Massive African Dust Storm Cooling Atlantic Hurricane Odds for Early August

By: Dr. Jeff Masters , 3:10 PM GMT on July 31, 2013

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A massive dust storm that formed over the Sahara Desert early this week has now pushed out over the tropical Atlantic, and will sharply reduce the odds of tropical storm formation during the first week of August. The dust is accompanied by a large amount of dry air, which is making the Saharan Air Layer (SAL) much drier than usual this week. June and July are the peak months for dust storms in the Southwest Sahara, and this week's dust storm is a typical one for this time of year. Due in large part to all the dry and dusty air predicted to dominate the tropical Atlantic over the next seven days, none of the reliable computer models is predicting Atlantic tropical cyclone formation during the first week of August.


Figure 1. A massive dust storm moves off the coast of Africa in this MODIS image taken at 1:40 UTC July 30, 2013. Image credit: NASA.


Video 1. The predicted movement through August 3 of this week's Africam dust storm, using the NOAA NGAC aerosol model. Image credit: NOAA Visualization Laboratory.

How dust affects hurricanes
Saharan dust can affect hurricane activity in several ways:

1) Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow. Ocean temperatures in the MDR are currently 0.7°F above average, and this anomaly should cool this week as the dust blocks sunlight.

2) The Saharan Air Layer (SAL) is a layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion--a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this "inversion" acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm's updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane.

3) Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as "condensation nuclei"--centers around which raindrops can form and grow--the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)


Figure 2. Map of the mean summer dust optical thickness derived from satellite measurements between 1979 and 2000. Maximum dust amounts originate in the northern Sahel (15° to 18° N) and the Sahara (18° to 22° N). The Bodele depression in Chad is also an active dust source. Image credit: Evidence of the control of summer atmospheric transport of African dust over the Atlantic by Sahel sources from TOMS satellites (1979-2000), by C. Moulin and I. Chiapello, published in January 2004 in Geophysical Research Letters.

Dust in Africa's Sahel region and Atlantic hurricane activity
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° - 22° N) and the northwestern Sahel (15° - 18° N) (Figure 3.) The dust from the Southwest Sahara stays relatively constant from year to year, but the dust from the Northwest Sahel varies significantly, so understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is. The level of drought experienced in the northwestern Sahel during the previous year is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. In 2012 (Figure 3), precipitation across the northwestern Sahel was much above average, which should result in less dust than usual over the Atlantic this fall, increasing the odds of a busy 2013 hurricane season.


Figure 3. Rainfall over the Northwest Sahel region of Africa was about 200% of average during the 2012 rainy season. The heavy rains promoted vigorous vegetation growth in 2013, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.

The future of African dust: highly uncertain
A September 2013 paper in the Bulletin of the American Meteorological Society by Joseph Prospero and Olga Mayol-Bracero, "Understanding the Transport and Impact of African Dust on the Caribbean Basin," discusses the large uncertainties on how African dust may change due to climate change. Over the past decade, there has been no clear relationship between African dust and climate indices such as rainfall in the Sahel or the El Niño/La Niña cycle, which "makes it difficult to predict how dust emissions and transport might change over the coming decades as climate changes. The problem is exacerbated by the inability of models (IPCC 2007) to agree on future rainfall trends over large areas of North Africa (including the Sahel) that are known to be major dust sources today and in the recent past."

Links
Saharan Air Layer Analysis from the University of Wisconsin

Atlantic dust forecast from the Tel-Aviv University Weather Research Center

The Saharan Air Layer (SAL) was first described in 1972, in this classic paper: Carlson, T. N., and J. M. Prospero (1972), The Large-Scale Movement of Saharan Air Outbreaks over the Northern Equatorial Atlantic, Journal of Applied Meteorology, 11(2), 283-297

Dr. Amato Evan published a study in Science magazine March 2009 showing that 69% of the increase in Atlantic sea surface temperatures over the past 26 years could be attributed to decreases in the amount of dust in the atmosphere.

