Baetsen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Baetsen Observatory clear sky chart from 2005-11-06 to 2024-01-14.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	15% 9% 3 7% 6% 3 5% 5% 5% 7% 34%	74684
	Night	16% 9% 4 6% 6% 3 5% 5% 5% 7% 35%	75068
Transparency	Day	47% 9% 14% 19% 10% 1	74395
	Night	48% 8% 13% 19% 10% 1	74891
Seeing	Day	18% 11% 29% 25% 13% 4	13330
	Night	19% 9% 30% 28% 12% 1	22789

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 15% 5% 8% 8% 3 4 4 5% 7% 26%	1517
	11th to 20th	Day	17% 17% 6% 9% 8% 4 5% 5% 4 5% 20%	1540
	21st to end	Day	13% 14% 6% 11% 9% 5% 6% 5% 4 5% 23%	1782
February	1st to 10th	Day	16% 14% 5% 12% 8% 5% 6% 5% 5% 5% 20%	1620
	11th to 20th	Day	13% 13% 6% 11% 7% 3 5% 3 4 7% 26%	1573
	21st to end	Day	19% 12% 6% 9% 6% 3 4 4 4 4 27%	1383
March	1st to 10th	Day	15% 10% 4 9% 8% 4 6% 5% 5% 6% 30%	1963
	11th to 20th	Day	14% 12% 6% 7% 6% 3 5% 3 4 6% 34%	1980
	21st to end	Day	20% 10% 3 6% 5% 3 3 4 4 5% 36%	2178
April	1st to 10th	Day	18% 9% 5% 8% 7% 3 5% 4 6% 6% 30%	2160
	11th to 20th	Day	20% 8% 3 6% 6% 2 4 4 5% 7% 37%	2156
	21st to end	Day	20% 9% 3 6% 6% 3 5% 5% 5% 5% 34%	2340
May	1st to 10th	Day	15% 9% 4 7% 5% 3 5% 5% 6% 7% 34%	2380
	11th to 20th	Day	14% 7% 2 5% 5% 3 5% 4 6% 7% 41%	2478
	21st to end	Day	11% 5% 3 5% 5% 4 4 4 6% 9% 44%	2725
June	1st to 10th	Day	16% 6% 3 5% 5% 3 5% 5% 6% 9% 38%	2520
	11th to 20th	Day	13% 6% 2 5% 5% 3 5% 5% 6% 10% 41%	2520
	21st to end	Day	13% 6% 2 5% 6% 3 6% 5% 6% 8% 41%	2520
July	1st to 10th	Day	8% 5% 2 4 4 2 7% 5% 7% 10% 46%	2520
	11th to 20th	Day	11% 4 2 5% 5% 4 6% 6% 6% 9% 42%	2520
	21st to end	Day	8% 5% 2 5% 7% 4 6% 6% 8% 9% 39%	2644
August	1st to 10th	Day	8% 6% 2 5% 6% 4 6% 6% 7% 8% 42%	2327
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 6% 7% 9% 42%	2303
	21st to end	Day	11% 6% 2 5% 5% 3 6% 5% 7% 9% 42%	2444
September	1st to 10th	Day	13% 5% 2 5% 6% 3 5% 6% 5% 8% 43%	2160
	11th to 20th	Day	12% 4 3 4 4 3 5% 5% 6% 8% 47%	2110
	21st to end	Day	13% 5% 3 6% 6% 4 6% 5% 5% 7% 41%	1908
October	1st to 10th	Day	17% 9% 3 6% 5% 4 6% 6% 5% 6% 34%	1800
	11th to 20th	Day	20% 10% 3 6% 7% 3 4 5% 5% 6% 31%	1800
	21st to end	Day	23% 10% 5% 7% 6% 4 5% 4 5% 6% 25%	1980
November	1st to 10th	Day	16% 8% 3 8% 6% 4 5% 4 6% 7% 33%	1595
	11th to 20th	Day	16% 9% 6% 8% 6% 4 6% 6% 6% 7% 27%	1520
	21st to end	Day	20% 14% 4 6% 6% 3 4 4 5% 8% 25%	1520
December	1st to 10th	Day	18% 14% 6% 9% 8% 5% 6% 5% 5% 5% 20%	1520
	11th to 20th	Day	18% 12% 5% 8% 5% 3 5% 5% 4 7% 27%	1906
	21st to end	Day	28% 16% 5% 9% 7% 3 5% 4 3 4 15%	2772
January	1st to 10th	Night	18% 13% 6% 8% 8% 4 5% 4 4 6% 25%	2649
	11th to 20th	Night	20% 15% 5% 9% 7% 3 5% 5% 4 5% 21%	2492
	21st to end	Night	13% 12% 5% 10% 7% 4 6% 5% 4 6% 28%	2574
February	1st to 10th	Night	15% 15% 5% 9% 7% 4 7% 5% 5% 5% 23%	2340
	11th to 20th	Night	16% 12% 5% 8% 7% 3 5% 4 5% 6% 28%	2273
