Weather and Climate Flashcards
Weather vs Climate
Weather is the atmospheric conditions in a place at a particular point in time while climate is a 20 year rolling average of weather taking into account seasonal variation.
Structure of Atmosphere
Troposphere up to Tropopause at 10,000m where temperatures can be -60. 50% of air is within 5.6km of earth. Stratosphere up to stratopause at 46km. 99% of air within 40km. Mesosphere and then the ionosphere. Temperature inversions present. ELR: 6.5degrees/1km.
Atmospheric Composition (at surface)
78% nitrogen, 21% Oxygen. 1% Nobel gasses. 0.02% CO2.
Tropopause
Acts as a convective lid on weather systems due to the temperature inversion. Sits at varying heights (between 9 and 17km) where the pressure is always 300mb. Where its height changes, there are massive pressure gradients leading to jetstreams.
ITCZ
Forms the rising limb of the Hadley cell. Usually sits around the equator (always between 5-10 degrees N/S of it). Drives the tri-cellular model. Furthest N on 22nd June and furthest S on 22nd December.
Coriolis Force
Opposes the pressure gradient force. Leads to a perpendicular geostrophic wind formation at 750m above ground level where there are no other influences such as friction. In ground level, winds cut the isobars by 10-20 degrees. Is lesser closer to the equator. Causes winds to be deflected R in the N, L in the S.
Polar Front Jetstream
A 400km wide band of wind in the upper atmosphere which blows from W–>E at between 180 and 300km/hr.
No friction so blows perpendicular to PGF. Interference with this by the rocky mountains causes Rossby waves to form. These cause lows which seed depressions as when they move toward the poles, particles move further apart and air is drawn up into the low pressure sections within the jetstream, causing low pressure areas below into which warm and cold air cyclonically spirals, creating embryo depressions. When the Rossby waves turn toward the equator the opposite happens and high pressure is created. These ridges intersperse the depressions.
Rossby Wave Breakdown
When Rossby waves become so heavily sinuous that they back bite and cut themselves off static areas of low and high pressure are left. Highs especially can lead to blocking anticyclones while lows can lead to large rainstorms.
Embryo Depression
Two distinct fronts formed either side of a central low. At cold front Pm air undercuts Tm air and is forced upward while - at warm fronts - warm Tm air advances over Pm air. Rising warm air contributes to the central low pressure.
Mature Depression - Warm Front
Warm air gently squeezed up over the cold air. It cools at the saturated adiabatic lapse rate leading to condensation and cloud formation. Clouds are initially high cirrus, descending through cirrostratus, altostratus to nimbostratus which are rain bearing. Rain is consistent and steady along a 150km band going into the warm sector. As position relative to the low changes and there are large pressure and temperature gradients winds are gusty and veer from S–> SW. Visibility poor as there is mist and fog where humid Tm air meets cold Pm air. Pressure falls from 1008mb to 990mb in the warm sector. Temperatures rise 6-8 degrees.
Mature Depression - Warm Sector
Air stable but warm with a low pressure of 990mb. It gives some broken stratus cloud at low levels leading to some drizzle and poor visibility (fogs form as the air is warm giving it a lot of capacity for water vapour and it has a long sea track to collect this water vapour).
Mature Depression - Cold Front
Cold Pm air is moving faster than the air at the warm front, catching it and undercutting it, forcing it up rapidly along a 50km band. This rapid rising and adiabatic cooling to dewpoint leads to cumulonimbus clouds of 8 Okta cover and extremely heavy precipitation. Large temperature and pressure gradients lead winds to be gusty and squally. Pressures rise back up to around 1010mb and temperatures fall back to about 1 degree in the winter. Air is generally dry although there will be the odd shower cumulus cloud (less than 1 Okta of cover) leading to clearing showers.
Variation in Depressions
Deep depressions give large pressure gradients and very strong winds (15th October 1987; 122mph peak wind speeds and 85mph hourly mean at Shoreham from 957 mb event). In the winter, the polar front will be positioned over the UK so precipitation could be as snow and sleet. The track over the UK is usually fairly straight. If it is not and the UK catches anafronts (central;heavy) or katafronts (peripheral;light) only weather will be different as it will for ‘pile ups’ caused by blocking anticyclones. Also, speed. On October 30th 2000 depressions piled up over the UK causing widespread flooding as there were 3 major rain storms in 3 days.
Occluded Fronts
This is the breakdown stage. The faster moving cold front ‘catches up’ the warm front and forces all warm air up from the ground as the depression is ‘infilled’ by Pm air. Rain here is heavy however as the warm, humid air has been forced to rise. Strong winds and heavy showers. Cumulonimbus to an extent of over 6000m.
