In my message the other night about noticing bird migration on the radar I did not explain how it works or what to look for, so naturally a few people asked what I was talking about.
First, about the birds. Many birds species migrate at night. Among other advantages this allows them to burn energy in powered flight when they can't feed because it's dark out (using stars and magnetism to navigate) and spending the daylight hours to forage and replenish energy stores to continue the journey on another night. Birds won't fight a strong headwind to migrate a long distance, nor will they migrate in the rain or snow. Instead they will wait for a night which has winds which will carry them the direction they want to go (generally from the south in spring, generally from the north in the fall), or air that is calm, or at least winds that are not over a few mph against them. If conditions are good, they take off shortly after sunset and unless they run into precipitation or significant headwinds (which will ground them) or a major obstacle such as the Great Lakes (which they are reluctant to tackle without a full tank of fuel), they continue until dawn, when they descend and seek a good place to spend the day or a few days while fattening up again.
Now, about the radar. The NexRad weather radar I linked to is designed to detect precipitation - bits of liquid or frozen water in the air. The radar map may be blank if there's no precipitation in the area. If there is precipitation, it will show up as distinct irregular green blobs (or yellow where precipitation is more intense) corresponding to the storm clouds. You can watch these storm clouds moving either by using the "Animate Map" feature which shows the last 45-minutes or so of radar data, or simply by updating the still image from time to time.
The rotating radar beam is sent out approximately level (or at least the bottom of it is), so the information it gathers is like a flat disk. However, the earth is spherical, so the land below the radar beam curves away, and the radar is examining an increasingly greater altitude farther out from the radar source.
Now, put the birds and radar together. Birds are mostly water, so the radar detects them. Single birds aren't significant, but a lot of birds in the air can resemble a light rain.
On evenings when the season and winds are right, birds take flight from wherever they individually spent the day, and each starts migrating. This is like a vast dispersed cloud of water drops rising from the ground toward the birds' cruising altitude somewhere between a few hundred and a couple thousand feet in the air. As they ascend, the first of them to enter the radar's view are closest to the radar source. As they continue to ascend, birds farther from the radar source get high enough to be detected. Birds even farther away from the radar source never get high enough to be detected because the radar is examining an altitude above their cruising altitude. While individual birds fly beyond (thus below) radar range by traveling away from the radar source (leaving to the north in spring), other birds at a similar altitude enter the radar range by traveling toward it (approaching from the south in spring), so what the radar shows can be constant through much of the night. What it looks like on the radar is a fuzzy green disk centered on the radar source. This disk grows outward from the radar source at dusk and shrinks back toward the radar source at dawn.
Another bit of info I neglected to mention is that the radar can detect how fast the precipitation or birds are moving toward or away from the radar source. On that view, the disk of birds will be orange on the half which is leaving, green on the half which is coming toward the radar source, divided by a wedge of white where the birds are neither moving toward nor away but are moving sideways past the radar source. If there is a cloud of insects in the air at night, they will be more passive than powered and will not show so strong a doppler effect greater than the wind speed as birds do.
One other detail: There are a number of distinct stationary points of green or yellow which show up around the range fairly close to the radar source. I believe these are wind turbines, whose moving arms are difficult to program the radar to ignore, unlike stationary features such as buildings and landforms.
To review where I was looking for radar info, start at Weather Underground:
http://www.wunderground.com/
At the top, hit the button called "Maps & Radar" then click the top choice, "Radar". This brings up a US map with the radar detected precipitation (& birds) on it, and little plus signs (+) indicating radar stations, which you can click on to get the local more detailed view.
I choose Binghamton because it's the closest to us and covers Tompkins County and the Cayuga Lake basin well. But a really cool one to look at during migration is Buffalo, because of its location between Lakes Erie and Ontario. Check Buffalo radar at dusk on a migration night and you can watch the birds take off only from the land, leaving the water blank, then travel over the water from the one side and leave a gap on land on the far side where the birds are all leaving the lakeshore going inland with no birds to replace them for the first hour or so until the birds arrive from the far side.
At the top left of the map you can click "Animate Map" for the quick review of the previous 45-minutes' data and "Stop Map" to revert to the most current image.
For the doppler information, click "Select Radar Type" just above the map on the right. This brings up a list of choices, of which "Base Radial Velocity 0.50° Elevation" is a good choice.
By the way, the site I use to find the weather conducive to migration (and weather forecasts in general) is on the National Oceanic and Atmospheric Administration (NOAA) National Weather Service site:
http://www.weather.gov/
On the upper left is a box to enter the name of the city and state you are interested in.
This will produce a map locating the place for which a seven-day-&-seven-night summary written forecast is listed.
What I like better though, is one of the "Additional Forecasts & Information" options farther down the page on the right, particularly the "Hourly Weather Graph."
This shows, hour by hour, 48 hours at a time, with options to jump forward 2 days, a graph showing the predicted:
* temperature (with heat index or wind chill if applicable),
* dewpoint,
* wind direction and speed (including gusts if applicable),
* relative humidity,
* percent cloud cover,
* percent chance of precipitation,
* type of precipitation (snow, freezing rain, sleet, rain, thunderstorms), and
* quantity of precipitation.
It's all very precise and easy to see what's going to happen. Keep in mind, though, that it's only generalized for the time and place you select. So for downtown Ithaca where I live I add 5°F. And if a front is coming through, but the forecasters don't know exactly when at the time they make the forecast (and I don't know how often or how far in advance they do this), then the graph may show raised precipitation chances over many hours when in fact the storm will be brief, and the winds may be shown shifting gradually over several hours, when in reality they may shift more rapidly at some point. To get the details of when the precipitation is going to hit me personally, I go back to that radar site I started out talking about.
Tonight with predicted winds from the NW about 7mph to start, I'm thinking not much migration will occur, but winds are supposed to slack off to 1mph overnight. If in fact they are low enough when the birds would pick up and move, maybe we will get some migration. But tomorrow night the winds are supposed to start from the NE at an almost insignificant 3mph, then clock around to a favorable 6mph from the south, so I think the chances of seeing migration on the radar are better for tomorrow night. This assumes that there will be birds in the area ready to move from here or to here, and that they are not prevented from getting here by adverse conditions farther south, and that they have not left the area already, factors which I haven't checked.
--Dave Nutter
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