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 2024-11-25.

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% 4 5% 4 6% 7% 35%	101671
	Night	16% 9% 3 7% 5% 4 5% 4 5% 7% 35%	97801
Transparency	Day	45% 8% 14% 21% 10% 1	105271
	Night	46% 8% 14% 20% 11% 1	102289
Seeing	Day	19% 11% 29% 25% 12% 4	14504
	Night	19% 10% 30% 28% 12% 1	23847

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% 4 8% 6% 5% 4 3 4 6% 30%	2517
	11th to 20th	Day	11% 20% 6% 15% 7% 5% 4 4 4 5% 19%	2652
	21st to end	Day	21% 19% 7% 10% 6% 5% 4 3 4 4 19%	3124
February	1st to 10th	Day	15% 15% 5% 13% 7% 6% 5% 3 5% 5% 22%	1710
	11th to 20th	Day	13% 15% 6% 12% 6% 4 4 3 4 7% 26%	1663
	21st to end	Day	19% 13% 5% 10% 6% 4 4 3 5% 4 28%	1467
March	1st to 10th	Day	20% 10% 3 8% 5% 4 4 3 4 6% 31%	3418
	11th to 20th	Day	11% 14% 5% 10% 5% 5% 4 4 5% 6% 30%	3480
	21st to end	Day	17% 8% 2 6% 4 4 4 4 5% 6% 40%	3815
April	1st to 10th	Day	18% 11% 4 8% 6% 4 4 4 5% 6% 30%	2280
	11th to 20th	Day	20% 8% 3 7% 5% 3 4 3 5% 6% 36%	2278
	21st to end	Day	19% 9% 3 7% 5% 5% 4 4 5% 5% 34%	2470
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 4 4 6% 7% 32%	4190
	11th to 20th	Day	15% 10% 3 8% 5% 5% 4 3 6% 8% 33%	4398
	21st to end	Day	12% 6% 2 6% 4 4 4 3 6% 9% 46%	4837
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 4 4 6% 8% 36%	2660
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 40%	2660
	21st to end	Day	14% 6% 2 6% 5% 4 5% 4 7% 8% 40%	2660
July	1st to 10th	Day	9% 5% 2 6% 4 4 6% 4 7% 11% 42%	4440
	11th to 20th	Day	11% 4 2 6% 4 5% 4 4 7% 9% 45%	4440
	21st to end	Day	6% 5% 2 6% 5% 6% 6% 5% 8% 9% 43%	4633
August	1st to 10th	Day	15% 8% 2 6% 4 5% 5% 4 5% 7% 39%	4104
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 5% 7% 11% 38%	4044
	21st to end	Day	9% 6% 2 5% 4 4 5% 4 7% 9% 44%	4290
September	1st to 10th	Day	13% 5% 2 5% 5% 4 5% 4 5% 7% 44%	2280
	11th to 20th	Day	11% 4 2 4 4 4 4 4 7% 7% 48%	2227
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	2014
October	1st to 10th	Day	16% 9% 2 6% 4 5% 6% 4 5% 6% 36%	1900
	11th to 20th	Day	19% 10% 2 8% 5% 4 4 3 5% 6% 32%	1900
	21st to end	Day	22% 10% 5% 8% 5% 5% 5% 3 5% 6% 26%	2090
November	1st to 10th	Day	15% 9% 3 9% 5% 6% 4 4 6% 6% 33%	1680
	11th to 20th	Day	15% 11% 5% 9% 5% 5% 5% 4 7% 7% 27%	1600
	21st to end	Day	20% 15% 3 7% 6% 5% 4 3 5% 7% 24%	1552
December	1st to 10th	Day	17% 15% 5% 10% 7% 5% 6% 4 5% 5% 20%	1520
	11th to 20th	Day	18% 14% 4 9% 5% 5% 5% 4 4 7% 27%	1906
	21st to end	Day	28% 17% 5% 10% 5% 4 5% 3 3 4 15%	2772
January	1st to 10th	Night	18% 15% 5% 9% 6% 5% 4 3 5% 6% 24%	4429
	11th to 20th	Night	15% 13% 4 13% 9% 5% 5% 4 5% 5% 21%	4307
	21st to end	Night	20% 13% 5% 9% 5% 5% 5% 3 4 5% 24%	4532
