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 2026-01-17.

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 8% 5% 5% 5% 4 5% 7% 34%	127682
	Night	16% 9% 3 8% 5% 4 5% 4 5% 7% 34%	123978
Transparency	Day	47% 7% 13% 21% 11% 1	131268
	Night	48% 7% 13% 20% 12% 1	128478
Seeing	Day	20% 11% 29% 24% 12% 4	15882
	Night	19% 10% 30% 28% 12% 1	25310

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	14% 12% 4 11% 6% 5% 5% 3 5% 6% 30%	3677
	11th to 20th	Day	10% 19% 7% 18% 9% 5% 4 3 4 4 17%	3818
	21st to end	Day	16% 17% 7% 13% 6% 5% 5% 3 4 4 20%	4466
February	1st to 10th	Day	15% 15% 5% 12% 7% 6% 5% 3 5% 5% 22%	1800
	11th to 20th	Day	13% 15% 7% 12% 7% 5% 4 3 4 6% 25%	1753
	21st to end	Day	18% 13% 6% 10% 6% 4 4 3 5% 4 27%	1542
March	1st to 10th	Day	20% 10% 4 9% 5% 4 4 3 4 5% 32%	4904
	11th to 20th	Day	12% 12% 5% 9% 5% 5% 5% 4 5% 8% 30%	4980
	21st to end	Day	19% 10% 3 7% 5% 4 3 3 4 5% 37%	5465
April	1st to 10th	Day	18% 11% 4 8% 6% 4 4 4 5% 6% 30%	2400
	11th to 20th	Day	20% 9% 3 7% 5% 3 4 3 5% 6% 36%	2400
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 4 4 6% 34%	2600
May	1st to 10th	Day	20% 9% 3 8% 5% 4 4 3 5% 6% 31%	6012
	11th to 20th	Day	16% 10% 3 8% 5% 4 4 3 5% 7% 35%	6318
	21st to end	Day	18% 7% 2 7% 4 4 4 3 6% 8% 38%	6949
June	1st to 10th	Day	15% 7% 2 6% 5% 5% 4 4 6% 8% 36%	2800
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 40%	2800
	21st to end	Day	14% 7% 2 6% 5% 4 5% 4 6% 8% 39%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 7% 10% 42%	6360
	11th to 20th	Day	9% 4 2 6% 4 4 4 4 6% 9% 48%	6360
	21st to end	Day	6% 4 2 6% 4 5% 5% 5% 7% 9% 47%	6661
August	1st to 10th	Day	15% 8% 2 6% 5% 5% 5% 3 5% 7% 39%	4234
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 7% 10% 38%	4174
	21st to end	Day	9% 6% 2 5% 4 4 5% 4 7% 8% 44%	4426
September	1st to 10th	Day	13% 5% 2 5% 5% 4 5% 4 5% 7% 45%	2400
	11th to 20th	Day	11% 4 2 4 4 4 5% 4 6% 7% 49%	2344
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	2120
October	1st to 10th	Day	16% 8% 2 6% 4 5% 6% 4 5% 6% 38%	2000
	11th to 20th	Day	19% 10% 2 8% 5% 4 4 3 5% 6% 33%	2000
	21st to end	Day	22% 10% 5% 8% 5% 5% 5% 3 5% 6% 27%	2200
November	1st to 10th	Day	16% 9% 2 9% 5% 5% 4 3 5% 6% 33%	1765
	11th to 20th	Day	15% 11% 5% 9% 6% 5% 5% 4 7% 7% 27%	1680
	21st to end	Day	19% 15% 4 8% 6% 5% 4 3 5% 7% 24%	1680
December	1st to 10th	Day	18% 17% 5% 12% 7% 6% 6% 3 5% 5% 16%	2680
	11th to 20th	Day	19% 14% 5% 10% 5% 4 5% 3 3 5% 25%	3066
	21st to end	Day	25% 16% 6% 11% 6% 4 4 3 3 4 17%	4048
January	1st to 10th	Night	15% 13% 5% 10% 6% 5% 4 3 5% 6% 27%	6489
	11th to 20th	Night	13% 14% 5% 13% 9% 6% 5% 4 6% 5% 20%	6198
	21st to end	Night	15% 13% 5% 11% 6% 5% 5% 4 5% 6% 25%	6490
February	1st to 10th	Night	14% 15% 4 9% 7% 6% 6% 4 5% 5% 25%	2600
	11th to 20th	Night	15% 13% 5% 10% 7% 4 5% 3 5% 6% 28%	2533
