Ask The Weather Expert!

[phw115wvwx]

A sea breeze occurs during the day when the land is warmer than the water, which will create a small low pressure over land and a small high pressure over water.  Since air wants to go from high pressure to low pressure, air will move onshore in this situation.  When a sea breeze goes past you, it will become cooler (anywhere from 5 to 15 degrees Fahrenheit) but more humid as you enter a marine air mass.  Showers and thunderstorms can develop or be enhanced on the leading edge of the sea breeze as it acts like a small frontal boundary to provide extra lift.  When any convection tries to go through the sea breeze, the more stable marine air at the lower levels eventually choke off the updrafts and suppress the storms.

At night, the entire process reverses due to the land cooling off so much to where the water is now warmer than the land.  Now, you get a small high pressure over land and a small low pressure over water, so air will move offshore in this case.  The cooling effects from the passage of the land breeze boundary would be felt over the ocean this time.  It's a lot harder to generate showers and thunderstorms on the leading edge of the land breeze due to the fact it's nighttime, but it could happen if conditions over the water are unstable.

Everything depends on having a significant temperature difference between land and ocean, or nothing will happen.  Sea breezes and land breezes can be enhanced by the prevailing flow, which would help them move further away from the shoreline.  Overall, they greatly affect your local temperature and wind patterns.  They also are important in determining thunderstorm chances during the warmer times of the year.  Thus, meteorologists have to greatly consider them when making forecasts for your area.  Look up some images on Google about lake and sea breezes, and you'll see each one is a circulation that creates the patterns I described above.  You'd need more advanced Meteorology classes to understand why it's true.  Hope this explanation helps you!

[TWCCraig]

Sorry to keep you busy in this thread, but can you explain to me what exactly is the difference between "stable" air and "unstable" air? I'm quite curious about this, I would also like to know how higher instability favors stronger thunderstorms. Thanks Pat. 

[phw115wvwx]

Stable means if you displace something that it will always come back to you.  Unstable means it will never return.  In terms of the atmosphere, we are concerned about whether pushing air upward will cause it to come right back down or continue upward.  Thus, it all comes down to density as the atmosphere is like a fluid.  Colder air is more dense than warmer air as air molecules in cold air are closer together to create more mass in a volume.  Also, if two air masses have the same temperature, dry air is more dense than moist air as water vapor is lighter than the main components of dry air, which are nitrogen and oxygen.  A Chemistry class will reveal why it's true.

When you lift a bubble of air upward, it will naturally cool at certain rates depending on if the water vapor inside has condensed or not.  Those rates are something you will learn in advanced Meteorology classes.  However, the background environment temperature drops with increasing height at varying rates in the troposphere.  As the air bubble goes up, we check to see if it has become more or less dense than the surrounding environmental air.  If we lift a cold and dry air bubble, it will become more dense than the surrounding environment, which will cause it to sink back to the ground.  This setup happens in high pressure as more air is pushing down on you than can be taken away elsewhere, and the atmosphere is deemed as stable.  Blue skies are all that you will see as you can't lift anything upward to form a cloud.

However, if we lift a warm and moist bubble of air upward, it could reach a point where the surrounding environment has cooled off more than the bubble of air.  That bubble is now less dense and will continue to rise.  Eventually, the water vapor in it will condense and form clouds and precipitation.  This setup happens in low pressure as more air is lifting away from you than can be replaced, and the atmosphere is deemed as unstable.  If the atmosphere becomes really unstable, we create strong updrafts of warm and moist air that will fuel towering cumulonimbus clouds that become our thunderstorms.  Of course, instability is just one factor that determines the risk of severe weather, but you can't have any thunderstorms if you don't have any unstable air.

There's your lesson on atmospheric stability.  How to determine the state of the atmosphere's stability and quantify it from upper-air soundings will be something you will learn once you enter your first Meteorology classes.  Hope this explanation helps you!

[TWCmatthew]

Hey Patrick, I have a couple questions about microbursts.

Does higher 0-6km speed shear (or any strong winds in the atmosphere for that matter) tend to be bad for strong microburst formation? I know that stronger winds in the mid-levels helps the storm to move faster, and I believe that shear, combined with potentially greater instability, forms stronger cells that stay "alive" a long time and don't die quick enough to produce a vigorous downdraft, but could produce otherwise straight-line winds during its mature stage.

