Climatology Flashcards
What is created over the city due to warm air rising?
Heat dome
Why, besides being near a colder ocean, are the conditions on the west coast more dry?
- There is decending air at the horse lattitudes
- This results in warm air above the cold ocean.
- Consequently, there is a temp inversion and no large-scale rising of air can take place.
- Anticlockwise rotation
- SW winds
- Retain moisture (colder to warmer lattitudes)
- // to coast and are not onshore
Describe the weather conditions experienced at the cold front of a mid-lattitude cyclone
- Temperature falls rapidly
- Pressure increases quickly
- Warm air rises sharply
- Cumulonimbus clouds form
- Heavy showers over a small area
- Decrease in humidity
- Wind “backs”
- Later, clouds clear but it is still cold
Where is the angle of incidence measured from?
The ground
Why must there be divergence in upper air in order for tropical cyclones to form?
Removes air at high altitudes which maintains surface LP
What do the westerlies in a mid-lattitude cyclone become?
Northwesterlies
What is the term for “backing” in the context of the NH?
Veer / veering
What is the wind-direction in the warm sector of a mid-lattitude cyclone?
NW
Name the three HP cells around South Africa
- South Atlantic HP
- South Indian HP
- Kalahari HP
In which season is the effect of aspect the greatest?
In winter
Where do gales occur in the formative stage of a tropical cyclone?
50 km from eye
What does a cold snap on the plateau look like on a synoptic chart?
What happens once cold air collects at the bottom of the valley?
A temperature inversion occurs
What season is it if there is a interior HP?
Winter
Describe the movement of air associated with a LP cell
Rising air
Where is pressure the lowest in a tropical cyclone?
In the eye
What is unstable air?
A pocket of air that rises
Warmer than surrounding air
Why must there be a steep pressure gradient in order for a tropical cyclone to form?
- Strengthens Coriolis force
- Causes air to rotate
Why does a anti-cyclonic temperature inversion occur?
Subsiding air is warmer than the air on the plateau which is rising and cooling
During which months do mid-lattitude cyclones form in the SH?
April to October
What is the movement of warm air at both the warm front and the cold front?
Warm air rises
What is the average pressure in the eye of a tropical cyclone?
940 hPa
Where does the pollution done lie during the day?
Higher altitudes
The heating of the city causes the pollution to rise
What else can a mid-lattitude cyclone be called?
- Temperate cyclones
- Extra-tropical cyclones
- Temperate depressions
- Frontal depressions
Why is there local air circulation over cities?
Warmer air is less dense and rises and cooler air from rural areas blows in
Why is there more cloud cover over a city?
- Warm unstable air rises easily
- Winds that converge on the city (due to it being hotter) bring moisture in from rural areas
- More condensation nuclei over the city as a result of pollution
- Higher levels of smog (which can be almost like cloud cover)
What is the speed of winds circulating in the system of a tropical cyclone?
200 -300 km/h
Describe the cloud formation in the immature stage of a tropical cyclone
Spiral bands of clouds
Is the direction that mid-lattitude cyclones travel dependent on the hemisphere they are in?
No. Mid-lattitude cyclones ALWAYS travel from W to E
What speed are the winds within 50 km of the eye in the immature stage of a tropical cyclone?
Hurricane speed winds
In which season the Kalahari HP (and thus the inversion layer) rise?
Summer
Why is there often fog over the West Coast of SA?
Warm air (from subsiding air) is cooled over the cold ocean
What is the only way in which areas fallling in the shadow zone of a slope heated?
By reflection
What is a tropical cyclone called in its immature stage of development?
A tropical storm
Define temperature inversion
When the temperature of the atmosphere is colder than the temperature of the atmosphere higher up
Why is the air of a berg wind hot?
The air of a berg wind heats adiabatically as it moves down the escarpment
Why does wind “back” in a mid-lattitude cyclone?
Air rotates around the LP in a clockwise direction in the SH
What kind of wind (local winds) does aspect influence?
Anabatic winds
Where do tropical cyclones develop?
- Over tropical oceans
- 5° N&S of the equator
What are anabatic winds?
Upslope winds
How long does one experience the conditions of the eye in a tropical cyclones
For up to two hours
Why should faactories not be constructed in valleys?
