RTS Weather Station

Welcome to RTS Weather Station

Welcome to the official web site for the ATSC/RTS weather station. To learn more about ATSC, the weather station and Kwajalein, click here.

Detailed information about our products and the systems that produce them, along with archived information and links to other interesting sites are provided. Near real time radar, satellite and forecasts are also available.

Feel free to email us with any questions or comments.

Latest News

Why Was February So Wet?

February 2014 has gone down in the history books as the wettest February for Kwajalein, in a precipitation record that dates back to 1945. 13.82" of rain was recorded at the Kwajalein Weather Station for the month, besting the previous record of 11.79" set in 2011. What is amazing about this total is that February is typically the driest month of the year, with an average rainfall of only 3.27". In fact, 13.82" would be about 2" above normal in October, the climatologically wettest month of the year. So, why was this February so abnormal?

The subtropical ridge is an area of high pressure that normally sits north of Kwajalein Atoll during the dry season. It creates a stable atmosphere in which the development of precipitation is difficult. However, when the subtropical ridge is displaced to the east, typically by mid-latitude flow that push it to the International Dateline and beyond, the monsoon trough in Micronesia may become established in the southern Marshall Islands. This feature may be stationary and heavy rainfall around it can persist for days. While this pattern occurs over the region in other months, it had not occurred in February since 1944.

The graphic below shows the meteorological pattern just described. The subtropical ridge ("A") is well northeast of Kwajalein Atoll, displaced from its typical location near of Wake Island. The circulation ("C") centered near Chuuk anchors the monsoon trough (dashed line), which extends to the east through Kosrae and into the southern Marshall Islands. Kwajalein Atoll sits on the northeastern side of the trough, in an area where northeast winds meet east to southeast winds. The convergence of these winds produces long-lasting, widespread, heavy rainfall, which is what we recorded on more than one occasion this February.

NOAA Graphic

Kwajalein Weather Factoids (previously published in the Kwaj Hourglass as "Weather Balloons")

What is nocturnal instability? (5/13/2014)

On clear nights, light showers will often pop up overnight and dissipate by mid-morning. What causes this to happen? It is a phenomenon we call "nocturnal instability".

After sunset, radiational cooling occurs in the lower atmosphere, especially when there is little to no cloud cover. At the same time, air at the ocean surface does not cool as much because the water itself cools very little. Water has a higher specific heat than air; that is, water requires more energy to heat and, inversely, more slowly releases heat that it has absorbed.

The air at the surface rises into a lower atmosphere which is now cooler than it was before, thanks to radiational cooling. The rising air is more positively buoyant, so its upward motion is enhanced. When warm, moist air rises, and rises at a faster rate, clouds and rain showers form.

In the morning, a few hours after sunrise, the heat of the sun warms the lower atmosphere more rapidly than the ocean surface, which reduces the positive buoyancy of rising air and effectively cancels the nocturnal instability. Most of the showers rain out and low-level cloud cover dissipates.

Why are rain drops different sizes? (7/6/2014)

Rain drops come in a variety of different sizes. Drizzle and mist are characterized by very small drops; heavy rain is compromised of large drops. Even within a single rain shower, we find a distribution of rain drop sizes, or a drop-size distribution, that runs the gamut from roughly 0.5 mm to 5 mm in diameter. Why is this? To understand drop-size distribution, we must first understand the formation of cloud and rain drops.

Obviously, precipitation starts with a cloud. Cloud drops form by the condensation of water vapor on tiny particles called condensation nuclei - these can be salt, dust, soot, etc. The nuclei come in a range of sizes, and thus cloud drops start that way, as well. Cloud drops collide and coalesce, forming larger cloud drops and widening the spectrum of cloud drop size.

Eventually, some of the cloud drops exceed 0.5 mm in diameter and become classified as rain drops. As the rain drops continue to collide and coalesce, they become too heavy to be suspended by upward motion and fall to the ground. Heavier rain typically comes from clouds with stronger upward motion, so the rain drops have spent more time colliding and coalescing, and are therefore larger.

To summarize, we see a range in rain drop size because 1) there is a spectrum of size for condensation nuclei, the building blocks for cloud and rain drops, and 2) the rate of coalescence is variable. Another consideration is that rain drops sometimes break apart as they fall to the ground.

Dry October (11/1/2013)

October 2013 was the driest October on record, dating all the way back to 1945. Only 4.17" of rain was measured at the Kwajalein Weather Station, easily breaking the previous record of 5.04" set in 1969.
Statistically, October 2013 was approximately two standard deviations below the average of 11.83".

