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Frequently Asked Weather Questions - WCAX.COM Local Vermont News, Weather and Sports-

Frequently Asked Weather Questions
 

Q: What is dew point? --Gary's Answer:

A: The technical definition of the dew point (or dew point temperature) - is that it is temperature at which water vapor in the air will condense into liquid form if the air temperature lowers to match that dew point temperature. The dew point can tell us many useful things about the weather.

Water vapor is a gas, and is one of several gases that make up the atmosphere, with nitrogen and oxygen making up the bulk of it. The water vapor content in the air can vary, depending on the weather conditions. The dew point is an indication of just how much moisture is in the air. If the dew point is high (like in the 60s or 70s), then that indicates a lot of water vapor in the air. If the dew point is low (like in the 40s or lower), then that indicates less water vapor, hence the air is drier.

The dew point is either lower or the same as the air temperature. It is never higher. When the air temperature comes down to the dew point, the water vapor will condense into tiny water droplets. That is how clouds form!

With weather balloons, we can measure the air temperature and the dew point at any level in the atmosphere. Where the air temperature is the same as the dew point, that will tell us at what level the clouds are. If that happens right near the ground, then a cloud will form right on the ground - and that's called fog! So, knowing the air temperature and the dew point can tell us if fog will form.

Since the air temperature cannot go lower than the dew point, then the dew point is often a good indicator of what the low temperature will be. If the dew point is below the freezing mark (32°), that can indicate the possibility of frost, which is important to farmers and gardeners in the spring and fall.

The difference between the air temperature and the dew point can tell us what the relative humidity is. A complicated formula is used to figure it out exactly, but when the air temperature and the dew point are the same, then the relative humidity is 100%. And that means fog.

The lower the relative humidity is, the more drying power the air has. This can be a good indicator of brush fire danger, if the RH (relative humidity) is low for a long period of time.

The dew point can also be used to measure human comfort in the summer. When the dew point is in the 60s or higher, there is so much water vapor in the air that it is difficult for our bodies to stay dry. Our natural cooling process through perspiration does not work very well with high dew points. When the dew point is in the 50s or lower, the air is more comfortable. But if it gets too low, then that can indicate very dry air, which can lead to dry skin and scratchy throats.

And of course, the dew point indicates when dew will form on the grass (or frost, when it is cold enough). [TOP]


Q: What is the difference between sleet and freezing rain? --Sharon's Answer:

A: That's a good question. It helps to remember that the precipitation has a long ways to travel, from the cloud to the ground, and there can be layers of warmer and colder air between the ground and the cloud, which changes the form the precipitation will be in.

Even rain usually starts off as snow when it falls from the clouds, but it melts as it runs into warmer air, and falls as raindrops if it is warm all the way through to the ground.

Freezing rain occurs when the snow coming out of the cloud falls through a warm layer, melts into rain, but the ground itself is colder than 32 degrees. Then the rain freezes on contact and forms a layer of ice.

Sleet forms when the layer of colder air near the ground is a little thicker, ...The rain drops have a chance to freeze into ice pellets BEFORE they hit the ground, and they bounce around a bit, instead of forming a smooth layer of ice.

When it is snowing....the air is generally colder than 32 degrees from the cloud, all the way to the ground, so the snow never melts, and it never has to refreeze.

But it can be warmer than 32 degrees, and the snowflake can still survive.

This can happen if the air is dry enough. The snowflake falls into the warmer air, and they begin to melt. However, if the air is dry, the water quickly evaporates, cooling the surrounding air, and slowing the melting of the snowflake.

