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Posts Tagged ‘rain’

Weather Word Wednesday : Nor’easter

January 12th, 2011 No comments

I just read a Twitter post from @reedtimmerTVN that said another Nor’easter was bearing down on the East Coast of the United States today. It occurred to me that I really don’t know what a Nor’easter is, so I figured that would be the perfect word for Weather Word Wednesday.

According to Wikipedia, a Nor’easter is “is a type of macro-scale storm along the East Coast of the United States and Atlantic Canada, so named because the storm travels to the northeast from the south and the winds come from the northeast, especially in the coastal areas of the Northeastern United States and Atlantic Canada.”

Specifically, a Nor’easter describes a low pressure area who’s center of rotation is just off the East Coast and who’s leading winds in the left forward quadrant rotate onto land from the northeast. Nor’easters can cause coastal flooding, coastal erosion, hurricane force winds and heavy snow. While they can occur at any time of the year, they are most frequent in the winter months. These systems are known for bringing down extremely cold arctic air from the north.

Satellite image of the intense nor'easter responsible for the North American blizzard of 2006. Note the hurricane-like eye at the center.

Nor’easters generally affect the United States, from Virginia to the New England cost, as well as Quebec and Atlantic Canada. They tend to bring massive amounts of precipitation, high winds, large waves, and marginal storm surges to coastal areas. In general, though, people tend to call any strong rain or snow storm in the Northeast a Nor’easter. The very name brings feelings of dread and anticipation to people in the region!

For more information, visit this Wikipedia article on Nor’easters.

Water Vapor Imagery

August 24th, 2010 No comments

I recently “discovered” on the local NWS website the Satellite Water Vapor Imagery data for our area, and I’m quite fascinated by it! The official description of this data is as follows:

Water vapor satellite imagery depicts moisture content in the middle and upper layers of the atmosphere. Lower level moisture is not depicted in these images. Moisture transport over large distances generally happens through the middle and upper layers of the atmosphere. Hence these images will depict moisture coming into the Southwest from the Gulf of Mexico or across Texas. Additionally, weak disturbances from the east are best tracked through water vapor imagery.

The imagery looks like this when viewed online. The is a screen capture of one frame of data:

Water Vapor Imagery

Water Vapor Imagery

Basically, what this shows is the amount of moisture that is in the upper atmosphere. This may be in the form of clouds that you see, but often it is not. Water vapor literally surrounds the earth, but is unevenly distributed due to things like oceans, lakes, rivers, deserts, arid areas, etc. For instance, in the image above, you can see a lot of water vapor coming up from the southwest into places like Arizona and New Mexico, which are traditionally dry regions. Areas like the Pacific Northwest, which are usually wet, have much less than normal. This is all due to seasonal shifts in winds and weather patterns, which change the flow of moisture around the planet. By looking at an image like this, you can get an idea of how much moisture is available in your area…moisture that could turn to rain or snow!

Vertical Development

June 30th, 2010 1 comment

When you start seeing vertical development in storm clouds, there’s a good chance rain is on the way. Here’s a picture of some clouds outside right now, and the associated radar image from my iPhone.

Categories: Clouds, Monsoon, Weather Tags: , ,

How To Forecast Weather Without Gadgets

June 2nd, 2010 No comments

I stumbled across this today, and thought it was worth sharing. There are lots of ways to forecast the weather without relying on gadgets. Knowing how to do it old school is a great skill to have, and helps you understand the fundamental properties of weather. Check out this graphic, and have fun learning!

How To Forecast Weather

What Is Base Reflectivity?

May 21st, 2010 3 comments

The other evening I was checking out the NWS page for Tucson, and I clicked on the local Radar image. This is what I saw:

Tucson Base Reflectivity

Tucson Base Reflectivity

At first glance, it would appear that it was raining outside in the areas indicated in blue. But, having spent the evening outside coaching a softball game, and having checked the skies before heading in for the evening, I knew that, if anything, the only thing in the sky was some high clouds. So, why would the radar indicate that it was raining? I decided that it was time to get a better understanding of what Base Reflectivity was all about.

Turning to Google, I found a link to the National Weather Service page on Base Reflectivity. I started reading the FAQs on weather radar, and it became very apparent that there is more than meets the eye when reading weather radar output.

To begin, we need to understand how weather radar works. Basically, the Next Generation Radar (NEXRAD) obtains weather information (precipitation and wind) by measuring returned energy. The radar sends out a burst of energy (green), and if the energy strikes an object like rain drops, bugs, birds, etc., the energy is scattered in all directions (blue). A small fraction of that energy gets directed back to the radar. How Radar Works The radar has a listening period, in which it collects and analyzes the signals that it receives. The whole process to analyze the signal is super fast, and occurs around 1300 times per second! In an average hour, the radar spends about 7 minutes sending signals, and 53 minutes listening for them. Based on some geeky physics stuff, the analysis can tell the “phase shift” of the signals it receives, which lets it know in what direction, and how fast the object it got bounced off of is going. Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to “see” the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice.