February	1st to 10th	Night	14% 16% 4 9% 7% 6% 6% 4 5% 5% 24%	2470
	11th to 20th	Night	15% 13% 5% 9% 7% 4 5% 3 5% 6% 28%	2403
	21st to end	Night	16% 10% 4 9% 5% 5% 5% 4 5% 6% 30%	1986
March	1st to 10th	Night	18% 10% 3 8% 6% 4 5% 3 5% 5% 33%	3421
	11th to 20th	Night	15% 10% 4 8% 5% 5% 5% 4 5% 6% 34%	3479
	21st to end	Night	17% 6% 3 5% 4 4 4 3 5% 6% 44%	3803
April	1st to 10th	Night	19% 10% 3 7% 4 4 5% 3 4 7% 34%	1900
	11th to 20th	Night	17% 8% 3 6% 5% 4 6% 3 5% 6% 38%	1866
	21st to end	Night	17% 9% 3 6% 4 5% 5% 4 5% 7% 35%	1710
May	1st to 10th	Night	19% 8% 2 6% 4 4 4 4 5% 6% 39%	2748
	11th to 20th	Night	14% 8% 3 7% 5% 4 5% 4 7% 7% 36%	2495
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 49%	2738
June	1st to 10th	Night	13% 6% 3 6% 5% 4 7% 4 6% 8% 40%	1520
	11th to 20th	Night	10% 4 2 7% 5% 4 5% 4 7% 9% 44%	1520
	21st to end	Night	12% 7% 3 6% 4 5% 4 4 6% 8% 42%	1520
July	1st to 10th	Night	7% 5% 2 5% 5% 4 5% 5% 6% 9% 47%	2520
	11th to 20th	Night	11% 5% 1 5% 3 4 3 4 6% 8% 50%	2520
	21st to end	Night	9% 4 2 4 4 3 4 4 7% 8% 52%	2917
August	1st to 10th	Night	11% 5% 2 6% 4 3 5% 4 5% 8% 48%	2825
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 5% 7% 50%	2785
	21st to end	Night	10% 5% 2 5% 3 4 4 4 6% 8% 49%	3332
September	1st to 10th	Night	12% 6% 2 5% 4 4 5% 3 5% 7% 48%	1900
	11th to 20th	Night	10% 5% 1 4 3 4 4 4 5% 7% 53%	1950
	21st to end	Night	16% 7% 3 6% 4 3 4 3 4 6% 44%	2165
October	1st to 10th	Night	15% 7% 3 7% 5% 4 4 3 5% 7% 40%	2280
	11th to 20th	Night	20% 8% 3 7% 5% 5% 4 4 5% 7% 33%	2280
	21st to end	Night	20% 10% 3 7% 4 4 5% 4 5% 6% 31%	2508
November	1st to 10th	Night	17% 8% 2 6% 4 5% 4 4 6% 7% 37%	2617
	11th to 20th	Night	17% 11% 3 7% 5% 5% 5% 4 6% 8% 30%	2800
	21st to end	Night	21% 12% 3 7% 5% 5% 5% 5% 6% 7% 25%	2723
December	1st to 10th	Night	19% 14% 4 11% 6% 4 4 4 5% 6% 22%	2660
	11th to 20th	Night	16% 16% 4 8% 6% 5% 5% 4 6% 5% 24%	3288
	21st to end	Night	30% 15% 4 8% 4 4 5% 3 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	52% 13% 17% 11% 6% 1	2518
	11th to 20th	Day	68% 9% 12% 6% 4	2652
	21st to end	Day	68% 8% 11% 9% 4 1	3124
February	1st to 10th	Day	64% 7% 12% 14% 3 1	1710
	11th to 20th	Day	58% 8% 11% 15% 8% 1	1672
	21st to end	Day	57% 7% 12% 17% 7%	1467
March	1st to 10th	Day	51% 8% 14% 17% 9% 1	3421
	11th to 20th	Day	49% 10% 15% 17% 9% 1	3480
	21st to end	Day	40% 9% 11% 21% 16% 2	3815
April	1st to 10th	Day	52% 7% 12% 17% 10% 2	2280
	11th to 20th	Day	47% 8% 9% 20% 14% 1	2278
	21st to end	Day	49% 8% 11% 18% 12% 2	2470
May	1st to 10th	Day	46% 10% 12% 17% 13% 2	4202
	11th to 20th	Day	46% 12% 15% 17% 9% 1	4402
	21st to end	Day	33% 9% 15% 25% 15% 3	4837
June	1st to 10th	Day	43% 9% 14% 20% 13% 1	2660
	11th to 20th	Day	37% 8% 15% 23% 15% 2	2660