That is why on some occasions, it can be warmer than 32 degrees, and we will still find some big fat wet snowflakes that survive and build up on the ground. [TOP]


Q: Can it hail in the winter - is hail the same as sleet? --Gary's Answer:

A: Hail usually falls during severe thunderstorms in the summer. Rain drops are pulled back up high into the atmosphere because of powerful thunderstorm updrafts. The upper atmosphere is always very cold, even in the summer. So, when the rain drop gets high enough in the thunderstorm cloud, it freezes into a ball of ice. Then it falls again, with part of the ice ball melting. It can then be pulled back up high into the cloud, freezing again, and becoming slightly larger. The hailstone can go through this cycle several times before it gets heavy enough to finally fall to the ground as a hail stone.

Thunderstorms are rare in the winter, but they CAN happen. But usually, the thunderstorm clouds are not as tall as they are in the summer, so it is less likely that the hail process can form. But it IS possible to hail in the winter - just not very likely. But balls of ice often fall in the winter. These are actually ice pellets, or sleet. It is different than hail because it does not go through the hail forming process. Sleet happens when snow from a cloud falls through a warmer layer in the atmosphere, then melts into a raindrop, but then falls through a colder layer again, freezing it into an ice pellet.

So, if you think that it is hailing in the winter, it is most likely sleet, not hail. [TOP]


Q: What are Heating Degree Days? --Gary's Answer:

A: Heating Degree Day (or HDD) is a unit that helps us understand how much energy must be used to heat (or cool) our homes to a comfortable level. It is assumed that 65° Fahrenheit is a comfortable indoor temperature. So, if the outdoor temperature is colder than 65°, then you will need to use some energy to heat your house up to that 65° level. The number of HDD will indicate how much energy you will need to use.

The formula for determining HDD is simply subtracting the average outdoor temperature for the day from 65. It looks like this: HDD=65-T, where T indicates the average outdoor temperature. T is determined by simply taking the high temperature for the day, adding it on to the low temperature for the day, and dividing by 2. It looks like this: T=(high temperature + low temperature)/2.

The higher the HDD, the more heat it took to heat your home to a comfortable level. The lower the number, the less heat.

You can tell if you are using more or less energy during the winter by checking the number of HDD used, and comparing it with average HDD levels from years past.

HDD is very important to fuel companies, who must provide heating fuel, and be prepared by ordering more, or less, fuel depending on how much energy is being expended based on the HDD numbers.

The summertime equivalent of HDD is CDD - Cooling Degree Days! It follows the same idea as HDD, except you subtract 65 from the outdoor temperature, instead of the other way around. [TOP]


Q: How can I find out what the weather was on . . . . . .? --Gary's Answer:

A: There is a wealth of past weather information on the website of the Burlington office of the National Weather Service. Go here: http://www.erh.noaa.gov/btv/index.php.

From the menu on the left of the page, click on "Local" under "Climate." Under "Observed Weather Reports," click on "Preliminary Climatology Data" under "1. Product." Then pick the town you want under "2. Location." Click on "Archived Data" and then the month and year you want (going back about 5 years). Then hit "GO" under "4. View." [TOP]


Q: I'm getting married in June (or October, or March, etc.) . . . what's the weather going to be? --Gary's Answer:

A: That's a tough one! Forecasts are usually very accurate for one or two days in advance. Accuracy starts to drop off a bit by Days 3 through 5. And long range forecasts, going out a month or more, are more general in nature, just giving indications of general temperature and precipitation trends. So, it is almost impossible to give a specific forecast for a specific date in the far future. But just remember, no matter what the weather, your wedding day will always be terrific! [TOP]


Q: What are those little rainbows on either side of the sun, and what are those rings around the sun/moon? --Sharon's Answer:

A: Many of us are lucky enough to see what are often described as beautiful splashes of light, or rainbow types of displays in the sky! But....no rain, so what are they?

These are halos, and sundogs! The halos are more common, and occur when there is a very thin sheet of cirrostratus clouds. This layer is very thin, and the sun shines through them, but the ice crystals, which make up the cirrostratus clouds, bend the sun light at a specific angle which makes the halo effect. You can see this around the moon sometimes too.

