Formation de la précipitation

Présentation au sujet: "Formation de la précipitation"— Transcription de la présentation:

1 Formation de la précipitation
Images

2 Comment se forme la précipitation
Goutte de pluie typique : D = 2 mm Une goutte de pluie typique est 1 million de fois plus grande qu’une goutte de nuage!!! Goutte de nuage typique : D = 20 micromètres

3 Comment se forme la précipitation
Il y a plus de 1000 noyaux de condensation dans un centimètre cubique d’air!                     

4 Comment se forme la précipitation

5 Comment se forme la précipitation
Picture a hot "muggy" summer day and a cold, snowy winter day:  each one of these days may have a relative humidity of close to 100% but because the summer day is warmer, the air contains much more moisture and you can feel the wetness in the air.  A snowy winter day, on the other hand feels drier because the cold air holds much less moisture.  100% Relative Humidity simply means that the air is holding its maximum amount of moisture for a particular temperature. Therefore as the air becomes colder, water vapor must condense into liquid water because the air cannot hold as much water vapor as it could just moments before when it was at a higher temperature.  The water vapor molecules condense on the surface of the CCN until the air is in equilibrium and there are no more water vapor molecules available for condensation without making the air surrounding the drop too dry.  Before you know it, a cloud droplet has formed.  In fact, many billions of cloud droplets can form in a matter of minutes causing a large cloud to form. * NOTE:  Without cloud condensation nuclei, supersaturation must reach relative humidities of several hundred percent in order for water vapor molecules to condense freely!!!

6 Finalement! Nous avons un nuage avec de milliards de gouttelettes de nuage.
The cloud droplets remain suspended in the air by slight upward air currents and some heavier droplets actually fall out of the bottom of the cloud evaporating very quickly in the drier air beneath the cloud. Now we can take a look at how a humble little cloud droplet grows to be 1 million times larger! It is not easy for a cloud droplet to continue to grow into a form of precipitation.  Several conditions must exist within the cloud in order for the droplet to grow large enough to fall to the ground as precipitation.  We will talk about these conditions as they apply to the 2 major mechanisms for precipitation development: The 2 major mechanisms that explain how cloud droplets become large enough to fall to the ground without evaporating are: 1)  The Bergeron or "Ice Crystal" Process 2)  Growth by Collision and Coalescence

7 Processus de Bergeron Temperature du nuage Hydrométéores?
Supérieure à 0 C (32 F) Eau liquide -10 to 0 C (12-32 F) Eau surfondu -40 to -10 C (-4-14 F) Eau et glace (mixed clouds) Below -40 C (-4 F) Cristaux de glace (glaciated clouds) The Bergeron Process, named after its discoverer, Tor Bergeron, involves supercooled water droplets.  The term supercooled, refers to water that is still a liquid at temperatures below the melting point of 0 degrees Celsius or 32 degrees Fahrenheit.  Sounds impossible right? Actually, studies have shown that water in very small drops, such as the size of a cloud droplet, can exist at temperatures well below freezing (as low as -40 C)! HERE IS SOME ADDITIONAL INFORMATION YOU WILL NEED TO KNOW BEFORE WE LEARN ABOUT THE BERGERON PROCESS: An interesting and useful fact is that these supercooled water droplets will freeze almost instantly if they come into contact with a solid particle that resembles an ice crystal.  This solid particle is called a freezing nucleus. Also they will freeze if agitated; sometimes just by coming into contact with one another causes ice crystals to form. Many of the puffy cumulus clouds that you see in the sky may be made up entirely of supercooled liquid water droplets!   Clouds at certain temperatures can also be comprised of both supercooled water and ice crystals and are classified as Mixed Clouds: When a cloud is comprised of both Supercooled Water and Ice Crystals, The Bergeron Process can begin! The Bergeron Process explains how ice crystals grow at the expense of liquid cloud droplets within a mixed cloud: There are more water molecules surrounding the water droplets than there are surrounding the ice crystals.  This occurs because the saturation vapor pressure over a water surface is greater than that over an ice surface at the same [subfreezing] temperature.  Saturation vapor pressure describes how much water vapor is needed to make the air saturated at any given temperature and in effect, is the pressure that the water vapor would exert if the air were saturated with respect to a given temperature.  The supercooled liquid droplets are more readily able to evaporate and contribute to the vapor pressure in the surrounding air than the ice crystals are able to sublimate and contribute to the vapor pressure.  Therefore, when ice and liquid coexist within a cloud, water vapor must evaporate from the drop and flow toward the ice crystal in order to maintain equilibrium.  As this water vapor diffuses toward the ice crystal, the droplet must evaporate more in order to keep the vapor pressure in equilibrium with its surroundings.  Therefore, what happens, is a viscious cycle of water vapor evaporating from the drop, collecting on the ice crystal, and freezing so that the crystal continuously grows at the water droplet's expense.  

