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| Frequently Asked Questions |
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Frequently Asked Questions |
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Are winds one minute or ten minute mean winds (or equivalents there of)?
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The satellite images are instantaneous snap-shots of a 25 km x 25 km area. This is probably more comparable to an 8 - 10 minute wind (if you consider the amount of space measured by a storm moving past a fixed point making observations for 8 minutes).
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We validate satellite winds with buoy winds, most of which are temporal averages ranging from 6 minutes (TAO) to 8 minutes (NDBC), and generally obtain wind speed differences of less than 1 m/s.
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How are the 3-day, weekly and monthly average data maps made?
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The radiometer (SSM/I, TMI, and AMSR-E) and scatterometer (QuikScat, SeaWinds) data are all available as 3-day, weekly and monthly averaged data. These files are created from the earth gridded daily pass data. Each grid cell contains the average (mean) value of all valid geophysical data points in that cell for all daily passes within the averaging time period. For grid locations where no data exist within the averaging period (occasionally occurs in 3-day maps), a value of 254 (missing data) is assigned.
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Ice (a value of 252) is assigned to the grid cell if ice was present more often than valid data within the averaging period.
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Why do I see data on daily maps with tomorrow's date, when tomorrow has not yet occurred?
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Each daily map is for ascending or descending passes of a satellite on that date at Greenwich, England. For example, data may be collected in the New York area at approximately 8:30 pm local time. 8:30 pm in New York City is 1:30 am in Greenwich, England (8:30 + 5 hour time difference). If it is August 17th, local time, it must be August 18th in Greenwich (the date of the map). Thus, data collected on Aug 17th, local time, may appear on the August 18th map.
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How can I tell when a satellite collected data over a particular spot on a daily data map?
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The time parameter can be extracted from each daily binary data file using read routines available on our FTP site (see the Data Description for each instrument). A visual method for quick approximations is discussed in: Support / Crossing Times / Swath Time Labels.
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