6 Precipitation

Question 1

Why do cloud droplets remain suspended in the air?

Answer 1

for a falling object, gravity is always countered by wind resistence and drag

-large objects should fall faster, but the larger an object the more resistence

Question 2

drag

Answer 2

• drag is dependent on the rate of motion (ie, the force of gravity) and the size of the object • so, a large object is pulled more by gravity, but it also experiences more wind resistance

-eventually the forces balance and the object reaches terminal velocity

Question 3

Terminal velocity is based solely on _____

Answer 3

size

so larger objects have a greater terminal velocity

• cloud droplets are extremely small so they have low terminal velocity
• easily countered by updraft

Question 4

• in order to get a cloud droplet to fall, we must encourage gravity to overcome the _______ – we do this by increasing the size of the droplet

Answer 4

rising updraft

Question 5

• a typical cloud droplet is about __ - __ um

Answer 5

• a typical cloud droplet is about 5 - 10 um across – in order to get them to fall we must increase this by 100 times • note: as we increase the radius of the droplet, we increase the volume by a factor of 3 –a radius increase of 100 equals a volume increase of 1,000,000 • this produces a larger and much more massive object, which will have a terminal velocity capable of overcoming an updraft –a raindrop

Question 6

How to clouddroplets become rain droplets?

Answer 6

1• first, cloud droplets can grow through condensation
• initial condensation forms on condensation nuclei, but if there is enough water vapour, the available condensation nuclei may be used up
•any further condensation must occur onto an already developed cloud droplet
• however, due to the abundance of nuclei, and the relative low amount of water vapour in most clouds, this increases the size to no more than 20 m – there just isn’t enough water to go around to create large droplets • therefore, condensation acts more as a trigger for rain drop formation, and less as a precipitation process

2. most clouds that produce precip are warm
   they have a temperature above freezing, and the droplets are liquid • also, the cloud is composed of a variety of droplet sizes, each small enough to be suspended, but still moving at different terminal velocities • larger droplets are falling at a faster rate than smaller droplets • eventually, the larger droplet – the “collector” – will catch up to the slower moving droplets and a collision is imminent • but, what happens next is dependent on the relative size of the collector drop and the smaller droplets

3.collision efficiency
• if the collector drop is much larger than the other droplets, the collision efficiency will decrease, because compressed air beneath the collector will push the small droplets away • collision efficiency is also low when the collector is a similar size to the other droplets, since they will be travelling roughly the same speed
-collisions and coalescene is the primary way in which raindrops from in tropical regions where clouds are warm

3.b)• in mid-latitude and polar regions, clouds tend to be colder and composed of supercooled droplets or solid crystals
• cold clouds have temperatures below 0 °C throughout
.cool clouds have temperature above 0 degrees at their base and below 0 degrees at their top
-• a typical cumulus cloud will have a warm bottom (liquid droplets), a cool middle (supercooled droplets) and a cold top (ice crystals) • the coexistence of liquid droplets and solid crystals is vital to the development of precipitation in most clouds • the process of raindrop formation in these clouds is known as the Bergeron proces

Question 7

collision effeciency

Answer 7

is the % of collisions between objects

Question 8

coalescence

Answer 8

• when a collector drop does collide with a smaller droplet, they can either bounce off each other or, more commonly, they can combine into a larger droplet – known as coalescence

Question 9

coalescence efficiency

Answer 9

the rate of coalescence vs collisions is the

-most collisions result in coalescence

Question 10

Bergeron Process

Answer 10

: the saturation vapour pressure over ice is less than the saturation vapour pressure over water

• if there is just enough water vapour in the air to keep the supercool droplet from evaporating, then there is more than enough water vapour to keep the crystal from sublimating
• if the vapour pressure is balanced for the droplet, then it must also favour deposition of vapour onto the crystal, causing it to grow
• at the same time, the growth of the crystal reduces the amount of vapour in the air, leading to evaporation of the droplet
• but evaporation from the droplet produces more water vapour in the air, which leads to more deposition onto the crystal
• the crystal grows and the droplet shrinks in a continuous cycle

.eventually the crystal becomes large enough that its terminal velocity exceeds the updraft force, and it falls as precip