8 Processus de Bergeron Des gouttelettes d’eau surfondue et des cristaux de glace entourés de millions de molécules d’eau (en rouge) The Bergeron Process, named after its discoverer, Tor Bergeron, involves supercooled water droplets.  The term supercooled, refers to water that is still a liquid at temperatures below the melting point of 0 degrees Celsius or 32 degrees Fahrenheit.  Sounds impossible right? Actually, studies have shown that water in very small drops, such as the size of a cloud droplet, can exist at temperatures well below freezing (as low as -40 C)! HERE IS SOME ADDITIONAL INFORMATION YOU WILL NEED TO KNOW BEFORE WE LEARN ABOUT THE BERGERON PROCESS: An interesting and useful fact is that these supercooled water droplets will freeze almost instantly if they come into contact with a solid particle that resembles an ice crystal.  This solid particle is called a freezing nucleus. Also they will freeze if agitated; sometimes just by coming into contact with one another causes ice crystals to form. Many of the puffy cumulus clouds that you see in the sky may be made up entirely of supercooled liquid water droplets!   Clouds at certain temperatures can also be comprised of both supercooled water and ice crystals and are classified as Mixed Clouds: When a cloud is comprised of both Supercooled Water and Ice Crystals, The Bergeron Process can begin! The Bergeron Process explains how ice crystals grow at the expense of liquid cloud droplets within a mixed cloud: There are more water molecules surrounding the water droplets than there are surrounding the ice crystals.  This occurs because the saturation vapor pressure over a water surface is greater than that over an ice surface at the same [subfreezing] temperature.  Saturation vapor pressure describes how much water vapor is needed to make the air saturated at any given temperature and in effect, is the pressure that the water vapor would exert if the air were saturated with respect to a given temperature.  The supercooled liquid droplets are more readily able to evaporate and contribute to the vapor pressure in the surrounding air than the ice crystals are able to sublimate and contribute to the vapor pressure.  Therefore, when ice and liquid coexist within a cloud, water vapor must evaporate from the drop and flow toward the ice crystal in order to maintain equilibrium.  As this water vapor diffuses toward the ice crystal, the droplet must evaporate more in order to keep the vapor pressure in equilibrium with its surroundings.  Therefore, what happens, is a viscious cycle of water vapor evaporating from the drop, collecting on the ice crystal, and freezing so that the crystal continuously grows at the water droplet's expense.  

9 Processus de collision et coalescence
First, lets take a look at some important factors in the growth of a droplet through this process: 1)  There must be a high liquid water content within the cloud. 2)  There must be sufficiently strong and consistent updrafts within the cloud. 3)  A large range of cloud droplet sizes is very helpful. 4)  The cloud must be thick enough so that the cloud droplets have enough time to gather surrounding smaller droplets. 5)  The electric charge of the droplets and the electric field in the cloud and its effects are still being studied. Unlike the Bergeron Process, where precipitation forms under supercooled conditions, the Collision and Coalescence Process typically occurs within relatively warm clouds with tops warmer than -15C. As you can tell by the name of this process, the collision of falling and rising droplets is what allows them to grow large enough to fall to the ground as precipitation.   *This process usually does not involve ice crystals, but there are circumstances that we will learn about where ice crysals can collide and become larger (coalesce) as well. The liquid water content of the cloud is important for an obvious reason, can you guess it? -Thats right!  Without sufficient liquid water to form droplets, there will be no liquid precipitation!   Within the warm cloud there is an updraft of air caused by air coming together or converging at a point beneath the cloud.  After the air converges, it is forced upward.  This process is what initially helps to build the cloud and now that it has formed, it continues and carries smaller cloud droplets up into the cloud while larger droplets stay suspended within the cloud or even fall downward slowly.  As you might guess, with billions upon billions of cloud droplets hanging out in the cloud, some of them are bound to bump into each other!  This is where the term, "collision" comes into play!

