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Ask Matt a Weather Question

Meteorologist Matt Baylow answers your weather questions. Click here to send him one.



What is "MicroClimate" forecasting? And why is it so important?
While the San Diego Chamber of Commerce has convinced the rest of the country that it's always sunny and 72F degrees here, you don't have to live in San Diego too long to know that's not the case! The term "MicroClimate" in MicroClimate forecasting refers to the fact that while it may be cloudy and 60F at the beach, just a few miles inland at Clairemont it might be sunny and ten degrees warmer. . .closer to 70F. And in the East County, it could be hazy and more like 85F. Yet a few miles further, up in the mountains, it's only 53F degrees. So the "one forecast fits all" approach to forecasting used by many other big cities just doesn't work in San Diego.


Isn't the weather pretty boring here? Doesn't it stay the same all the time?
San Diego has really two times during the year that, typically, we don't get a lot of weather. There's about a month around April, and another month around October. The rest of the year, however, it's anything BUT boring.

San Diego's "rainy season" is in full swing during the winter months. We get about 90% of our yearly rainfall during the five months from November through March. Tornadoes are rare, but do occur (11/10/00, Poway) and hail, small in diameter, can be enough to cover the ground for hours--and make it look like snow (2/27/01, Scripps Ranch)!

"El Nino" and "La Nina" weather patterns affect San Diego's long term weather pattern more than just about anything else. . .and can cause quite a lot of damage along the coastline when storms, strong surf, and high tides combine.

"May gray" and "June gloom" are terms used to refer to the constant low cloud cover along the beaches in late spring and early summer. If you live in Mira Mesa or La Mesa, for example, it can be sunny and warm on a summer day. . .but if you head to the beach you'll find it's overcast and cold. So even on a rather uneventful day, the weather isn't the same from MicroClimate to MicroClimate.


But how hard can it be to forecast the weather here?
San Diego doesn't have the extreme kinds of weather that other locations around the country have; we don't see blizzards in the wintertime, or supercell thunderstorms in the summer. It's never below zero here, and big temperature variations from day to day are rare.

But there are weather conditions that are difficult to forecast. Most notably: the low clouds known as "June gloom." When it's sunny inland, and warm on a summer day, every San Diegan wants to know "Which beach will be sunny--and when?" Yet the clearing of those low clouds is the hardest thing to predict. Another example would be winter storms. When winter storms move down the coastline, they tend to weaken quite a bit--and slow down. It took less than one winter for me to figure out that if you delay the onset of a winter storm by about 12 to 18 hours, and cut the forecasted precipitation amount in half, you're usually a lot more accurate. Coastal eddys that "pop up out of nowhere" can mess up a forecast, too. The fact that San Diego is bounded by an ocean also makes some forecasting difficult, as weather stations to our west are, well, rather sparse.


Why does it get so foggy at night?
First of all, fog is simply a cloud that is sitting on the ground. There are two types of fog, and we get both of them in San Diego--the first is radiation fog, and the second is advection fog. Both are the same thing--fog--but they're named differently because of how that fog forms. Fog is caused when the air temperature reaches the dewpoint temperature; and the dewpoint temperature is the temperature of the atmosphere at which water condenses. For example, if the dewpoint temperature is 65F degrees, everything that's colder than 65F degrees, like a can of pop out of the fridge, will have water condense on it.

Radiation fog develops when heat at the Earth's surface radiates out into space. This usually happens at night, especially in the summer time. The dewpoint temperature doesn't change much during the day or night--so when the air temp is warm (say, 75F) and the dewpoint is, say, 65F during the day, there's no fog. But at night, when the air temp cools down to 65F, which is the same as the dewpoint temp in this example, voila! They're the same temperature (in this case, 65F), and you've got fog.

Advection fog develops when moisture is advected into the forecast area, or "moved" by winds. This usually happens along the coastline, when coastal eddys pump up moisture into the beach communities. In this case, the atmosphere has so much moisture pumped into it that it can't hold any more of it--and so it releases the excess moisture in the form of condensation--which is the fog.


What's the difference between the dewpoint temperature and relative humidity?
The dewpoint temperature is an absolute measure of moisture in the atmosphere (if it's 65F, then everything colder than 65F will have water condense on it). Relative humidity is a relative measure of moisture in the atmosphere compared to what it can hold. If the relative humidity is 50%, that means the air is "half saturated." It's holding about half the water vapor it can hold at that temperature.


What causes "Santa Ana" winds?
Winds that come from the northeast or east travel over several small mountain ranges before arriving in San Diego. As wind goes up over a mountain, it cools and condenses--leaves some clouds on the windward side, and that's moisture coming out of the atmosphere. As that air travels back over the mountain and down into the next valley, it warms up due to compression--and it's drier(since it lost that moisture)--so it's a hotter, drier air flow. Each time the air goes up and over, this happens--so by the time it reaches us it's extremely warm and usually very dry with relative humidites of less than 15%.


I understand lightning is an electrical discharge; but how does thunder happen?
Lightning bolts are EXTREMELY hot (20,000C degrees). When you heat something, it expands as a rule--and the air shaft around a lightning bolt expands rapidly when it is heated by the bolt. Then, as the bolt lasts only a split second, the air cools almost as instantly. The air around the lightning bolt, as it expands and then contracts back onto itself, is the "clap" of thunder that you hear.