Sun dogs are those brighter spots on each side of the halo. We see these when the sun is fairly low in the sky, and there must also be some of those high thin clouds which contain ice crystals that the light can refract off of. For sundogs, they have to be just the right kind of ice crystals, ....6 sided and shaped sort of like short pencils.

When those crystals float straight up and down in the air, it refracts, or bends the light in just the right way to cause a sundog. Sundogs do tend to be brightest in the winter, because that is when this type of ice crystal is most common, ...But they can be seen any time of year.

And if you see a sun pillar,....which almost looks like a search light beam coming up from the sun...those are caused by ice crystals that look like tiny 6 sided flying saucers. [TOP]


Q: What are the differences between winter weather advisories, winter storm warnings, and winter storm watches? --Sharon's Answer:

A: Watches and warnings are issued by the National Weather Service. These are some definitions that you should become familiar with:

Winter Storm Warning: Issued when hazardous winter weather in the form of heavy snow, heavy freezing rain, or heavy sleet is imminent or occurring. Winter Storm Warnings are usually issued 12 to 24 hours before the event is expected to begin.

Winter Storm Watch: Alerts the public to the possibility of a blizzard, heavy snow, heavy freezing rain, or heavy sleet. Winter Storm Watches are usually issued 12 to 48 hours before the beginning of a Winter Storm.

Blizzard Warning: Issued for sustained or gusty winds of 35 mph or more, and falling or blowing snow creating visibilities at or below ¼ mile; these conditions should persist for at least three hours.

Lake Effect Snow Warning: Issued when heavy lake effect snow is imminent or occurring.

Lake Effect Snow Advisory: Issued when accumulation of lake effect snow will cause significant inconvenience.

Wind Chill Warning: Issued when wind chill temperatures are expected to be hazardous to life within several minutes of exposure.

Wind Chill Advisory: Issued when wind chill temperatures are expected to be a significant inconvenience to life with prolonged exposure, and, if caution is not exercised, could lead to hazardous exposure.

Winter Weather Advisories: Issued for accumulations of snow, freezing rain, freezing drizzle, and sleet which will cause significant inconveniences and, if caution is not exercised, could lead to life-threatening situations. [TOP]


Q: What is the difference between snow flurries and snow showers? --Sharon's Answer:

A: Snow Flurries: Light snow falling for short durations. No accumulation or light dusting is all that is expected.

Snow Showers: Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

Snow Squalls: Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant.

Blowing Snow: Wind-driven snow that reduces visibility and causes significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind. [TOP]


Q: Why does it snow more in the mountains? --Gary's Answer:

A: Clouds are formed when air rises, and then cools as it gains altitude. When the air temperature cools down to the dew point, the water vapor in the air will condense and form a cloud. And a cloud can drop rain or snow.

There are a few ways to get air to rise. An approaching cold front can push the air upwards. That is why it usually rains or snows when a cold front comes through. Also, air that is pushing together near the ground, called "low-level convergenge," has nowhere to go but up. And also, when the air is from the right direction, it can hit a mountainside, and the mountain itself forces the air to rise. This is called "orographic lift." So, the mountains can create their own weather by forcing air upwards, which then form clouds, which then drops rain or snow. Yet it can be dry in the valleys at the same time. [TOP]


Q: How come you never mention my town? You always focus on Burlington. --Dan's Answer:

A: We really do try to reach everyone in all parts of our region. We've been doing the "Around the Region" forecast, which airs in the first ten minutes of the 6pm show, as well as during the 11pm, morning and weekend broadcasts.

In that section, we divide the region up into four sections, each covering a different part of northern New England. Our viewers in the Northeast Kingdom, the Adirondacks, northern New Hampshire and southern Quebec all wish we would spend more time covering their regions, as well as our viewers in southern VT and NH.

