LEC.175 Atmosphere, Weather, and Climate Flashcards
How much cloud cover is there across the globe?
~1/2
What is the most abundant gas in the atmosphere (%)?
Nitrogen (78%)
What do units ppmm and ppmv mean?
ppmm = parts per million by mass
ppmv = parts per million by volume
By how much does CO2 increase per year in ppmv?
~20 ppmv/yr
What are 5 reasons why the atmosphere is important?
- Warms Earth’s surface by ~33°C
- Mixes heat vertically by convection
- Mixes heat meridionally (equator to pole) by advection
- Redistributes water via hydrological cycle
- Absorbs most energetic solar radiation (external UV shield)
What is the difference between convection and advection?
Convection is movement of air by heating whilst advection is movement of air by pressure differences
What are the 5 spheres in the atmosphere from closest to furthest from the Earth’s surface?
Troposphere –> stratosphere –> mesosphere –> thermosphere –> exosphere
Which sphere in the atmosphere do most weather systems occur in?
Troposphere
What is the homosphere and heterosphere?
Homosphere = troposphere –> mesosphere edge (homopause), homogeneous mixture of gases
Heterosphere = 100km –> 500km, heterogeneous mixture of gases
Does pressure increase or decrease with increasing altitude in the atmosphere?
Decrease
What is the mean free path in a gas?
Average distance travelled by a molecule between collisions with other molecules (dependant on T and P)
At homopause in the atmosphere, what is the mean free path of gases approximately equivalent to?
The average scale of turbulent motion (below = mixing by turbulence, above = mixing by diffusion)
Approximately at what altitude is the critical level in the atmosphere and what is mean free path approximately equal to at the critical level?
~500km, the distance to escape the Earth’s gravitational field
What causes overturning/mixing/an unstratified atmosphere?
Hot, less dense layers below cold, more dense layers (not stable)
Why does temperature increase in the stratosphere?
Due to the presence of ozone which absorbs solar radiation
Why is the stratosphere stratified?
Temperature increases throughout layer so no turbulence/overturning/mixing
What are 4 features of the troposphere?
- ‘Overturning sphere’
- Vertical motion
- Contains almost all atmospheric water
- Most weather systems occur in the troposphere
Why does overturning occur at the planetary boundary layer?
Wind velocity decreases to 0 at surface so friction causes overturning
What are 2 reasons why weather forecasting is difficult?
- Sun heats atmosphere but mostly indirectly by heating Earth’s surface
- Atmosphere is sensitive to change
…So no simple cause and effect
What is used to collect and transmit weather data?
Radar/station data/satellite data/civil aircraft/sondes –> national meteorological centre –> regional meteorological centre
What are the 2 types of weather stations used to make observations in the UK?
- Synoptic stations (send data directly to Met Office) e.g manned Met Offices, part manned stations, automatic stations
- Daily climate stations e.g. voluntary climate observers
In which direction do winds move around high and low pressure systems?
High pressure systems: Clockwise
Low pressure systems: Anti-clockwise
What are cold and warm air fronts and how are they represented in a forecast?
Cold air front: Cold air mass, triangles
Warm air front: Warm air mass, semi-circles
What are 2 ways weather observations are made consistent?
- Every detail of station observation is regulated by Met Observer’s Handbook in UK
- Observations are referenced to mean sea level pressure as pressure differs with altitude
What are the 2 types of thermometer used to measure temperature manually and how do they work?
Mercury-in-glass or alcohol-in-glass thermometers, work via expansion + contraction of liquid
At what time is air temperature always measured and what other measurements are taken?
0900, maximum and minimum temperatures since previous observation
What is used to protect thermometers from external factors and how does it work?
Stevenson screen - naturally ventilated + shielded from solar/terrestrial radiation + rain
What are the units of absolute humidity and saturation vapour pressure?
Absolute humidity: pw
Saturation vapour pressure: ew
What is saturation vapour pressure?
The maximum amount of water vapour that air, above liquid/solid water, can contain at equilibrium (depends very sensitively on T)
What is relative humidity (%RH) and what is the equation for %RH?
Ratio of partial pressure to vapour pressure of water/ice, %RH = 100 (pw/ew)
How does a psychrometer measure relative humidity?
Ordinary thermometer (‘dry bulb’) is matched with thermometer covered in wet muslin cloth (‘wet bulb’) and diff. in temp. is related to relative humidity e.g. when Twet < Tdry, heat has been used to evaporate water from cloth so T decreases
What meteorological instrument is used to measure wind speed and how does it work?
Calibrated cup anemometer, counts no. rotations per minute
What scale is used to subjectively measure wind speed when instruments aren’t available?
Beaufort scale (0-12, e.g. Force 8 = gale)
Why is wind speed measured at a standard height of 10m?
Friction reduces wind speed close to the ground
What are 2 ways of measuring wind direction?
- Wind vane (measures angle from north, clockwise, in °)
- Run of wind (cumulative wind measured by connecting anemometer to a mile-ometer, standard height = 2m)
What causes winds?
Pressure differences (winds travel from high –> low pressure
What instrument is used to measure atmospheric pressure and how does it work?
Barometer, reads station pressure and converts to mean sea level pressure using station altitude