Reader Q&A 11/10/16

What's the difference between a frost advisory, a freeze warning? And what's a "hard freeze"? I also thought it was to be freezing for frost to form in the first place, hence I don't get the difference.

Question from Kitty Kat

That is a great question.  Hang in there with me and i will eventually answer it but we have some background work to do. It all has to do with where the temperature is recorded.  Most official weather observations for temperature are taken at about 5 ft. above the ground.  Did you know however that the temperature right above the ground can actually be a few degrees colder than the air at 5 ft. above the ground.  It has to do with the way in which the ground loses heat to the atmosphere on clear, calm nights.  So, now to Frost Advisory and Freeze Warning. A Freeze Warning is simple, it is issued when the National Weather Service office thinks the temperature at the official recording height will get down to 32 degrees.  But you can get nights when the temperature at 5 ft. is warmer than 32 yet the temperature right above the ground is at or below freezing.  So, it could only get down to 34 officially but if the air just above the ground is at 31 then you will get ice crystals/frost develop on the grass and any tender plants could be damaged by the frost.  Finally, the growing season for a location is said to officially end when we get the first occurrence of 32 degrees.  Hope this helps…

Tom Niziol - Winter Weather Expert

How do you determine where a dry line is?

Question from Jeff Tedder

The dryline is found on the western edge of the Gulf of Mexico moist flow where and when it contrasts sharply with drier air coming in from the southwest.   There is often a shift in winds from southeast to southwest at the surface as you move westward across the dryline.  We use surface weather maps to see it, and it can often be identified in visible satellite imagery.

Greg Forbes, Severe Weather Expert

In RI,is there any specific month when we are more likely to get a hurricane/tornado?

Question from Gordon Dwane

The peak month for tornadoes in RI is August.   There could be a tropical storm or hurricane from June through October, but August and September are probably most vulnerable.

Greg Forbes, Severe Weather Expert

Why does The Weather Channel focus on East coast weather? I live in Portland, Oregon, and we have interesting and important weather here also! When the remnants of Typhoon Songda came up the West coast, you barely spent any time on it.

Question from Jennifer Berry

Coming from Portland, Oregon myself, I can attest to that! The Pacific NW does see a lot of significant and interesting weather. I don’t know if you’ve noticed, but the past few weeks, we have highlighted the Pacific NW several times. We do cover the entire nation, but we make an effort to cover as much important weather as possible. 🙂

Liana Brackett, On-Camera Meteorologist

Has New York City or New York ever had a tornado

Question from Nicholas Bradbury

There have been 3 tornadoes in history in Manhattan Borough, the most recent an F0 in 1995.   The others were from anecdotal information in 1854 and 1858.  I don’t remember if these tornadoes went right across the heart of the city.    There have been tornadoes in the other boroughs that make up the metro, including an F2 in Brooklyn in 2007.  Tornadoes are rather uncommon in the metro relative to places in the central US and South.

Greg Forbes, Severe Weather Expert

I watched a small debris cloud (20' x 30'?) coming down the road & then come onto the river in downtown Beaufort,SC , then form into a waterspout. My question is did I witness a weak tornado before it became a waterspout?

Question from Chris Tipton

it sounds that way to me, especially if it lasted more than a few seconds as a waterspout.

Greg Forbes, Severe Weather Expert

Can u have 3 tornados or 2 merge together to form one big tornado? I ask because my cousin wanted to know how torandos form and can more than one form 1 big one.

Question from Elizabeth Curtis

It is very rare for separate tornadoes to merge.  What CAN happen, though, is that as a tornado is forming it may start as several funnels in close proximity revolving about a common center.  This multi-vortex character is not uncommon while a tornado is in progress, breaking down from one wide funnel into several smaller ones rotating about the tornado center.  We call these suction vortices.

Greg Forbes, Severe Weather Expert

I was watching Mike Seidel this morning while he was in Rhode Island. I noticed there was a lot of foam lying on the beach. What conditions cause all that foam?

Question from Lisa Ihnenl

it’s from the agitation of the water by waves when the water has dissolved organic matter in it

Greg Forbes, Severe Weather Expert

How can we get a snow squall when there isn't a cloud in the sky?

Question from Christie Moody

Well, I might want to hear more about how one may have experienced that.  You can get ice crystals which form in “clear” skies if the temperature is cold enough.  We sometimes refer to those types of ice crystals as “Diamond Dust” because they glisten in any type of light.  To get an actual snow squall, a brief heavy fall of snow that reduces visibility, you need to have a source for that snow, and that could be a cloud bank that is some distance away.  The wind can actually cause the falling snow to drift a few miles downwind, but I would say the skies would not necessarily be “cloud free”.  Finally if there is snow on the ground, and it is cold enough outside so that the snow can be easily wind blown, you can get reduced visibility when strong winds pick that snow up and blow it around.  This is known as a ground blizzard and it could occur without a cloud in the sky.