There is stable air above the temperature inversion
Chimney stacks of factories in valleys should extend above the inversion layer
_____ can form on the valley floor.
Frost pockets
Where does the worst weather occur in the mature stage of a tropical cyclone?
In the SW in the SH
In the NW in the NH
The position of the HPs that affect SA vary with ____ and ____.
The seasons and the position of the overhead sun
How does a line thunderstorm develop?
- A trough of low presssure develops (NW to SE)
- The dry and moist air masses converge
- Moist NE air is lighter (so rises and cools)
- Rain falls to the NE of the trough axis
Which direction do tropical cyclones move?
From East to West
Why do mountain tops cool rapidly?
Due to terrestrial radiation
Name some negative impacts of tropical cyclones on people
- Flooding of rivers
- Storm surges
- Ships destroyed
- Soil salty (bad for agriculture)
- Tourism negatively affected
- Fertile soil is eroded
- Vegetation uprooted (loss of habitat)
Where do mid-lattitude cyclones occur?
In the middle, or temperate lattitudes
What is a tropical cyclone called in its formative stage of development?
A tropical depression
What do the polar easterlies become in a mid-lattitude cyclone?
Southwesterlies
What does it mean if the difference between air temp and dew point temp. is large?
The air is dry
What are tropical cyclones named according to?
People’s names
Starting with A
What are tertiary (or local) winds caused by?
The heating and cooling of slopes
What is the approximate pressure in the eye of the mature stage of a tropical cyclone?
940 hPa
Why must there be no friction in order for tropical cyclones to form?
- Causes the winds to reach great speeds
- Enables Coriolis force to remain active (which maintains rotation)
What does a deflection of a mid-lattitude cyclone look like on a synoptic chart?
What would a station model look like showing berg wind conditions?
- IT WOULD BE ON THE COAST
- No clouds
- Low dew point temperature
- High air temperature
- Wind direction from land
Why do coastal LPs occur?
They lie between the two HP cells
What are some strategies to prepare for the event of a trropical cyclone?
- Monitor by weather bureaus
- Observed via:
- Satellite
- Reconnaissance aircraft
- Weather reports
- US National Hurricane Center
- Vacate low ground
- Board up windows
- Move to concrete storm shelters
- Sand bags in coastal areas
- Disaster management schemes
- Conservation of wetlands
- Concrete sea walls
What is the precipitation in the immature stage of a tropical cyclone?
Light rain
Which slopes (warmer or colder) are preferred for settlement?
Warmer
What season is it if there is a coastal low?
Summer
Describe the weather conditions experienced in the warm sector of a mid-lattitude cyclone
- Temp increases to a maximum
- Pressure reaches its lowest just before the cold front arrives
- Scattered cloud
- Decreasing rain
- Increasing sunshine
- Wind drops
- Wind - NW
What happens to the temperature inside the eye of a tropical cyclone?
It increases slightly
Does moist air have a high or low dew point temperature?
High dew point temperature
Which direction does Coriolis Force deflect air?
To the RHS in the NH
To the LHS in the SH
Name the four stages of development of a tropical cyclone
- Formative stage
- Immature stage
- Mature stage
- Dissipation stage
Explain the development of a berg wind
- Air subsides onto the plateau
- Air heats, warm and dry
- Moves down the escarpment to converge on the LP that is over the ocean
- This wind warms adiabatically (and all moisture evaporates)
Approximately how far towards the poles do tropical cyclones travel?
Approximately 25° - 30° N&S
In which season in the berg wind most common?
In winter
Describe the vertical movement of air in the formative stage of a tropical cyclone
The air rises
What is the life-span of a mid-latitude cyclone?
4 - 14 days
In which season does fog most regularly occur?
Winter
How do buildings affect the wind in an urban climate?
- Friction (slows down wind speeds)
- Cause gusty winds
- Channel the wind down streets (very strong here)
In which season are mid-lattitude cyclones best developed?
In winter and spring
What can the heat dome be compared to, on a global scale?
The greenhouse effect
Why does a temperature inversion lead to stable air?
The cold dense air is unable to rise as the air above it is warmer and lighter
Where does air subside in the mature stage of a tropical cyclone?
In the eye
This causes the eye to be cloudless
What are EIGHT conditions required for the formation of a tropical cyclone?
