# Presentation: 559434

## Moisture

- Gulf of Mexico – near surface moisture
- Gulf of California and Pacific Ocean – mid to high level “monsoon” moisture

**Notes:**
You need three basic elements to produce thunderstorms:
Moisture
Lift, and
Instability
The moisture sources for Colorado are either the Gulf of Mexico, which typically supplies low level moisture and the Gulf of California and Pacific Ocean which typically supply mid and high level moisture.  This higher moisture is called “monsoon” moisture and produces high based storms that can bring little or no rain and consequent dry lightning.

## Lifting mechanisms

_**Cold Front**_** ****	****	****	**cold air undercuts warm air
_**Warm Front**_** ****	****	****	**warm air overruns cold air
_**Stationary Front**_** ****	****	**a stalled front
_**Dry Line**_** ****	****	****	**dry air undercuts moist air
_**Low Pressure Areas**_**  ****	**draws air in and lifts it
_**Thunderstorm Outflow**_**  ****	**cold air flowing out from a** **storm 				undercuts warm air

_**Hills / Mountains**_

**Notes:**
There are many ways to lift air.  The most efficient is terrain.

## Upper Level Disturbance

***L***

- Upper-level disturbance  and jet streak draws air skyward.

**Notes:**
Upper disturbances cause an ascent of air in advance of them.  This ascending air can cause a mid level warm air (cap) to be eroded away, which can allow thunderstorms to rapidly develop. 

## The ground heats the air...

**Notes:**
The atmosphere is warmed from the ground up.  The ground heats the air near it, and the heated air is mixed upward.  A more direct sun angle in the summer heats the ground much more readily than the shallow angle “weaker” sunshine in the winter.

## Buoyancy

**The ****tendency or capacity to remain afloat**** in a liquid or rise in the air.**
**The ****upward force that a fluid exerts**** on an object less dense than itself.**

## Buoyancy

**Notes:**
This man and his gear are floating because the combined density of him and his gear is less than the density of the water.  He and his gear are buoyant with respect to the water.

## Buoyancy

**Notes:**
The Titanic was buoyant with respect to sea water until holes were ripped in its side.  Then, when water rushed into it, the ship was more dense than the surrounding sea water, and it tragically went down.

## Buoyancy

**Notes:**
With respect to atmospheric gases, a hot air balloon floats because the density of the warm air in the balloon and the balloon is less than the density of the air outside of the balloon.  It is buoyant because the more dense air outside the balloon exerts an upward force on the less dense warm air inside of it and the basket.

## Buoyancy

**Notes:**
The atmosphere exerts an upward force on the balloon and basket  if the density of the balloon and the basket is lower than the density of the surrounding air.  A balloon is brought down by letting the air cool down in the balloon or by letting some of the warm air in the balloon escape.  Then the balloon can become neutrally buoyant, moving along at a constant elevation, or become negatively buoyant and descend slowly to the earth.

## Buoyancy

**Notes:**
The air over this fire is very hot, and is very buoyant with respect to the surrounding air.  This is what also happens when a cloud tower develops.  The packet (parcel) of air (analogous to a hot air balloon) rises because it is slightly warmer than the surrounding air.  The parcel of air will continue to rise until it becomes colder than its surroundings.

## BuoyancyWhy storms DON’T form on some warm and humid days

**In cloud temps**

**Environmental temps**

**Notes:**
The rate at which the atmospheric temperature drops with increasing altitude varies from day to day.  On some days the temperature drops off gradually, and on other days, the temperature drops off quickly.  When temperature drops off gradually, the atmosphere is said to be stable.  When temperature drops off quickly, the atmosphere is said to be unstable.  But stable or unstable with respect to what?  Parcels of air which rise within the general environment!  When a parcel of air rises within the general environment, either lifted by a front or terrain, it cools at a predictable rate.  During its dry ascent, the parcel will cool at around 5 degrees F per 1000 feet.  When a cloud forms (water vapor condenses in the parcel), the ascent becomes moist, and the temperature will fall off around 3.5 degrees F per 1000 feet.  The above chart shows a case were air over a darker field is heated 4 degrees F higher than the surrounding air.  That parcel is less dense than the surrounding air, so it rises and cools at a predictable rate of 5 degrees F.  Meantime, the surrounding air cools at a varying rate with increasing height.  The air parcel will continue to rise until its temperature is the same or cooler than the surrounding air.  In the example above this occurs around 5,000 feet above ground.  The parcel stops rising, and only shallow clouds develop.

