Can't remember exactly... wait, of course, I've got the manual for Weather Link on my work computer!!
Under "Setup", "Station Config" (in WeatherLink), there's a checkbox for Download Options -- "Clear archive memory after download"
(From the manual)
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You may have the software automatically clear the archive memory whenever it downloads data. Clearing after each download will decrease the time it takes to download because there will be less “accumulated” data in the archive memory. Not clearing after each download allows you to download data into multiple databases (if more than one person uses a
single weather station, for example). To clear data in the archive memory
after each download, select the check box.
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Also of possible interest:
(From the manual)
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The archive memory is the weather information storage area in the WeatherLink itself. At each archive interval the WeatherLink stores one record to archive memory. The WeatherLink has room in the archive memory for approximately 1 day of weather data for each minute in the archive interval. For example, if you use the 1 minute archive interval, the WeatherLink may store approximately 1 day of data.
If you use the 30 minute archive interval, the WeatherLink may store approximately 30 days of data. If you use the 2 hour archive interval (120 minutes), the WeatherLink may store approximately 120 days of data.
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If you use 1 minute intervals, and you lose power to your computer for more than a day, you will lose data... I used to use 1 minute intervals, now I have it set to 5 minutes. There's a thread somewhere on this issue...