Dave's Weather Facts

What is the Weather like during the Month of August in Hampton Roads?

2010-08-03T10:55:00.002-04:00

The month of August is a very warm and humid month in Hampton Roads, with very few breaks from the humidity. One reason for the high humidity is the temperature of coastal waters. Water temperatures are usually at their highest in August and this causes more evaporation into the atmosphere. A wind off the water isn’t as refreshing in August as it is in June or early July, but it still can give some relief from an intense heat wave. Thunderstorms are quite common during August, especially during the first half of the month. As the length of daylight decreases and the sun’s intensity lessens slowly over the course of the month, the average temperature begins its decline which continues as we head towards winter. However, August can be (and usually is) a brutally uncomfortable month. Hurricane season begins to ramp up in earnest as the Cape Verde storms usually make their appearance this month. These are the long tracked tropical cyclones that develop way out in the central or eastern North Atlantic Ocean. Home grown storms are also possible during August as the water temperature off the Southeast Coast is warm enough for warm core storm development. August is the second wettest month on average for Hampton Roads. (July is the wettest month on average). This is primarily due to the heavy rainfall produced by thunderstorms and is partly due to tropical cyclones and their remnants that move through Hampton Roads in August. August is a great time to hit the beaches as the water temperatures are normally great for swimming. Of course, jelly fish can be prevalent this month due to the warm water temperatures.

What is the Weather like during the Month of July in Hampton Roads?

2010-08-03T10:55:00.001-04:00

July is the warmest month of the year and believe it or not, it is on average, the wettest month of the year. Although the summer solstice has passed and the days begin to get shorter in July, the daylight hours are very long and the intensity of the sunlight remains very high throughout the month. Humidity levels are normally moderate to high which leads to those hot and steamy days. On occasion, cooler and less humid air masses make it to Hampton Roads bringing relief from the heat and humidity, but these “polar invasions” are a rarity. Thunderstorms are very common in July due to the heat and humidity, mostly occurring during the late afternoon and evening. This is why July is the wettest month of the year on average. Thunderstorms can dump a lot of rain in a short period of time. As the water temperature warms, sea breezes become less effective in bringing relief. Also, since the water temperature is nearing its warmest point, a breeze off of the water can bring in more humidity. Tropical cyclone season in the Atlantic is slowly on the increase during this month, but most days are quiet with just a few exceptions. In the past, Hampton Roads has been affected by tropical cyclones during July, but most are just heavy rain makers. August is when things ramp up quickly and the potential threat from the tropics builds. July is probably the best month to spend your beach vacation in Virginia Beach since the weather is normally warm enough for swimming and the water temperature is normally warm enough for most of us to enjoy the surf.

What is the Weather Like During the Month of June in Hampton Roads?

2010-08-03T10:54:00.000-04:00

June begins the three month period known as “Meteorological Summer” which ends on August 31st. Since the summer solstice occurs in June, the sun is obviously at its strongest point of the year. This combined with the length of daylight can make for some very warm days. June 1st also marks the beginning of hurricane season in the Atlantic Basin. Heat, high humidity levels, and thunderstorms become common in June but every so often we get a break for a few days. Cold fronts sometimes get hung up across the region or just south of here. The reason is that the Bermuda High is normally very strong during June which basically blocks the approach of these cooler and drier air masses. Upper-level winds play a role in the stalling frontal systems. But, as I said earlier, every so often we do get relief from the heat and humidity when these air masses make it down into our part of the world. Although June is the first month of hurricane season, tropical cyclones are quite rare during this month. However, on very rare occasions, a tropical cyclone (or its remnants) does affect Hampton Roads. The normal position of the jet stream is now well to the north so low pressure systems usually pass well to the north. Every so often the jet stream does take an unusual dip down into our region bringing with it some storminess. Overall, June is a great time of year to plan your summer vacation here in Hampton Roads.

What is the Weather Like During the Month of May in Hampton Roads?