10 Processus de collision et coalescence
Unlike the Bergeron Process, where precipitation forms under supercooled conditions, the Collision and Coalescence Process typically occurs within relatively warm clouds with tops warmer than -15C. As you can tell by the name of this process, the collision of falling and rising droplets is what allows them to grow large enough to fall to the ground as precipitation.   *This process usually does not involve ice crystals, but there are circumstances that we will learn about where ice crysals can collide and become larger (coalesce) as well. The liquid water content of the cloud is important for an obvious reason, can you guess it? -Thats right!  Without sufficient liquid water to form droplets, there will be no liquid precipitation!   Within the warm cloud there is an updraft of air caused by air coming together or converging at a point beneath the cloud.  After the air converges, it is forced upward.  This process is what initially helps to build the cloud and now that it has formed, it continues and carries smaller cloud droplets up into the cloud while larger droplets stay suspended within the cloud or even fall downward slowly.  As you might guess, with billions upon billions of cloud droplets hanging out in the cloud, some of them are bound to bump into each other!  This is where the term, "collision" comes into play! As the cloud droplets experience millions of collisions, they sometimes join together (or coalesce) and form larger cloud droplets.  The larger cloud droplets then fall faster (because they have a higher terminal velocity, click here to learn more) and collide with smaller droplets in their path.  Studies done in laboratories have shown that not all collisions result in coalescence, that is to say, that some of the drops break apart after colliding.  The studies have shown that "coalescense appears to be enhanced if colliding droplets have opposite (and, hence attractive) electrical charges... especially in thunderstorm precipitation coalescence where strongly charged droplets exist in a strong electrical field" Ahrens 1994.

11 Processus de collision et coalescence
The diagram above shows a droplet as it grows larger by collision/coalescence and eventually becomes too large to remain in one piece.  Smaller cloud droplets are shown with some upward vertical velocities (red arrows) to indicate that they may be caught in an updraft within the cloud. Part B of the above diagram shows a cloud droplet that initially moves toward the drop as if it were going to barely collide, but then it moves away taking a quick right hand turn, do you see it?  What is happening here?   This droplet is getting caught up in streamlines that form around a falling larger drop.  These streamlines are where air has been "plowed" away by the bottom of the drop so that it turns away from the drop, carrying smaller droplets out away and keeping them from colliding, and thus coalescing with the larger drop.  Picture a snow plow (if you have ever seen one!) and how the snow piles up along the front of the plow and is pushed off to the side into the snowbank, it is a similar process! The above diagram shows "y", the critical distance between the centers of the droplet with radius R2 and the collector drop with radius R1.  This critical distance is the farthest distance that the centers can be from one another so that the droplet can still make contact with the collector drop.  If a droplet is beyond this distance it will not collide with the collector drop and thus, will have no chance of coalescing and aiding in the collector drop's growth into a bonified precipitation droplet! There is an equation that is used to determine the chances that a droplet will collide with a collector drop.  This equation gives us the collision efficiency (E) and looks like this: The collector drop has a hard time growing because of this streamline effect.  If droplets are too small, the "y" distance is much smaller and they will most likely be caught up in the streamline of air around the drop.  Also, on the other hand, if the droplet is close in size to the collector drop, then their terminal velocities (fall speeds) will be close to the same and they will fall side by side to that they do not collide. OK, now we have talked about how a droplet collides with a collector drop, but even if it collides, who is to say that it will "stick" to the collector drop and coalesce? Perhaps you have seen water droplets bounce when they collide with another water surface?  Such as when raindrops hit a lake surface and splash back upward so that the water droplet is not instantly absorbed or "coalesced" into the lake?   There is a coefficient that looks at the fraction of collisions that result in coalescence and it is called the coalescence efficiency.  The coalescence efficiency is obtained by doing laboratory experiments in which the experimental measurements are compared to the theory. Once you have both the Collision and the Coalescence efficiencies, you can multiply them together to get the Collection Efficiency of a drop.  This is very important because it will tell you exactly how well a droplet will be able to grow into a precipitation drop. *It is also possible for a snow crystal to form initially by the Bergeron Process and then coalesce with other ice crystals to become larger.  This is usually evident during a warm, wet snow event when you see snow flakes the size of half dollars (2-3 inches!) or larger.  When the snow flakes become warmer, some of their surfaces may melt just enough to allow the re-freezing (Aggregation) of another flake to their outer surface.  Flakes can also become large enough that they too are broken apart by the drag force of the air hitting it from below.  Once the flake has has been broken up into smaller crystals, the smaller crystals are free to begin the Bergeron process or aggregate with other crystals all over again!