Kaufman, Y. J., I. Koren, L. A. Remer, D. Rosenfeld, and Y. Rudich, 2005a: The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proc. Natl. Acad. Sci. USA, 102, 11 207–11 212.

Wang, Chunzai, Shenfu Dong, Amato T. Evan, Gregory R. Foltz, Sang-Ki Lee, 2012, Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes, J. Climate, 25, 5404–5415.
doi: http://dx.doi.org/10.1175/JCLI-D-11-00413.1

Jeff Masters

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Quoting 3244. Abacosurf:
Radar shaping up...going to be nice to watch the evolution.


Definitely get a better handle on how things are shaping up now that it is able to be viewed in full on radar.
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Quoting 3262. JLPR2:


1009mb, pretty low when compared with the pressure readings in the area.


That's from the Nassau airport too so I would tend to think it's pretty accurate.
Member Since: August 19, 2005 Posts: 5 Comments: 6026
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3273. Grothar
Quoting 3258. BaltimoreBrian:


I knew that would get your little mind going. :)
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Ex Dorian doesn't look like much to me.
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Quoting 3206. Droab:
Is there any intensity forecast or models out?

Not sure if this was already scheduled, but while driving in central and north palm beach county, saw 3 cranes clearing canals of plant vegetation. 2 of the 3 were close to spillways.


The LTAM model is out, and it says it will become the first worldwide Category 6.
Member Since: Posts: Comments:
3270. GetReal
Quoting 3230. Grothar:
Almost all the models now have 91L just skirting the Florida east coast and staying over water the entire time, as it rides up the east coast.








IMO this system remains too weak to make connection with the trough to the north. Additionally the continental high will be building back into SE U.S. turning the flow to the north of the blob will be turning NE to SW in 48 hours, or less.
Member Since: Posts: Comments:
3269. JLPR2
Quoting 3257. Chicklit:
Evening, folks. Does ex-Dorian have an ACE?

Also, quite the dust cloud over the MDR.


Praying I get to play golf Saturday morning.
what a week...


Should reach the Northern Lesser Antilles tomorrow and Puerto Rico at night into Saturday. I can almost imagine my allergies.

Cursed allergy giving sand. :|
Member Since: Posts: Comments:
Quoting 3251. AussieStorm:
DISORGANIZED THUNDERSTORMS CONTINUE ALONG A TROUGH OF LOW
PRESSURE...THE REMNANTS OF DORIAN...LOCATED BETWEEN SOUTH FLORIDA
AND THE BAHAMAS. SURFACE PRESSURES ARE FALLING...AND SOME
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE BEFORE UPPER-LEVEL WINDS
BECOME UNFAVORABLE IN A DAY OR TWO
. THIS DISTURBANCE IS FORECAST TO
MOVE TOWARD THE NORTH AND NORTHWEST...BRINGING CLOUDINESS...
SHOWERS...AND GUSTY WINDS TO PORTIONS OF THE NORTHWESTERN BAHAMAS
AND THE COASTAL WATERS OF SOUTHEASTERN FLORIDA ON FRIDAY AND
SATURDAY. AFTER THAT...THE DISTURBANCE IS EXPECTED TO MOVE TOWARD
THE NORTH-NORTHEAST AND BECOME ASSOCIATED WITH A FRONTAL TROUGH
OFFSHORE OF THE EASTERN UNITED STATES. THIS SYSTEM HAS A MEDIUM
CHANCE...30 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS...AND A MEDIUM CHANCE...30 PERCENT...OF BECOMING A
TROPICAL CYCLONE DURING THE NEXT 5 DAYS.



so if the NHC thought ex-Dorian was going to develop in the next 5 days why didn't they say 50%. All the prognostications going on in here about 91L is laughable.