Also, is it better to have only a smaller amount of positive CAPE for microburst formation? I know that there has to be at least some positive buoyancy and limted (or no) negative buoyancy above the LCL for convection of any nature to form. But if there's too much CAPE, can the storm strengthen and sustain life for a longer period of time, and it may not die quick enough for rapid acceleration of rain cooled air toward the ground? However, I guess it would be hard for large CAPE to be present with a lot of dry air/negative buoyancy below the LCL aka DCAPE, unless mid level lapse rates are near dry adiabatic.

Sorry if I went a little too overboard.

[phw115wvwx]

Matthew, microbursts have been reported in supercells and the beginning stages of bow echoes, which occur in highly-sheared environments.  So, the answer to your first question is actually no.  Having high wind shear can actually increase microburst formation as you can transport more momentum to the ground in a downdraft.  You can still produce vigorous downdrafts in highly-sheared environements, but it can be spread out over larger areas.  Microbursts are defined as powerful downdrafts in an area less than 2.5 miles in diameter.  Going larger than 2.5 miles in diameter would change the name to a downburst, but it's still the same physics just occurring over a larger area.

For your second question, I hope you now realize that there are two ways to make a microburst:  precipitation loading and downward momentum transport.  Microbursts can form at high CAPE values with a storm producing strong updrafts to lift moisture into a dry air layer aloft and strong downdrafts to force this rain-cooled air to the surface.  There are also microbursts that form without any precipitation as seen in dry thunderstorms, where the CAPE values could actually be quite small.  Those examples are wet and dry microbursts, respectively.  Hope this helps you, and there are lots of articles on microbursts as they are a popular research topic.

[TWCmatthew]

Thank you very much, Patrick! Great explanation.

[TWCCraig]

Hey Patrick,

You may not know the answer to this question, because it's mainly not a meteorological question, but, when I looked at today's airport observations at KISP, I noticed that the low temp was 66.2°F according the observations. However, according to the NWS, the low was rounded up to 67°F. I've always thought that the low gets rounded to the nearest degree, I don't understand why it would be rounded up in this case. The same thing also happened yesterday. Yesterday, the observations went down to 64.4°F, but the actual low was 65°F. I'm using this data from Wunderground. I noticed that on Wunderground, it has some observations between the standard 1-hour observations, those observations have the 66.2°F reading which would make the low 66°F vs. the NWS's 67°F low. I noticed that on http://w1.weather.gov/data/obhistory/KISP.html it doesn't have the observations between the hour so the low would round 67°F. Are the observations between the hour considered invalid, because the NWS doesn't seem to take into consideration the in between hour observations. 

[phw115wvwx]

Here's a really long answer, but I can perfectly explain this discrepancy as it's a weather reporting issue.   First, temperatures are always rounded to the nearest degree in Fahrenheit in the United States, but Celsius is the preferred unit worldwide.  Every temperature reading matters in determining the high and low, and your airport actually does it every second but only puts out a routine observation every hour, which is known as a METAR (Meteorological Terminal Aviation Routine) weather report.  You can rest assured that the high and low refer to the highest and lowest temperatures taken by the second each day.  Special METAR observations will also go out in between the hourly observations if changes in the weather that would threaten aviation like falling visibilities or a thunderstorm.  Weather Underground will show those special observations, but the official hourly observations are what you see on the NWS.  What you have uncovered is a flaw in Weather Underground, so let's explore why that site is wrong:

Here's what your METAR observation looked like at 4:21 AM (Z time is also UTC, which is currently 4 hours ahead of EDT, so it's 0821Z) when it was 66.2°F according to Weather Underground:

KISP 040821Z 00000KT 2SM BR BKN003 OVC014 19/19 A3010 RMK AO2

The temperature is the first line before the slash in Celsius, which I have bolded.  It was 19°C at that time.  If we convert to Fahrenheit, we get 66.2°F, which is what Weather Underground uses.  But there's a huge problem here:  That 19°C is just the temperature rounded to the nearest Celsius!  Remember, the worldwide standard is Celsius, so all airport observations reflect this fact and round to the nearest degree.  What you should consider is that 66°F = 18.9°C and 67°F = 19.4°C.  Thus, both of those temperatures in Fahrenheit round to 19°C, so it could be either one of those!  Weather Underground fails to account for this possibility.  You're looking at one of the special observations above as it was issued off the hour due to the fog lowering visibilities during that time.  Now, let's look at the normal hourly observation back at 3:56 AM (0756Z):

KISP 040756Z 00000KT 3SM BR FEW002 OVC014 19/19 A3010 RMK AO2 SLP194 T01940194

Again, you see the same temperature in Celsius, but note the numbers I bolded.  The actual temperature in Celsius without rounding is 19.4°C.  The zero out in front means that the temperature is positive in Celsius.  It would be a 1 if it was negative.  That temperature group only shows in the hourly observations.  Weather Underground sees this group and converts 19.4°C directly to 66.9°F, which rounds to 67°F.  The NWS only uses that temperature group, so it will always be correct.  So, what was Islip's low this morning?  Every six hours (00, 06, 12, and 18 UTC), we get the high and low from the past six hours.  Here's your observation at 7:56 AM (1156Z):

KISP 041156Z 00000KT 2SM BR BKN008 OVC015 20/19 A3012 RMK AO2 SLP198 T02000194 10200 20194 51005

The numbers in bold are the high and low for the past six hours.  A 1 precedes the high, and a 2 precedes the low.  The high was 20.0°C (again, 0 out in front as it's positive), and the low was 19.4°C.  Thus, the low was 67°F.  Unfortunately, Weather Underground looks at all the observations it gathered without considering the rounding issue I have explained above.  Thus, it wrongly thought the low was 18.9°C, or 66°F.  The NWS uses the six-hour groups above to determine the high and low, so it always gets the correct answer unless the weather station fails.

I hope this long explanation helps you, Craig!  The moral of the story is simple:  Look at the actual observations from the source itself.  The NWS is your best source for now, but you'll eventually learn how to read METAR reports when you take your first Meteorology classes.  There are tutorials online although it's quite complex at first until you understand the METAR code.  Once you have a grip on how it works, here's where you can look at METAR reports online by simply typing in your airport's ID (Islip, NY is KISP) and choosing how far back in time you want to go with the drop-down box (only goes back the past 36 hours):

http://aviationweather.gov/adds/metars/

[Mr. Rainman]

Excellent explanation, Patrick! 

[Zach]

Excellent explanation, Patrick! 

I certainly agree! Great job!

[TWCCraig]

Thanks a lot Patrick, very well explanation. 

[Localonthe8s]

This is such a general question and it's not directed towards any specific member here but what do you think is your outlook for severe weather this season? Will it be just an average, uneventful year with a big outbreak or two and nothing else? Will it be much like last year where activity was strong during the early spring months but waned off after May? Or how about close to 2011?

[phw115wvwx]

This is such a general question and it's not directed towards any specific member here but what do you think is your outlook for severe weather this season? Will it be just an average, uneventful year with a big outbreak or two and nothing else? Will it be much like last year where activity was strong during the early spring months but waned off after May? Or how about close to 2011?

That's something you can't predict this far out.  Seasonal forecasts are just not accurate as so many factors come into play.  Furthermore, we're not in a La Nina or El Nino event, so I really can't tell you any preference for above or below normal with severe weather.

[toxictwister00]

We'll have to see if a Nino or Nina episode makes a return by Spring and what potential effects it could have on Severe Wx Season. I'm not sure which one has the better chance of elevating the frequency of severe weather, but I'm naturally assuming it's La Nina.

[WeatherWitness]

So after the snowfall in Dallas the other day, and looking at the NWS daily weather summary for the day, does the precipitation report include snowfall?  I would think snowfall is considered precipitation, so it should be included, but I know they have a separate section for snowfall and precip.  I ask this because the snowfall report for DFW Airport was .3 inches, but the precip report was only 0.05 inches.  I read somewhere online where you need to divide by 10 to convert a snowfall total to a rainfall total, but it appears there was also rain from that day too.