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Is Erika a Threat to Florida?

After bringing gusty winds and flooding rains to portions of the Lesser Antilles, Virgin Islands, and Puerto Rico, Erika is passing south of Hispaniola this afternoon. As of 2pm EDT, Erika was centered just off the south coast of the Dominican Republic, and still stubbornly heading westward at 16 mph.

Satellite loop of Tropical Storm Erika from Midday August 28, 2015.

Erika remains weak, with maximum sustained winds near 50 mph. Strong wind shear and its proximity to the mountainous terrain of the Dominican Republic are keepING the storm weak.

For several days, computer models have been forecasting Erika to turn toward the west-northwest and eventually northwest, around a ridge of high pressure anchored over the central Atlantic Ocean. Erika has defied the models, and continues moving toward the west. As Erika nears the western edge of the high pressure ridge, and a trough of low pressure enters the Gulf of Mexico, Erika should finally start to turn more towards the northwest over the next 24 hours. Eventually, a turn toward the north should happen, bringing Erika closer to the state of Florida. Since Erika has traveled much farther to the west than anticipated, the risk to the Southeast has been greatly diminished. The main question now is – what part of Florida is at the greatest risk?

Computer model forecasts for Tropical Storm Erika's future track. — Computer model forecasts for Tropical Storm Erika’s future track.

Most of the models are now showing a threat to the Gulf Coast of Florida. However, most of these models also show Erika spending a lot of time over parts of Hispaniola and/or Cuba. Tropical systems depend on warm water (over 26 degrees C) to maintain their strength. A track over land could result in the dissipation of Erika before it even reaches Florida. Of the models that do keep the circulation of Erika intact, most of them keep it at tropical storm strength. As a result, it appears as though the biggest threat to Florida would be heavy rainfall and flooding. This is both good and bad, depending on which part of Florida you are talking about. Much of South Florida is in a severe drought, so rainfall would be welcome across the area. Meanwhile, the west coast of Florida, especially near Tampa Bay, has seen severe flooding and record rainfall this summer, so additional heavy rain would not be welcomed.

Erika has also produced flooding across portions of the eastern Caribbean. Over 12 inches of rain was reported in Dominica, with devastating flooding resulting in at least 12 deaths on the island. Across Puerto Rico and the Virgin Islands, which have also been in a severe drought this year, rainfall totals were mainly under 2 inches. Erika did produce strong winds across the region, however. Some of the highest reported winds were as follows:

Location	Peak Sustained Wind	Peak Wind Gust
St. Croix, US Virgin Islands	38 mph	62 mph
Sint Maarten, Netherlands Antilles	36 mph	52 mph
St. Thomas, US Virgin Islands	33 mph	48 mph
Santo Domingo, Dominican Republic	25 mph	43 mph
Guadeloupe	25 mph	40 mph
Punta Cana, Dominican Republic	25 mph	40 mph

Elsewhere in the Atlantic, a tropical wave will move off the coast of Africa this weekend, and conditions could be favorable for it to slowly develop as it makes its way across the ocean.

The Pacific, especially the Central Pacific, remains active. Tropical Storm Kilo is centered about 160 miles west-northwest of Johnston Island, moving toward the west at 10 mph. Maximum sustained winds are near 70 mph, and additional strengthening is expected. A tropical storm warning is in effect for Johnston Island, but conditions should start to improve over the next 12-24 hours as Kilo pulls away. Kilo is expected to turn more towards the northwest and strengthen into a hurricane as it heads across open waters towards the International Date Line.

Hurricane Ignacio is centered about 840 miles east-southeast of Hilo, Hawaii, moving toward the west-northwest at 10 mph. Maximum sustained winds are near 90 mph, and some additional strengthening is expected over the next 24-36 hours. After that, Ignacio should start to weaken and continue west-northwestward. The current track brings Ignacio just north of the Hawaiian Islands early next week. Residents of Hawaii should monitor Ignacio’s progress this weekend.