	21st to end	Night	17% 10% 4 8% 6% 3 5% 5% 5% 6% 30%	1872
March	1st to 10th	Night	14% 8% 4 8% 8% 3 5% 4 5% 5% 35%	1983
	11th to 20th	Night	18% 10% 5% 7% 5% 3 4 4 4 5% 35%	1979
	21st to end	Night	16% 7% 3 6% 6% 3 4 3 4 6% 40%	2177
April	1st to 10th	Night	19% 9% 3 6% 5% 3 5% 4 4 7% 34%	1800
	11th to 20th	Night	16% 7% 3 6% 5% 3 6% 4 5% 7% 38%	1767
	21st to end	Night	18% 8% 3 6% 5% 4 5% 5% 5% 7% 35%	1620
May	1st to 10th	Night	18% 5% 3 5% 5% 3 5% 5% 6% 7% 39%	1577
	11th to 20th	Night	12% 5% 3 5% 5% 3 5% 4 6% 7% 45%	1415
	21st to end	Night	10% 4 3 4 5% 2 4 4 6% 8% 50%	1550
June	1st to 10th	Night	12% 5% 3 5% 6% 3 6% 6% 6% 8% 40%	1440
	11th to 20th	Night	10% 4 3 5% 6% 3 5% 5% 7% 9% 45%	1440
	21st to end	Night	12% 6% 3 6% 4 4 5% 4 6% 9% 43%	1440
July	1st to 10th	Night	6% 4 2 4 5% 3 5% 4 5% 8% 54%	1440
	11th to 20th	Night	10% 5% 2 5% 4 3 4 5% 5% 9% 49%	1440
	21st to end	Night	9% 4 2 4 5% 2 4 6% 7% 9% 49%	1671
August	1st to 10th	Night	9% 4 2 5% 5% 2 4 5% 6% 8% 50%	1611
	11th to 20th	Night	7% 4 2 5% 5% 3 5% 5% 6% 8% 49%	1598
	21st to end	Night	10% 5% 2 5% 4 3 5% 5% 5% 7% 48%	1892
September	1st to 10th	Night	12% 6% 2 4 5% 3 6% 4 6% 7% 46%	1800
	11th to 20th	Night	11% 5% 1 4 4 3 3 5% 5% 7% 52%	1847
	21st to end	Night	16% 6% 3 4 5% 3 4 4 4 6% 45%	2051
October	1st to 10th	Night	16% 7% 3 7% 6% 4 4 4 5% 7% 39%	2160
	11th to 20th	Night	20% 8% 3 6% 6% 4 4 5% 5% 7% 32%	2160
	21st to end	Night	21% 10% 3 6% 5% 4 5% 5% 5% 7% 30%	2376
November	1st to 10th	Night	17% 8% 3 5% 4 4 5% 5% 5% 7% 37%	2482
	11th to 20th	Night	18% 9% 4 6% 5% 4 5% 5% 5% 8% 30%	2660
	21st to end	Night	20% 11% 3 7% 6% 4 6% 5% 6% 8% 25%	2660
December	1st to 10th	Night	19% 13% 5% 10% 7% 3 5% 5% 5% 6% 22%	2660
	11th to 20th	Night	17% 15% 4 7% 6% 4 5% 5% 5% 6% 24%	3288
	21st to end	Night	30% 14% 4 7% 5% 3 5% 4 5% 6% 16%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	56% 10% 15% 14% 5%	1518
	11th to 20th	Day	66% 8% 11% 9% 6%	1540
	21st to end	Day	62% 8% 11% 12% 6% 1	1782
February	1st to 10th	Day	66% 7% 11% 12% 3 1	1620
	11th to 20th	Day	59% 7% 11% 14% 8% 1	1582
	21st to end	Day	59% 7% 11% 17% 6%	1383
March	1st to 10th	Day	51% 8% 15% 15% 9% 1	1966
	11th to 20th	Day	51% 6% 11% 17% 13% 2	1980
	21st to end	Day	50% 6% 9% 17% 16% 3	2178
April	1st to 10th	Day	52% 7% 12% 18% 10% 2	2160
	11th to 20th	Day	46% 7% 9% 21% 15% 1	2156
	21st to end	Day	49% 7% 11% 18% 12% 2	2340
May	1st to 10th	Day	48% 8% 10% 18% 13% 3	2380
	11th to 20th	Day	40% 8% 13% 24% 14% 1	2482
	21st to end	Day	37% 8% 16% 24% 12% 2	2725
June	1st to 10th	Day	42% 8% 14% 20% 14% 1	2520
	11th to 20th	Day	37% 9% 15% 24% 14% 2	2520
	21st to end	Day	39% 9% 17% 22% 13% 1	2520
July	1st to 10th	Day	30% 9% 15% 26% 18% 2	2450
	11th to 20th	Day	35% 9% 17% 28% 10% 1	2445
	21st to end	Day	36% 12% 17% 24% 11% 1	2496
August	1st to 10th	Day	35% 12% 17% 23% 12% 1	2314