Great Storm (15th October 1987)
Weather forecast deny reports of a ‘hurricane’ due to hit the UK. 953mb low in the Bay of Biscay tracks over the UK overnight where it rises to only 957mb. Winds peaked at 122mph with a sustained hourly mean of 85 mph recorded at Shoreham. 13 people were killed. £1.4bn damage was done and subject to 1.3m insurance claims. Lloyds of London issued a profit warning. Black Monday stock market collapse came 5 days later. After 24 hours, 1 million Londoners were without power. Took 3 days to restore it to rural areas. Busses stopped across 1/3 of London for 24 hours, the railways took 48 hours to recover. At Toys hill in Kent, 97% of trees were destroyed. A ship carrying refugees ran aground in Harwich. National Grid, Power and Telecoms had a lot of damage which they were underprepared to fix. The Met office investigated and increased Atlantic monitoring infrastructure.
‘Sources’ of high pressure that can lead to anticyclones.
- Ridges between depressions.
- High pressure areas when Rossby waves break down.
- Extension of the Azores high.
- Blocking anticyclones over Scandanavia and N Europe can be caused by an extension of the Pc Siberian High.
Anticyclones
Pressures greater than 1020mb. Air swirls outward from a central high pressure area.
Anticyclones - Radiation Fog
Little cloud cover means that overnight, when the ground loses a lot of the thermal energy it has gained from insolation (is released as latent heat) it becomes very cold which leads to it cooling the air above it by conduction and radiation. Water vapour cools here to dew point making fogs that extend to above 50m above the ground but can be as low as 2m. Temperature inversion acts as a convective lid. Carbon particulates in smoke can worsen this by acting as condensation nuclei.
Anticyclones - Anticyclonic Gloom
A mat of stratiform cloud around 0.5miles in altitude present in up to 90% of winter anticyclones. Moist air penetrates the circulation of the anticyclone and is cooled to dewpoint. There is no pressure gradient (it is uniformly high pressure) so there are no winds to shift it.
Summer Anticyclones
No cloud to act as insulation and bounce solar short wave radiation back into space so temperatures can reach 25 degrees C. This cools rapidly at night back to around 15 degrees. Low level ozone and photochemical smog are big issues in industrial areas. Again, no winds prevents this from being blown away. Convective lid again. In the SE especially, heavy heating of the ground leads to convective thunderstorms.
Winter of 1963
The Big Freeze. 1040mb anticyclone over Iceland. For 2 months Am and Ac winds were forced over the UK. Max temps were below freezing for a month from 22nd December. Mins only consistently became above freezing on 4th March. Depressions were deflected over, mixing with the cold air and bringing heavy snowfalls. Meltwater floods followed.
5th-9th December 1952: London Killer Smog
For 5 days visibility was below 50m. Anticyclone of 1036mb forced people to heat their homes with coal leading to carbon particulate emissions that caused smog. 4000 deaths from respiratory problems can be attributed.
1953 Storm Surge
980mb depression over the N Sea caused a storm surge of c.4m and 60mph winds. 260 killed in the East of the UK.
30th October Storm
Depressions/occluded fronts piled up over the UK causing 3 rain storms in 3 days.
Heatwave 1994
1030mb anticyclone led temperatures to rise to 37 degrees C in Cheltenham. 197 hours of Sunshine in July.
Urban Heat Island
Needs winter anticyclone (no wind; wind would disturb) the thermal blanket. Homes and office blocks are heated, this latent heat is radiated out into the atmosphere. Pollutants emitted act as a thermal blanket keeping this in. The effect is increased when low albedo surfaces such as concrete, brick and tarmac absorb heat energy from the sun and surrounding buildings, storing it and slowly radiating it upward. Also, glass buildings reflect solar insolation. It is accentuated by the fact that, in rural areas, heat is not kept in by a thermal blanket and a greater plant coverage means that more heat is lost through higher evapotranspiration. Surfaces in rural areas tend to have higher albedos.
Solar Radiation (Rural vs Urban)
Solar radiation is lower in urban areas than rural as the aerosolised particulate blanket acts as condensation nuclei forming clouds that are of a high albedo and reflect solar radiation back out into space.
Relative Humidity (Rural vs Urban)
In urban areas the relative humidity is lower than in rural areas. Les evapotranspiration in urban areas leads to lower absolute humidity. For the same absolute humidity the higher temperatures in urban areas give a greater relative humidity.