When Will the Doldrums End? (9/1/2013)

"Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean."
- Rime of the Ancient Mariner

September is the month of Kwajalein's lowest normal wind, averaging 9 mph. However, long periods of doldrums are unusual. This year, Kwajalein has had an extended period of doldrums which began in August and has continued into mid-September. Doldrums like these last occurred in 2006, when they didn't break until the end of October. In September 2006, the mean wind speed was 5 mph, the lowest in the last 30 years.

Did someone kill an albatross? Look for the doldrums to break by the end of October.

Noon Shadows Vanish At Kwajalein in August (8/1/2013)

At Kwajalein, the sun is located on the north side of the sky during late spring and early summer.

The sun makes the transition from the north side of the sky to the south in August and will be directly overhead on two days and very close to overhead for several weeks.

Sun overhead (slightly north): 8/31/2013 - 12:50 PM Sun overhead (slightly south): 9/01/2013 - 12:49 PM

Place a can or some other object on the ground while the sun is overhead, you'll see it has no shadow.

Watch the shadow of an object on the ground between 11AM and 2PM, you'll see it vanish, and then reappear.

Quick Facts About El Nino At Kwaj (8/21/2012)

In El Nino, the sea surface temperature in the Pacific Ocean near the equator is at least 0.5 C warmer than normal.
An "El Nino Watch" from the NOAA Climate Prediction Center is now in effect. This means sea surface temperatures have become warmer than usual and an El Nino is likely to occur by Sep 2012.
The Last El Nino: Was from Jun-Jul-Aug, 2009 to Mar-Apr-May 2010. This El Nino "modoki" had little effect on us.
A Previous El Nino: From Jun-Jul-Aug, 2004 to Dec-Jan-Feb 2005 spawned Tropical Storm Talas which tracked within radar range of Kwajalein.

The Strongest El Nino since 1944: Was from Apr-May-Jun 1997 to Mar-Apr-May 1998. Typhoon Paka tracked within radar range of Kwajalein during it.

Our last four tropical cyclones (Roy 1988, Zelda 1991, Paka 1997, and Talas 2004) occurred during El Nino conditions.

Due to El Nino, at Kwaj, expect higher than normal rain in Sep, Oct, and Nov followed by a drier than normal dry season in 2013.

40th Anniversary of Kwajalein's Wettest Day - 16/17 December 1972 (12/12/2012)

Violet formed near Majuro as a Tropical Depression on Dec 12, 1972. The system moved north to Maloelap Atoll, east of Kwaj, where it became a weak Tropical Storm. By the time the storm turned west towards Kwajalein it had weakened to a Tropical Depression. Very light rain fell at Kwajalein between midnight at 6 am on Dec 16, 1972. Between 6am Dec 16 and 6am Dec 17, rain fell, the likes of which had never been seen since US Army soldiers captured the island in 1944, or has been seen since.

At total of 17.15" of rain fell in that 24 hour period. The second highest rainfall was a mere 8.82" on June 2, 1995.

Six hour rain totals at Kwajalein for December 16-17, 1972:
12am - 6am:    .08"
6am - 12pm: .74"
12pm - 6pm: 4.90"
6pm - 12am: 5.47" 11.19" on Dec 16, 1972.
12am - 6am: 6.04"
6am - 12pm: .63"
12pm - 6pm: .43"
6pm - 12am: .02" 7.12" on Dec 17, 1972.

1972 was a very strong El Nino year, and the ITCZ trough was very active and persistent over the Marshall Islands throughout the rainy season.

Hourglass Weather Balloon 2011 Index:

  • 2011-1 February Rainfall Record Shattered
  • 2011-2 March Tropical Cyclones Near Kwajalein
  • 2011-3 Kwajalein Tsunami Records
  • 2011-4 Noon Shadows Vanish
  • 2011-5 Do April Showers Bring May Flowers at Kwaj?
  • 2011-6 Have You Seen A Waterspout Lately?
  • 2011-7 Is It Going To Rain Today?
  • 2011-8 June 20-26, 2011 Is Lightning Safety Week - Lightning Myths
  • 2011-9 Kwajalein Atoll Lightning Facts
  • 2011-10 Warm Kwajalein Rains
  • 2011-11 Is A Green Flash In Your Future?
  • 2011-12 "Normal" Weather Shifts
  • 2011-13 Why Have We Had So Much Rain in 2011?
  • 2011-14 An October Warning
  • 2011-15 Strange Kwajalein Clouds
  • 2011-16 Take Cover When You Hear Thunder

February Rainfall Record Shattered

  • New February rainfall record: 11.79" / Feb 2011
  • Old February rainfall record: 10.21" / Feb 1996
  • The new record is a 15% increase.
  • Normal February Rainfall: 3.73" (1971-2000 Average)
  • Lowest February Rainfall: 0.04" / Feb 1977
  • Five wettest Februaries: 2011/11.79", 1996/10.21", 1976/9.05", 2000/8.13", 1974/7.90"

Does the new record mean 2011 will be a wet year? 50/50 chance. In years with the ten wettest Februaries, five had "above normal" annual rainfall and five were "near normal".