Tom Niziol

What causes clear air turbulence (CAT) and how can I better identify it using my WX charts? I'm a pilot and I use ForeFlight (onboard navigational tool with WX). On one particular day, I checked my WX, including all imagery that includes Low and Max CAT charts and turbulence was no factor. But on the flight heading north from FL to MS from 4,500 to 11,500', there was CAT. Actually, that was the weekend there were a couple commercial flights effected by CAT, where a few passengers where injured. I understood there was two fronts converging; a warm and cold front. Please educate me. ;)

Question from Stacy Everitt

Clear-air turbulence (CAT) is often found in the strong vertical shear layers above and below the jet stream, associated with an upper-level frontal zone.    It isn’t simple to identify on ordinary weather maps and usually takes special analyses to deduce.

Greg Forbes, Severe Weather Expert

Mysteriously something continues to pump massive quantities of very cold cP Siberian air into the upper. levels of the troposphere,denying N America of cP air at the surface (and therefore strong cold fronts), that we normally expect to see at this time of year. Forecast models in the short term pick up on this cP air in the short term only to have this Siberian air immediately kicked upstairs and then denying N America of strong cold fronts. Exactly what it is that is kicking this Siberian air into the upper.layers of the troposphere remains a mystery.

Question from Steven Reubens

There is  a LOT of research that is looking into the role that high latitude forcing has on the overall hemispheric jet stream pattern during the winter season.  One of the foremost researchers is Dr. Judah Cohen, from M.I.T. who now works for AER.  I am no expert in this area but if you have not read his blog, this might be a great starting point for you.  It is at   I would bet however from the gist of the question, you may already know of him.  Enjoy the winter !!

Tom Niziol - WInter Weather Expert

Does all lightning come from a thunderstorm? I am a Jr. weather geek who is 10 years old and I live in Winter Springs, FL.

Question from Tyler Hedges

all natural lightning comes from thunderstorms.   There can be triggered lightning when objects travel through charged layers of the atmosphere and carry different amounts of charge into more charged layers.  For example, sometimes rockets launched upward into clouds can trigger lightning because they bring ground charge into contact with clouds charge.

Greg Forbes, Severe Weather Expert

Hi Alex! What's it like to be a meteorologist on the weather channel? I might go to college and study meterolgy..any tips?!

Question from Lisa Brazeika

Hi there!  It’s a blast and a dream come true.  You’re surrounded by the best of the best and some of the greatest experts in the field.  My advice is to take as many science and math classes as possible before college – because the meteorology major is very math/physics intensive.  Don’t be afraid to ask questions and get a tutor – the coursework can get intense and tough!  But it’s so interesting it’s totally worth it.

Alex Wilson

Hi folks. Thanks for the opportunity. Was able to get a photograph of a water spout on Lake Erie last Sunday. What are the conditions that are conducive for the formation of those. Seems like the local meteorologists talk about them possibly occurring in spring or fall only. Lake effect happens a lot more often !!!! Thanks Ms. Wilson, Mr. Bettes, Ms. Carfagno and the whole team. TWC is fantastic.

Question from Jim Bilek

for Lake Erie they would be much more common in fall when water is warm than in spring when water is cold.  It’s that warm water that helps drive instability, especially when combined with some colder air coming in aloft.  If there is some kind of shifting winds in the vicinity, the updrafts of the thunderstorms can sometimes form waterspouts.

Greg Forbes, Severe Weather Expert

What is a pressure area, and how does it work? Thank you

Question from dianne holle

Weather systems have low pressure centers, high pressure centers and intermediate values of pressure in between.  The low pressure centers occur below locations where there is less air in the column and/or warmer, lighter air.  High pressure centers have more air and/or colder, heavier air above them.   Winds that pull apart or come together high in the atmosphere can remove or add air to the column to help make the low and high pressure areas, respectively.  Near the surface, air tends to spiral counterclockwise (in the Northern  Hemisphere) toward the center of low pressure areas, with that converging air being forced to rise and create clouds and precipitation.  Air spirals outward from highs and sinks, tending to bring “fair” (clear) weather with them.

greg forbes, severe weather expert

When do you think a winter storm will hit Virginia?

Question from Taylor Myers

There is nothing in the near future that suggests a “winter storm”.  However, the next couple weeks will likely see a gradual change in the overall weather pattern in the eastern U.S. that will feature somewhat cooler air across Virginia.  The high elevations of western VA could be in store for a little snow if this forecast change in the weather pattern holds and we can get a Low to track through that cold air. Stay tuned…

Tom Niziol

Do tornados always spin in the same direction?