Question 11

riming

Answer 11

ice crystals make great ice nuclei, so if a crystal collides with a supercooled droplet, the liquid will freeze to the crystal – known as riming

Question 12

aggregation

Answer 12

• when crystals collide, they may stick together –aggregation • this is more effective if the crystal has a slight coating of water, which helps adhesion • therefore, aggregation is more common when the temperature is close to 0°C

Question 13

process of moving from ice nucleation to a full sized snowflake takes around __ minutes

Answer 13

30

Question 14

snow (3 types)

Answer 14

snowis the primary result of ice crystal growth through deposition, riming and aggregation

dendrite=regular snowflake

plate=snowflake filled out

column=column shape

• the structure of the snowflake is dependent on the conditions under which the crystal formed – studying snowflakes tells us about the conditions in the cloud
• as an ice crystal falls through the cloud, it might encounter different temperature and moisture conditions
-each temperature and moisture conditions will favour a different type of crystal growth
-• crystals that fall through a uniform cloud will have a very simple structure, while crystals that fall through many different conditions will have very complex structures

Question 15

Snow(forms)

Answer 15

• typically, plates form in warm conditions, about 0 - -3°C
• from -3 - -8°C, hollow columns and needles for
• from -8 - -22°C, the structure reverts back to plates, often featuring dendritic features

below -22°C, the structure is columnar

• but consider a snowflake that forms as a column, but falls into conditions favourable for plates

Question 16

• a typical snowflake is composed of 10^__ molecules of water

Answer 16

18

Question 17

• in fact, all branches of a dendrite(snow) form at __° angles

Answer 17

60

-when these molecules combine, the specific bonds between molecules occur at 60° angles, giving all snowflakes a 6-sided structure

Question 18

Snowfall

Answer 18

• the pattern of snowfall in Canada generally follows the principles of temperature and uplift
• orographicuplift on the west coast leads to high snowfall amounts
• the rainshadow effect leads to reduced snowfall over the prairies
-increased snowfall in the east dure to winds blowing off the atlantic ocean
• reduced snowfall in the Arctic since the air is cold and holds little moisture overal

Question 19

Lake effect snow

Answer 19

is common downwind of large bodies of water that remain partially unfrozen during the winter (eg, Great Lakes)
• lake effect snow is produced when cold and dry air blows over the relatively warm water source, leading to evaporation – this produces instability, rising air, and cloud formation
-when the air is forced to rise again over the land, snow is produced and falls in tremendous quantities

Question 20

Rain(tropics)

Answer 20

• in the tropics, cloud temperatures are not below freezing, so raindrops form and fall to the surface as rain • an exception is in high altitude regions, where air temperature drops below freezing and precipitation is in the form of snow

Question 21

Rain(midlats)

Answer 21

• in mid-latitude regions, most precipitation starts as snow that forms by the Bergeron process and, as it falls, melts to form rain
• in most rain showers, there is a mix of large and small rain drops
• most showers start with large drops, then turn to smaller drops – this is because the large drops fall faster(terminal velocity) so we experience them first
• also, smaller drops evaporate faster(think curvature) so in the first few minutes of a shower, they are lost to evaporation

Question 22

Raindrop curvature

Answer 22

• Raindrops are not tear drop shaped
• all raindrops start out spherical, but as they fall the wind resistance at their base flattens them
• as they grow by coalescence, their speed increases as does wind resistance and the bottom becomes concave
• eventually, wind resistance exceeds the surface tension that holds the drop together –it breaks apart, forming smaller drops which then grow by coalescence

Question 23

Graupel

Answer 23

recall that ice crystals can grow by the process of riming, where supercooled droplets freeze onto the ice crystal
-graupel obscures the typical 6 sided crystal

• graupel can reach 5mm in size, and consequently have high terminal velocities • occasionally, especially when updrafts are strong, graupel is held in the cloud and provide hailstone nuclei

-hailstones are produced by a cycle of falling, melting, uplift, and freezing

Question 24

Hail

Answer 24

• since hailstones grow to large sizes, they require strong uplift forces to keep them suspended – this is why hail is mainly associated with severe thunderstorms • severe thunderstorms are generated by intense convection, and vertical winds speeds exceeding 100 km hr-1