- Sea temp. >27 °C
- High humidity
- Unstable air
- Very low pressure
- Steep pressure gradient
- Coriolis force
- Divergence in upper air
- No friction
What is a berg wind?
Hot, dry winds occuring when there is a steep pressure gradient between the KHP and depression
Why do line thunderstorms occur in summer?
The inversion layer has risen above the escarpment and the moist air is able to reach the plateau
Where does air divergence take place in a tropical cyclone?
In the upper air
In which stage is a tropical cyclone given a name?
In the mature stage
Which way do tropical cyclones tropical cyclones curve when travelling polewards in each hemisphere?
- SH
- Travel polewards
- Curve to left
- NH
- Travel polewards
- Curve to right
Why are HP cells associated with cloudless skies?
Warming air causes the clouds to evaporate
Why is temperature hotter in a city?
- There are activities in the city which generate surplus heat
- Concrete, tar and steel absorb more heat
- High-rise buildings
- larger surface area
- more direct rays on the sides
- Multiple reflection, less heat being radiated out to space
- Pollution limits abount of heat lost by terrestrial radiation
- Fewer water sources which absorb heat
- Less evapotranspiration which cools down the city
- Greater CO2 levels cause greater absorption of terrestrial radiation
What is the pressure in the formative stage of a tropical cyclone?
More than 1000 hPa
Drops in this stage
How does a trough of low pressure develop in summer in South Africa?
Land is heated and warm air rises
What two ways can rain occur on the plateau?
- LP over the land causes moist air (from Indian) to converge
- Line thunderstorms can develop
Where can a berg wind develop?
On either the W or E coast
Why do storms last longer in cities?
The air has a greater water vapour capacity
Why does an occlusion happen in a mid-lattitude cyclone?
The cold front moves more quickly:
- it displaces the warm air ahead of it easliy.
- it only uses energy to move horizontally - not to rise
What is a vortex?
The area where air is circulating around the core of low pressure
Which side of continents do tropical cyclones affect?
The east coast
When do katabatic winds occur most frequently?
After noon
What is a moisture front?
A line marking the boundary between the moist NE air and the dry SW air
What are katabatic winds?
Downslope winds
Give TWO reasons for why the plateau is cloudless in winter
- Air descends from the Kalahari HP and warms adiabatically
- It is cold in winter (no rising convection currents)
What is the warmest part of the valley called?
Thermal Belt
Name the four stages of development of a mid-lattitude cyclone
- Initial
- Mature
- Occlusion
- Dissipation
What precipitaition is associated with cumulonimbus clouds?
Torrential rain, hail, thunder and lightning
Do mid-lattitude cyclones develop closer to SA in summer or winter?
Winter
Why is there less latent heat released as a tropical cyclone reaches over land?
There is less moisture
What is the study of valley climates referred to as?
Microclimatology
What kind of crops should be planted on the valley floor?
Frost-resistant crops
Why does air above the slope at night move downwards?
It is cold and dense
Which pressure belt are the three high pressure cells that affect SA found in?
The sub-tropical HP belt (30° N&S)
When is the heat dome most noticeable?
At night
In winter
In what season do tropical cyclones occur?
late summer to early autumn
What is the diameter of a tropical cyclone?
500 km
What will happen to tropical cyclones as global warming increases?
The frequency and intensity of tropical cyclones will increase
Describe the movement of air associated with a HP
Descending air
Does air diverge from a HP or LP?
HP
Why does a cold snap happen?
The SAAC “ridges in” behind the cold front, thus reinforcing the SW air behind the cold front
Why must there be Coriolis force in order for tropical cyclones to form?
Causes rotating winds which drops the pressure
What is the pressure in the immature stage of a tropical cyclone?
Less than 1000 hPa
What is smog?
A comination of fog and smoke
What are some strategies that could be used to reduce the urban heat island effect?
- Use reflective building materials
- Increase vegetaion
- plants on buldings insulate
- plants cool environment
- shade on buildings (no air-con)
- reduce CO2
- give tax rebates to people if a certain % of property includes plants
- Reduce the energy consumption
What is aspect
The direction in which the slope faces
What usually happens, a warm front occlusion or a cold front occlusion?
A cold front occlusion
How fast do tropical cyclones move?
20 - 25 km/h
How big is a tropical cyclone in its immature stage of development?
