Tuesday, June 24, 2014

What in the World Cup is the Wet Bulb Globe Temperature Index?

Location of Manuas in Brazil
The World Cup match between the U.S. and Portugal this past weekend was played in Manaus, the capital city of the state of Amazonas in northern Brazil. It is located in the middle of the Amazon rainforest. It has a tropical monsoon climate, with temperatures fairly constant and of course tropical humidity.  The U.S.-Portugal game began about 6:30 p.m. local time.  The temperature at game time, about 30 minutes after sunset, was about 85°F and the dewpoint was a stifling 73°F. You may have read or heard a reference to the Wet Bulb Globe Temperature index this weekend in relation to the U.S.-Portugal game and anticipated conditions. To be honest this was the first time I had heard of this, and that sent me searching for some information.

Most of us are familiar with the Heat Index used here in the U.S. to describe the combined effects of temperature and humidity as an "equivalent temperature".  The Wet Bulb Globe Temperature index (WBGT)is a more elaborate and complex method of measuring heat stress. It measures heat stress in direct sunlight which takes into account temperature, humidity, wind speed, sun angle and solar radiation.

There are three temperature measurements that are part of the WBGT calculation. The wet-bulb temperature is measured using an exposed thermometer with its bulb covered by a cotton wick wetted by distilled water. This measures evaporative cooling which in turn is affected by wind, humidity, and solar radiation.

The second measurement is the black globe temperature. This consists of a thermometer centered in a 6-inch black globe. This measurements represents the integrated effects of solar radiation and wind.

The third measurement is the standard air temperature measured by a shielded thermometer (in a radiation screen). This represents the temperature "in the shade" and is the standard air temperature most of us are familiar with.

These three measurements are used to calculate the WBGT as follows:

Tw = wet-bulb temperature
Tg = black globe temperature
Ta = air temperature

WBGT = (0.7 × Tw) + (0.2 × Tg) + (0.1 × Ta)

Indoors, or when solar radiation is not a factor such as at night, the following formula is used:

WBGT = (0.7 x Tw) + (0.3 x Tg)

Since the formula just weights the contribution of the respective temperatures, temperatures can be in either Celsius or Fahrenheit.

The WBGT was developed in the late 1950s by the U.S Department of the Navy in response to heat stroke cases at the U.S. Marine Corps Recruit Depot at Paris Island, South Carolina. It was recommended as an international standard to measure workplace heat stress in 1989.

One reason we haven't heard much about the WBGT is that it is not easily measured. There are ways to calculate it using wind, solar radiation, temperature and humidity measurements, but wind and especially real-time solar radiation measurements aren't readily available. The Heat Index used here in the U.S. uses temperature and humidity, two easily and regularly measured parameters. However, it represents conditions "in the shade" and does not account for wind and sunshine, both of which can make a significant difference on the heat stress on the body.

WBGT values are comparatively lower than corresponding heat index values. Here is a table of comparisons between the WBGT and the Heat Index. Note how when the wind is stronger the WBGT is a little lower, accounting for the cooling effect of the wind. Solar radiation is approximated by percent of sky cover.
Comparison of WBGT and Heat Index for various weather conditions.
Credit: NWS Tulsa

When the WBGT is 80 to 85°F, working or exercising in direct sunlight will stress the human body after 45 minutes.When the WBGT is above 90°F, heat stress will occur after only 15 minutes.
There are commercially available instruments available to measure WBGT.  They run about $200 and up, a relatively small expense for the workplace or for a large sporting event such as the World Cup. The Occupational Safety and Health Administration (OSHA) has guidelines for workplace heat stress using WBGT.

Devices for measuring WBGT

Thursday, June 19, 2014

Large Tornadoes, Heavy Rain, and Nebraska Twin Tornado Update

The severe weather on Monday from Nebraska eastward to southern Wisconsin was far from the last of it this week. The day after one of the "twin tornadoes" in northeast Nebraska leveled the town of Pilger, another supercell Tuesday evening spawned at least two huge tornadoes. These were unusual in several respects. They were both "wedge" tornadoes - menacing looking storms that were perhaps a half mile wide at times. The most unusual aspect was that a second large tornado (there probably were several tornadoes generated by this cell over its four hour lifetime) was on the ground almost spinning in place for well over an hour. It first set down north of Laurel, NE in Cedar County, and moved very slowly to the southeast. Here are few radar images of the storm when this tornado was on the ground.

The base reflectivity image at 10:07 p.m. CDT (left) from the Sioux Falls, SD radar. The hook echo pattern is evident on the west side of the cell. The right image is the storm relative velocity display at 10:02 p.m. A circulation is located where the red/yellow and green/blue colors are adjacent to each other.

This is a wider view of the radar at 10:19 p.m. The hook echo is clearly evident on this image. The red box is the tornado watch that was in effect.