	21st to end	Night	15% 11% 4 9% 5% 5% 5% 4 5% 6% 30%	2087
March	1st to 10th	Night	17% 10% 4 9% 6% 4 5% 4 5% 5% 32%	4921
	11th to 20th	Night	11% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	4979
	21st to end	Night	21% 7% 3 6% 4 4 3 3 4 6% 40%	5453
April	1st to 10th	Night	19% 10% 3 7% 5% 4 5% 3 4 7% 33%	2000
	11th to 20th	Night	16% 8% 3 7% 4 4 6% 3 4 7% 37%	1965
	21st to end	Night	17% 8% 3 7% 4 5% 5% 4 5% 6% 35%	1800
May	1st to 10th	Night	25% 8% 3 6% 4 3 4 4 5% 5% 34%	3926
	11th to 20th	Night	14% 7% 3 7% 5% 4 5% 4 8% 7% 37%	3575
	21st to end	Night	18% 6% 2 6% 4 4 4 4 6% 7% 39%	3926
June	1st to 10th	Night	13% 5% 3 6% 6% 4 7% 4 6% 7% 40%	1600
	11th to 20th	Night	9% 4 2 7% 5% 4 5% 4 7% 9% 44%	1600
	21st to end	Night	11% 6% 3 6% 4 5% 4 4 6% 8% 42%	1600
July	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	3600
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 49%	3600
	21st to end	Night	9% 4 2 4 3 3 4 4 6% 8% 53%	4188
August	1st to 10th	Night	10% 4 2 6% 4 3 5% 4 5% 8% 49%	2915
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 50%	2875
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 8% 49%	3437
September	1st to 10th	Night	11% 6% 2 5% 4 4 5% 3 6% 7% 48%	2000
	11th to 20th	Night	10% 5% 1 4 3 4 4 4 5% 7% 53%	2053
	21st to end	Night	17% 7% 3 6% 4 3 4 3 4 6% 44%	2279
October	1st to 10th	Night	14% 6% 3 7% 5% 4 4 3 5% 7% 42%	2400
	11th to 20th	Night	20% 8% 3 7% 5% 5% 4 4 5% 7% 34%	2400
	21st to end	Night	20% 10% 3 7% 4 4 5% 4 5% 7% 32%	2640
November	1st to 10th	Night	16% 8% 3 6% 4 5% 4 4 6% 7% 37%	2751
	11th to 20th	Night	16% 11% 4 7% 5% 5% 5% 4 6% 8% 29%	2940
	21st to end	Night	20% 12% 3 8% 5% 5% 5% 5% 6% 7% 24%	2940
December	1st to 10th	Night	17% 15% 4 10% 6% 4 5% 5% 6% 7% 21%	4720
	11th to 20th	Night	17% 16% 4 9% 5% 5% 5% 4 6% 6% 24%	5348
	21st to end	Night	25% 14% 5% 11% 5% 5% 5% 3 4 5% 16%	7150

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	52% 9% 12% 17% 9% 1	3678
	11th to 20th	Day	72% 6% 10% 7% 4 1	3850
	21st to end	Day	67% 6% 10% 9% 8% 1	4466
February	1st to 10th	Day	63% 7% 11% 14% 4 1	1800
	11th to 20th	Day	59% 7% 10% 14% 8% 1	1762
	21st to end	Day	59% 7% 11% 17% 6%	1542
March	1st to 10th	Day	52% 6% 13% 14% 14% 1	4907
	11th to 20th	Day	49% 7% 17% 16% 8% 2	4980
	21st to end	Day	48% 6% 9% 20% 16% 1	5465
April	1st to 10th	Day	53% 7% 12% 17% 10% 2	2400
	11th to 20th	Day	48% 7% 9% 20% 14% 1	2400
	21st to end	Day	49% 7% 11% 17% 12% 3	2600
May	1st to 10th	Day	51% 7% 12% 15% 11% 4	6021
	11th to 20th	Day	48% 10% 12% 14% 16% 1	6322
	21st to end	Day	44% 6% 13% 23% 11% 2	6949
June	1st to 10th	Day	43% 8% 14% 20% 14% 1	2800
	11th to 20th	Day	37% 8% 15% 24% 14% 2	2800
	21st to end	Day	40% 8% 16% 22% 13% 1	2800
July	1st to 10th	Day	37% 4 15% 27% 15% 1	6290
	11th to 20th	Day	34% 4 13% 34% 14% 1	6285
	21st to end	Day	32% 6% 15% 32% 15%	6470
August	1st to 10th	Day	43% 7% 15% 22% 13% 1	4221