Since our weathercasts are only about three minutes long, we do our best to highlight any active weather going on, as well the complete forecast for the next few days. [TOP]


Q: Why do leaves turn in the fall? -- Dan's Answer:

A: The fall colors of yellow, orange, and red are actually always in the leaves, even during the spring and summer. But during the spring and summer, the leaves are full of chlorophyll, the chemical that converts sunlight into energy for the trees. Chlorophyll is the dominant pigment during the warm months and keeps the leaves looking green, hiding the other colors. But as the daylight hours shorten in the fall, and the colder weather arrives, then the production of chlorophyll decreases, and the yellow, orange and red pigments underneath are revealed.

Some years, the foliage is more brilliant than in other years. Often, this is due to the weather. The yellows and oranges are produced by carotenes, and the darker reds are produced by anthocyanin, which is only present in certain trees under certain conditions. The best weather conditions during the foliage season for the brightest colors are sunny days, with a daytime temperature above 50°, and nighttime temperatures in the 30s and low 40s. [TOP]


Q: How often does Lake Champlain freeze? Every year? --Sharon's Answer:

A: Lake Champlain does not freeze over every year, although back in the 1800's it froze over almost every year. The last 10 years, it has frozen over 5 times, in 1996, 2001, 2003, 2004 and 2005. It's not likely to freeze over this year! [TOP]


Q: When is does Lake Champlain most often flood? --Sharon's Answer:

A: The typical cycle of Lake Champlain has the lowest level of water in the fall, in September or October, when the average level is 95.5 feet above sea level. Then there is a slow rise to 97' in early March, followed by a rapid rise to high water in April or May, at 100.5 feet, and then a slow fall through the summer to the low level in the fall.

But as we all know, ....the weather is often anything but typical around here! With the mild winter we've had, and frequent rains, the level of Lake Champlain has been quite high all winter, and near flood stage in late November and in late January. Does that mean we are guaranteed spring flooding? Not necessarily. That will depend on whether we have a gradual warm up into spring in April and March, with little rain, or whether we will see more heavy rain and a quick warm up to melt the remaining snow in the mountains very quickly. Heavy rains and a quick warm up could spell trouble along the lakeshore. [TOP]


Q: When is maple season - what is the best weather for the best flow? --Sharon's Answer:

A: Town Meeting day is traditionally the time to get the first runs of the season in, and the season usually lasts about a month. But any seasoned maple producer will tell you, every season is a little different. The weather that triggers the best sap runs are those beautiful late winter days, with bright sunshine and temperatures in the 40s, with crisp, chilly nights with temperatures in the 20s. [TOP]


Q: What is the difference between partly cloudy, partly sunny? --Gary's Answer:

A: When there isn't a cloud in the sky, it's sunny! When clouds cover the whole sky, it's cloudy! But what about all those in betweens? We have different terms to describe different stages of cloud cover in the sky.

When there are just a few clouds, it's Mostly Sunny (or Mostly Clear at night).

When there are more than just a few clouds, but still more in the way of clear skies, then it's Partly Cloudy.

When the clouds become more numerous than the sunny areas, then it Partly Sunny.

When there are clouds almost everywhere, but still a few, small sunny breaks, then it's Mostly Cloudy.

Notice that Partly Cloudy indicates sunnier conditions than Partly Sunny! So, even though the word, "cloudy" is in "Partly Cloudy" and the word, "sunny" is in "Partly Sunny," it does not mean that Partly Sunny is sunnier than Partly Cloudy. It's just the other way around.