12 Pluie ou bruine? Meterologists define rain as liquid water drops that have a diameter of at least .5 millimeters.  Drops smaller than this are considered drizzle. The temperature of the air that precipitation falls through is what determines its form. In fact, precipitation always starts out within the cloud as either liquid drops or snow crystals. It is the temperature and winds beneath the cloud that will determine whether this precipitation will change into one of many forms that eventually hit the earth's surface. In warmer clouds such as those over the tropics, the precipitation begins as rain and continues to grow through collision/coalescence and falls all the way to the surface as raindrops.  In much of the world though, rain begins as some form of ice and melts as it falls through warmer air near the surface.

13 Bruine Drizzle is defined as precipitation that consists of fine uniform liquid drops, which are less than .5 millimeters in diameter.  Drizzle usually falls from low level clouds, called Stratus.  It is a mist that falls out of lower clouds when there are not sufficient updrafts to keep the smaller water droplets up within the cloud.

14 Bruine Drizzle usually falls from low level clouds, called Stratus.
It is a mist that falls out of lower clouds when there are not sufficient updrafts to keep the smaller water droplets up within the cloud.

15 Bruine It can also form as larger raindrops fall into
dry air and evaporate, becoming smaller until they are less than .5mm in diameter.

16 Profil de température de l’atmosphère
Below is a diagram that meterologists use to show the vertical structure of the atmosphere.  This is very helpful in determining what form precipitation will take on as it falls through a layer with specific temperature and wind profiles.  This profile was made by a balloon that carries sensors up into the atmosphere.  This diagram is called a Skew T Log P Diagram and is also known as a "Sounding". The sounding below is a typical rain sounding.  The air is moist, and this is indicated by the dashed black line showing that the dewpoint temperature and actual air temperature are almost the same throughout the lower levels of the atmosphere*.  [The dewpoint temperature is the temperature at which water vapor will condense and the air will be saturated, that is to say it will hold all the water vapor that it can at that particular temperature.  Any extra water vapor will be condensed into liquid water and thus, we have clouds & precipitation!]  Notice that the maroontemperature line is above freezing at levels beneath the cloud layer, this will cause any preciptation to melt and fall as rain.  It may help to picture the precipation following the maroon line all the way to the surface, thus encountering a changing environmental temperature as it falls. *If the layers beneath the cloud were drier, (if the black dashed line were farther to the left of the maroon line), then the falling precipitation may evaporate totally before hitting the surface.  When this happens it is called Virga, sometimes virga can be seen as a shaft or darker area of precipitation that seems to be hanging overhead.

17 Neige We have now learned that precipitation often begins as ice crystals and or snow, and sometimes melts inside of warmer layers of air beneath the cloud.  What must occur within the atmosphere for it to remain as snow until it reaches the surface? The layers beneath the cloud must be below freezing for most of the way to the surface. Below is a typical snow sounding where the air is moist and the temperature remains below freezing (the maroon line remains to the left of the freezing line in red) all the way to the surface. It is also important to note that snowflakes can often survive and remain frozen up to 300 meters below the melting level in the atmosphere and these flakes are often the wet, sticky variety that can aggregate easily with one another to become very large!

18 Sleet Sleet consists of clear or translucent particles of ice that form when ice crystals melt partially or totally and become more like raindrops and then re-freeze within a colder layer of air near the surface.  The thickness of this cold layer is very important because it is what distinguishes between sleet and freezing rain.   If the cold layer is sufficiently thick, the raindrop will re-freeze and fall to the surface as a raindrop sized piece of ice called sleet. If the cold layer is not thick enough and it is confined to the lowest level near the ground, the raindrops will not freeze until they have splashed onto the ground, thus covering the surface with a glaze of ice; this is called freezing rain.  [If you recall, colder air is more dense and tends to sink toward the lowest levels of the atmosphere, thus colder air may remain near the surface keeping temperatures on the ground below freezing]. Below are two soundings that illustrate Sleet and Freezing Rain scenarios within the atmosphere.  Can you guess which one is the typical sleet sounding and which is the typical freezing rain sounging? is a typical profile for sleet.  You can see how the layer of colder air goes up higher in the atmosphere than it does in B, which is a typical profile for freezing rain.