Satellite photo and forecast track for Hurricane Ignacio.

In the Eastern Pacific, Hurricane Jimena is centered about 1075 miles southwest of Cabo San Lucas, Mexico, moving toward the west at 12 mph. Maximum sustained winds are near 90 mph, and Jimena could strengthen into a major hurricane over the next 24-48 hours. Jimena will remain over open waters, and is no threat to any land areas.

Erika Remains Weak in the Caribbean, Is Ignacio a Threat to Hawaii?

Tropical Storm Erika crossed the Leeward Islands early Thursday, and remains a weak system in the northeastern Caribbean Sea.

The center of Tropical Storm Erika passes in between Antigua and Guadeloupe around daybreak on Thursday. On Antigua, sustained winds of 23 mph with gusts as high as 39 mph were reported, while Guadeloupe reported sustained winds as high as 25 mph with gusts to 40 mph. On Saint Martin, a sustained wind of 31 mph with a gust to 47 mph was reported this afternoon.

The bigger story, however, has been the rainfall. In Dominica, nearly 13 inches of rain has been reported, leading to widespread severe flooding across the island.

The rain should slowly end across the island by tonight, but the cleanup will take considerably longer.

As for Erika’s future, it remains a weak tropical storm that should pass just south of the Virgin Islands tonight, then head towards Puerto Rico and Hispaniola. The exact track that it takes near these islands will have a huge impact on its future. The longer it stays over the islands, the weaker it will be, and it could even dissipate completely. If it were to pass north of Hispaniola, it could survive, and then start to become better organized as it heads towards the Bahamas. Upper-level conditions should become much more favorable for Erika if it can survive the journey into the Bahamas. Most of the forecast models show Erika strengthening into a hurricane over the Bahamas, then turning northwestward or northward (see image below). By late this weekend, assuming Erika survives, it could be a significant threat to Florida and the Southeast. Residents from Florida to North Carolina, as well as the Bahamas, should keep a close eye on Erika’s progress.

Model forecasts for the track of Tropical Storm Erika.

Meanwhile, in the Pacific, Tropical Storm Jimena is located about 900 miles southwest of Cabo San Lucas, Mexico, moving toward the west-northwest at 17 mph. Jimena has top winds near 45 mph, and is expected to steadily strengthen into a hurricane over the next 3-5 days while moving across the open waters of the Eastern Pacific.

The bigger threat is Hurricane Ignacio. Ignacio is centered about 1135 miles east-southeast of Hilo, Hawaii, moving toward the west-northwest at 13 mph. Ignacio has top winds near 90, and is expected to continue strengthening over the next 24-36 hours, before a weakening trend begins. The current forecast brings Ignacio just north of Hawaii by early next week. While several storms have threatened Hawaii this season, none have actually had any impacts on the state as of yet. Despite the good luck, residents of Hawaii and those will plans to head there over the next several days should keep a close eye on Ignacio’s progress.

Forecast for track for Hurricane Ignacio as of Thursday afternoon. — Forecast for track for Hurricane Ignacio as of midday Thursday August 27, 2015.

Elsewhere, the tropical remain quiet, which is good news, especially in the Western Pacific, where numerous storms have brought widespread damage and flooding to portions of the Philippines, Taiwan, China, and Japan over the past few weeks.

Where Will Erika Go? How Strong Will Erika Get?

As Tropical Storm Erika continues to approach the Lesser Antilles, the questions remain the same as they were a few days ago: Where will Erika go and how strong will it get?

The first question is somewhat easier to answer in the storm term. As of 2pm EDT on Wednesday, Erika was centered about 245 miles east of Antigua, moving toward the west at 17 mph. It seems fairly likely that Erika will impact the islands of the northeastern Caribbean. As a result, Tropical Storm Warnings remain in effect from Puerto Rico and the Virgin Islands eastward to Montserrat, and a Tropical Storm Watch is in effect for Guadeloupe. A warning means that tropical storm conditions are likely within the next 36 hours, while a watch means that tropical storm conditions are possible within the next 48 hours.