Why don't we get hurricanes in San Diego?
Hurricanes require warm water for their fuel. Ocean surface temperatures of 80F degrees or warmer are required for hurricanes to live and thrive. . .that's why when they land on a continent, while they do catastrophic damage, they fall apart. . .their fuel is cut off.

If you've been to the beach lately, you know the water is much colder than 80F. That's why we'll never have a hurricane in San Diego--our water is just too cold.

There have been occasions when a hurricane off the southern coast of Mexico worked its way throught the Gulf of California. By the time these storms have crossed what little land is between San Diego and the Gulf of California, they were downgraded to Tropical Storms. But we have had about five tropical storms roll through town.

Why is our high temperature usually before noon, while the rest of the country is warmest around 3 or 4 in the afternoon?

Hey, good question! San Diego's weather is modified by that huge pool of water to the west (you know it as the Pacific). It keeps coastal communities from getting as cold as inland MicroClimate communities in the winter, and keeps the beaches from getting as hot as inland valleys in the summer.

When air heats up inland, it rises (heat rises, remember). As that air moves upward in the atmosphere, it is replaced by air from out over the ocean. That's known as our "afternoon sea breeze." When that sea breeze kicks in, it stops any real warming from the sun, since this air is coming from off of 60F to 70F degree water. In other words, it acts like a giant air conditioner. So even though the sun is still heating the earth, and some of that heat is being released BY the earth, the cool air off the ocean offsets it.


Is global warming really happening?
The concept of global warming is a real one; in theory, gasses can act like a big greenhouse and help keep the earth warmer over time. In reality, early global warming models did not take into account that a warmer ocean would provide more clouds--and these clouds will reflect some of the sun's short-wave radiation. Also, temperatures in most cities across the country are taken at the airport. . .and when airports were built, they were at the edge of town. But in the past fifty years, towns have grown around these same airports, and the "heat island" effect has increased temperatures all by itself. (The "heat island" effect is heat released by concrete and buildings).

The bottom line is that the meteorological community is not convinced global warming is a threat at this time; as the American Meteorological Society says, "the jury is still out." That doesn't mean that it isn't happening, or that it won't happen. . .that just means it's too early to say that it IS happening.


Why is it that when I get out of a pool in the summertime, I feel cold -- even when it's really hot outside?
That has to do with a process called "evaporative cooling." When water evaporates, it requires heat to do so. That's why when it rains outside, the air temperature usually gets colder. Raindrops that are falling do evaporate as they fall--and they take heat out of the atmosphere.

You've probably seen those "misters" at restaurants and water parks in the summertime. Those water misters work in the same way--the mist evaporates before it reaches you standing in line. . .and takes some heat out of the air.

Ok, so you and the pool. When you get out of the water, you're wet--and as the water on your skin evaporates, it takes heat to do so. . .just like the raindrops that fall from the sky. But here, the heat comes from your skin. So you feel colder. Even if it's hot outside.


How come the forecast doesn't always work out right?
Until we can control the weather, we won't forecast accurately 100% of the time. An analogy I like to use with sportscasters is to try to get them to "forecast" the winners of this weekend's games. They can't control the games, and so periodically teams that should win and win big, don't.

But as to weather, think of the atmosphere as a huge ocean of air, constantly moving and churning. . .just like the Pacific. If the computer forecast models make it look like tomorrow will be sunny, and I think those models have a pretty good track record of late, I'll forecast a sunny day. Then, after you go to bed at night (and I'm off the air), a coastal eddy could develop overnight off our coastline. Coastal eddys spin up moisture, and so when you wake up in the morning and skies are overcast, there's a reason why the forecast didn't work out.

A good meteorologist can at least explain why it was supposed to be sunny and it ended up being cloudy; bad weather reporters usually make up some lame excuse. . .and people are usually a lot more forgiving when you can explain why it was supposed to be one way, and ended up being another.


Just what is a "coastal eddy?"
That's one of the terms you'll hear frequently during weather forecasts in San Diego. Coastal eddys are basically just small low pressure systems that develop some nights out over the Pacific off San Diego's shoreline. This small low pressure system spins up moisture from out over the water eastward and onto our coastline--the result is usually a cloudy start to the upcoming day.

There are some indicators that a coastal eddy will develop. A strong south wind at Lindbergh Field during the early evening hours is one sign of a developing eddy. . .and the computer models usually pick up on atmospheric conditions that lead to their development.


I'm having a wedding next October. What will the weather be like?
If you're not having a wedding, you can substitute "graduation" or "outside party" or "picnic." Forecasters in San Diego get this kind of question all the time! But did you know that NO meteorologist, no matter how accurate they are, can predict the weather with any real accuracy beyond six days or so? It's true. Even the National Weather Service 30-day outlooks are right only about 60% of the time--and that's only slightly better than chance (50%). So when someone asks about how the weather will be weeks or months away, there's just no way to give a definite answer. . .and be confident of being correct.

One thing I can do, though, is provide some climatology for the event. So, for example, if it rains once every six days on average in San Diego, there's a pretty good chance your wedding will be dry. Sunrise and sunset times don't change from year to year, and the average high and low temperatures for the day in question can give you a pretty good idea of what sort of temperatures to expect. Of course, when the event gets a lot closer, even 6 to 10 day outlooks can provide a glimpse of the weather.