	11th to 20th	Day	39% 6% 16% 29% 10%	4213
	21st to end	Day	36% 5% 14% 35% 9% 1	4426
September	1st to 10th	Day	37% 8% 14% 29% 10% 1	2400
	11th to 20th	Day	32% 7% 16% 30% 14% 1	2344
	21st to end	Day	41% 9% 16% 24% 10%	2120
October	1st to 10th	Day	39% 5% 13% 28% 14% 1	3260
	11th to 20th	Day	40% 5% 17% 22% 15% 1	3240
	21st to end	Day	50% 6% 16% 20% 8%	3538
November	1st to 10th	Day	48% 8% 14% 19% 9% 1	1765
	11th to 20th	Day	53% 10% 14% 15% 8% 1	1680
	21st to end	Day	59% 8% 13% 15% 4	1680
December	1st to 10th	Day	69% 6% 14% 9% 2	2680
	11th to 20th	Day	62% 7% 10% 12% 8%	3066
	21st to end	Day	69% 8% 7% 9% 6%	4048
January	1st to 10th	Night	57% 8% 11% 16% 7%	6489
	11th to 20th	Night	63% 10% 12% 9% 6% 1	6241
	21st to end	Night	60% 8% 10% 10% 11% 1	6490
February	1st to 10th	Night	59% 8% 13% 15% 4 1	2600
	11th to 20th	Night	56% 7% 12% 16% 8%	2535
	21st to end	Night	52% 7% 14% 19% 7% 1	2087
March	1st to 10th	Night	53% 6% 10% 16% 12% 3	4931
	11th to 20th	Night	42% 8% 18% 21% 10% 1	4980
	21st to end	Night	44% 5% 10% 21% 17% 2	5453
April	1st to 10th	Night	50% 6% 11% 19% 13% 1	2000
	11th to 20th	Night	47% 7% 9% 19% 17% 2	1965
	21st to end	Night	46% 7% 12% 18% 15% 2	1800
May	1st to 10th	Night	51% 6% 12% 13% 13% 5%	3933
	11th to 20th	Night	42% 9% 14% 20% 13% 2	3576
	21st to end	Night	42% 7% 14% 22% 14% 2	3927
June	1st to 10th	Night	40% 9% 11% 21% 16% 3	1600
	11th to 20th	Night	35% 8% 13% 22% 20% 2	1600
	21st to end	Night	39% 7% 13% 24% 15% 1	1600
July	1st to 10th	Night	35% 4 16% 23% 20% 1	3560
	11th to 20th	Night	33% 3 13% 32% 18% 1	3552
	21st to end	Night	30% 5% 13% 31% 20% 1	4064
August	1st to 10th	Night	34% 5% 15% 28% 16% 2	2904
	11th to 20th	Night	34% 5% 16% 31% 13% 1	2902
	21st to end	Night	33% 5% 12% 34% 15% 1	3437
September	1st to 10th	Night	34% 7% 15% 29% 13% 1	2000
	11th to 20th	Night	29% 7% 14% 32% 17% 1	2053
	21st to end	Night	41% 7% 12% 27% 13% 1	2279
October	1st to 10th	Night	34% 4 10% 28% 23% 1	3920
	11th to 20th	Night	41% 6% 14% 30% 9%	3899
	21st to end	Night	41% 6% 15% 27% 10% 1	4252
November	1st to 10th	Night	44% 8% 14% 22% 12% 1	2751
	11th to 20th	Night	51% 9% 14% 16% 9% 1	2940
	21st to end	Night	56% 10% 16% 14% 4	2940
December	1st to 10th	Night	61% 5% 16% 14% 4	4720
	11th to 20th	Night	61% 8% 11% 11% 9%	5348
	21st to end	Night	70% 7% 10% 9% 5%	7150

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	27% 19% 34% 14% 6% 1	220
	11th to 20th	Day	28% 22% 31% 13% 5% 1	255
	21st to end	Day	25% 22% 30% 19% 3	330
February	1st to 10th	Day	26% 27% 31% 13% 3	300
	11th to 20th	Day	27% 21% 37% 14% 1	300
	21st to end	Day	30% 20% 36% 13% 1	306
March	1st to 10th	Day	24% 16% 38% 18% 4	500
	11th to 20th	Day	22% 21% 34% 19% 4 1	500
	21st to end	Day	32% 11% 33% 17% 7% 1	550
April	1st to 10th	Day	24% 14% 35% 21% 5% 1	500
	11th to 20th	Day	29% 13% 30% 17% 8% 3	492