How strong Erika will be when crossing the islands is still a big question. Currently, Erika has winds of 45 mph. The official forecast from the National Hurricane Center, is for Erika to maintain its current strength or strengthen slightly over the next 48 hours. However, there are some indications that it may actually weaken before then.

Satellite loop of Tropical Storm Erika from Wednesday afternoon August 26, 2015

In the last few frames of the satellite picture, if you look closely, you can see the lower-level circulation of Erika pulling away from the western edge of the thunderstorm activity. This is a tell-tale sign that wind shear is still impacting the storm. For Erika to strengthen, the wind shear will need to lighten up, and allow the thunderstorm activity to redevelop near the center of the storm. Until that occurs, Erika will not gain strength. Some of the computer models are showing the possibility that Erika degenerates into a tropical wave as it nears or passes the Leeward Islands, similar to Tropical Storm Danny a few days ago. This would bring some much-needed rainfall to the drought-stricken region, without the sustained strong winds that normally accompany a tropical storm.

In the longer term, things become a bit murkier in terms of the track and strength of Erika. It seems fairly likely that what’s left of Erika will continue on a west to west-northwest track and could start to strengthen (or redevelop, if Erika dissipates), as it nears the Bahamas this weekend. At this point, there is no consensus as to what will happen.

Forecast track from the National Hurricane Center as of 2pm on August 26.

The National Hurricane Center forecast (pictured above) brings the storm into South Florida as a weak hurricane early Monday. However, on the various forecast models, the solutions range from a track into the Eastern Gulf of Mexico to one the keeps it near or east of the Bahamas and then out to sea. There are some that show a potential threat to Florida or the Southeast, but since this is still at least 5-7 days away, the odds are fairly low at this point. Still, residents of the region should keep an eye on Erika, as it could have some impact early next week.

In addition to the track questions, there are plenty of questions as to how strong Erika will get. Erika should remain a weak tropical storm for the next 48-72 hours (unless it dissipates into a tropical wave). Beyond that, the uncertainty becomes large. Many of the forecast models keep it a tropical storm, while some strengthen it into a hurricane, in some cases a very powerful hurricane (see image below). Assuming it survives the journey toward the Bahamas, conditions should be favorable for Erika to start to strengthen towards the end of the week. The track it ultimately takes will determine if it is able to get there.

Intensity forecast from various models for Tropical Storm Erika as of 8am EDT August 26, 2015.

If Erika does make landfall in Florida as a hurricane, it will be the first storm to do so since Hurricane Wilma did on October 24, 2005, nearly 10 years ago. In the hurricane database that goes back to 1851, the 10-year drought for Florida is the longest on record. The previous record was 6 years, between 1979 and 1985. While some may argue that the lack of landfalls is a good thing, in many ways, it’s also bad. Reports have indicated that over a million people have moved to Florida since 2005, and the vast majority of those people have likely never experienced a hurricane, let alone a major hurricane. They may not know how to prepare for a storm that could leave them without power for a week or more. They may not know what a mandatory evacuation means or if they are in a mandatory evacuation zone. They may not know where the nearest shelter is located. Now would be a good time for residents of the region to brush up on this, in case Erika does threaten the region this weekend.

8-26-2015 2:41pm EDT

Severe Weather in the Northeast and Another Tropical Threat for the Caribbean?

A strong cold front will move across the Northeast on Tuesday, producing showers and thunderstorms across the region, and bringing much drier air into the region.

Showers and a few thunderstorms are expected across the region in the morning. If enough clearing develops after these move out, the atmosphere could destabilize enough to trigger another round of showers and thunderstorms in the afternoon, and some of these could become strong to severe. The Storm Prediction Center is forecasting a marginal risk of severe storms across much of central and southern New England for Tuesday. The main threats with any storms that develop will be heavy rainfall, strong winds, and frequent lightning.