19 sleet Sleet consists of clear or translucent particles of ice that form when ice crystals melt partially or totally and become more like raindrops and then re-freeze within a colder layer of air near the surface.  The thickness of this cold layer is very important because it is what distinguishes between sleet and freezing rain.   If the cold layer is sufficiently thick, the raindrop will re-freeze and fall to the surface as a raindrop sized piece of ice called sleet. If the cold layer is not thick enough and it is confined to the lowest level near the ground, the raindrops will not freeze until they have splashed onto the ground, thus covering the surface with a glaze of ice; this is called freezing rain.  [If you recall, colder air is more dense and tends to sink toward the lowest levels of the atmosphere, thus colder air may remain near the surface keeping temperatures on the ground below freezing]. Below are two soundings that illustrate Sleet and Freezing Rain scenarios within the atmosphere.  Can you guess which one is the typical sleet sounding and which is the typical freezing rain sounging?

20 verglas The layer of cold air in B is much more shallow and therefore, for a falling rain droplet, it would not have time to freeze while it is still in the air, but instead would freeze as it makes contact with the below freezing surface of the earth.

21 Formation de la grêle Hail consists of pieces of ice that can be transparent or partially opaque and can range in size from as small as peas to as large as grapefruits. Hail forms inside of cumulonimbus clouds (cumulonimbus clouds are anvil shaped and usually thunderstorm-producing clouds) when there is a strong updraft to carry graupel pellets back up into the cloud.  [Graupel is simply frozen raindrops, similar to sleet].  The strong updraft carries raindrops and ice crystals alike back up into the cloud where temperatures are below freezing and raindrops will freeze into sleet or graupel if there is a freezing nucleus available.  The graupel is then carried up through the cloud where millions of supercooled water droplets collide with the ice surface and are instantly frozen on causing the graupel to become larger.  [This process is called accretion].  When the now larger graupel or hail stone reaches the top of the cloud, it begins to fall back downward on the outer edge of the cloud where the updraft is weaker.  The hail continues its descent until it falls back down into an area where the updraft is stronger and this cycle begins again with the hail stone growing another ring of ice.  This cycle will continue and the hail stone will become larger until finally it becomes too heavy for the updraft to carry upward.  At this point it falls out of the bottom of the cloud, sometimes causing damage to whatever it lands on.

22 Formation de la grêle The layers of a hailstone sometimes differ in color from transparent to opaque, this is due to the temperature and the amount of supercooled water droplets within the cloud.  If there is a very high number of supercooled water droplets, they may not all have time to freeze onto the stone before it enters a warmer layer and then back up into the sub-freezing layer.  When the stone re-enters the sub-freezing layer, the liquid water that is covering it freezes as more of a clear glaze of ice.

23 Mesures Rain and drizzle are the easiest forms of precipitation to measure.  Rain gauges are used to measure liquid water depth and can be as simple as an open bucket with a consistent cross section throughout.  Meteorologists however, use more accurate instruments and slightly more sophisticated gauges to measure rainfall.     Remember that a measurement of precipitation is taken on a flat surface, so that if 1 inch of rain has fallen, it would leave a puddle 1 inch deep in the bottom of a flat open pan or bucket with a consistent cross sectional area.  [This means the bucket can not be tapered]. Generally amounts of rainfall less than 1 hundredth of an inch (.01") are reported as a trace of rainfall.     Below is a standard bucket rain gauge, which magnifies the amount of rainfall through a simple cross sectional area conversion so that more precise measurements can be made, especially for very small amounts of rain.   Typically the cross sectional area of the collector funnel is 10 times the cross sectional area of the inside tube.  This allows for rainfall to be magnified 10 times. EXAMPLE:  0.1 inches of rain would fill the inside tube with 1 whole inch of water, and a scaled ruler would measure this as 0.1 inches of actual rainfall. The larger cross section on the outer can allows for a large amount of rain to be collected and then the smaller cross section on the inside cylinder allows a scaled ruler to be used to measure the exact amount of rainfall down to the nearest hundredth of an inch. If more than 6 inches of rain were to fall, the overflow would fall into the larger gray cylinder and would have to be poured into the smaller cylinder to be measured and added to the 6 inches that filled the inner cylinder. *Imagine how hard it would be to measure to the nearest .01 of an inch in a regular bucket of water with a standard ruler!        