I guess since there is nothing else to watch, we might as well watch a group of thunderstorms that have 30% chance of becoming a TD in the next 5 days. LOL
Hey Aussie....this could be the difference between me going surfing this weekend or not. Of course it's important!!
Member Since: Posts: Comments:
Quoting 3265. hurricanes2018:
Link 80 mph!! wow

nice video!!
Member Since: Posts: Comments:
Quoting 3251. AussieStorm:
DISORGANIZED THUNDERSTORMS CONTINUE ALONG A TROUGH OF LOW
PRESSURE...THE REMNANTS OF DORIAN...LOCATED BETWEEN SOUTH FLORIDA
AND THE BAHAMAS. SURFACE PRESSURES ARE FALLING...AND SOME
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE BEFORE UPPER-LEVEL WINDS
BECOME UNFAVORABLE IN A DAY OR TWO
. THIS DISTURBANCE IS FORECAST TO
MOVE TOWARD THE NORTH AND NORTHWEST...BRINGING CLOUDINESS...
SHOWERS...AND GUSTY WINDS TO PORTIONS OF THE NORTHWESTERN BAHAMAS
AND THE COASTAL WATERS OF SOUTHEASTERN FLORIDA ON FRIDAY AND
SATURDAY. AFTER THAT...THE DISTURBANCE IS EXPECTED TO MOVE TOWARD
THE NORTH-NORTHEAST AND BECOME ASSOCIATED WITH A FRONTAL TROUGH
OFFSHORE OF THE EASTERN UNITED STATES. THIS SYSTEM HAS A MEDIUM
CHANCE...30 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS...AND A MEDIUM CHANCE...30 PERCENT...OF BECOMING A
TROPICAL CYCLONE DURING THE NEXT 5 DAYS.



so if the NHC thought ex-Dorian was going to develop in the next 5 days why didn't they say 50%. All the prognostications going on in here about 91L is laughable.


I guess since there is nothing else to watch, we might as well watch a group of thunderstorms that have 30% chance of becoming a TD in the next 5 days. LOL
I don't really see what is so funny??,we are here in South Florida with a potential tropical system to our East and you are 50,000 miles away in Australia!,so we have to be careful and paid attention,the NHC is not perfect by any means!!.
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Link 80 mph!! wow
Member Since: March 12, 2013 Posts: 154 Comments: 129933
3264. ncstorm
fried alligator..
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3263. beell
Just what are those track models modeling?
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3262. JLPR2
Quoting 3233. ProgressivePulse:
One reading out there. West Andros "The Flats" is a protected area so there is not much there.





1009mb, pretty low when compared with the pressure readings in the area.
Member Since: Posts: Comments:
3261. Patrap
Quoting 3251. AussieStorm:
DISORGANIZED THUNDERSTORMS CONTINUE ALONG A TROUGH OF LOW
PRESSURE...THE REMNANTS OF DORIAN...LOCATED BETWEEN SOUTH FLORIDA
AND THE BAHAMAS. SURFACE PRESSURES ARE FALLING...AND SOME
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE BEFORE UPPER-LEVEL WINDS
BECOME UNFAVORABLE IN A DAY OR TWO
. THIS DISTURBANCE IS FORECAST TO
MOVE TOWARD THE NORTH AND NORTHWEST...BRINGING CLOUDINESS...
SHOWERS...AND GUSTY WINDS TO PORTIONS OF THE NORTHWESTERN BAHAMAS
AND THE COASTAL WATERS OF SOUTHEASTERN FLORIDA ON FRIDAY AND
SATURDAY. AFTER THAT...THE DISTURBANCE IS EXPECTED TO MOVE TOWARD
THE NORTH-NORTHEAST AND BECOME ASSOCIATED WITH A FRONTAL TROUGH
OFFSHORE OF THE EASTERN UNITED STATES. THIS SYSTEM HAS A MEDIUM
CHANCE...30 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS...AND A MEDIUM CHANCE...30 PERCENT...OF BECOMING A
TROPICAL CYCLONE DURING THE NEXT 5 DAYS.



so if the NHC thought ex-Dorian was going to develop in the next 5 days why didn't they say 50%. All the prognostications going on in here about 91L is laughable.