Behind the front, much drier air will settle in, after more than a week with dewpoints of 70 degrees or higher. Temperatures will remain seasonably warm though, unlike areas to the west, where much cooler conditions were noted behind the front. In Rapid City, South Dakota, the low temperature dropped to 41 degrees Monday morning, breaking the old record of 43 degrees, set in 1972. Cheyenne, Wyoming dropped to 39 degrees Monday morning, which also broke a record. The old record low was 40, set 119 years ago in 1896. Several locations in Montana dropped below freezing Sunday morning, with frost reported across parts of the area.

Meanwhile, the tropics remain active. Tropical Storm Erika has formed about 955 miles east of the Leeward Islands. Erika has maximum sustained winds near 45 mph, and is moving toward the west at 20 mph. Erika is expected to slowly strengthen over the next 24-48 hours while turning a bit more towards the west-northwest. The current forecast brings Erika very close to the Northern Leeward Islands Wednesday night or early Thursday. Watches will likely be issued for some of the islands early Tuesday. Beyond that, there is plenty of uncertainty as to where Erika will head and how strong it will be. Some of the models have Erika dissipate over the eastern Caribbean, similar to what happened to Danny, over the next few days, others strengthen it into a hurricane and send it towards the Bahamas.

Forecast track for Tropical Storm Erika as of 11pm EDT August 24, 2015

Since Danny fizzled while it was crossing the Leeward Islands, it brought only a few showers to the Virgin Islands. This region has been in a severe drought for the past several months, and it was hoped that Danny would bring some much -needed rain to the region to help put a dent in the drought. St. Croix, USVI has received only 7.31 inches of rain in 2015, a little over 12 inches below the normal total of 12.33 inches. Things haven’t been much better in San Juan, Puerto Rico, where 21.8 inches of rain have fallen this year, 10.38 inches below the normal for the year-to-date.

Meanwhile, in the Eastern Pacific, Tropical Depression 12-E formed late Monday. The depression was centered about 1535 miles west-southwest of the southern tip of Baja California with top winds near 35 mph. The depression was drifting toward the west at 3 mph, but should start to turn more towards the west-northwest and pick up speed over the next few days. The system should strengthen into a tropical storm over the next day or so and head out over the open waters of the Pacific, not threatening any land areas. Another tropical wave south of Mexico could develop into a tropical depression later this week as it moves across open waters.

Forecast track for Tropical Depression 12-E as of 8pm PDT August 24, 2015.

In the Central Pacific, Tropical Depression Kilo has top winds near 30 mph and is centered about 150 miles east of Johnston Island. It is expected to drift toward the north then turn toward the west over the next few days with some slow strengthening expected. Kilo is not expected to impact any land areas for the next few days. Meanwhile, Hurricane Loke is located over 1000 miles west-northwest of the main Hawaiian Islands, crossing the Pearl and Hermes Atoll with top winds near 75 mph. Loke is expected to turn towards the north-northwest and head out over the open waters of the North Pacific while losing tropical characteristics over the next few days.

In the Western Pacific, Typhoon Goni has crossed southwestern Japan and is moving into the Sea of Japan Tuesday afternoon. Goni still has top winds near 95 mph, but is expected to steadily weaken as it crosses the Sea of Japan and heads toward far eastern Russia. Goni produced a wind gust to 102 mph in Makuazaki when it made landfall on Monday. On Sunday, Goni crossed the Ryukyu islands, southwest of the Japanese mainland. As the storm passed directly over Ishigakijima, it produced a wind gust to 159 mph.

Weather is Always Happening

Tropical Storm Danny is the main weather story for today. However, after registering a category 3 rating for winds in a previous hurricane hunter recon mission, it is only a tropical storm and continues to decrease in strength. This is evidenced by the storm’s central pressure rising to just above 1000 mb this afternoon, significantly higher than 2 days ago. While the forecast track is still a little uncertain, the current forecast from the National Hurricane Center (NHC) is for the storm to take a track to the west, just south of Puerto Rico overnight Monday. This is just after a recent track change from toward the northwest to toward the west. The storm will continue to weaken and should be a tropical depression after crossing the Leeward Islands according to NHC’s forecast. Currently the NHC has posted Tropical Storm Warnings for Antigua, Barbuda, Montserrat, St. Kitts, Nevis and Anguilla and Tropical Storm Watches for Saba and St. Eustatius, St. Maarten, Puerto Rico, Vieques, Culebra, and the US Virgin Islands.