	21st to end	Day	40% 8% 16% 22% 13% 1	2660
July	1st to 10th	Day	35% 5% 15% 27% 15% 1	4370
	11th to 20th	Day	34% 5% 15% 36% 10% 1	4365
	21st to end	Day	34% 8% 16% 25% 16% 1	4485
August	1st to 10th	Day	44% 7% 15% 21% 13% 1	4091
	11th to 20th	Day	39% 6% 16% 29% 10%	4083
	21st to end	Day	36% 5% 14% 35% 9% 1	4290
September	1st to 10th	Day	37% 9% 14% 28% 11% 1	2280
	11th to 20th	Day	32% 8% 16% 29% 14% 1	2227
	21st to end	Day	41% 9% 16% 24% 10%	2014
October	1st to 10th	Day	40% 5% 13% 28% 14% 1	3160
	11th to 20th	Day	40% 5% 17% 23% 15% 1	3140
	21st to end	Day	51% 6% 16% 20% 7%	3428
November	1st to 10th	Day	48% 8% 15% 19% 9% 1	1680
	11th to 20th	Day	52% 10% 14% 15% 8% 1	1600
	21st to end	Day	58% 9% 14% 15% 4	1552
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	59% 12% 14% 12% 3	4429
	11th to 20th	Night	61% 15% 14% 6% 3 1	4307
	21st to end	Night	61% 11% 12% 11% 4 1	4532
February	1st to 10th	Night	59% 8% 13% 15% 4 1	2470
	11th to 20th	Night	55% 8% 12% 17% 8%	2405
	21st to end	Night	52% 8% 15% 19% 7% 1	1986
March	1st to 10th	Night	51% 8% 12% 21% 7% 1	3431
	11th to 20th	Night	45% 10% 15% 19% 9% 1	3480
	21st to end	Night	37% 8% 12% 24% 18% 2	3803
April	1st to 10th	Night	49% 7% 11% 19% 13% 1	1900
	11th to 20th	Night	47% 7% 10% 18% 17% 2	1866
	21st to end	Night	46% 8% 12% 17% 15% 2	1710
May	1st to 10th	Night	43% 9% 14% 16% 15% 3	2755
	11th to 20th	Night	42% 12% 15% 19% 9% 2	2496
	21st to end	Night	30% 10% 15% 23% 21% 2	2739
June	1st to 10th	Night	40% 10% 12% 21% 15% 2	1520
	11th to 20th	Night	35% 8% 13% 21% 21% 2	1520
	21st to end	Night	39% 7% 13% 24% 15% 1	1520
July	1st to 10th	Night	33% 5% 15% 25% 21% 2	2480
	11th to 20th	Night	33% 4 14% 33% 15% 1	2472
	21st to end	Night	29% 6% 16% 31% 16% 1	2824
August	1st to 10th	Night	35% 5% 14% 28% 16% 2	2814
	11th to 20th	Night	34% 5% 16% 31% 12% 1	2812
	21st to end	Night	33% 5% 12% 33% 15% 1	3332
September	1st to 10th	Night	35% 7% 15% 27% 14% 2	1900
	11th to 20th	Night	30% 7% 15% 30% 18% 1	1950
	21st to end	Night	41% 7% 12% 27% 13% 1	2165
October	1st to 10th	Night	35% 4 11% 27% 23% 1	3800
	11th to 20th	Night	41% 6% 14% 30% 9%	3779
	21st to end	Night	42% 6% 15% 27% 10% 1	4120
November	1st to 10th	Night	44% 8% 14% 22% 12% 1	2617
	11th to 20th	Night	50% 9% 14% 16% 9% 1	2800
	21st to end	Night	55% 11% 17% 13% 4	2723
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	25% 23% 32% 14% 5% 1	218
	21st to end	Day	24% 21% 31% 21% 3	297
February	1st to 10th	Day	23% 28% 32% 14% 3	270
	11th to 20th	Day	22% 21% 40% 14% 1	270
	21st to end	Day	29% 22% 35% 14% 1	279
March	1st to 10th	Day	23% 15% 38% 19% 5%	460
	11th to 20th	Day	21% 20% 35% 20% 4 1	460
	21st to end	Day	29% 10% 34% 18% 8% 1	506
April	1st to 10th	Day	22% 14% 36% 22% 6% 1	460