2010-08-03T10:53:00.002-04:00

The weather in the month of May in Hampton Roads varies from year to year as with any other month. The weather can be very changeable with wide swings in temperatures. The sun is very strong in May and with the lengthening daylight hours; temperatures can be quite warm (or even hot) at times. Back door cold fronts can be a common phenomenon in May bringing a sudden drop in temperatures as the wind shifts to the NE coming off the cooler ocean water. Sometimes low clouds accompany the wind shift along with some coastal fog. Thunderstorms sometimes pop up during this month, especially when frontal systems and low pressure troughs move through the Mid-Atlantic Region. Humidity levels on average slowly climb during May bringing some rather uncomfortable days. However, long term heat waves are still rare and a breeze is usually blowing to counter the heat and humidity. With the ocean water temperature still on the cool side, sea breezes can be very refreshing in May. May can, and usually does, have some of the top ten best weather days of the year for our area. On the other hand, there can be some cloudy, gloomy and even rainy days in May. All day rain events are rare in May but they do occur on occasion. All in all, May is a very good month for many outdoor activities. With some of the flowers and trees reaching full bloom, May is a very scenic month.

What is the Weather Like During the Month of April in Hampton Roads?

2010-08-03T10:53:00.001-04:00

The weather during the month of April can be just as changeable as March, but the cold doesn’t last as long, nor is it as intense as it was in March. The cold air masses moderate as they move south and east due to the increasingly strong April sun. Also, the weather patterns change which don’t bring as many arctic or polar air masses into the U.S. The average position of the jet stream is a little farther north in April than it was in March. April can begin quite chilly and winds tend to be strong during this month due to the same reasons that March is windy. (The strong sun destabilizes the atmosphere bringing the strong winds aloft down to the surface and the strong pressure systems that move across the country). With the warmer temperatures and increased humidity levels, thunderstorms make their return in April. However, thunderstorms are not anywhere near as frequent as they are in the summer. Tornado season increases in earnest during April across the Mid-West and South and every so often, severe weather occurs in Hampton Roads. Back door cold fronts become more frequent in April. These are the fronts that come down from the north and northeast turning our winds into the northeast cooling us down big time. Remember, the ocean and bays are still very chilly so when the wind shifts from off of the water, temperatures can drop quite a bit in a very short amount of time. April is usually the first month when you will hear the weathermen say that it will be cooler along the coast on certain days. Coastal fog is also more common in April due to the cool water temperatures. The fire season usually peaks in April as humidity levels can still be quite low at times and if the rainfall has been below normal, the threat of brush/forest fires can increase during windy periods. After a long and dreary winter, April is a welcome month for most folks as they can resume outdoor activities, which includes yard work, of course.

What is the Weather Like During the Month of March in Hampton Roads?

2010-08-03T10:50:00.000-04:00

The weather during the month of March is usually very changeable with wide swings in temperature. It is a transitional month as winter slowly loses its grip on the Mid-Atlantic States. Due to the increasing solar radiation, the average high temperature climbs from the low to mid 50’s at the beginning of the month to the low 60’s by the end of the month. Weather systems can be quite strong during March causing intense storm systems to develop. This is due to the extreme differences in air masses during March and the strong upper-level winds. The jet stream is normally quite strong and frequently dips far to the south across the U.S. The increasing temperatures across the south cause a very large temperature gradient since the cold air masses coming down from Canada and the Arctic can still be every bit as cold as they were during the middle of winter. This sometimes results in extreme temperature differences across the strong frontal boundaries that frequently develop. March is a windy month due to primarily two reasons. The first is the strength of the pressure systems that move across the country. The second reason is that the stronger sun destabilizes the atmosphere allowing stronger winds aloft to mix down to the surface. In other words, the sun heats the ground and then the air at the lowest layer becomes warmer than the air above it. This warmer and lighter air rises up and cooler air has to come down to replace it. This, in effect, causes the strong winds aloft to come down to the surface. This is the reason why the afternoon is normally the windiest time of day. The chance of snow rapidly decreases during March. So, all in all, March can be a volatile month, but there can be some really nice days, too.

