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FITS stands for Flexible Image Transport System and was introduced by the gentlemen D.C. Wells, E.W. Greisen and R.H. Harten in the journal Astronomy & Astrophysics Supplement Series from June 1981, pages 363 to 370. The FITS file format was developped to facilitate the exchange of data between universities and institutions without having to care about operating systems and hardware (7 and 9 track tapes; diskettes, CDs and DVDs were not yet invented).

The file header consists of one or more sets of 36 cards. Each card has 80 characters. It starts with a keyword of 7 characters. An equal sign (=) is at column 9. The value is noted right adjusted at column 30. Column 32 may hold a slash (/), from column 34 to 80 there may be a comment. Each card is padded with blanks.

There are at least 6 cards needed with these mandatory keywords: SIMPLE, BITPIX, NAXIS, NAXIS1, NAXIS2 and END.

SIMPLE is T (true), if the data is organised in 8, 16, 32, etc. bits.

BITPIX indicates, how many bits make up a data point.

NAXIS informs how many dimensions of data there are in the file. 2 means a picture (width & height), 3 may mean multiple images.

NAXIS1 is the size of the first dimension (e.g. image width in pixels).

NAXIS2 is the size of the second dimension (e.g. picture height in pixels).

END marks the last card in the header.

Cards from the 36 card set not used are padded by blanks (32, 20h). The file header size is therefore always 36 x 80 bytes = 2,880 bytes or a multiple of 2,880 bytes.

Payload data follow immediately after the file header. They use 2,880 bytes or a multiple thereof. Records are padded by zeroes (0, 00h).

The file header is 3 x 36 x 80 bytes = 8,640 bytes up to 1999. From 2000 hence, 4 x 36 x 80 bytes = 11,520 bytes.

Up to and including the year 1999, the following probe data are stored in this order (multiply the index by 366 x 48 and add the header length to the result gives the file position of the first value of that probe in the file):

Temperature inside,
Temperature outside,
Temperature outside, highest value measured,
Temperature outside, lowest temperature measured,
Barometer
Humidity inside
Humidity outside,
Windspeed average in 30 minutes,
Windspeed peak within 30 minutes,
Wind direction,
Precipitation (unusable).

Beginning with the year 2000 three additional derived measurements are recorded:

The header for the pre-2000 years contains 3 sets of cards with 36 cards per set. The probe data use 11 x 366 x 48 bytes = 193,248 bytes, this makes up 67.1 records with 2'880 bytes each. 0.9 record is padded by zeroes. (68 x 2,880 = 195,840). The complete file size becomes 8,640 + 195,840 = 204,480 bytes.

Starting with the year 2000, more cards are needed and there are 4 sets of 36 cards each. There are also 3 additional probe data (3 probes x 366 days x 48 ½ hours). The 14 probes use up 85.4 x 2,880 bytes and are extended to 86 x 2,880. Hence, the file size grows to 11,520 + 247,680 = 259,200 bytes.

You may scrutinize the individual file headers by having them displayed here. Just select the year and start the query.