EAE 02 Humidity

Question 1

What are the 3 types of thunderstorms?

Answer 1

• Air Mass Thunderstorm
• Supercell Thunderstorm
• Mesoscale Convective System/Complex (MCS or MCC)

_{EAE 2aa}

Question 2

What are the 3 key features of a

What are the 3 key features of an Air Mass Thunderstorm?

Answer 2

• Short lived
• Heavy rain, downbursts, hail and land/water spouts
• Low wind shear

_{EAE 2ab}

Question 3

What are the 3 key features of a Supercell Thunderstorm?

Answer 3

• Long-lived rotating updraught
• Heavy rain, downbursts, damaging hail and tornados
• High wind shear

_{EAE 2ac}

Question 4

What are the 3 key features of a Mesoscale Convective System/Complex?

Answer 4

• Long-lived, weakly or non rotating updraught
• Heavy rain, downbursts, hail and weak tornados
• High wind shear and linear trigger

_{EAE 2ad}

Question 5

Where do Air Mass Thunderstorms occur?

Answer 5

• 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.

_{EAE 2ae}

Question 6

What is primarily responsible for Air Mass Thunderstorms?

Answer 6

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.

_{EAE 2af}

Question 7

What is the mechanism for an Air Mass Thunderstorm?

Answer 7

• 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

_{EAE 2ag}

Question 8

Describe Supercell Thunderstorms?

Answer 8

• 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.

_{EAE 2ah}

Question 9

Describe Mesoscale Convective Systems

Answer 9

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

_{EAE 2ai}

Question 10

What is the structure of Mesoscale Convective Systems

Answer 10

The actual structure of an MCS can vary considerably

• mesoscale convective complexes
• squall line thunderstorms

_{EAE 2aj}

Question 11

How do Mesoscale Convective Complexes behave?

Answer 11

• 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.

_{EAE 2ak}

Question 12

What are Squall Line Thunderstorms?

Answer 12

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.

_{EAE 2al}

Question 13

What are Tornadoes?

Answer 13

Tornadoes, in general, are microscale phenomena with diameters ranging from tens of meters to a kilometre or two.

_{EAE 2am}

Question 14

How are tornadoes classified?

Answer 14

Tornadoes are classified as

• Non-supercellular (i.e. landspouts/waterspouts)
• Supercellular

_{EAE 2an}

Question 15

What are the main ingredients for a thunderstorm?

Answer 15

• Moisture
• Instability
• Trigger

Wind shear is a secondary component but important!

_{EAE 2ao}

Question 16

What is required for latent heating processes?

Answer 16

We need moisture in the atmosphere for latent heating processes to occur.

_{EAE 2ap}

Question 17

What is meant by ‘Saturation’

Answer 17

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.

_{EAE 2aq}

Question 18

What does ‘Saturation’ depend on?

Answer 18

• Saturation depends on temperature.
• Saturation does NOT depends on the presence of any other gas.

_{EAE 2ar}

Question 19

How does the temperature relate to saturation vapour content.

Answer 19

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

_{EAE 2as}

Question 20

What is the relation between moisture content and density?

Answer 20

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

_{EAE 2at}

Question 21

What is knowing how much water vapour is present useful for?

Answer 21

• 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.

_{EAE 2au}

Question 22

What is relative humidity?

Answer 22

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.

_{EAE 2av}

Question 23

What are the characteristics of relative humidity?

Answer 23

• 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

_{EAE 2aw}

Question 24

Why does relative humidity change during the day?

Answer 24

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°

_{EAE 2ax}

Question 25

**What is the dewpoint temperature?**

Answer 25

The temperature at which the water vapour in the air would condense into liquid. ## Footnote 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). _{EAE 2ay}

Question 26

**What are the characteristics of the dewpoint temperature?**

Answer 26

* 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*! ## Footnote _{EAE 2az}

Question 27

**What is knowing the dewpoint temperature useful for?**

Answer 27

A very useful forecasting tool ## Footnote The dew point can be used as a good approximation of the overnight minimum temperature. _{EAE 2ba}

Question 28

**How does dew point impact overnight temperature?**

Answer 28

* 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. ## Footnote _{EAE 2bb}

Question 29

**How does dew point explain desert temperatures?**

Answer 29

If the dew point is much lower than the air temperature, the rate of cooling at night is very fast. ## Footnote This is why dry regions (high plains, deserts) have such large temperature ranges between day and night. _{EAE 2bc}

Question 30

**What does dew point tell us about clouds?**

Answer 30

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. _{EAE 2bd}

Question 31

**What is the 'lapse rate'**

Answer 31

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. _{EAE 3aa}

Question 32

**What is the major determinant of Lapse Rate?**

Answer 32

The decrease in temperature with height as we lift a parcel depends on moisture: > **Dry** = no water vapour in it is condensing > > **Moist** = 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. _{EAE 3ab}

Question 33

**What is DALR?**

Answer 33

Dry Adiabatic Lapse Rate ## Footnote 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). _{EAE 3ac}

Question 34

**What is the LCL?**

Answer 34

Lifting condensation level The height when water vapour condenses to form clouds. ## Footnote * 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. _{EAE 3ad}

Question 35

**What happens to an air parcel above the LCL?**

Answer 35

It will further cool but not at the DALR. ## Footnote 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. _{EAE 3ae}

Question 36

**What is the MALR?**

Answer 36

Moist Adiabatic Lapse Rate ## Footnote 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). _{EAE 3af}

Question 37

**What is meant when a lifted air parcel is 'unstable'?**

Answer 37

It will continue to rise. ## Footnote The air parcel's temperature drops with height **less** than the ambient termperature gradient _{EAE 3ag}

Question 38

**What is meant when a lifted air parcel is 'stable'?**

Answer 38

It will sink back down to the ground. ## Footnote The air parcel's temperature drops with height **more** than the ambient termperature gradient _{EAE 3ah}

Question 39

**What is meant when a lifted air parcel is 'conditionally unstable'?**

Answer 39

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) _{EAE 3ai}

Question 40

**What are CCN?**

Answer 40

Cloud Condensation Nuclei ## Footnote 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. _{EAE 3aj}

Question 41

**When can clouds form?**

Answer 41

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. _{EAE 3ak}

Question 42

**What is lifted condensation level?**

Answer 42

The level at which a parcel of moist air lifted dry-adiabatically would become saturated. ## Footnote _{EAE 3al}

Question 43

**What are the relative size of 'raindrops' and 'cloud droplets'?**

Answer 43

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. _{EAE 3am}

Question 44

**What mechanis allow cloud droplets to grow into rain drops?**

Answer 44

* Condensation of water vapour onto an existing cloud droplet. * Collision-coalescence of existing cloud droplets. ## Footnote _{EAE 3an}

Question 45

**What is the collison-coalescence process?**

Answer 45

* 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. ## Footnote _{EAE 3ao}

Question 46

**What does the collison-coalescence process mean for the size of raindrops?**

Answer 46

Storms produce much larger raindrops than other kinds of cloud because of the multiple opprotunities to grow. ## Footnote _{EAE 3ap}

Question 47

**How does hail form?**

Answer 47

* 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. ## Footnote _{EAE 3aq}

Question 48

**How does electrical charge separate in the atmosphere?**

Answer 48

There is no scientific consensus on the exact way that electrical charge separates in the atmosphere. ## Footnote **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. _{EAE 3ar}

Question 49

**What is thunder?**

Answer 49

* 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). ## Footnote _{EAE 3as}