EAE 02 Humidity Flashcards
What are the 3 types of thunderstorms?
- Air Mass Thunderstorm
- Supercell Thunderstorm
- Mesoscale Convective System/Complex (MCS or MCC)
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What are the 3 key features of a
What are the 3 key features of an Air Mass Thunderstorm?
- Short lived
- Heavy rain, downbursts, hail and land/water spouts
- Low wind shear
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What are the 3 key features of a Supercell Thunderstorm?
- Long-lived rotating updraught
- Heavy rain, downbursts, damaging hail and tornados
- High wind shear
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What are the 3 key features of a Mesoscale Convective System/Complex?
- Long-lived, weakly or non rotating updraught
- Heavy rain, downbursts, hail and weak tornados
- High wind shear and linear trigger
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Where do Air Mass Thunderstorms occur?
- These storms usually occur well away from fronts or cyclones. They only occupy a small space for a short time.
- They are commonly observed in Melbourne during summer months.
- They are quite common in the tropics year round.
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What is primarily responsible for Air Mass Thunderstorms?
Daily solar heating is primarily responsible for these storms. The short wave radiation warms the boundary layer through the day and eventually the air becomes warm enough that it will penetrate into the free troposphere.
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What is the mechanism for an Air Mass Thunderstorm?
- Strong updraught leads to the formation of heavy precipitation
- Precipitation weighs down on the updraught, forcing it into a downdraught
- Updraught and precipitation formation is thus stopped
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Describe Supercell Thunderstorms?
- Like an air mass thunderstorm, a supercell thunderstorm is a single storm rather than a grouping of storms.
- Much larger than an air mass thunderstorm.
- A supercell thunderstorm may be up to 50 km in diameter making it a mesoscale phenomena.
- These phenomena are far more severe than air mass thunderstorms.
- Commonly they are associated with tornadoes and are observed to have rotation.
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Describe Mesoscale Convective Systems
Organised thunderstorms that exist on a scale of up to a few hundred kilometres.
Can be affected by the Coriolis force
Form through up-scale growth of thunderstorms
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What is the structure of Mesoscale Convective Systems
The actual structure of an MCS can vary considerably
- mesoscale convective complexes
- squall line thunderstorms
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How do Mesoscale Convective Complexes behave?
- Within an MCC, individual thunderstorms form and dissipate.
- The outflow of one storm directly aids nearby updrafts.
- The dissipation of one storm leads to the strengthening of an existing storm or possibly even the formation of a new storm.
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What are Squall Line Thunderstorms?
A squall line is a collection of thunderstorms aligned in a line.
In many regions they are found ahead of cold fronts.
The line is parallel to the cold front and moves ahead of it.
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What are Tornadoes?
Tornadoes, in general, are microscale phenomena with diameters ranging from tens of meters to a kilometre or two.
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How are tornadoes classified?
Tornadoes are classified as
- Non-supercellular (i.e. landspouts/waterspouts)
- Supercellular
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What are the main ingredients for a thunderstorm?
- Moisture
- Instability
- Trigger
Wind shear is a secondary component but important!
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What is required for latent heating processes?
We need moisture in the atmosphere for latent heating processes to occur.
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What is meant by ‘Saturation’
When the evaporation rate of water equals the condensation rate, the atmosphere is said to be saturated.
It is not correct to say “the air cannot hold any more water vapour” e.g. the air can be supersaturated.
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What does ‘Saturation’ depend on?
- Saturation depends on temperature.
- Saturation does NOT depends on the presence of any other gas.
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How does the temperature relate to saturation vapour content.
When the temperature of air changes, the saturation vapour content changes
- As the temperature of an air parcel increases, the saturation vapour content increases
- As the temperature of an air parcel decreases, the saturation vapour content decreases
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What is the relation between moisture content and density?
Higher water vapour reduces density.
Water vapour is lighter than air, so an air parcel with a high water vapour content is less dense than a dry air parcel (if both have the same temperature and pressure).
Molecular mass
* Dry air = 28.6 * With 10% water vapour = 27.5
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What is knowing how much water vapour is present useful for?
- Predicting amounts of rainfall.
- Forecasting the advection (transport) of water vapour.
- Determining evaporation and condensation rates that relate to things like
* Cloud formation * Bush fires * Heat stress * etc.
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What is relative humidity?
Measure of the amount of water vapour in the air relative to the saturation point of the current temperature.
I.e. the % of molecules currently in the gas phase compared to the maximum that is physically possible.
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What are the characteristics of relative humidity?
- Air that is saturated has a relative humidity of 100%. Relative humidity
- It is NOT a measure of the amount of water vapour in the air.
- The relative humidity changes with the density and temperature of air.
> E.G.
* Air parcel at 10° & 100% humidity
* Same parcel at 20° & 52% humidity
* Same parcel at 30° & 28% humidity
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Why does relative humidity change during the day?
The relative humidity can vary from 100% (dew formation) to under 50% over the course of a day purely due to changes in temperature, even though the actual amount of water vapour present in the air does not change.
Reading from graph:
* 95% humidity at 6° * 25% humidity at 26°
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What is the dewpoint temperature?
The temperature at which the water vapour in the air would condense into liquid.
Found by hypothetically cooling a parcel of air until it reaches the saturation point for that given amount of water vapour (and thus possibly forms dew).
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What are the characteristics of the dewpoint temperature?
- Dewpoint temperature is a measure of the amount of water vapour in the air.
- Does not change with the density and temperature of air.
- Gives information about when saturation will occur (i.e. Dew!).
- Is much simpler to interpret than RH and is what meteorologists use!
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What is knowing the dewpoint temperature useful for?
A very useful forecasting tool
The dew point can be used as a good approximation of the overnight minimum temperature.