[Image of 5-day forecast of predicted track, and coastal areas under a warning or a watch]

The storm itself is about 165 miles east-southeast of Guadeloupe and 210 miles east-southeast of Antigua. The maximum sustained winds within the storm are 40 MPH and will continue moving west at 16 MPH. Threats for this storm will be for rain from 2 to 4 inches of rain through Tuesday and wind up to tropical storm force in warned areas. A tropical storm watch means that tropical storm conditions are possible within the next 48 hours. However, a tropical storm warning means that tropical storm conditions are expected within the next 36 hours and action to prepare for for the storm should be taken.

The main problem with this storm, along with dry air nearby, has been that there has been a strong field of shear that has eroded the southwestern side of any strong thunderstorms.

Other systems are tagged by the NHC in the Atlantic Ocean closer to Africa. The one with the best chance of developing is located 800 miles west of the Cape Verde Islands and has a chance for development into a tropical depression toward mid-week. Whether it has any impact on land is yet to be determined. This system continues to move to the west at 20 MPH.

A storm just southeast of the East Coast is also tagged by the NHC with a possibility of development and is currently affecting portions of the East Coast with cloud cover and humidity at the very least. Showers and a few storms are moving inland to the US from the storm itself as well. This storm has 0% chance of development.

In addition to the tropical weather, severe weather is possible today as the front that produced severe weather over the Northern Plains yesterday moves over Michigan and portions of the Ohio River Valley today. A tornado or severe thunderstorm watch has not been issued over this area, however, strong winds, a tornado and hail are all possible with today’s storms. It must be said that SPC’s chances of the above threats over Michigan and portions of the Ohio River Valley are much lower than yesterday’s.

Also, afternoon showers and storms with the threat of strong winds may occur over portions of Southeastern US. Similar to yesterday, strong winds follow the front and will reside around an adjacent low pressure system as hinted at by wind advisories over portions of the Northern Plains.

In addition, dry weather continues over portions of the Western United States and Alaska as forest fires continue to burn. Larger fires are in California, Washington, Idaho, Oregon and Montana. This year, according to the National Interagency Coordination Center, 7,448,217 acres are burning, which is above the year-to-date 10 year average of 5,350,800 acres. Much of this has been caused by warm and dry weather over the Western US along with thunderstorms that have a lack of rain, but plenty of fire-sparking lightning.

(As of August 23 2015)

Severe Weather in the Plains, Tropical Storm Watches in the Caribbean

A strong cold front will help to trigger strong to severe thunderstorms across portions of the Plains States and Mississippi Valley today. Showers and thunderstorms are already moving across parts of the North Dakota and Minnesota, with a few reports of hail up to one inch in diameter. As the front marches eastward, conditions will become more favorable for severe thunderstorms from the Upper Mississippi Valley into the Central and Southern Plains. The main threats are large hail, wind gusts to 70 mph or more and tornadoes.

Severe Weather Outlook for August 22, 2015 from the Storm Prediction Center.

Behind the front, strong northwest winds will usher unseasonably cool air into the Northern and Central Rockies and western portions of the Plains states. Wind advisories and high wind warnings are in effect for much of the region as wind gusts of up to 50-60 mph are likely today and Sunday. Some record low temperatures are possible Sunday morning in parts of Montana, Wyoming, and western portions of the Dakotas and Nebraska as low temperatures are expected to tumble into the 30s and lower 40s. Frost advisories have been issued for parts of western Montana.