## BuoyancyWhy storms DON’T form on some warm and humid days

**Notes:**
If there is a wedge of warm air in the mid levels of the atmosphere, rising parcels that encounter the warmer air will stop rising, because they become negatively buoyant with respect to the surrounding atmosphere.  This warmer wedge of air, called an inversion (environmental temperature increases with increasing altitude), can be caused by a high pressure system and its sinking air (which warms air layers), or by advection (horizontal movement of warmer air at that level).  This warmer layer or wedge of air is called a cap or a lid.  Rising parcels which hit the cap or lid become negatively buoyant with respect to the environment, so they cease to rise. Cloud development is stopped.  Meteorologists carefully monitor the atmosphere each day for the presence of instability, and the presence of a cap (lid).

## Buoyancy

- 20,000 ft   0F
- 15,000 ft   25F
- 10,000 ft   40F
- 5,000 ft     62F
- 0 ft            85F

-  9
- 27
- 45
- 63

- 5F / 1000 Ft

- At 3000 Feet Parcel Temp drops to 70F .

- A cloud forms...so the parcel will cool more slowly at a rate of 3.5F/1000 Feet.

**Notes:**
In this example, a parcel rises into the environment.  Note that the temperature inside the parcel is always higher than the temperature in the general atmospheric environment.  Therefore it will be positively buoyant with respect to the environment.  It will continue to rise UNTIL the temperature in the parcel becomes equal to or colder than the temperature of the environment.  This could be 40,000 to 60,000 feet above the ground! 

## Buoyancy

**Environmental temps**

**In cloud temps**

**Notes:**
In this example the temperature of the rising cloud tower is warmer than the temperature of the surrounding environment for many thousands of feet.  The atmosphere with respect to the parcel (cloud tower) is unstable.  When the temperature of the cloud tower and the surrounding environment become equal (5 degrees F) the cloud tower becomes neutrally buoyant, then above that, negatively buoyant.  However, since the cloud tower updraft has so much upward momentum, it will continue to rise above that level, and a cumuloform top can be seen above the spread out anvil of the storm.  The greater the difference in temperature between the cloud tower air and the surrounding environment, the greater the instability, and the faster the air will rush upward.  In extreme cases, the updraft, caused by the difference in temperature between cloud tower and surrounding environmental air, can exceed 100 mph in strength.  This magnitude of updraft speed can cause large hailstones and damaging downdrafts to develop.

## Overshooting tops

**Notes:**
As seen from satellites, overshooting tops are the result of high momentum air overshooting the stable layer at the top of the thunderstorm.  The stable layer (where the cloud tower air becomes negatively buoyant) is manifest by the anvil of the storm.  Overshooting tops mean very strong updrafts, which cause severe hail and winds.

## Buoyancyas buoyancy INCREASES,updraft strength INCREASES

**Notes:**
The photo on the left shows a tower with a somewhat fuzzy appearance, characteristic of a weak or moderate updraft.  The photo on the right shows a tower with a crisp, hard appearance, towering to great heights, characteristic of a strong updraft. 

## Weather Radar VIL

**Weather Radar ****VIL**

**VIL is a measure of the updraft strength and a good hail indicator.**

**VIL = **

**63**

**Notes:**
The doppler radar derives a product called Vertically Integrated Liquid.  At its fastest rate, the radar spins around, looking at 14 different slices of the atmosphere (from 1⁄2 degree above the horizon to about 15 degrees above the horizon) in just 5 minutes.  A VIL value represents reflectivity values converted into equivalent liquid water values in a square vertical column (a little over 2 miles in length on each side) from the lowest elevation scan to the highest. All the values from each elevation scan are integrated (added up) to come up with the one VIL value for each 2 mile square column from the bottom of the storm to the top.  Hail is an excellent reflector of radar energy, so the higher the VIL value, the more hail there may be in the storm.  Very high VIL values may mean very large hail and very heavy rain.  There is no “magic” VIL number that identifies severe hail.  VIL numbers as low as the teens can yield severe hail in the early Spring when the freezing level is very low.  VIL numbers in the 80s during the hot mid-summer may not yield severe hail.  This is why it is important for spotters to call us even when very small hail is seen.  We can then look at what hail size caused what VIL number, and then we can come up with the “VIL number of the day” for severe hail.  The units of VIL are kilograms of liquid water per square meter.  In the example given in the slide, the value of 63 is for the square column (2 miles on a side) from near the bottom of the storm to the top.