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How does dew point impact overnight temperature?
- Once air temperature drops to the dew point, condensation begins to occur.
- Latent heat must be released to the air when water vapour condenses to liquid.
- This heating offsets some (often all) further cooling.
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How does dew point explain desert temperatures?
If the dew point is much lower than the air temperature, the rate of cooling at night is very fast.
This is why dry regions (high plains, deserts) have such large temperature ranges between day and night.
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What does dew point tell us about clouds?
The actual temperature and the dew-point temperature vary with height through the atmosphere.
When the temperature and the dew point temperature are the same and the air is likely to be cloudy.
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What is the ‘lapse rate’
Since atmospheric pressure decreases with height, a warm parcel of rising air will cool.
We call the rate of cooling with height the lapse rate.
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What is the major determinant of Lapse Rate?
The decrease in temperature with height as we lift a parcel depends on moisture:
> Dry = no water vapour in it is condensingMoist = it is saturated, meaning that water vapour in it is condensing into liquid
The reason that these values are different is because in the moist case (where water vapour condenses into liquid) we have to include the latent heating caused by condensation, which offsets the cooling due to lifting.
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What is DALR?
Dry Adiabatic Lapse Rate
Provided air is unsaturated (no cloud), the rate of cooling of air with height has a value very close to 1 degree celcius per 100 meters (1°C /100 m).
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What is the LCL?
Lifting condensation level
The height when water vapour condenses to form clouds.
- As we lift a parcel it cools, and will reach a point where the air becomes saturated (remember the saturation vapour content decreases with temperature).
- When we reach the height that saturation occurs, water vapour condenses into liquid and clouds form.
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What happens to an air parcel above the LCL?
It will further cool but not at the DALR.
Saturated air cools more slowly than the DALR.
This is because as we lift the parcel past the LCL, excess water vapour in the air condenses into liquid (clouds and rain), and latent heating from this condensation heats the parcel and offsets the cooling due to expansion.
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What is the MALR?
Moist Adiabatic Lapse Rate
For a saturated air parcel, the lapse rate has a value of about 0.6 degree C per 100 meters (0.6 °C /100 m).
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What is meant when a lifted air parcel is ‘unstable’?
It will continue to rise.
The air parcel’s temperature drops with height less than the ambient termperature gradient
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What is meant when a lifted air parcel is ‘stable’?
It will sink back down to the ground.
The air parcel’s temperature drops with height more than the ambient termperature gradient
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What is meant when a lifted air parcel is ‘conditionally unstable’?
A parcel of unsaturated it cools faster then the environmental temperature and so drops down again (i.e. Is stable)
A parcel of saturated air cools more slowly and so continues to rise (i.e. Is unstable)
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What are CCN?
Cloud Condensation Nuclei
Small particles (microns in size) that water vapour molecules can “stick” to and begin to form a cloud drop.
CCN consist of many types of aerosol including:
* Amoke particles from fires or volcanoes * Salt from spray * Tiny particles of wind-blown sand or dust * Sulphates and carbon emissions from human activity such as factories and vehicle exhausts.
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When can clouds form?
Clouds can start to form once air is lifted to its lifted condensation level
Cloud formation at saturation happens much more easily when cloud condensation nuclei are present.
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What is lifted condensation level?
The level at which a parcel of moist air lifted dry-adiabatically would become saturated.
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What are the relative size of ‘raindrops’ and ‘cloud droplets’?
A typical cloud droplet is 10~15 microns in diameter 10~15×10⁻³mm
A typical raindrop is more than 1 mm
* If a raindrop is more than 4mm it will typically split * A raindrop can be as small as 0.5 mm
This means that cloud droplets are 10-1000X smaller than raindrops.
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What mechanis allow cloud droplets to grow into rain drops?
- Condensation of water vapour onto an existing cloud droplet.
- Collision-coalescence of existing cloud droplets.
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What is the collison-coalescence process?
- Large droplets begin to collide with other droplets as they fall through the cloud.
- As they bump into each other, some of them end up coalescing (sticking together and becoming a single larger droplet).
- The strong updraughts in thunderstorms can transport a droplet upward many times, allowing it multiple opportunities to fall back through the cloud and collide and coalesce with other drops.
- Eventually the raindrop becomes large and heavy enough that it falls to the ground.
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What does the collison-coalescence process mean for the size of raindrops?
Storms produce much larger raindrops than other kinds of cloud because of the multiple opprotunities to grow.
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How does hail form?
- Temperatures near the top of thunderstorm clouds are very cold (-50 to -60°C).
- As cloud droplets are transported in the thunderstorm updraughts towards these very cold regions, they can start to freeze into small balls of ice. (Generally, they need to come into contact with small particles such as dust or salt crystals in order for freezing to occur.)
- The top of a thunderstorm cloud is made of small ice crystals, not cloud droplets.
- Once some ice crystals form, they collide with other water droplets in the cloud and cause them to freeze.
- Eventually some can become large and heavy enough to fall to the ground as hailstones.
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How does electrical charge separate in the atmosphere?
There is no scientific consensus on the exact way that
electrical charge separates in the atmosphere.
It is agreed that rapid convective motions are necessary
- In the updraught region of a storm, rising ice crystals collide with falling frozen particles (snow, hail).
- This leads to the rising ice crystals becoming positively charged and the falling particles becoming negatively charged.
- The upper part of the thunderstorm cloud becomes positively charged and the middle to lower part becomes negatively charged.
- Eventually this charge buildup leads to electrical discharge in the form of lightning.
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What is thunder?
- The lightning strike momentarily heats the atmosphere up to 15,000°C (hotter than the sun), rapidly expanding the air. Then, the atmosphere recompresses the air column.
- The rapid expansion and contraction of the atmosphere produces a sound wave (thunder).
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