Naked Science Forum
Life Sciences => The Environment => Topic started by: Simulated on 16/06/2008 14:59:43
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Why is there lots of loud thunder and bright lightning first? Why does the rain come down very heavliy afterwards? Most storms I see around here are like that. Just wondering why.
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The simple answer is to look at a picure of a cumulonimbus cloud, or a diagram may be better, you will notice that it is vaguely triangular. The rain is released at the middle and towards the rear of the cloud. So first you see the approach with its thunder and lightning then as the cloud passes over you, you have the rain.
Edit:
If you want to learn the basics then pop down to your library and find books by Alan Watts, 'weather wise' is a good enough place to start. Or listen to MPR (minnesota public radio, which is next to you...i think) they have a good meteorlogical segment on wednesdays or thursdays.
you could even pop down to the University of Minnesota in St Paul, if you are thinking of meteorology as a career, for a look around the Dept of Soil, Water, and Climate
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Soo why's there a big patch of fast moving, dark clouds before the thunder and lightning
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Do you mean the storm moving in?
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When the storm is moving in yes
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The simple answer is to look at a picure of a cumulonimbus cloud, or a diagram may be better, you will notice that it is vaguely triangular. The rain is released at the middle and towards the rear of the cloud. So first you see the approach with its thunder and lightning then as the cloud passes over you, you have the rain.
Edit:
If you want to learn the basics then pop down to your library and find books by Alan Watts, 'weather wise' is a good enough place to start. Or listen to MPR (minnesota public radio, which is next to you...i think) they have a good meteorlogical segment on wednesdays or thursdays.
you could even pop down to the University of Minnesota in St Paul, if you are thinking of meteorology as a career, for a look around the Dept of Soil, Water, and Climate
Nice information Paul! Thanks!
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So the easy answer was not enough, eh!
What you describe as a big patch of fast moving dark cloud, is one of the signs of an approaching thunderstorm. Perhaps you know someone who is a SKYWARN (http://www.skywarn.org) spotter, i believe that this is one of the signs they are trained to spot.
The clouds may also look brownish, yellowy, or green, these colours and the darkness of the cloud are caused by the storms massive size and by it blocking out the sunlight. I seem to remember that the green colouration is caused by the hail - although i forget the reason why.
As you have noted the storm seems to come at you with great speed, maybe you can now understand why severe weather warnings are generally only given minutes before the weather event is upon you. The typical size of a storm is only between 5 and 25 miles in diameter so they can very quickly be on top of you. Perhaps you also need / want to know how the storms are born? This will help in your own perdictions, weather spotting and safety.
Essentially, there are three things needed to produce a thunderstorm.
1. Moisture: If you are watching humidity levels you will (should) notice that the humidity changes.
2. Instability: The air above you is either stable or unstable, for a thunderstorm you need unstable air. Cumulus clouds are associated with unstable air, if you see their heigh getting bigger than their base, then this is another sign.
3. Lift: This is the trigger that starts air rising and focuses storms. I don't think there are any mountains near you (could be wrong) so you would need air colliding with a front, cool air blowing from an ocean or lake )i know you have lakes) or the gust front front from one storm that causes another.
over to NOAA
The thunderstorm life cycle:
1) Towering cumulus stage - Imagine a parcel of air like a balloon. If the air in the balloon is warmer than the environment around it, it will rise. As the balloon (air parcel) rises, the air cools, eventually cooling to its condensation point. A cloud becomes visible. As the air condenses, heat is released which helps the air parcel remain warmer than its surrounding environment, and so, it continues to rise, building up speed. This rising air forms the updraft, a thermal. A towering cumulus cloud has grown with crisp, hard edges forming a puffy or cauliflower look to the cloud. The height of the cloud is usually equal to or greater than the width of the cloud's base.
2) Mature thunderstorm stage - The warm air continues to rise until eventually it has cooled to that of its surrounding environment. This is often not until it hits the tropopause and the more stable air of the stratosphere. The storm may now have reached a height of 5 to 10 miles above the ground. The rising air has been moving at speeds near 40 mph. Now as it slows, the upper level winds begin to fan out the cloud forming the anvil. With strong winds aloft and longer lasting storms, anvils can spread 100 miles downwind.
A thunderstorm's updraft can carry 8000 tons of water aloft per minute! The water vapor condenses to cloud droplets which collide and grow in the rising updraft. Eventually, the weight of the droplet overcomes the rising air and it falls. The falling rain droplets begin to drag the air down around them and a downdraft forms. The rain also is falling into unsaturated air and so some evaporation occurs. Evaporation is a cooling process (your body cools when sweat evaporates from your skin). This rain-cooled air is now cooler than its surrounding environment and it sinks, helping to form and intensify the downdraft. A thunderstorm with concurrent updrafts and downdrafts is considered mature. As little as 20 minutes has elapsed since the cloud began to form.