Meanwhile, Hurricane Danny is starting to steadily weaken while approaching the Caribbean. As early early Saturday afternoon Danny was centered a little more than 600 miles east of the Leeward Islands, moving toward the west-northwest at 12 mph. After peaking as a Category 3 hurricane on Friday with winds near 115 mph, dry air and increasing wind shear have weakened Danny to a Category 1 with top winds near 85 mph, and additional weakening is expected. Tropical Storm Watches have been issued for much of the Northeastern Caribbean, including the islands of Antigua, Anguilla, Barbuda, Montserrat, St. Kitts, Nevis, Saba, St. Eustasius, St. Maarten, Guadeloupe, St. Barthelemy, and St. Martin. A tropical storm watch means that tropical storm conditions are possible within the next 48 hours.

Current forecast track for Hurricane Danny

Danny is expected to weaken into a tropical storm before crossing the islands. It should continue to weaken as it moves towards the US Virgin Islands and Puerto Rico. Some of the forecast models are showing the possibility that it could dissipate as it nears Puerto Rico. Others keep it as a very weak storm which then moves near Hispaniola and into the Bahamas, where conditions could become more favorable for it to start to strengthen again.

While the future track and strength are still very uncertain, one thing that is more likely is that Danny will bring beneficial rainfall to portions of the US Virgin Islands and Puerto Rico. The region has been in the midst of a severe drought for much of the year, with less than half of the normal yearly rainfall in some areas. While Danny won’t bring drought-busting rainfall to the region, every little bit that falls is beneficial.

Elsewhere in the Atlantic, a weak area of low pressure about 150-200 miles north-northwest of Bermuda will likely head out to sea over the next few days. There is a slight chance it could acquire tropical characteristics and become a tropical storm, but this seems unlikely. Much farther to the southeast, a tropical wave passing south of the Cape Verde Islands is producing disorganized shower and thunderstorm activity. As it crosses the Atlantic over the next several days, conditions should become more favorable for the system to develop. Yet another tropical wave will move off the Coast of Africa this weekend. This system could also start to get better organized as it crosses the Atlantic over the next week.

In the Central Pacific, both Kilo and Loke weakened to Tropical Depressions overnight, but they should both re-strengthen into tropical storms over the next day or two. Loke should remain a tropical storm while heading northward, possibly impacting Midway Island early next week. Kilo is the more immediate threat. Kilo should head northwestward while slowly strengthening over the next 24 hours. Sometime after that, a turn toward the north and eventually northeast is expected. Exactly where the storm makes the turn is critical to determining if there is a threat to parts of Hawaii. Right now it looks like there will only be a threaat to some of the westernmost islands, but if it turns earlier, that will change things. Conditions are favorable for Kilo to become a hurricane early next week.

Current satellite phot and expected track of Tropical Depression Kilo from the Central Pacific Hurricane Center.

Farther west, there are still two typhoons in the Western Pacific Ocean. Typhoon Atsani is expected to steadily weaken while heading out to sea well south and east of Japan. Tyhpoon Goni will bring gusty winds and heavy rain to parts of Taiwan today, before setting its sights on Okinawa and southwestern Japan tomorrow and Monday.

Winter Redux? Record Heat in the West, Damp and Chilly in the East

For much of this past winter, the weather pattern across the United States featured a ridge of high pressure across the West, and a trough of low pressure across the East. As a result of this, most of the West had a record-warm and very dry winter, while the East, especially the Northeast, suffered through record cold and snowfall. As we head into the final weekend of June, that pattern is returning, at least for a little while, across the county.

Upper level forecast for Sunday evening.

Temperatures have soared well over 100 for the past few weeks across the Southwest and into interior California, as is typical for summer. That heat will spread northward this weekend, with temperatures rising well above the century mark as far north as interior portions of Oregon and Washington, as well as parts of Idaho, western Montana, and even southern Canada. Across the deserts of eastern Washington and Oregon and Western Idaho, some readings may exceed 110 degrees. The heat will even extend west of the Cascades, with highs rising into the 90s for Seattle and Portland. Numerous records are likely to be broken across the Northwest. Temperatures may drop a few degrees as we head into the start of next, but the heat will remain in place for the next 7-10 days at least.