Still small in size
Give SIX negative impacts of mid-lattitude cyclones on human activities
- Snow on Cape Fold Mountains can endanger livestock
- Very cold conditions can disrupt human activities (power cuts etc.)
- Berg winds experienced ahead of a cold front can cause veld fires
- SW winds can cause stormy conditions (danger to ships)
- Heavy rain causes poor visability
- Snow on mountain passes
When does a Southeaster occur?
When the SAAC ridges to the south of South Africa
Over what range do squall line thunderstorms occur?
They occur over hundreds of kilometers
What are temperature inversions caused by in valley climates?
Cold air collecting at the bottom of the valley
What can a person call a city, in terms of climate?
A heat island
Describe the cloud formation when you see a tropical cyclone approaching (first vortex)
- Cirrus
- Cumulus
- Cumulonimbus
Why does the inversion layer over the interior of SA rise in summer?
Convection currents from the plateau being warm lift the descending air of the HP
Give TWO positive impacts of mid-lattitude cyclones on human activities
- Winter rainfall - cultivation of grapes
- Replenishes water in dams
Give the weather conditions that change due to the presence or an urban climate
- Temperature
- Humidity
- Cloud cover
- Precipitaiton
- Wind
When do katabatic winds occur?
- Usually after midnight
- Cold, clear and calm nights
What is the diameter of a mid-lattitude cyclone?
3000 km
In which months do tropical cyclones occur in the SH?
Jan - April
What is the shape of the isobars surrounding a mid-lattitude cyclone?
Oval-shaped
In which wind belt do tropical cyclones travel?
Tropical Easterly winds
They are then picked up by the Westerly wind belt
Why does a mid-lattitude cyclone dissipate?
All the warm air has been displaced
Thus, only the cold air is on the surface
And all pressure differences are reduced
What TWO things happen at the Initial stage of the development of a mid-lattitude cyclone?
- A front exists
- Friction develops and rotary air movement begins (wave stage)
When does frost form in a valley climate?
At night, when the air directly above the earth’s surface cools to below freezing
Why must there be a high humidity for tropical cyclones to form?
Latent heat is released in the process of condensation (this provides energy)
What is the angle at which the sun’s rays strike a slope?
The angle of incidence
What is the gradient of a warm front of a mid-lattitude cyclone?
1:150 - 1:300
What is a storm surge?
Large waves caused by the very strong winds and low pressure in a tropical cyclone
What is the speed of the katabatic wind mainly caused by?
The steepness of the slope
Why must the sea temp be greater than 27 °C for the formation of tropical cyclones?
- Evaporation occurs
- Thermal energy is provided
- Convection currents (when warm air rises)
- Low pressure (when warm air rises)
Where is the polar front found?
Where the warm westerly winds and the cold polar easterly winds converge at the sub-polar LP
When is there the most pollution in a city, summer or winter? Why?
Winter
- More artificial heating
- Inversion layer is lower down (stable air which does not rise)
How does fog form?
If there is light wind and the earth’s surface is cold.
What is a tropical cyclone called in its mature stage of development?
A tropical cyclone
Give THREE reasons why frost is common over the Highveld in winter
- Cloudless nights (terrestrial radiation)
- Calm conditions (no mixing of warm and cold air)
- Low humidity (little absorption of terrestrial radiation)
What kind of sun rays do slopes facing the poles recieve?
Oblique rays
What is a squall line thunderstorm?
Thunderstorms accompanied by sudden and violent gusts of wind
When does dew form?
When the earth’s surface cools to above freezing
Why must there be unstable air for tropical cyclones to form?
- Air must rise
- Rising air causes condesation (release of latent heat)
What is the wind direction behind the cold front of a mid-latitude cyclone?
SW
What kind of crops will be found above the inversion layer (in the thermal belt) in a valley climate?
Frost-sensitive crops
Can a berg wind be caused by a frontal depression or a coastal low?
Both
What is different about the weather experienced in the first vortex and the second vortex of a tropical cyclone?
The wind direction is reversed
What THREE important factors are there to consider when working with valley climates?
- Aspect
- Tertiary (local) winds
- Temperature inversions
The lattitude where mid-lattitude cylones form is dependent on the position of …
the ITCZ
Is the pressure gradient of a mid-lattitude cyclone steep?