Why was Feb so wet? To be answered in a coming article


March Tropical Cyclones near Kwajalein (within 300 miles).

5 total since 1950. Georgia in 1951, Wilda in 1955, Rita in 1969, Freida in 1981, and Mamie in 1982.

Typhoon: 126 mph, Typhoon Georgia, March 1951 - During La Nina conditions. Georgia formed just east of Kwajalein and moved east, was at maximum intensity east of Maloelap Atoll. Georgia did not cause Tropical Storm strength winds at Kwaj.

Four Tropical Storms: 1 in La Nina, 1 in El Nino, 2 in normal conditions.
March maximum rainfall: 24.33", 1951 - due to Typhoon Georgia. March maximum wind: South at 39 mph in 1999, were thunderstorm winds, not a tropical storm.

The last March Tropical Cyclone was 29 years ago.

2011 is a La Nina year. Ocean temperatures are slightly above normal near Kwaj for the week 1-7 March.

Kwajalein Tsunami Records

21.6 inches: Mar 11, 2011; Sendai, Japan; Mag 8.9 - Rank #5
15.0 inches: May 22, 1960; Valdivia, Chile; Mag 9.5 - Rank #1
11.8 inches: Mar 9, 1957; Andreanof Is, AK; Mag 8.6 - Rank #13
9.8 inches: Nov 4, 1952; Kamchatka, USSR; Mag 9.0 - Rank #4
9.4 inches: Jul 14, 1971; Solomon Is; Mag 7.9 - Doublet
9.4 inches: Jul 26, 1971; Solomon Is; Mag 7.9 /
7.9 inches: Oct 13, 1963; Kurile Is, USSR; Mag 8.5 - Rank #16
5.9 inches: Mar 28, 1964; Prince William Sound, AK; Mag 9.2 - Rank #2

No Tsunami was detected at Kwajalein for the #3 and #6 ranked earthquakes - Indonesia 2004 and Chile 2010.

Noon Shadows Vanish At Kwajalein in April

At Kwajalein, the sun is located on the north side of the sky during late spring and early summer.

The sun makes the transition from the south side of the sky to the north in April and will be directly overhead on two days, and very close to overhead for several weeks.

Sun overhead slightly South: 4/12/2011 - 12:49 PM Sun overhead slightly North: 4/13/2011 - 12:49 PM

Place a can or some other object on the ground while the sun is overhead, you'll see it has no shadow.

Watch the shadow of an object on the ground between 11AM and 2PM, you'll see it vanish, then reappear.

Do April Showers Bring May Flowers at Kwaj?,\p> Spring rains of April are famous in the United States. But what about Kwajalein? April is a transition month at Kwaj as the dry-season ends and tropical rains start to return. The normal rainfall for April is 7.63", though 20.09" fell in 1971, and the severe drought of 1983 saw only 0.20".

In near-normal Aprils at Kwaj, much of the month's rain falls on a single day:
2005: 3.12" total, 0.96" on 4/18.
2006: 5.81" total, 2.48" on 4/2.
2008: 5.24" total, 1.55" on 4/19.
2009: 5.29" total, 1.76" on 4/9.
2010: 1.72" total, no big rain events.

2007 was a wet April with 11.06" total and five days with more than 0.80".

The increase in April rain after the dry season will usually bring flowers in May. In 2011 we've had a very wet dry-season, and April is drier than usual. What are your flowers doing? Are they begging for rain?

The return to rainy season weather brings the chance of waterspouts. Have you seen one?

There have been eight official waterspout sightings at the Kwajalein Weather Station since 2000:
Aug 28, 2000
Sept 21, 2001
Sept 28, 2001
Oct 29, 2001
June 24, 2004
Sept 9, 2006
Oct 13, 2007
Mar 25, 2008
There have been at least three other, unofficial, sightings of waterspouts on Kwajalein since March 2008.
Unlike tornadoes, waterspouts typically form in weak wind situations under moderately developed cumulus clouds, and dissipate quickly over land. There are two types of waterspouts: Kwajalein waterspouts are like "dust devils", usually weak and ephermeral, though they can become strong enough to damage small storage buildings; outside of the tropics there are tornadic waterspouts, these are tornadoes over water and are as dangerous as any other tornado, however Kwajalein never experiences them.
Well developed waterspouts may contain a small area of dangerous wind shear and should be avoided.