Storm relative velocity image at 10:51 p.m. CDT. The circulation is still evident but is weaker. The violet polygon is a tornado warning and the green polygon is a flash flood warning.

Tornado on the ground near Coleridge, NE.
Photo: Matthew Higgins via Twitter

The National Weather Service in Omaha is still in the process of completing a survey of this storm and noted that several tornado tracks were found. The good news, if you can call it that, is this storm stalled over a sparsely populated area or otherwise damage could have been much worse.  It will take a few days to determine the tracks, times, and number of tornadoes that occurred.Here is a preliminary assessment they released today.

For more information and future updates, see the Coleridge Area June 17 tornado page on the NWS Omaha/Valley, NE web site.

The NWS Weather Prediction Center put together this excellent video which shows the development of the storms on satellite, overlaying the surface weather and radar images.

Earlier in the afternoon another large tornado touched down in southeastern Montana in Carter County near the South Dakota border. This had the distinction of being the first ever EF-3 tornado in southern Montana.  What caught my eye about this storm was this spectacular wide shot by Roger Hill, a professional photographer. The tornado can be seen just to the right of the bright area on the left side of the storm.

Carter County, MT tornado on June 17. 
Photo by Roger Hill via Facebook
More information on this storm can be seen at the NWS Billings, MT web site.

On Wednesday there were additional tornadoes in South Dakota, and damaging winds from a complex of storms from southern Wisconsin to western Pennsylvania.  However, heavy rain was a major occurrence from southeastern South Dakota on Tuesday to Minnesota, Wisconsin, and northern Illinois on Wednesday and Wednesday night.  On Monday the slow-moving storms in northeast Nebraska and southeastern South Dakota produced torrential rain. Canton, SD measured 8.43 inches of rain as of 6:00 a.m. CDT Tuesday, and many more locations in southeastern South Dakota measured from 3 to more than 5 inches of rain.  24-hour rainfall amounts in southern Minnesota topped 3 inches in many areas, and totals for the past four days have exceed 6 inches in some locations. Minneapolis set a new calendar record for rainfall for any day in the month of June today (June 19) with 3.95 inches of rain as of 1:30 p.m. There was more rain after that time.

24-hour rain fall for June 18 (L) and June 19 (R) ending at 7:00 a.m. CDT
Finally, the NWS released an update on the "twin tornadoes" in northeastern Nebraska on Monday. There were five tornadoes in northeastern Nebraska.  Four tornadoes, including the one that hit Pilger, NE were rated EF-4, and the remaining tornado was rated an EF-0. A summary of these tornadoes can be seen at the NWS Omaha/Valley web site.

Monday, June 16, 2014

Twin Tornadoes in Nebraska Highlight Another Day of Severe Weather in the Plains

It has been a turbulent few days in the Central Plains. For the second time in three days severe thunderstorms have hammered the area. On June 14 numerous storms producing damaging winds and a few tornadoes swept across southern Nebraska and northern Kansas. 

Storms returned again today, but this time the focus was on eastern Nebraska across northern Iowa.

The surface weather map this morning had a warm front extending southeast from a low over northern Wyoming along the Kansas-Nebraska border and east across northern Missouri. South of the front was very warm and humid air and strong south to southwest winds aloft. The area along and north of a warm front like this is often a prime location for severe storm development.
Surface weather map at 7:00 a.m. CDT, June 16, 2014
 In anticipation of the potential for significant severe weather, the Storm Prediction Center placed an area from eastern Nebraska across northern Iowa in a Moderate Risk for severe weather today.

Convective outlook issued at 8:00 a.m. June 16, 2014
At 7:00 a.m. this morning the upper air sounding, which profiles temperature, moisture, and wind vertically through the atmosphere indicated weak instability. Below is what is called a Skew-T plot of the sounding. The temperature is the red line, dew point the green line, and the brown dashed line (generally to the right of the temperature line in this plot) indicates the ascent of a saturated parcel of air. In this plot the line indicates that the saturated parcel is warm than the environment aoound, meaning it is more bouyant and will continue to rise. The area between the brown dashed line and the temperature line represents CAPE, or the Convective Available Potential Energy, or CAPE. In basic terms, CAPE is a measure of instability in the atmosphere, expressed in Joules per kilogram of air. The higher the CAPE, the more unstable the atmosphere.  The CAPE in this sounding (based on a parcel of air rising from the surface) was 468 J/kg, indicating weak instability at that time. However, the atmospheric profile was expected to modify during the day and conditions were expected to become more unstable.

Skew-T plot for 7:00 CDT sounding from Omaha, NE
At 1:00 p.m. CDT the warm front extended from north-central Nebraska to just south of Omaha.