Where Cold Fronts Come to Die

2009-08-08T19:46:00.001-04:00

During the summer months, cold fronts usually have trouble moving through our part of the world and many times they stall and dissipate over our region or just south and east of here over the coastal Carolinas. This is due to several factors including the positioning of the jet stream and the strength of the Bermuda High. Also, the cool/dry air masses and their associated high pressure areas are not that forceful at this time of year. That is why the heat and humidity usually win out in Hampton Roads. Every so often though, a cool air mass will have enough of a push to make it here bringing a brief break to the heat and humidity. When the fronts do make it out to the Gulf Stream off the Southeastern Coastline, there is that possibility that a tropical cyclone could spin up if conditions are right. If there is an unusually strong high pressure area to the north, this could increase the possibility of tropical cyclone development. Once we get into September and October, the cool air masses start to become stronger and have more push, so they often begin to make it through our area with no problem. However, it all depends on the weather pattern at the time and the direction that the upper-level winds are blowing. If they are blowing SW to NE or parallel to the cold front, then the front will most likely stall out. If the upper-level winds are blowing NW to SE, the front should be able to push off the coast with no problem. There are times when the jet stream is too far north to push the fronts through the area.

Why are nights with a clear sky sometimes colder than nights that are cloudy?

2009-01-02T07:55:00.000-05:00

Clouds act like a blanket and actually prevent the heat gained from the previous day from escaping out to space. Humidity levels throughout the atmosphere play a role, also. The lower the humidity, the more heat will radiate out into space. Other factors such as wind speed affect the temperature. On windy nights, the cold air (which is heavier than warm air) cannot collect at the surface. The wind keeps the air mixed preventing the heat from radiating into space. Now, if an arctic air mass is moving into your area, the advection of the cold air can be just as effective in bringing down temperatures to frigid levels. However, once the winds subside as the high pressure area moves in, the following night will usually be colder. Snow cover plays a role in surface temperatures as it reflects the sun’s light and heat during the day and produces ideal conditions for rapid heat loss at night.

Why does the size of snowflakes vary?

2009-01-01T09:43:00.001-05:00

Did you ever wonder why snowflakes can be large and featherlike or very small? The reasons for the variability are complex but I’ll try to give you the basic reasons. First and foremost, if temperatures are marginal to create snow, the flakes will stick together due to the flakes being wetter and more “sticky”. Also, if humidity levels are high, they will tend to be larger. Wind speeds or the speed that the flakes are falling could also be a factor as stronger winds will tend to keep the flakes from sticking to one another. Electrical conditions in the atmosphere can play a role, also.

Did You Know that Cold Air is heavier than Warm Air?

2008-07-23T02:58:00.000-04:00

Cold air is much heavier than warm air and this is the basis for much of what we call weather. Did you ever notice after taking a hot shower, that when you first open the door to the bathroom, the cooler air from outside the bathroom comes in at the lowest level? Or how about when you open your freezer, you could feel the cold air just rush downward towards the floor. Cold air in the atmosphere behaves the same way and this is why it stays separate from air that is warmer. Why is cold air heavier than warm air? Cold air is denser than warm air. The molecules are packed closer together. The amount of water vapor in the air also affects the density of the air. The more water vapor that is in the air, the less dense the air becomes. That is why cold, dry air is much heavier than warm, humid air. You may have heard that a baseball or a golf ball will travel further on a warm, humid day than it would on a cold, dry day. Since the warm, humid air is less dense, the ball travels through it with less friction. Please read my posts on cold fronts and warm fronts which describe how the different air masses interact and the weather that is caused by them.

Dave

Why the United States Ranks Number One in Tornado Frequency in the World

2008-07-22T19:42:00.004-04:00

There is no other country in the world that has more tornadoes per year than in the United States of America. There is a huge supply of cool, dry air that comes down from Canada, warm moist air is readily available to come up from the south, and dry air is sometimes another ingredient that is almost always present in the southwestern states and in Mexico. That is what makes the U.S. the leader in tornado activity. There is nowhere else in the world that you have better conditions more conducive to tornado development than in the heartland of the U.S. The lack of any mountain ranges running west to east allows the cold air from Canada and the Arctic region to flow south and the warm air is free to flow north unobstructed from the Gulf of Mexico. The way the continent extends all the way into the Arctic allows the cold air masses to build and to extend southward, without being modified quickly by large bodies of water. The Gulf of Mexico provides the warmth and the moisture for these storms. The dry air from the southwestern states and Mexico is needed to produce the so-called dry-line, which causes numerous tornadoes and severe thunderstorms. Strangely enough, the dry air that infiltrates the low pressure center actually makes tornadoes more likely in certain situations. Upper level winds are very often favorable in "tornado alley" during the late winter and spring. Winds that blow from different directions at different altitudes can cause the spin necessary to produce a tornado. Note that tornadoes can form (and usually do) during any month of the year.

