Question: How do meteorologists figure out the wind chill and heat index? Does the wind speed have anything to do with the figuring?

Answer:  When you venture outside on a cold, windy day or a hot, humid day, you may notice that it feels colder or warmer than the temperature WeatherBug is showing in your tool bar.  This is because the human body’s perception of temperature changes with increased wind or humidity.

For example, in the winter, wind removes heat from the body (especially through the head) making us feel colder.  On the contrary, on a sunny winter day when the wind is calm, the temperature may feel 10-15 degrees warmer than it really is since the calm air forms an insulating layer around your skin. 

In the summer, the wind can still cool us down as it helps evaporate our sweat faster, but humidity can make the temperature feel much warmer.  The combination of temperature and relative humidity creates a “feels like” temperature called heat index. 

Both heat index and wind chill are calculated by mathematical formulas – there is no weather sensor that measures these parameters.  The National Weather Service has developed convenient charts for both indices to save us all from having to manually calculate the feels like temperatures. 

You can find the heat index and wind chill temperatures on your WeatherBug when you click the “More Observations” link in the “So Far Today” box near the lower left-hand side of your main WeatherBug screen.  When the outdoor temperature is 65 degrees or warmer, WeatherBug will calculate heat index.  When the outdoor temperature is 64 degrees or cooler, it will show wind chill. 

Question: What was the weather like on July 4, 1776?

Answer: Back in the days before weather stations automatically recorded daily weather conditions, people used to keep weather journals.  One of the most famous weather observers in the early days of our country was a man by the name of Thomas Jefferson.  Yes, the same Thomas Jefferson who was one of the signers of the Declaration of Independence on July 4, 1776!

Jefferson’s journal shows that on July 4, 1776, Philadelphia was 68°F at 6am, 72.25°F at 9am, 76°F at 1pm and 73.5°F at 9pm.  From the weather journal of Mr. Phineas Pemberton, a prominent Philadelphian, we also learned that the sky clouded up in the afternoon as winds shifted from the north to the southwest and the barometric pressure decreased. 

With a high temperature almost 11 degrees cooler than normal, I’m sure at least one of our Founding Fathers mentioned that the nice weather capped off this historic day! 

Question: Right before a tornado or hail storm I have noticed the sky and clouds being an eerie “green”.  What causes this?

Answer: I saw this same phenomenon with a severe thunderstorm near McCook, Nebraska during the WeatherBug Storm Chase this spring.  The green sky was definitely eerie and made the storm look very ominous.  In fact, this particular storm did drop large hailstones and a funnel cloud.

To answer this question, we need to remember what we learned about the colors of the rainbow and the spectrum of visible light in school.  Visible light is made up of 7 colors – red, orange, yellow, green, blue, indigo and violet - which can be remembered by the phrase ROY G BIV. 

As objects come into contact with sunlight, different colors are absorbed and reflected due to the different wavelengths of each color (reds have long wavelengths, while blues and violets have short wavelengths).  If an object absorbs all colors, it appears black.  If an object absorbs no colors, it appears white.  The sky appears blue because dust particles and atmospheric gasses scatter back only blue light. 

There are two theories about why the sky turns green during some severe thunderstorms.  The first is that during thunderstorms around sunset, the blue hue from the sky gets mixed with the red hue from the setting sun which can also create a greenish color. 

The second theory, which I personally subscribe to, is that the hailstones, which are made of ice, scatter green light.  To create large hailstones which can scatter lots of green light, a thunderstorm needs to have strong updrafts.  Strong updrafts are also necessary for tornado development.  Since hail can cause severe damage and injury, it’s best to seek shelter immediately if you see green skies with a thunderstorm.

Question: Is there such a thing as heat lightning? 

Answer: This week, we celebrate the official beginning of summer, as well as Lightning Awareness Week.  Summer is the peak time for lightning which is the second most frequent weather related killer in the US after floods.

Heat lightning is indeed a form of lightning, but instead of a bolt, it appears as a reddish-colored glow that illuminates the sky and seems to have no thunder associated with it.  The lack of thunder is due to the large distance between the strike and the observer.  If you were near the source of the lightning strike, you would see the familiar bluish-white bolt and hear its crack of thunder.

Heat lightning gets its name because it typically occurs on warm, humid summer nights.  Typically, the sky directly overhead the observer is clear adding to the illusion that the hot air is generating its own lightning.

Question: How likely is it that Florida will be hit by a hurricane this year?

Answer: After last year’s recording-breaking hurricane season where four hurricanes struck Florida in just six weeks, the big question on everyone’s mind is what’s going to happen this year. 

Unfortunately, both the National Weather Service (NWS) and Dr. William Gray (a professor at Colorado State University and renowned hurricane forecaster) are predicting another above-normal hurricane season for the Atlantic.  The consensus is that there will be 12-15 tropical storms, with 7-9 becoming hurricanes, and 3-5 of these becoming major hurricanes.  Dr. Gray indicates that if El Nino conditions don’t develop, those numbers may need to be forecasted upwards.

In addition to monitoring the status of El Nino to forecast tropical storm development, Dr. Gray’s team analyzes sea surface temperatures, pressure and winds from around the globe, as well as matches the weather patterns from this pre-hurricane season with past years since hindsight has proven to be a strong indicator. 

And while Dr. Gray and the NWS are constantly improving their forecasts of the number and type of storms the Atlantic may encounter, there is still no proven model that forecasts what paths these storms are going to follow.  We can only hope that Floridians will be spared this year so they can continue to focus on repairing their homes and businesses from last year’s devastation.

Hurricane season by name begins on June 1 and ends on November 30, with the peak months for tropical storm development in the Atlantic being August through October. 

Question: What is dew point temperature? 

Answer: This is by far the most popular question I am asked by WeatherBug users.  Dew point is the temperature to which the air must be cooled to condense into either water droplets (dew) or ice crystals (frost) as long as barometric pressure and amount of moisture in the air remain constant.  Technically, when the dew point is below 32 degrees, the correct terminology is “frost point”.

Dew point is not measured by a weather sensor.  Instead it is calculated based on the air temperature and humidity.  When the air temperature and the dew point temperature are within five degrees, the air is considered saturated and the humidity will be near 100%.  Dew/frost is likely to occur at this point especially if the wind is calm.  When the air temperature and dew point are further apart, the air is drier and less humid. 

Additionally, whereas air temperature (and humidity) will rise and fall in response to heating from the sun, dew point remains relatively steady and will never be higher than the air temperature.  This makes it a great tool for predicting the next morning’s low temperature.  Try it the next time you are under a consistent weather pattern like a high pressure system that brings clear, calm weather.

You can find the definition of dew point and many more weather terms in the WeatherBug Glossary.  To access it, first click on the “Weather Fun” button on the left-hand side of your WeatherBug, then go to the “Trivia & Facts” tab.  The Weather Glossary is the second link on the list at right.