Surface map at 1:00 p.m. CDT June 16, 2014

The National Weather Service in Omaha launched a special extra upper air sounding at 2:00 p.m. to determine the latest atmospheric profile. As expected the atmosphere had become very unstable, with surface CAPE at 5765 J/kg, well into the extreme instability category. Note also how the winds turn from the SSE at the surface to west at 10 kilometers. This is directional shear.

Skew-T plot of 2:00 p.m. upper air sounding at Omaha, NE

At 3:10 p.m. the Storm prediction Center issued a Tornado Watch for northeastern Nebraska. This was a PDS watch (Particularly Dangerous Situation), indicating the possibility for supercells with large hail and damaging tornadoes.

A supercell developed southwest of Norfolk, NE between 2:00 and 3:00 p.m. and moved northeast. The first tornado was reported south of Norfolk about 3:40 p.m. The storm moved east of Norfolk and approached Stanton and Pilger, NE. Two distinct and separate tornadoes were on the ground about a mile apart (according to the National Weather Service). Here is the radar image at about the time the tornado struck Pilger, NE.

Radar image at 4:04 P.m. CDT from Omaha, NE. The left image is the base reflectivity, and the image on the right is the Storm Relative Velocity image. The location of the tornado circulation can be seen where the blue color is adjacent to the red.
Below are some images of the tornadoes, including a brief video (last).

June 16, 2014 twin tornadoes in northeast Nebraska.
Photo credit: Tony Laubach/Facebook
Twin tornadoes in Nebraska.
Screen capture of video from StormChasingVideo.com/Facebook
View of twin tornadoes from Wisner, NE.
Photo credit: Taryn Vanderford on Twitter

One of the tornadoes destroyed 50 to 75 percent of Pilger according to the county sheriff.  As of the time of this post there has been one fatality reported and more than 16 people injured, many of them critically. Tornadoes also caused damage in Cuming and Wayne counties.

Here is the map of storm reports for June 16 as of  8:50 p.m. CDT.

Wednesday, June 4, 2014

The Climate of Puerto Rico

On June 1st we welcomed the Commonwealth of Puerto Rico to the CoCoRaHS community, expanding the reach of CoCoRaHs from the tropics of the Pacific (Hawaii) to the tropics of the western Atlantic and eastern Caribbean.

Somewhat rectangular in shape, Puerto Rico is the smallest and most eastern of the Greater Antilles Islands. Puerto Rico's mountainous terrain comprises about 60 percent of its 3,425 square miles. Surrounding the mountainous interior, where the largest peak is Cerro La Punta at 4,390 feet, there are coastal lowlands which extend in to the interior 8 to 12 miles in the north and 2 to 8 miles in the south.  Most of the major population centers are located along the coasts, and so it's likely that most of the CoCoRaHS observers will be located along the coasts as well.

If you place an island with significant terrain in a tropical ocean you should expect quite a variation in precipitation across that terrain. Puerto Rico is no exception.  Average annual rainfall ranges from less than 30 inches to more than 170 inches. Within that range, there is a lot of variation with the highest amounts found in the higher elevations and lowest amounts along the southern coast.

There is no "dry" season in Puerto Rico, although February to April tends to be when rainfall is the lowest. During the fall season westerly moving tropical waves bring showers and thunderstorms to the island, and of course it is also hurricane season.  Puerto Rico has had many brushes with tropical cyclones, but the last hurricane to cause major damage to the island was Hurricane Georges in September 1998. In 2011 Tropical Storm Emily produced torrential rains over the island causing flooding and landslides.

The wettest location on the island is Pico Del Este, a mountain peak on the eastern end of Puerto Rico at an elevation of 3,408 feet in the Caribbean National Forest. The average monthly rainfall ranges from 10.27 in March to 18.61 inches in November, with an annual average of 171.06 inches.  The driest location is Magueyes Island, located just a stone's throw off of the southwest coast. Average annual precipitation here is 29.24 inches, ranging from 1.14 inches in March to 4.58 inches in September.

Location of Pico del Estes (left) and Magueyes Island (right)

Temperatures do not vary much from month to month and generally range between 70°F and 90°F. In San Juan, for example, the average maximum is 83.1°F in both January and February, rising to 88.9°F in September.  Lows range from 70.7°F in January 76.8°F in August. In the mountains lows range from the upper 50s to low 60s and highs from the mid 60s to low 70s.  The warmest weather is typically found along the southern coast up and around the east coast to the northeast coast near San Juan.

Temperature means and extremes for San Juan.

Puerto Rico is on Atlantic Standard Time and does not observe Daylight Savings Time.
An excellent web site to learn more about Puerto Rico,from its history and culture to geography and economy is Welcome to Puerto Rico!

More information about the climate of Puerto Rico can be found at the Southeast Regional Climate Center website