24 Mesures Another form of rain gauge is the tipping bucket gauge.  This gauge uses a double sided scoop or bucket that pivots when full to dump out the rainfall.  An electronic sensor feels every time the bucket tips each way and records the number of times that the buckets tip.  The buckets hold a known amount of rainfall so that the amount of precipitation can be recorded along with the time of occurence and intensity (amount of rain per unit of time). It is often useful to know when the most intense rainfall occured and exactly how intense it was.  For example this information can allow meteorologists to get a feel for how much rain in a short amount of time will cause a flash flood. Yet another form of rain gauge measures the weight of rain that has fallen and translates this to a vertical depth.  This is called a weighing gauge and is very similar in design to the standard bucket rain gauge except for a weighing-scale located at the base

25 Mesures Snow and sleet are measured several different ways.  One method is to simply take a ruler or measuring stick and measure the depth of the snow on a flat board called a snowboard.  The snowboard consists of a perfectly flat, white (reflective) surface.  The snowboard is white so that it does not absorb solar radiation and heat up, thereby melting snow that falls on its surface.  This measurement is done very soon after the snow has fallen so as to get a measurement before any settling takes place, and with snow a very large amount of settling can potentially take place depending on the temperature at which the snow formed.  If it formed at very cold temperatures, it is more likely to have a higher settling rate because the crystals are more dendritic in nature and tend to allow much more room for air to gather in between flakes as it lands on the surface. Dendritic is a word that means "tree-like" and this is very much what a dendritic crystal or dendrite looks like.  The tip of a dendrite  grows and side branches grow outward as supercooled water freezes onto the existing ice.

26 Mesures Other forms that snow/ice crystals can take on include: needle, plate, and columns.  As you can see from the figure below, there is much less space for air to become trapped among accumulated crystals of these types.  Therefore, the effect of settling on snow comprised of these crystals is much less than the settling that would occur within a dendritic snow.

27 Mesures Snow and sleet can also be measured for their liquid water equivalence.  This is done by collecting the snow in a bucket similar to a rain guage except it is heated to melt the snow as it lands so that its depth can be measured as if it were rain.  This method can be much more useful and is typically a more consistent method----it allows meteorologists to determine a liquid to snow ratio, which can be as high as 15:1 for very cold snow and as low as 1.1:1 for sleet that piles up with much less room for air in between the particles. In a standard rain gauge where measurement of liquid equivalence is taken with a scaled ruler, anti-freeze or oil is used to prevent the liquid from freezing.

28 Mesures Another instrument used to measure the liquid equivalence of snow depth is a snow pillow.  A snow pillow usually consists of around 4 stainless steel panels that are plumbed so that they are all level with eachother.  The panels are filled with an antifreeze solution so that snow melts into them and the added weight due to the liquid puts pressure on the panels, sending the fluid to a pressure transducer that converts the pressure into a liquid equivalence and liquid depth.  Snow pillows are sometimes used at remote sensing stations so that the signal from the pressure transducer is sent via antenna to a computer that can analyze the data.

29 Mesures The sonic range sensor is an instrument used on weather observation towers to measure snow depth.  This sensor sends out pulses of ultra-sonic sound waves (sound waves that can not be heard by the human ear) and records how long it takes for the echoes to come back to the sensor.  Knowing the speed of sound allows the distance from the sensor to the top of the snow to be determined.

30 Mesures Freezing rain is measured using a ruler or measuring stick to determine its vertical depth.  Sometimes radial or diameter of ice measurements are taken for ice that accumulates on tree branches and powerlines.  Power companies are often very interested in radial depths of ice because it could be important relative to how much weight from ice that a powerline can support. Freezing rain that falls into a heated rain gauge or one with anti-freeze is easily measured for liquid equivalence just as if it were regular rain.

31 Mesures Hail is the only form of precipitation that is commonly measured according to each individual hailstone's size. It is typically measured according to an individual diameter of a particular hailstone, perhaps the largest one that is observed, so that a report is given stating that hail as large a particular diameter was observed in a certain location.  This measurement is then often compared to everyday objects such as pea-size, golf ball size, or baseball size!! Since hail can be a very devastating form of precipitation, it is very important to note the size of the hail so that it can be related to resulting damages to nature and property. Hail can be measured for its liquid equivalence by melting an accumulated area of hailstones.  It is important that hailstones are collected in an area that is large enough account for bouncing and rebounding.  [Hail stones are not collected efficiently by a typical funnel such as that on a rain guage because of this bouncing effect as it collides with a surface at relatively high speeds.] It can also be of some importance to measure the depth of hail accumulation.  This can be done with the use of a simple ruler or measuring stick immediately after the hail has fallen, because it tends to melt very quickly. You have now reached the end of this tutorial!  I hope that your learning experience hasn't been too precipitous!