	21st to end	Day	23% 13% 37% 18% 7% 1	500
May	1st to 10th	Day	22% 10% 30% 24% 9% 4	503
	11th to 20th	Day	17% 9% 27% 29% 14% 3	527
	21st to end	Day	14% 7% 25% 31% 18% 5%	573
June	1st to 10th	Day	18% 8% 31% 29% 13% 2	530
	11th to 20th	Day	13% 9% 28% 29% 18% 3	530
	21st to end	Day	21% 5% 27% 29% 14% 4	530
July	1st to 10th	Day	10% 5% 23% 33% 23% 6%	525
	11th to 20th	Day	14% 5% 24% 30% 20% 7%	524
	21st to end	Day	9% 4 30% 37% 16% 5%	576
August	1st to 10th	Day	12% 4 23% 33% 22% 7%	526
	11th to 20th	Day	11% 3 23% 31% 23% 8%	519
	21st to end	Day	13% 5% 26% 31% 19% 7%	583
September	1st to 10th	Day	14% 7% 23% 28% 19% 9%	530
	11th to 20th	Day	9% 4 21% 31% 21% 14%	530
	21st to end	Day	15% 7% 29% 28% 14% 6%	530
October	1st to 10th	Day	19% 11% 26% 24% 15% 5%	530
	11th to 20th	Day	19% 17% 33% 22% 7% 1	530
	21st to end	Day	27% 16% 30% 21% 6% 1	583
November	1st to 10th	Day	28% 15% 28% 20% 8% 1	328
	11th to 20th	Day	22% 22% 35% 15% 4 1	220
	21st to end	Day	30% 17% 33% 17% 3	220
December	1st to 10th	Day	25% 17% 36% 15% 6%	220
	11th to 20th	Day	28% 24% 32% 13% 3	220
	21st to end	Day	29% 19% 36% 15% 1	242
January	1st to 10th	Night	22% 18% 38% 16% 5%	840
	11th to 20th	Night	28% 15% 34% 19% 4	825
	21st to end	Night	20% 13% 36% 24% 6%	880
February	1st to 10th	Night	21% 17% 40% 20% 2	800
	11th to 20th	Night	21% 17% 38% 21% 3 1	783
	21st to end	Night	20% 16% 40% 21% 4	660
March	1st to 10th	Night	19% 14% 37% 26% 4	800
	11th to 20th	Night	20% 15% 35% 25% 4 1	800
	21st to end	Night	20% 12% 33% 26% 10%	880
April	1st to 10th	Night	23% 12% 33% 27% 5%	800
	11th to 20th	Night	19% 11% 33% 26% 10% 1	781
	21st to end	Night	19% 9% 31% 30% 11%	600
May	1st to 10th	Night	21% 6% 26% 31% 13% 3	551
	11th to 20th	Night	14% 5% 28% 38% 14%	394
	21st to end	Night	12% 4 21% 39% 21% 2	433
June	1st to 10th	Night	13% 3 26% 43% 15% 1	400
	11th to 20th	Night	11% 4 24% 42% 19% 1	400
	21st to end	Night	12% 4 25% 41% 18% 1	400
July	1st to 10th	Night	6% 2 19% 50% 22% 2	390
	11th to 20th	Night	10% 6% 15% 42% 26% 1	388
	21st to end	Night	11% 2 20% 43% 22% 2	547
August	1st to 10th	Night	11% 2 15% 38% 31% 3	597
	11th to 20th	Night	10% 1 15% 39% 30% 4	594
	21st to end	Night	12% 3 23% 35% 25% 2	783
September	1st to 10th	Night	13% 5% 21% 35% 23% 4	800
	11th to 20th	Night	12% 4 18% 32% 27% 7%	800
	21st to end	Night	19% 7% 19% 29% 21% 5%	800
October	1st to 10th	Night	18% 6% 28% 27% 19% 2	800
	11th to 20th	Night	22% 12% 30% 26% 9% 1	800
	21st to end	Night	24% 11% 33% 27% 5%	880
November	1st to 10th	Night	22% 12% 26% 29% 10% 1	820
	11th to 20th	Night	21% 16% 33% 23% 6% 2	840
	21st to end	Night	26% 13% 33% 22% 6%	840
December	1st to 10th	Night	27% 15% 35% 19% 4	840
	11th to 20th	Night	25% 14% 36% 21% 4	840
	21st to end	Night	31% 14% 35% 17% 2	924

Page generated at 2026-01-14 01:14:30. Version ac07

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