While the West bakes this weekend, an usually strong storm system for late June will bring gusty winds, heavy rain, and cool temperatures to the Northeast. The storm system will cross the Ohio Valley tonight and Saturday, and across Southern New England Saturday night and Sunday. To the south and east of the storm and it’s associated warm front, strong to severe thunderstorms are expected. The threat of severe weather shifts from the Ohio and Tennessee Valleys today into the Mid-Atlantic states and the Carolinas on Saturday. Damaging winds, hail, torrential downpours and a few tornadoes are possible in these areas as thunderstorms move through.

To the north, heavy rain and unseasonably cool conditions are expected from the Great Lakes into New England. Rainfall totals of 1-3 inches are expected across much of the region, with some isolated heavier amounts possible, especially across the Great Lakes. Flood watches are in effect for much of the region. While too much rain can be a bad thing, portions of this region are still in a drought after a very dry spring, so the rainfall will be welcome in some areas, especially the Northeast.

With the cloud cover and rain, temperatures will be as much as 15 to 25 degrees below normal this weekend. High temperatures will stay in the 60s across much of New York and Pennsylvania on Saturday, with some spots staying in the upper 50s all day. The chilly air will shift into central and northern New England on Sunday. Across southern New England, highs on Sunday will depend on the exact track of the storm system and how far north the warm front is able to get. A track south of Long Island will keep temperatures in the upper 50s and 60s for places like Boston and Providence on Sunday, where as a track right across southern New England could allow temperatures to rise into the 70s.

Drier and more seasonable conditions will return to the Northeast on Monday as high pressure builds in behind the departing storm system.

Relative Humidity vs Dewpoint – Which is More Informative?

We’ve all heard people talking about a hot and humid day by saying that it was “90 degrees with 90% humidity outside”. The truth is, that except for a few places in the Middle East, it is nearly impossible to have that combination of heat and humidity, especially in the United States. To show why this can’t happen, we first will define what “relative humidity” is.

The air in our atmosphere can only hold a certain amount of water vapor. The warmer the temperature is, the more water vapor that the air can hold. Relative humidity is just the ratio between the amount of water vapor currently in the air compared to the maximum amount of water vapor that the air can hold. When the relative humidity is near 100%, you will usually have precipitation (rain or snow) or fog. A more accurate representation of how humid the air is can be found by using the dewpoint. The dewpoint is simply the temperature you’d have to cool off to so that dew can form. The higher the dewpoint, the more humid the air. humidity

When you combine the effects of the temperature and the dewpoint, you get the “Heat Index”, also known as the “Apparent Temperature”.

heatindex

As you can see from the chart above when the temperature is 85 with a dewpoint of 75, the heat index is 94. This is roughly the same as a temperature if 95 with a dewpoint of 60. If the first case it’s warm and very humid, while in the second case its hot, but not nearly as humid. A dewpoint above 80 degrees is fairly rare in the United States, only occurring near the Gulf Coast (due to the warm waters of the Gulf nearby), and occasionally near some of the cornfields of the Midwest, as moisture evaporates from the fields. Our initial example of “90 degrees with 90 percent humidity” would equate to a dewpoint of 86, a figure that has only been observed a couple of times, and usually close to the Red Sea or the Persian Gulf, where water temperatures can reach the 90s in the middle of summer. More likely, that humid day is 90 degrees with a dewpoint in the middle 70s, which is around 60% relative humidity.

The higher the heat index, the more you should limit your exposure outside and take necessary precuations. If you do need to be outside, make sure you drink plenty of liquids, wear sunscreen (SPF 15 or higher) and stay in the shade as much as possible. Prolonged exposure can lead to severe sunburn, heat exhaustion, or even heat stroke. You should also never leave children or pets inside a closed vehicle for even a few minutes.