Why the mornings are normally less windy than the afternoons?

2008-07-22T19:41:00.003-04:00

The reason is this. During the afternoon, the sun's heating is the strongest and this heating destabilizes the atmosphere. The sun heats the earth's surface, so the atmosphere is really heated from the bottom upwards. So, the air at the lowest level of the atmosphere is heated first. The warm air rises since warm air is lighter and less dense than the cooler air aloft. This "mixing" of the air helps to bring down stronger winds blowing at higher levels in the atmosphere. These stronger winds are blowing during the morning hours above the surface, but the air is many times too stable to allow the fast moving air to get to the surface. During the morning hours, an "inversion" is usually present which means that the lowest level of the atmosphere is cooler than the air aloft. This stable layer of cool air literally prevents the winds higher up in the atmosphere from mixing down to the surface. As soon as the sun heats the ground, this inversion begins to break down and the winds mix down to the surface. However, there are some exceptions: During stormy periods, the mornings can be just as windy as the afternoons. Also, if a strong cold front moves through your area in the early morning hours, the winds may be stronger immediately following the frontal passage and then die down in the afternoon or evening.

Why it's hotter during a drought?

2008-07-22T19:40:00.002-04:00

Did you know that the dryness actually helps to add to the higher temperatures during a drought? How? In normal conditions, some of the sun's energy is used up to evaporate moisture in the ground, vegetation and other various surfaces. However, when the ground is extremely dry, the sun's energy that is normally used to evaporate this moisture in the soil, goes towards adding heat to the atmosphere. In other words, during a drought, nearly all of the sun's energy goes to making it warm instead of drying the ground and various surfaces.

Why is March such a windy month?

2008-07-22T19:39:00.002-04:00

You may wonder why March is such a windy month compared to the other months of the year. While there are several reasons, I will highlight the major reasons. The extreme contrast in temperatures causes major storms to develop on occasion. Cold air masses can be every bit as cold as they were during the middle of the winter. The barometric pressure in the core of these air masses increases to very high levels. At the same time, the increasing sunshine over the temperate zones produces warm air masses to begin developing and moving north. The low pressure areas combined with departing high pressure areas produce the southerly winds that bring this warm air in contact with the still very cold air that dives down from the north. The pressure differences between the strong high pressure areas and the deep low pressure areas make for some windy days in March. Another reason it is windy in March is the increased atmospheric instability caused by the increasingly strong sunshine. This heats the earth's surface and since warm air is lighter than cold air, the air rises. This process results in more "mixing" of the air, which tends to bring down the stronger winds from the upper atmosphere. The very different air masses that move into the U.S. cause the huge fluctuations in temperatures making one day feel like summer and the next day feel like winter. Once we get into the month of April, the cold air masses moderate and while there could still be some large contrasts in temperatures and large intense storms, the warm air begins to start winning the battle. Note that November tends to be a windy month also, as we see the extra-tropical storms begin to strengthen and the difference in air mass temperatures increase.

Why is fog more common during the fall?