No.
Where do grasses thrive? On slopes facing the equator or on slopes facing the poles?
On slopes facing the equator
Describe the temperature and humidity in the formative stage of a tropical cylone
Hight temp
High humidity
Winds stronger than ___ cause the heat island to dissipate
20 km/h
What can occur at either the cold or the warm front of a mid-lattitude cyclone?
A frontal inversion
Which season is it if there is a tropical cyclone present?
Summer
Why do katabatic winds occur most frequently after noon?
- Sun’s rays warm the slops during the day
- Air warms and becomes lighter
What two things can cause a tropical cyclone to dissipate?
- When they reach cooler lattitudes
- When they cross the land
When does orographic rain occur?
When air of the warm sector is caused to rise up the mountains
What are the different names that tropical cyclones are known by in different parts of the world?
- Hurricane
- North America
- Willy-willies
- Eastern Austrailia
- Typhoons
- North Pacific and around China
- Tropical Cyclones
- Madagascar
What does it mean if the ITCZ is further S?
It is summer
Explain what is meant when it is said that mid-lattitude cyclones occur in “families”
The cold from of an older depression is joined to the warm front of a younger depression
Where does the pollution dome of a city lie during the night?
Low altitude
- Due to inversion that develops during the night
- Little rising air (concentrated at surface)
Describe the effect of line thunderstorms to the precipitation over the interior of SA
- Continual rainfall
- Large area
- Most of summer rainfall
When is the effect of aspect least noticeable?
At noon
Where are hurricane winds found in the mature stage of a tropical cyclone?
Within 160 km of the eye
What causes the coastal LP to move around the coast of SA?
The movement (ridging) of the SAAC draws the coastal low with it
What is often is followed after a costal low pressure?
A mid-lattitude cyclone often follows a berg wind
Which direction does air move around a HP cell in the SH?
anticlockwise
What is the diameter of the eye of a tropical cyclone?
30 - 50 km
What direction is the wind blowing on a station model?
FROM the direction of the arrow
Why does pressure rise as a tropical cyclone reaches land?
Friction weakens Coriolis force and air converges on the low pressure
What season is it if there is a cold front close to the land?
Winter
In what way do mid-lattitude cyclones travel (besides going from W to E)?
First, travel towards the equator
Then, curve towards the poles
What is another term for a front?
A discontinuity
What is the vertical extent of the cloud wall in a tropical cyclone?
15 km
Where do gales occur in the immature stage of a tropical cyclone?
500 km from the eye
Does air converge or diverge at the surface of a tropical cyclone?
Converge
What is the speed of movement of a mid-lattitude cyclone?
30 - 50 km/h (from W to E)
What is the pattern of the isobars around a tropical cyclone?
circular
What are the lattirudes in which the sub-tropical high pressure cells are found sometimes referred to?
Horse lattitudes
Describe the clouds in the mature stage of a tropical cyclone
Cumulonimbus clouds
Describe the weather experienced in the warm front in a mid-lattitude cyclone
- Temp ahead is low (rises gradually)
- Pressure drops
- Cirrus clouds, then cirrostratus, then altostratus, then nimbostratus
- Gentle widespread rain
- Wind - NE (in SH)
Where do ferns thrive? On slopes facing the equator or on slopes facing the poles?
Slopes facing the equator
Where do mid-lattitude cylones form?
At the polar front (40° - 60° N&S)
In which wind belt do mid-lattitude cyclones travel?
In the belt of the westerly winds
Why is relative humidity lower in a city?
- Fewer open water sources
- Storm-water drains
- Warmer air (due to it being an urban heat island) has greater capacity to hold water vapour
What two winds can the SIAC cause?
SE and NE winds over the E parts of SA
Why can’t tropical cyclones develop within 5° N&S of the equator?
There is no coriolis force
What is a nocturnal plateau inversion?
Where the earth’s surface cools, causing the air above it to cool.
This happens becuase of terrestrial radiation
This traps warm air as follows:
- Cold air from the clear night-time conditions
- Warm air from the air which heated up adiabatically
- Cold air from loss of heat due to terrestrial radiation
What is the gradient of the cold front of a mid-lattitude cyclone?
1:50 - 1:150
What is the difference between fog and mist?
Fog is more dense than mist