Base Reflectivity, which is what’s on the map above, is a display of echo intensity (reflectivity) measured in dBZ (decibels of Z, where Z represents the energy reflected back to the radar). “Reflectivity” is the amount of transmitted power returned to the radar receiver. Base Reflectivity images are available at several different elevation angles (tilts) of the antenna and are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries and determine hail potential.

Base Reflectivity ScaleWhen you look at the Base Reflectivity map, you’ll see various colors on it, and one of the scales that you see to the left of this text. If the radar is operating in “clear aid” mode, then the values range from -28 to +28 dBZ. If the radar is operating in “precipitation mode,” then the values range from 5 to 75 dBZ. Turns out, the map I was viewing was operating in clear air mode, which was something I had never heard of. Typically, light rain is falling when the values reach approximately 20 dBZ. As you can see from my map, I was nowhere near that. I suppose that the high clouds or other particulates in the air could have accounted for the return that I saw on the map, but it certainly wasn’t raid. Had I known about the two scales, and the 20 dBZ threshold, I wouldn’t have been confused!

There’s quite a bit to learn about weather radars and how they are used to predict the weather. I would highly encourage you to visit the NWS Radar Image WSR-88D Radar FAQs to learn more. By understanding the concepts, scales and technologies used to predict the weather, you can get a better understanding of the weather potential for your area. And, it never hurts to learn some geeky science!

Picture Of The Week

March 8th, 2010 No comments

Here is something to start your week with. I shot this in November 2008 from a friends house near Port Ludlow, WA. This is looking east across Puget Sound after a great rainstorm.

Picture of the Week - Puget Sound Rainbow

Picture of the Week - Puget Sound Rainbow

Categories: Photography, Weather Tags: , ,

Cirrus Clouds

March 5th, 2010 No comments

A couple days ago, as a storm was moving out of the area, we got some classic cirrus clouds in the upper atmosphere. These types of clouds are very high in the atmosphere, are traditionally thin with wispy strands, and can often herald the arrival of a storm (in this case they were remnants of a previous storm). Cirrus clouds like to live at levels above 26,000 feet (8000 meters), and are formed when water vapor freezes into ice crystals. The lack of moisture at such high altitudes is one of the reasons these clouds tend to present so thin and wispy.

Cirrus Clouds in Tucson, AZ

Cirrus Clouds in Tucson, AZ

A lot of times in Tucson we see hair like filaments of ice crystals precipitating out of the clouds in the form of what’s called virga. These streaks often indicate the difference in the motion of air between the upper part of the cirrus cloud and the lower air below it. It appears as though rain is coming from the cloud, but it’s actually ice crystals, most of which evaporates before it hits the ground (especially in Arizona). On some days, cirrus cloud development is so extensive that they become virtually indistinguishable from one another, forming what’s called cirrostratus clouds.

Another type of cirrus cloud that you’re already very familiar with is the condensation trails, or contrails,  seen in the sky coming from planes. These trails are basically artificial clouds formed by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals. On some days, with a high level of air traffic, you can see contrails crisscrossing the sky, and these contrails often hang around for some time. It’s pretty neat!

Clouds are very cool, and an obviously integral part of weather. I intend to eventually go through all the various cloud types, hopefully using images I’ve shot with my camera. If you understand why a particular cloud type is in the area, you have a good idea of what sort of weather might be on the way.

Chance of Rain

February 22nd, 2010 No comments

It’s raining outside in Tucson today, which leads me to think about what they mean when they say there is an X percent chance of participation. This morning, Weather Underground says there is an 80% chance of precipitation in Tucson. Does that mean 80% of the area will get rain? That it will rain 80% of the day?

I did some research, and the answer is pretty simple : out of 100 days where the weather conditions were exactly or similar to how they are today, it rained 80 times. Pretty simple!

The questions becomes, then, how do they get that data to make such a calculation? The answer lies with the National Weather Service. Each day the NWS releases several balloons into the atmosphere from locations across the country. Those balloons, called radiosondes, are released twice a day and the information they collect is radioed to the ground where it’s collected by the National Meteorological Center near Washington, D.C, where it’s processed by computer. All the information the radiosonde collected during its rise in the atmosphere – pressure, humidity, wind speed and direction, temperature – is fed into the computer and used, in conjunction with data from ground sources, to create a 3D model of the atmosphere. That model is evaluated against various laws of fluid mechanics to predict future conditions.

Chance of precipitation for 22 February 2010.

Chance of precipitation for 22 February 2010.

Unfortunately, given the nature of precipitation, these percentages are really just educated guesses based on previous conditions. The atmosphere is a very fluid and dynamic entity, and many things can trigger an unexpected change in conditions. A wet day can suddenly turn sunny, just like a sunny day can suddenly turn severe. Weather forecasters have a difficult job at best, and providing a chance of precipitation number is just one way that they can help you plan your day.

By the way…a 100% chance of rain does not mean that it’s raining right this moment (unless, perhaps, you live in the Pacific Northwest). Again, it simply means that out of 100 previous days where conditions have been similar to those today, it rained every time. And of course that begs the next question…what is rain? We’ll save that for a later post!