2008-07-22T19:38:00.002-04:00

Foggy mornings are much more common as we head into the autumn months and the reasons are quite simple. The nights are getting longer and longer which allows the temperatures to fall to the dew point. As the air cools, it cannot hold as much water vapor so the water vapor condenses into tiny moisture droplets that stay suspended in the air. This is what we call "fog". (See my weather fact on the dew point and why it is so important). The month of October can be especially foggy across Southeast Virginia, especially during the morning hours. October usually is the foggiest month of the year throughout most of the Mid-Atlantic region. Humidity levels can sometimes still be quite high during October. Once we get into the latter part of fall, humidity levels drop and there is a tendency for higher wind velocities. Therefore, the frequency of foggy mornings starts to decline on average. In Southeast Virginia, fog can occur at any time of year but it is far more likely in the fall. Note that during the early spring, coastal areas experience fog on occasion which is caused by the cold ocean waters cooling the air above the surface to the dew point. This phenomenon usually occurs later in the day through the mid to late morning hours, when the sun is less intense. This fog normally dissipates as it heads inland.

Why is August the most humid month of the year in Hampton Roads?

2008-07-22T19:37:00.001-04:00

Did you ever wonder why the humidity levels in August are so high? The reason is that the ocean temperature reaches its peak in August, which is usually around the 80 degree mark off of Virginia Beach. The warmer the temperature of the water is, the more water vapor evaporates from the surface of the water into the atmosphere, thus raising the humidity levels. The warm water temperature in August lessens the cooling effect of a sea breeze or a gradient wind coming from the ocean. In June, a breeze off the still cool ocean can bring much needed relief to coastal areas. However, in August, the cooling effect of a sea breeze lessens for two reasons. 1) Since the water isn't as cool as it was in June, the air temperature will not drop as much 2) Since the water is warmer and the evaporation rate is higher, the dew point will actually be higher right along the coast.

Why does a heat wave sometimes follow a cool spell in the spring and early summer?

2008-07-22T19:36:00.004-04:00

While strange but true, many a heat wave has come on the heels of a cool spell in the Mid-Atlantic Region. The reason is that the same cool, Canadian high pressure area that brings in the cool air moves off the east coast and then pumps up warm air on its back side. Remember, winds blow clockwise around high pressure areas in the northern hemisphere, so as the high settles off the east coast, winds turn southwesterly, and this brings in the warm and sometimes humid air. A very notable example of this occurred in April 1976, when the temperature reached 96 degrees on April 18th in NYC after a cool spell a week earlier.

Why does autumn have more foggy mornings in inland areas than any other season of the year?

2008-07-22T19:36:00.003-04:00

The main reason is the increasing length of darkness. This increases the time of radiational cooling thus causing the temperature to fall below the dew point. That is how fog is created. Warm air can hold more water vapor than cool air, so as the air cools overnight, the water vapor condenses into small droplets that are suspended in the air. This is what causes fog. You've probably noticed that during fall there is more dew on your vehicle than any other season. The temperature of the surface of your vehicle is normally cooler than the air, so condensation occurs on most mornings. Note that during the winter and spring, fog can be more common along coastal areas because the cool ocean waters cool the air below the dew point. This fog can blow inland on some days.

When is the Peak of Hurricane Season in the Atlantic?

2008-07-22T19:35:00.002-04:00

Based on averages since records have been kept on hurricanes, the peak of hurricane season is September 10th. On this day, the chance of there being a tropical cyclone spinning somewhere in the Atlantic Basin is the highest. Hurricane season begins June 1st and ends on November 30th. Tropical cyclones can occur outside of hurricane season, but they are quite rare.

*Image: NOAA

When is it Warmer Near the Water?

2008-07-22T19:34:00.003-04:00

On many days during the spring and summer months, you hear meteorologists say that the air temperatures will be cooler near the water. That is simply due to the fact that the water temperature is cooler than the air temperature during the spring and summer afternoons and when the wind blows from the water, it will be cooler near the shore. (See my weather fact on "Sea Breezes") However, during the fall and winter months, there are many mornings when the air temperature is warmer in the late night and early morning hours near the shoreline. That is because the water temperature is warmer than the air temperature on some occasions. This is especially true when arctic air masses move into the region. That is also why the date of the first killing frost is normally later near the Atlantic Ocean and Chesapeake Bay than it is away from the water. The reason for the temperature differences between the inland areas and the coastal region is that water takes much longer than land to heat up and to cool down. Keep in mind that the wind direction plays an important role in this process. On mornings when strong winds are blowing, there probably won't be that much of a temperature difference between inland and coastal areas.