I guess since there is nothing else to watch, we might as well watch a group of thunderstorms that have 30% chance of becoming a TD in the next 5 days. LOL


You can always re listen to the BB Show on the podcast too.
Member Since: July 3, 2005 Posts: 437 Comments: 134824
Quoting 3257. Chicklit:
Evening, folks. Does ex-Dorian have an ACE?

Also, quite the dust cloud over the MDR.


Praying I get to play golf Saturday morning.
what a week...


The highest ACE of the season so far.
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Member Since: March 12, 2013 Posts: 154 Comments: 129933
Quoting 3240. Grothar:


I do have a little bit of a life, Brian. :)
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Evening, folks. Does ex-Dorian have an ACE?

Also, quite the dust cloud over the MDR.


Praying I get to play golf Saturday morning.
what a week...
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3256. Patrap
Member Since: July 3, 2005 Posts: 437 Comments: 134824
3255. Grothar
Quoting 3247. aislinnpaps:
What a day, I'm exhausted. Orientation for the kids and parents went well. Teachers will all sleep very well tonight. It thundered, tons of lightening and POURED rain. I get home and stood in my garden with my hose. Not a drop here. : (


Breakfast better not be late.
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3254. ncstorm
Quoting 3248. will40:
well i told yall that until you see only blue water beneath him he aint dead


and my balloon is still an inflated balloon..no stick pins over here..
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3253. beell
Quoting 3234. 1900hurricane:

Yeah, it'll eventually get far enough away and the Upper Low will shear out, but the favorable window will be too brief for anything to develop in my opinion.


I'm right there with you...along with several others.

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Quoting 3215. Patrap:
I wonder if that is the center of circulation showin just west of Andro Island in the Bahamas??.
Member Since: Posts: Comments:
DISORGANIZED THUNDERSTORMS CONTINUE ALONG A TROUGH OF LOW
PRESSURE...THE REMNANTS OF DORIAN...LOCATED BETWEEN SOUTH FLORIDA
AND THE BAHAMAS. SURFACE PRESSURES ARE FALLING...AND SOME
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE BEFORE UPPER-LEVEL WINDS
BECOME UNFAVORABLE IN A DAY OR TWO
. THIS DISTURBANCE IS FORECAST TO
MOVE TOWARD THE NORTH AND NORTHWEST...BRINGING CLOUDINESS...
SHOWERS...AND GUSTY WINDS TO PORTIONS OF THE NORTHWESTERN BAHAMAS
AND THE COASTAL WATERS OF SOUTHEASTERN FLORIDA ON FRIDAY AND
SATURDAY. AFTER THAT...THE DISTURBANCE IS EXPECTED TO MOVE TOWARD
THE NORTH-NORTHEAST AND BECOME ASSOCIATED WITH A FRONTAL TROUGH
OFFSHORE OF THE EASTERN UNITED STATES. THIS SYSTEM HAS A MEDIUM
CHANCE...30 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS...AND A MEDIUM CHANCE...30 PERCENT...OF BECOMING A
TROPICAL CYCLONE DURING THE NEXT 5 DAYS.



so if the NHC thought ex-Dorian was going to develop in the next 5 days why didn't they say 50%. All the prognostications going on in here about 91L is laughable.


I guess since there is nothing else to watch, we might as well watch a group of thunderstorms that have 30% chance of becoming a TD in the next 5 days. LOL
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Quoting 3245. Grothar:


Right up the ole Gulfstream looks like.


5kts or less of shear and 85 degree waters.
Member Since: August 19, 2005 Posts: 5 Comments: 6026
3249. ncstorm
they have floating strip clubs?..I done seen everything..
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3248. will40
well i told yall that until you see only blue water beneath him he aint dead
Member Since: Posts: Comments:
What a day, I'm exhausted. Orientation for the kids and parents went well. Teachers will all sleep very well tonight. It thundered, tons of lightening and POURED rain. I get home and stood in my garden with my hose. Not a drop here. : (
Member Since: Posts: Comments:
Quoting 3236. Grothar:
These are the GFS concensus models. There may be a little surprise when the intensity models come out.