	11th to 20th	Day	33% 11% 18% 26% 12%	2303
	21st to end	Day	36% 9% 17% 25% 11% 1	2444
September	1st to 10th	Day	37% 9% 14% 28% 10% 1	2160
	11th to 20th	Day	33% 8% 16% 28% 14% 1	2110
	21st to end	Day	40% 10% 17% 24% 9% 1	1908
October	1st to 10th	Day	48% 9% 13% 18% 10% 1	1800
	11th to 20th	Day	52% 9% 15% 17% 6% 1	1800
	21st to end	Day	58% 8% 13% 14% 6% 1	1980
November	1st to 10th	Day	48% 9% 15% 18% 9% 1	1595
	11th to 20th	Day	52% 11% 15% 14% 7% 1	1520
	21st to end	Day	57% 9% 14% 15% 4	1520
December	1st to 10th	Day	63% 10% 14% 11% 2	1520
	11th to 20th	Day	57% 12% 14% 12% 6%	1906
	21st to end	Day	70% 11% 9% 9% 2	2772
January	1st to 10th	Night	60% 8% 12% 15% 5%	2649
	11th to 20th	Night	65% 9% 10% 9% 5% 1	2492
	21st to end	Night	57% 8% 12% 14% 8% 1	2574
February	1st to 10th	Night	61% 8% 13% 14% 4 1	2340
	11th to 20th	Night	56% 7% 12% 17% 8%	2275
	21st to end	Night	53% 7% 14% 18% 7% 1	1872
March	1st to 10th	Night	50% 7% 14% 19% 9% 2	1993
	11th to 20th	Night	49% 7% 11% 20% 11% 1	1980
	21st to end	Night	44% 6% 10% 21% 15% 3	2177
April	1st to 10th	Night	49% 7% 10% 20% 13% 2	1800
	11th to 20th	Night	46% 7% 10% 18% 17% 2	1767
	21st to end	Night	46% 8% 12% 17% 14% 2	1620
May	1st to 10th	Night	43% 8% 11% 19% 16% 3	1577
	11th to 20th	Night	37% 7% 11% 24% 16% 4	1416
	21st to end	Night	31% 8% 15% 23% 19% 4	1551
June	1st to 10th	Night	39% 10% 12% 21% 15% 3	1440
	11th to 20th	Night	34% 8% 13% 21% 21% 3	1440
	21st to end	Night	39% 8% 13% 24% 15% 1	1440
July	1st to 10th	Night	26% 9% 14% 25% 23% 3	1400
	11th to 20th	Night	32% 6% 15% 28% 18% 1	1392
	21st to end	Night	30% 8% 15% 29% 15% 2	1578
August	1st to 10th	Night	31% 8% 20% 27% 14% 1	1600
	11th to 20th	Night	30% 9% 17% 30% 13% 1	1598
	21st to end	Night	33% 9% 14% 28% 15% 1	1892
September	1st to 10th	Night	36% 8% 16% 26% 13% 2	1800
	11th to 20th	Night	30% 7% 15% 30% 17% 1	1847
	21st to end	Night	40% 7% 12% 28% 13% 1	2051
October	1st to 10th	Night	45% 7% 15% 19% 13% 1	2160
	11th to 20th	Night	50% 10% 13% 20% 7%	2160
	21st to end	Night	53% 9% 13% 14% 10% 1	2376
November	1st to 10th	Night	44% 9% 14% 22% 11% 1	2482
	11th to 20th	Night	50% 9% 15% 15% 9% 1	2660
	21st to end	Night	54% 11% 17% 13% 4	2660
December	1st to 10th	Night	61% 9% 14% 12% 4	2660
	11th to 20th	Night	59% 10% 14% 13% 4	3288
	21st to end	Night	68% 10% 12% 8% 2	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	24% 18% 33% 17% 7% 1	180
	11th to 20th	Day	26% 23% 31% 15% 5% 1	200
	21st to end	Day	23% 21% 31% 21% 4	264
February	1st to 10th	Day	26% 29% 32% 11% 3	240
	11th to 20th	Day	23% 21% 40% 15% 1	240
	21st to end	Day	30% 22% 34% 13% 1	249
March	1st to 10th	Day	22% 16% 39% 19% 5%	420
	11th to 20th	Day	21% 19% 35% 20% 4 1	420
	21st to end	Day	31% 10% 32% 18% 8% 1	462
April	1st to 10th	Day	20% 14% 35% 23% 6% 1	420
	11th to 20th	Day	26% 12% 29% 20% 9% 4	412