Question from Gerald Sullivan

99% rotate counterclockwise in the Northern Hemisphere.   Similar number but clockwise in the Southern Hemisphere.

Greg Forbes, Severe Weather Expert

What other lightning dangers are there besides the common ones?

Question from Shaun Fritts

lightning can hit and injure or kill people directly, but there can also be indirect strikes to people through their being in contact with electrical wiring, telephone lines, and plumbing even insides houses.  Lightning that is going down through a tree or through a fence can flash over and hit a person nearby, and lightning that has gone into the ground can enter a person through the feet if it has struck nearby.  Other impacts of lightning include fires, damage to equipment, etc.

Greg Forbes, Severe Weather Expert

Why does it seem that tornadoes take the same path when they hit? The recent tornado in Kokomo, Indiana took almost the same path as the one 2 years ago and I have seen this happen in recent stories after a storm.

Question from Ted Lisby

storms often move in approximately the same direction, so that sometimes they wind up with the same path.   But there are also lots of tornadoes that take tracks left or right of others and sometimes with different directions.

Greg Forbes, Severe Weather Expert

They say AVL is in a drought and short 6 " or so of Moisture .Who formulated the base Line ?? Did someone drill a hole in a granite rock and use a measuring stick - to measure how much water is in the hole ? Jist curious how they come up with " Oh we are short 3 inches or what ever "

Question from TOM EUTSLER

When the statistics refer to inches, they are referring to how many inches the year’s rainfall is below average.   So it’s just a simple subtraction.   It’s much more complex to determine whether an area is in severe or extreme drought, which involves things like how much rain is needed for agriculture and water supply needs.

greg forbes, severe weather expert

What is the dew point, and how does it affect the weather? I assume it has to do with moisture in the atmosphere, but how does it differ from the humidity figure that you folks post? Are there are some examples you could give how the same situation would be with a high dew point vs. A low dew point?

Question from John Larsen

Dew point is a measure of the actual amount of moisture in the air, whereas relative humidity (RH) is a relative amount compared to saturation.   100% RH would result in cloud/fog formation.    In warm air the dew point would have to be very high to get 100% RH, whereas it could be low in cold air.    It’s a combination of dew point and temperature that is used to compute RH.

Greg Forbes, Severe Weather Expert

I call myself a weather geek but I always questions... What exactly are ALL the ingredients for a tornado to form and how it forms. I watch TWC and WUTV daily, I have seen many explanations and illustrations but still I am curious about all the dynamics... Thanks, mattstelter

Question from Matt Stelter

It takes a combination of conditions unstable enough for strong thunderstorms to form AND favorable wind shear.  Usually that wind shear is a strong chance of speed and wind direction upward in the lowest few thousand feet above the surface.    It usually often takes a thunderstorm downdraft to bring down some of the mid-level thunderstorm rotation to the surface to trigger the tornado formation – but that aspect of tornado formation isn’t fully understood.

Greg Forbes, Severe Weather Expert


Question from JOHN GOTSCHALL

It can work the other way around.  The bow echo associated with the derecho can sometimes spawn tornadoes.   The derecho is much larger than an individual tornado.  Sometimes a big supercell thunderstorm can be the start of a growth of the bow echo and derecho.

Greg Forbes, Severe Weather Expert

Why can you see lightning before you hear it? I wanna know

Question from Payne Pham

Because light travels incredibly fast – about 186,000 miles per second – whereas sound travels much slower, about 5 miles per second.   So, you see the light from lightning almost instantaneously, whereas it takes the sound of the thunder about 5 seconds per mile to get to you.  Light and sound are different types of waves, so they travel at much different speeds.

greg forbes, severe weather expert

What does it mean when you can see the jet trails in the sky?

Question from Martha Manley

Those trails of clouds are called contrails (condensation trails).  The jet engines give off hot exhaust that is “loaded” with moisture as water vapor (along with carbon dioxide) formed as part of the fuel combustion.  That hot, moist exhaust mixes with the much-colder environmental air and cools.   If the environmental air was pretty humid and rather cold then that mixture can wind up causing the jet exhaust moisture to condense into a bunch of tiny cloud droplets to form the contrail.  If the environment was very dry then the contrail may not form or may evaporate quickly.

greg forbes, severe weather expert

Join the Discussion


  1. Why does it seem that 500 miles away in Boston from me, weather is beautiful and in Maine it is crappy. It just seems that jet stream always has to keep the nice weather south and we get cloudy, messy weather. Is their a reason this pattern seems to be persistent?