When does the Cape Verde season begin?

2008-07-22T19:33:00.002-04:00

The Cape Verde season, which is when tropical cyclones form in the Eastern Atlantic, begins around the 15th of August. At that time, upper-level winds and sea surface temperatures usually are right for storm development. Since these storms move across a large part of the Atlantic, they can become monster storms as they move westward. However, sometimes they turn harmlessly out to sea if there is a weakness in the vast high pressure system that extends to the stormís north. Other times, they move towards the Caribbean Sea, Gulf of Mexico, or the East Coast of the U.S.

*Image owned by NOAA

What was the only Tropical Storm that had its name retired?

2008-07-22T19:32:00.002-04:00

It was* Tropical Storm Allison that flooded the Houston, TX area in June of 2001. Rain amounts were absolutely incredible with this storm. Thirty-seven (37) inches of rain fell in one hard hit area. The storm killed twenty-two (22) people as it just stalled in one area and unleashed heavy rains for days.

Katrina has been added to this list.

What is Virga?

2008-07-22T19:31:00.002-04:00

Virga is precipitation that evaporates before it hits the surface of the earth. Virga is actually very common and can occur at any time of the year. Normally, it is a result of a dry lower atmosphere. Initially, when precipitation begins to fall from the thickening clouds as a low pressure area approaches, the light precipitation will evaporate as it falls to the earth. This process does two things. It moistens up the lower atmosphere and it cools the lower atmosphere from evaporational cooling. As the low pressure area (or warm front) move closer and the precipitation gets heavier, it eventually reaches the ground. Sometimes you could actually see virga. On certain days, when there is a lot of vertical instability in the atmosphere, you will have large cumulus and cumulonimbus clouds that can produce precipitation but it never hits the ground. And due to the normally good visibilities on days like that, you could actually see the precipitation under the base of the clouds against the blue sky in the background and then it fades away as it falls to the ground. The precipitation will appear like a veil of gray hanging from the cloud.

What is the jet stream?

2008-07-22T19:30:00.004-04:00

The jet stream is a river of fast moving air high in the atmosphere. It's normally near the boundary between the troposphere and the stratosphere (tropopause). It normally blows at speeds between 110 and 140 knots. The jet stream normally is situated in the middle latitudes. During the winter months, it usually blows across the U.S., undulating back and forth. During the summer, it lifts north well into Canada. Once in a while, the jet stream does unexpected things like dive far to the south in the spring bringing unseasonably bitter cold to parts of the U.S. On rare occasions, it can go far to the north during the winter, causing mild winter weather in the U.S. The jet stream can normally be found between the tropical air masses and the polar air masses. The jet stream is responsible for causing mid-latitude storms to intensify rapidly. It can also cause spring and summer thunderstorms to grow stronger and severe. The jet stream is responsible for moving weather systems along. Jet airliners can take advantage of the jet stream to travel faster west to east such as a flight from New York to London. They try to avoid it when they are going the opposite direction.

What is the wettest month of the year on average in Norfolk, Virginia?

2008-07-22T19:30:00.003-04:00

Many people might be surprised that the wettest month of the year in Norfolk, Virginia is July, with August being a close second place. It may seem like one of the winter months is wetter since we do have plenty of damp, dreary, cloudy days during the cooler months of the year. However, it's the summer storms that drop the most rain. A thunderstorm can easily drop several inches of rain in a very short period. During the winter, the precipitation is mostly light to moderate in nature. The other significant factor is tropical cyclones and their remnants. Hurricane season runs from June 1st through November 30th. The heavy rainfall associated with those systems increases the overall averages during the summer. Even if we don't get hit directly from a tropical cyclone or its remnants, the moisture that sometimes is brought into our region from a tropical cyclone enhances rainfall amounts under certain weather conditions. It's the overall weather pattern during the summer months that brings in copious amounts of moisture from the tropical Atlantic, Gulf of Mexico, and the Caribbean Sea. The Bermuda High is largely responsible for pumping in this moisture. In July, we average over four and a half inches of rain. November and December are our driest months here in the Hampton Roads area as they average less than three inches of precipitation.