The probability of precipitation, or PoP, is the most confusing part of a weather forecast. What does it mean?
A "measurable" amount of rain is 0.01". The forecast is for a 12hr period. The probability is for any point in the forecast area.
So, the precipitation forecast is the statistical probability that a measurable amount of rain will fall in the forecast area during the forecast time period. The forecast is composed from two statistics, certainty that precipitation will form or move into the area, and the areal coverage of that precipitation.
If there was a 100% certainty that precipitation will exist over the forecast area and cover 10% of it, the PoP is 10%.
What does it take to get a 100% PoP? 100% certainty and 100% areal coverage.
Categories describe the PoP:
PoP Category Example
80-100% Showers, Rain "Showers today."
60- 79% Likely, Numerous "Showers likely tonight."
30- 59% Chance, Scattered "Scattered showers tomorrow."
20- 29% Widely Scattered, Slight "Widely scattered showers overnight."
10- 19% Few, Isolated, Slight "A few showers today."
0- 9% No mention of precipitation.

And, yes, it is going to rain today.

June 20-26, 2011 is Lightning Safety Week - Lightning Myths

Myth: Lightning never strikes the same place twice.
Fact: Lightning often strikes the same place repeatedly, especially when it comes to a tall, isolated object. The Empire State Building is struck nearly 100 times per year, a sailboat on the open ocean will act in the same manner to attract a lightning strike.

Myth: Rubber tires on a car, and rubber-soled shoes, protect you from lightning by insulating you from the ground.
Fact: Most cars are safe from lightning, but it is the metal roof and sides that protect you, not the rubber tires. This does not apply to golf carts. Nor does it apply to tennis shoes, as current from a nearby lightning strike can still travel through the ground and into legs and body.

Myth: If you are in a house, you are safe from lightning.
Fact: A house is a safe place to be during a thunderstorm as long as you avoid anything that conducts electricity. This means staying away from corded phones, electrical appliances, TV cables and video games, computers, plumbing, metal doors and even windows with attached A/C units. It is also advised to not lean directly on BQ or old housing walls, as brick and concrete have the ability to conduct electricity.

Myth: Structures with metal, or metal on the body, i.e. jewelry, attract lightning.
Fact: Height, pointy shape, and isolation are the dominant factors controlling where a lightning bolt will strike. The presence of metal makes no difference on where lightning strikes, so there is no need to waste time removing metal objects when lightning is close. Your time will be better spent seeking immediate shelter.

More Lightning Myths Can be found on-line at: http://stormhighway.com/lmyths.shtml

Kwajalein Atoll Lightning Facts

Average Annual Thunderstorm Days: 10
Peak Lightning Months: Sep, Oct
Average Dry Season Thunderstorm Days: 0
Number of Lightning Warnings Issued Jan to May 2011: 51
Jan: 8
Feb: 9
Apr: 10
May: 9

2011 had 25 lightning days during the "dry season".

Lightning struck the Kwajalein Hospital on 1/29/2011 and did damage. Most dangerous place in a lightning storm? The Family Pool. Why? People do not pay attention to warnings there and cannot hear the phone during swim meets.

Low lightning strike density and low population density does not mean you are safe. Don't be the first person to die in a lightning strike at Kwajalein Atoll. Find shelter during lightning warnings, especially when you hear thunder or see lightning.

June 20-26, 2011 is National Lightning Safety Week


At Kwaj, clouds warmer than freezing may create rain. Those clouds look soft, ragged, and weak, how can they make rain? Tiny cloud drops collide and occasionally stick together. After millions of collisions, a drop may grow into a rain drop.

Diameter of a rain drop: .04-.12 inches Diameter of a cloud drop: .0004-.0019 inches Number of cloud drops in a rain drop: 1-3 million Time to make warm rain: 30 minutes

Is A Green Flash In Your Future?

A few people at Kwajalein have been fortunate to see the Green Flash.

The Green Flash occurs at the first moments of sunrise or the last moments of sunset when unusual atmospheric conditions bend sunlight so that a pure emerald green color can be seen.

Usually the Green Flash is seen on coasts where the viewer can get high above the ocean so the air is warmer below them. This can't happen at Kwajalein, where you see the Green Flash at sea level. What conditions might make the Green Flash visible at Kwajalein Atoll?

The wind speed should be low, to decrease turbulent mixing of the air and give the atmosphere a better chance to form layers that can bend light. A recent light rain shower in the direction of the sun will cool the air a little and also help the air form scattering layers.