Confused? It's just a matter of getting used to.[TOP]


Q: What is an Alberta Clipper? --Dan's Answer:

A: An Alberta Clipper is an area of low pressure that usually forms over the Canadian province of Alberta. During the winter, these weather systems will pass through the Great Lakes and New England regions. These storms are usually quick moving, and can drop some generally light snow. Cold air can follow the passage of an Alberta Clipper, but temperatures will usually moderate in a day or two. An Alberta Clipper may sometimes dip south across the Mid Atlantic region, and develop off the eastern coast of the United States. This could turn the storm into a Nor' Easter. [TOP]


Q: What is a Nor'easter? Why is it called that? --Dan's Answer:

A: A Nor' Easter is an area of low pressure that forms in the southeastern United States, or along the East Coast. These storms move north along the coast where they will usually interact with colder air moving south and east out of Canada. The two ingredients combine to create a winter storm that can drop several inches of snow in the northeast and New England states. The track of the low pressure is what usually determines how much snow we get in northern New England. If the storm is too far inland, the snow may mix with rain or sleet, and if the storm is too far off the coast, it could miss us completely. When the storm track is just right, we could see some of the heaviest snowfall totals the winter season can bring. [TOP]


Q: Can it thunderstorm in winter? --Dave's Answer:

A: Yes, thunderstorms can occur any month of the year anywhere in the United States. However, they are most frequent during the summer months. [TOP]


Q: Do we get tornadoes in Vermont? --Dave's Answer:

A: Vermont does get tornadoes, although they're relatively rare. The average number of tornadoes per year for Vermont is closer to zero than 1, and the tornadoes that do occur tend to be much weaker than the ones out in the Midwest. Even so, 34 tornadoes were reported in the state between 1950 and 1999. [TOP]


Q: Do we get hurricanes in Vermont? --Dave's Answer:

A: Hurricanes (with sustained winds 74 mph or greater) rarely reach as far inland as Vermont, though it has happened. Generally, a hurricane will weaken to a tropical storm or depression before it reaches Vermont. Vermont did receive damaging hurricane-force winds from the Hurricane of 1938. [TOP]


Q: Why are some thunderstorms tinted green? Does it indicate severe weather? --Dave's Answer:

A: Green thunderstorms do exist, but there's no concrete evidence as to why they're associated with severe weather, hail or tornadoes, or why they have the green color in the first place. One theory is that more massive thunderstorm clouds (cumulonimbus) will scatter more sunlight, giving a darker bluish color. If this occurs late in the afternoon when the western sky reddens during the setting of the sun, then the light scatters such that it appears green to the viewer's eye. Another theory is that green thunderstorms contain large hailstones, and those stones refract light differently than raindrops. Though some green thunderstorms are indeed severe, there's no proof that all green thunderstorms are severe, or that all severe thunderstorms are green. [TOP]


Q: I've heard that "fog eats snow." Is there an explanation for this? --Dave's Answer:

A: While fog doesn't literally eat snow or make it vanish before your eyes, it is true that the presence of fog will make the snow melt faster. Of course, the above-freezing air temperature will contribute to snowmelt. But when fog forms, condensation is taking place. The process of condensation releases energy in the form of heat, which gets released into the air. This added heat will increase the rate that the snow melts. Also, the water droplets from the fog itself will melt the snow to a certain extent. [TOP]


Q: How do you determine the time of sunrise and sunset? Due to topography, the sunrise today at my house was 7:10. --Sharon's Answer:

A: The sunrise and sunset times you see on the air on Vermont's Own Channel 3 News, are for Burlington, and can be found on charts at http://aa.usno.navy.mil/faq/docs/RST_defs.html#top

However, it's not surprising that you are noticing that the sunrise and sunset at your location is different. Sunrise and sunset are different for different latitudes. For example, today, sunset will be at 5:25 PM in Newport, but it is 5:32 PM in Bennington.

How much different can vary depending on the time of year as well. Even atmospheric conditions can make the appearance of sunrise or sunset seem off by a minute or more. Mountains in the distance, and the height of the observer clearly effect that even more.

Sunrise and sunset refer to the times when the sun would make an appearance on the horizon at sea level, where there is no obstruction of your view. Keep in mind, that the sun appears as a circular disk, not as a point of light. Sunrise refers to the time when the upper edge of the disc of the sun breaks across the horizon, and sunset also, when the upper edge of the disc slips back below the horizon.[TOP]

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