	21st to end	Day	23% 11% 38% 19% 8% 1	420
May	1st to 10th	Day	22% 9% 29% 24% 10% 5%	423
	11th to 20th	Day	18% 6% 29% 28% 15% 4	447
	21st to end	Day	12% 5% 25% 34% 19% 6%	485
June	1st to 10th	Day	17% 5% 30% 32% 14% 2	450
	11th to 20th	Day	13% 8% 25% 30% 20% 4	450
	21st to end	Day	18% 3 27% 32% 16% 4	450
July	1st to 10th	Day	9% 3 20% 36% 26% 6%	445
	11th to 20th	Day	14% 3 21% 32% 23% 8%	444
	21st to end	Day	8% 3 26% 39% 17% 6%	488
August	1st to 10th	Day	9% 3 21% 34% 24% 9%	446
	11th to 20th	Day	10% 3 21% 32% 25% 10%	439
	21st to end	Day	12% 5% 23% 31% 20% 7%	495
September	1st to 10th	Day	12% 5% 24% 28% 21% 9%	450
	11th to 20th	Day	10% 5% 22% 30% 19% 14%	450
	21st to end	Day	13% 8% 29% 26% 16% 7%	450
October	1st to 10th	Day	20% 10% 25% 24% 16% 5%	450
	11th to 20th	Day	19% 16% 33% 25% 6% 1	450
	21st to end	Day	27% 15% 29% 22% 6%	495
November	1st to 10th	Day	24% 15% 28% 22% 10% 1	278
	11th to 20th	Day	19% 22% 36% 17% 4 2	180
	21st to end	Day	25% 18% 34% 19% 4	180
December	1st to 10th	Day	21% 18% 37% 17% 6% 1	180
	11th to 20th	Day	24% 26% 31% 16% 3 1	180
	21st to end	Day	27% 18% 38% 16% 1	198
January	1st to 10th	Night	23% 16% 38% 17% 6%	760
	11th to 20th	Night	28% 13% 34% 21% 4 1	735
	21st to end	Night	19% 12% 36% 25% 7%	792
February	1st to 10th	Night	22% 17% 41% 19% 2	720
	11th to 20th	Night	20% 15% 39% 21% 3 1	703
	21st to end	Night	21% 15% 40% 21% 4	592
March	1st to 10th	Night	19% 13% 38% 26% 4	720
	11th to 20th	Night	21% 13% 35% 25% 5% 1	720
	21st to end	Night	19% 9% 34% 27% 11%	792
April	1st to 10th	Night	22% 12% 33% 28% 6%	720
	11th to 20th	Night	18% 11% 32% 27% 11% 1	703
	21st to end	Night	19% 8% 31% 30% 12%	540
May	1st to 10th	Night	20% 5% 26% 32% 14% 3	497
	11th to 20th	Night	15% 4 30% 37% 14%	354
	21st to end	Night	11% 3 21% 41% 23% 2	389
June	1st to 10th	Night	13% 2 25% 43% 16% 1	360
	11th to 20th	Night	11% 3 25% 41% 19% 1	360
	21st to end	Night	11% 3 23% 43% 19% 1	360
July	1st to 10th	Night	5% 1 17% 51% 23% 2	350
	11th to 20th	Night	10% 5% 16% 41% 26% 1	348
	21st to end	Night	10% 2 19% 44% 22% 3	491
August	1st to 10th	Night	11% 2 13% 39% 31% 4	537
	11th to 20th	Night	9% 1 14% 40% 31% 5%	534
	21st to end	Night	12% 3 21% 35% 26% 3	704
September	1st to 10th	Night	13% 4 21% 35% 22% 5%	720
	11th to 20th	Night	13% 4 19% 32% 25% 7%	720
	21st to end	Night	18% 8% 19% 28% 22% 5%	720
October	1st to 10th	Night	20% 5% 28% 26% 19% 2	720
	11th to 20th	Night	22% 12% 31% 26% 9% 1	720
	21st to end	Night	26% 9% 32% 28% 5%	792
November	1st to 10th	Night	22% 10% 26% 30% 11% 1	740
	11th to 20th	Night	21% 14% 33% 24% 7% 2	760
	21st to end	Night	25% 12% 33% 24% 7%	760
December	1st to 10th	Night	27% 15% 36% 19% 4	760
	11th to 20th	Night	23% 13% 37% 22% 4 1	760
	21st to end	Night	31% 14% 36% 17% 3	836

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