-a baseball sized hail stone weighs about 160 grams, which would have a terminal velocity of 144 km m/s

• hailstones contain a large amount of kinetic energy, and consequently can be very damaging • the kinetic energy of the hailstone is proportional to its radius raised to the 4th power, so a 1cm radius hailstone will have 16 times more kinetic energy than a ½cm hailstone

Question 25

sleet

Answer 25

• sleet is produced when falling raindrops freeze • in mid-latitudes, precipitation mainly starts as ice crystals (in flake form), then melt as they fall through warmer air • if those raindrops then encounter cold air, they will refreeze into ice pellets - sleet -this process requires an inversion and a thick cold layer

Question 26

Freezing Rain

Answer 26

* if the cold layer is not sufficiently thick to freeze the rain into sleet, then freezing rain is produced • a freezing rain event, often called an ice storm, is caused by liquid drops falling onto a surface that is below 0°C • this causes the liquid to freeze almost on contact, producing a thin film of ice • if the rain event is sustained, the ice film can become very thick * Note: freezing rain falls as liquid and only solidifies onto a surface

Question 27

Measuring Precipitation(3)

Answer 27

.precipitation rates vary quickly in time and space making point measurements unreliable 1• the general tool to measure rainfall is the rain gauge • rain falls onto the gauge, passes through a funnel, and is measured with a measuring stick • the funnel is used to increase the precision of the gauge 2• a tipping bucket rain gauge automates the standard gauge • when the water passes through the funnel, it hits a small bucket (of known volume) – when the bucket fills, the weight causes it to tip over and the water drains away, and a second bucket is exposed • the more the bucket tips, the more rain that is falling - using magenets, you can tell how many times the bucket has tilted 3. Radar:radar is particularly useful in measuring precipitation because rain, snow, sleet, and hail all reflect microwaves very effectively

Question 28

• of the major climatic elements (including temperature, pressure, wind), _______ is by far the hardest to measure

Answer 28

precip

Question 29

Snow Water Equivalent (SWE)

Answer 29

• in terms of snow, we are more interested in the snow water equivalent (SWE): if you melted the snow, how much water would you be left with? • dense, wet snow has a high SWE • light, powder snow has a low SWE • the ratio of snow depth to SWE can range anywhere from 4:1 to 50:1

Question 30

Niphershielded snow gauge

Answer 30

-the bell shaped shield reduces turbulence • the post is adjustable, so that the gauge can be lifted above the snow – always 5’ above snow surface • the inner chamber is removed and the snow is melted to determine SWE

Question 31

The most accurate method of measureing snow fall is the ______

Answer 31

snowcourse • researchers travel across the snow, stopping at regular intervals to measure snow depth (a typical snow course has 10 stops) • a hollow tube is used to measure depth, and also to extract a core of snow • the core is then weighed to determine the density and SWE of the snow • the average of the 10 readings is used to represent the snow cover • snow course measurements are done every day

Question 32

Snow courses are slowly being replaced by automatic _______

Answer 32

snow pillows • an anti-freeze filled pillow is placed on the ground and connected to a pressure transducer • as snow accumulates on top of the pillow, the weight applies pressure, which can be recorded, and transformed into SWE • snow depth is measured using a laser or ultrasonic range-finding device

Question 33

Humans have tried to control precipitation through _________

Answer 33

cloud seeding - to prevent too little water, drough - to prevent to much water(flood) * certain materials are injected into non-precipitating clouds in an effort to convert some of the supercooled droplets into ice, thereby starting the Bergeron process * planes flying through clouds can release dry ice (solid CO2; temperature -78°C) into the cloud, which reduces the temperature of the cloud droplets and promotes freezing • recall that supercooled droplets do not need ice nuclei below -40°C – this freezing is spontaneous • ice itself is a good ice nucleus, -so other supercooled droplets will freeze onto the ice crystal s and the droplets will grow Agl(silver iodide) can be released to do the same, but in higher temps

Question 34

in general the effectiveness of cloud seeding is _____

Answer 34

minimal