We are in a weather phase (transiting out of La Nina), which, historically, produces doldrums in July and August. You can hear a report of the current Kwajalein wind speed at 5-8161. Look for the Green Flash during doldrums (wind less that 8 knots) at sunrise and sunset, after showers have cooled the air; and remember to take a picture.

"Normal" Weather Shifts

Meteorologists redefine "Normal" weather every ten years. "Normals" are based on 30 years of measurements, which allows easy definition of "below normal", "near normal" and "above normal" weather conditions. For the past 10 years Kwajalein has used the 1971-2000 Normals, but in July, NOAA released the new 1981-2010 Normals. For Kwajalein, Normal Precipitation decreased, including a drop of more than 2 inches for May, Normal Temperature showed little change.

What caused the changes? The 1970s had several very strong El Nino episodes which caused high rainfall. Fewer weak El Ninos occurred between 2001 and 2010 to replace the 1970s events in the new normals.

Kwajalein Precipitation Normals 1981-2010 (inches)
        Jan   Feb   Mar   Apr   May   Jun   Jul   Aug    Sep   Oct   Nov    Dec  Annual
Amount 4.01 3.30 3.56 6.21 6.61 7.86 9.85 10.40 10.89 11.83 11.07 8.13 93.72
Change -1.11 -0.43 -0.26 -1.42 -2.01 -1.00 -0.39 -0.02 -0.93 +0.37 +0.33 +0.19 -6.68

Kwajalein Temperature Normals 1981-2010 (Fahrenheit)

          JAN    FEB    MAR    APR    MAY    JUN    JUL    AUG    SEP    OCT    NOV    DEC
Maximum 85.4 85.8 86.4 86.5 86.8 86.5 86.5 86.8 87.0 86.8 86.4 85.7
Average 81.7 81.9 82.3 82.5 82.8 82.5 82.3 82.5 82.6 82.5 82.3 82.0
Minimum 78.0 78.0 78.2 78.5 78.7 78.4 78.1 78.1 78.1 78.3 78.2 78.3

Why Have We Had So Much Rain in 2011?

Q: We are more that 21" above our normal rainfall for 2011. What's up with that? Is it El Nino?

A: Our wet February and March were caused by an unusual dry season weather pattern that created "shearlines" over the central RMI. That put our rainfall 17" above normal.

Since May 2011 we have been in a "neutral" or "transitional" El Nino situation, with near-normal ocean temperatures. Kwajalein often gets 20-30" of extra rain in the transitional years between La Nina to El Nino events. We were 2" below normal in April, so we've had another 6" above normal due to the transition out of La Nina.

An October Warning

October is normally Kwajalein's heavy weather month. October has our maximum normal monthly precipitation with 11.83", and it has a high occurrence of lightning warnings, west wind warnings, and warnings for winds stronger than 35 kts.

The reason for this is the sun has been nearly overhead in our region since April, and the ocean has had time to warm, so a great amount of potential energy is available for creating thunderstorms. However, this year we are returning to a weak La Nina and our ocean temperature is a little cooler than normal. In October 2010 we had 20 weather warnings across the atoll. At Kwaj, beware the ides of October!

Strange Kwajalein Clouds

Kwajalein's tropical location creates a type of cloud that is never seen in the United States, Europe or Asia. The lower boundary of the stratosphere is very high here, and the air temperature just below it is very cold, -80 degrees Celsius. Clouds that are colder than -40C are made entirely of ice and the ice crystals in them are usually hexagonal (they have six sides). However, the cold air above Kwajalein allows an unusual trigonal ice crystal to form. You can sometimes detect these clouds because they will make a bright yellow corona around the moon at night, within one moon's width of the moon.

If you see a bright yellow ring right around the moon here at night, you are witnessing something special.

Take Cover When You Hear Thunder

14 Lightning Warnings have been issued for Kwajalein or Roi-Namur since October 1, 2011.

Kwajalein has had a very active thunderstorm year in 2011 and it will continue until dry season weather appears. It is very important that people take cover during lightning warnings, especially when they hear thunder. People have remained at the pool or on the beach during these warnings, at great personal risk.

Please take cover during lightning warnings. If you hear thunder, do not stay at the pool or on the beach. Take cover in a building. Don't be the first lightning strike fatality at USAKA. It is not safe for children to play on the beach or at the pool during thunderstorms.

Current Conditions
Temp: 80°F (26.7°C)
Humidity: 74%
Dew Point: 71°F (21.7°C)
Wind: NE at 21 MPH
Pressure: 29.93" (1013 mb)
Sky: partly cloudy

Site Map | Contact Us | Copyright © 2007 Atmospheric Technology Services Company. All rights reserved.