixing is the primary factor determining atmospheric composition.
How do CFCs reach the stratosphere?
By mixing across the tropopause. CFCs do not reach the stratosphere because they are lighter than air; they mix to get there.
What is stability in the context of the atmosphere?
Likelihood of overturning a layer in the atmosphere.
What would make the atmosphere unstable (stratification and stability context)
Hot less dense layers beneath cold more dense layers - leads to overturning and mixing.
Describe the stratosphere
The ‘layered’ sphere. Vertical transport damped. Contains the ozone layer which is an important atmospheric heat source. Very dry. Almost no clouds
Describe the troposphere
The ‘overturning’ sphere. Vertical motion on small and large scales. Almost all atmospheric water contained within. Most weather systems capped by the tropopause
Describe the planetary boundary layer
Wind velocity decreases to zero at the surface. Resultant friction causes much overturning of air. Part of atmosphere in contact with the terrestrial heat source.