	11th to 20th	Day	28% 13% 29% 19% 9% 4	452
	21st to end	Day	23% 12% 38% 19% 8% 1	460
May	1st to 10th	Day	21% 10% 29% 25% 10% 4	463
	11th to 20th	Day	17% 7% 27% 30% 15% 4	487
	21st to end	Day	12% 6% 25% 33% 19% 5%	529
June	1st to 10th	Day	17% 7% 30% 31% 14% 2	490
	11th to 20th	Day	12% 8% 27% 31% 19% 3	490
	21st to end	Day	20% 5% 26% 30% 15% 4	490
July	1st to 10th	Day	10% 4 21% 35% 24% 6%	485
	11th to 20th	Day	14% 4 23% 30% 21% 7%	484
	21st to end	Day	9% 3 27% 39% 17% 5%	532
August	1st to 10th	Day	13% 4 21% 32% 22% 8%	486
	11th to 20th	Day	11% 3 22% 31% 24% 9%	479
	21st to end	Day	12% 5% 25% 31% 19% 7%	539
September	1st to 10th	Day	13% 6% 24% 27% 20% 9%	490
	11th to 20th	Day	10% 5% 20% 30% 21% 14%	490
	21st to end	Day	15% 8% 29% 27% 15% 6%	490
October	1st to 10th	Day	20% 11% 27% 23% 14% 5%	490
	11th to 20th	Day	19% 16% 33% 24% 8% 1	490
	21st to end	Day	27% 16% 29% 21% 6% 1	539
November	1st to 10th	Day	26% 15% 28% 21% 9% 1	303
	11th to 20th	Day	20% 23% 36% 16% 5% 2	200
	21st to end	Day	26% 18% 34% 19% 4	188
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	27% 14% 34% 20% 4 1	760
	21st to end	Night	19% 13% 36% 25% 6%	836
February	1st to 10th	Night	21% 17% 40% 21% 2	760
	11th to 20th	Night	20% 16% 39% 22% 3 1	743
	21st to end	Night	20% 16% 39% 21% 4	628
March	1st to 10th	Night	19% 13% 38% 26% 4	760
	11th to 20th	Night	21% 15% 34% 25% 4 1	760
	21st to end	Night	18% 11% 34% 27% 10%	836
April	1st to 10th	Night	22% 12% 33% 28% 5%	760
	11th to 20th	Night	19% 11% 33% 26% 10% 1	742
	21st to end	Night	18% 9% 31% 30% 12%	570
May	1st to 10th	Night	19% 5% 27% 32% 14% 3	524
	11th to 20th	Night	14% 4 29% 38% 14%	374
	21st to end	Night	11% 3 21% 40% 23% 2	411
June	1st to 10th	Night	13% 3 25% 43% 16% 1	380
	11th to 20th	Night	10% 4 24% 42% 19% 1	380
	21st to end	Night	12% 3 23% 42% 19% 1	380
July	1st to 10th	Night	6% 2 18% 51% 22% 2	370
	11th to 20th	Night	10% 5% 16% 42% 26% 1	368
	21st to end	Night	10% 2 19% 44% 23% 3	519
August	1st to 10th	Night	11% 2 14% 38% 30% 4	567
	11th to 20th	Night	10% 1 15% 40% 30% 5%	564
	21st to end	Night	12% 3 22% 35% 26% 3	744
September	1st to 10th	Night	13% 5% 21% 34% 22% 4	760
	11th to 20th	Night	13% 4 18% 32% 27% 7%	760
	21st to end	Night	19% 7% 19% 28% 21% 5%	760
October	1st to 10th	Night	19% 6% 28% 27% 19% 2	760
	11th to 20th	Night	22% 12% 30% 27% 9% 1	760
	21st to end	Night	24% 10% 32% 27% 5%	836
November	1st to 10th	Night	22% 11% 26% 29% 11% 1	780
	11th to 20th	Night	21% 15% 32% 23% 6% 2	800
	21st to end	Night	26% 12% 33% 23% 6%	779
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

Page generated at 2024-11-22 08:02:24. Version ac07

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