## Weather Radar VIL

**Weather Radar ****VIL**

**VIL is a measure of the updraft strength and a good hail indicator.**

**Notes:**
The image on the right shows the VIL product.  Each square is about 2 miles on a side.  The red squares mean higher VIL values, and the likelihood of very heavy rain and/or large hail caused by very strong updrafts in the cloud tower.

## Mid- and Long-distance cloud features

**Courtesy Evan Bookbinder**

**Notes:**
The anvil and overshooting top are easily seen.  The anvil is the level at which the cloud tower becomes negatively buoyant with respect to the surrounding atmosphere.  The overshooting top is a manifestation of the strong upward momentum of the updraft past the level of negative buoyancy.  

## Thunderstorm Spectrum

***Single Cell******	******Cluster******	******Squall Line******	******Supercell***

- Short-Lived					Severe
- Pulse Severe					Rotating Updraft

**Notes:**
Depending on the state of the atmosphere, thunderstorms can occur singly, in groups, or in lines.

## “Pulse” storms

**- ****Weak**** winds aloft.**
**- ****Very buoyant**** air can result in strong, ****nearly vertical**** updrafts.**
**- As the downdraft ****overwhelms**** the updraft (falls through the updraft), ONLY a short period of rain, wind, and possible hail occurs.**

**Notes:**
Pulse storms develop in an environment where winds are weak from the ground up.  If the atmosphere is very unstable with respect to the cloud tower (much cooler through a good depth of the atmosphere) the air will rush up almost vertically.  Rain and hail will develop in the mid levels of the storm and fall into the updraft.  The updraft will be killed, and the storm will bring short periods of rain, wind, and possible hail.  The stronger the updraft, the greater the potential for brief large hail and brief damaging winds.

## “Pulse” stormupdraft sequence 

**Initial                           Later                            Finally**

**Storm is initially dominated by the updraft,**
**Then after a short time...the downdraft overwhelms (falls through) the updraft**

## Multicell cluster

**Notes:**
Cloud towers (cells) also occur in clusters.  If two or more storms in a cluster are moving in slightly different directions or speeds, they can run into one another.  Two updrafts can combine into one stronger updraft, increasing the potential for the production of larger hail and higher winds.  Warning forecasters look for merging storms using radar and satellite imagery, to anticipate the development of severe thunderstorms.

## “Multicell” line stormsSquall lines

**Notes:**
Cloud towers (cells) also can line up along a weather front (cold, warm, dry line, etc.).  Storms in a line can move in different directions and at different speeds, so mergers of storms can take place.  If that happens, the updraft resulting from the combination of two or more updrafts will be stronger, and large hail and damaging winds will result.

## Squall line – shelf clouds

**Shelf clouds occur on the edge of the updraft- downdraft interface**
**Shelf clouds slope down AWAY from the rain**
**When you see a shelf cloud, think strong winds**

## Shelf cloud

- Cool Outflow

- Formed by 2 different air-masses interacting

**Notes:**
The shelf cloud is the interface between warmer air flowing into the storm and cool air flowing out of the storm.

## “Accessory” Clouds

** ****Often turbulent with small scale twisting and curling motions in clouds.**
**- Motion is neither organized nor persistent.**
**- Scud clouds may form and dissipate rapidly.**

**Notes:**
The shelf cloud, on the front edge of the storm, often is accompanied by turbulent motions.  Strong tornados rarely occur on the leading edge of a storm.  Look for strong or damaging straight winds, hail, and heavy rain on the forward edge of the storm.

## “Multicell” – Squall line  Radar View

**Notes:**
In this example two cells on a line in the center of the image have merged (collided) near Kim, Colorado.  The resulting stronger updraft produced hail the size of quarters and 70 mph downburst winds.  

**As winds aloft  accelerate toward the ground the squall line bows forward.**
**A bow echo is a radar signature strongly suggesting wind damage.**

** ****Bow Echoes / Squall Line**

## Macrobursts

**Notes:**
Damaging winds wider than 2 1⁄2 miles across are by definition “macrobursts”.  These damaging 90 mph winds on the forward edge of a severe thunderstorm were kicking dust hundreds of feet into the air in California. 

## Gustnadoes

**Notes:**
When air flows out on the edge of a storm, occasionally, small, shortlived wind eddies can develop.  These can cause some damage, and are known as gustnadoes.  

## End of Part Two

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**Notes:**
These dairy cows were killed by lightning along this barbed wire fence.