The vort/ center?/ blob circulation seems to be stationary or moving at a very slow pace NW?
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3245. Grothar
Quoting 3239. ProgressivePulse:


Evening Gro! Dorian found a groove spot if he wants to take advantage.


Right up the ole Gulfstream looks like.
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Quoting 3236. Grothar:
These are the GFS concensus models. There may be a little surprise when the intensity models come out.


Radar shaping up...going to be nice to watch the evolution.
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Quoting 3228. weatherman994:
I believe Hurricanes Andrew and Charley are the 2 strongest storms to hit Florida.
pluss the one in the keys in the 20s
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3242. Patrap
Member Since: July 3, 2005 Posts: 437 Comments: 134824
3241. sar2401
Quoting BaltimoreBrian:


Close enough for you to take pictures of Dorian's gray clouds?

LOL...I got that one. :-)
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3240. Grothar
Quoting 3235. BaltimoreBrian:


Close enough for you to take pictures of Dorian's gray clouds?


I do have a little bit of a life, Brian. :)
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Quoting 3230. Grothar:
Almost all the models now have 91L just skirting the Florida east coast and staying over water the entire time, as it rides up the east coast.




Evening Gro! Dorian found a groove spot if he wants to take advantage.
Member Since: August 19, 2005 Posts: 5 Comments: 6026
3238. sar2401
Quoting lobdelse81:
I wonder if ex-Dorian could pull a 2004 Alex?

Waters in the Gulf Stream are not above normal, there is a lot more shear, and the A-B high wasn't as dominant. I suspect that 91L will become Erin but much further south and then turn NE out to sea along the southern periphery of the A-B high.
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3237. Patrap
Member Since: July 3, 2005 Posts: 437 Comments: 134824
3236. Grothar
These are the GFS concensus models. There may be a little surprise when the intensity models come out.


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Quoting 3230. Grothar:
Almost all the models now have 91L just skirting the Florida east coast and staying over water the entire time, as it rides up the east coast.




Close enough for you to take pictures of Dorian's gray clouds?
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Quoting 3225. beell:


The upper low should start to shear out on Saturday as the the ridge to the north becomes the dominant feature.

Looks lovely by Monday morning, lol.


Yeah, it'll eventually get far enough away and the Upper Low will shear out, but the favorable window will be too brief for anything to develop in my opinion.
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One reading out there. West Andros "The Flats" is a protected area so there is not much there.



Member Since: August 19, 2005 Posts: 5 Comments: 6026
Quoting 3228. weatherman994:
I believe Hurricanes Andrew and Charley are the 2 strongest storms to hit Florida.


What about Wilma, Frances, and Jeanne?
Member Since: July 27, 2013 Posts: 0 Comments: 501
3231. Patrap
Member Since: July 3, 2005 Posts: 437 Comments: 134824
3230. Grothar
Almost all the models now have 91L just skirting the Florida east coast and staying over water the entire time, as it rides up the east coast.


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Convection over the Cuba is being pulled into the circulation. It looks primed for a good DMAX tonight. If it can get some good burst of deep convection then we might have a closed low. Tomorrow looks rainy for South Florida with the worst being tomorrow night and Saturday
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I believe Hurricanes Andrew and Charley are the 2 strongest storms to hit Florida.
Member Since: July 24, 2013 Posts: 0 Comments: 363
Quoting 3213. Grothar:
Beginning to fire up some convection. Starting early tonight.

The dream of Dorian reaching Florida looks to be coming into reality....Well in a way.
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3226. Patrap
00Z Guidance

Member Since: July 3, 2005 Posts: 437 Comments: 134824

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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