3) Dissipating stage - As the downdraft hits the ground, the rain-cooled air begins to spread out in all directions. Eventually, this more stable air (since it is cool) chokes off the warm inflow that was driving the storm's updraft. With no new fuel to keep the storm alive, it dies. The downdraft dominates and the storm rains itself out. Sometimes, all that is left it the anvil.
This entire thunderstorm life cycle from the growing cumulus cloud to the dissipated storm can take only 30 minutes. This is why thunderstorms can strike so quickly and with little if any warning. The National Weather Service predicts the likelihood of thunderstorms to develop, but does not warn for lightning nor general thunderstorms.
What are the types of severe thunderstorms?
The Single Cell Storm
A typical single cell thunderstorm develops on a hot humid day when there isvery little wind with height. They are the ones that cause those quick, short lived, summer storms that drench you in minutes. This type ofr thunderstorms form as a result of atmospheric instability. In other words" the air aloft is sufficiently cold enough to support a rising thermal of air from the ground". As the thermal rises, the air cools causing water vapor to condense into liquid droplets which forms the cumulus cloud. The cloud continues to grow if its environment is cold and moist enough to further support development. If the growing cloud reaches a warm dry layer in the upper atmosphere it will stop growing and begin evaporating. On a good day, the cumulus cloud will continue to grow, ice crystals will form at the top of the cloud and water droplets at the bottom. As electrical charges separate, lightning will be initiated. The thundershower begins to die when its updraft is no longer able to suspend the growing water droplets in the cloud. Heavy rain then falls dragging down cool air that is felt as a strong gust at the ground. The downdraft cools the air at the ground killing off the thunderstorm's source of warm moist air. The storm then dies out. The entire life span of the typical thunderstorm is about thirty minutes.
The Multicell Severe Storm
The multicell thunderstorm accounts for the majority of thunderstorms that occur worldwide. A small percentage of these multicell storms will produce large hail and damaging winds. Multicell thunderstorms form in unstable atmospheric environments where the down draft from a dying cell converges with warm moist air to force another updraft on the flank of the storm, which forms a new storm tower, perpetuating the system. As long as a supply of warm moist air continues in a region, a multicell thunderstorm system will continue to grow, lasting up to several hours. The atmospheric conditions that will cause a non-severe multicell storm to become severe include strong wind fields aloft, as well as a pocket of dry air in the midlevels of the atmosphere. As cloud and rain droplets fall into the pool of midlevel dry air, the droplets evaporate cooling the air. The rain-cooled air becomes heavy and rushes downward as part of the storm's downdraft, increasing the velocity sufficiently for it to produce damage when it hits the ground. Strong winds aloft may also be mixed down to the ground as part of the downdraft, further enhancing the surface wind speeds. Severe thunderstorm wind gusts out of multicell severe storms can exceed 100 mph producing tornado like damage. Multicell severe storms may last several hours producing periodic severe weather over localized areas.
The Supercell Thunderstorm
The supercell thunderstorm is the least frequent to occur but the most efficient and most violent. Giant hail, damaging surface winds, and in some cases, long lived major tornadoes, are all common prodigy of supercell thunderstorms. Strong atmospheric wind shear, which is a change in direction and speed of the wind with height, is necessary for a supercell thunderstorm to form. Normally, a supercell begins as a multicell storm, forming in a wind sheared environment. The wind shear begins twisting the updraft as it develops. As the updraft continues to intensify in an environment where the wind speed increases and changes its direction with height, it begins to spin, usually in a cyclonic direction. The spinning updraft and simultaneous evacuation of air under the developing storm forms an area of low pressure at the center of the storm, called a mesocyclone. The mature supercell will have a strong rotating updraft in conjunction with several down drafts. In the case of a supercell, the storm's down drafts of rain cooled air flowing out into the updraft region of the storm actually intensify the updraft sustaining the storm. Because of the steady state relationship between the supercells's updraft and down drafts, the storm may last for up to six of more hours producing continuous wind damage, hail, and tornadoes. Hail sizes up to grapefruits have been observed with supercell thunderstorms. And tornadoes form in approximately 60% of all supercell storms. The tornado typically forms at the location of the mesocyclone from a lowered cloud base called the wall cloud. The wall cloud is the spinning wall of clouds surrounding the mesocyclone center.
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Thank You Paul