Bayt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bayt Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	7% 4 1 3 2 2 2 2 3 5% 68%	134800
	Night	8% 4 1 3 2 2 2 2 4 5% 66%	131991
Transparency	Day	21% 3 6% 18% 27% 25%	129824
	Night	22% 3 7% 20% 26% 21%	127944
Seeing	Day	11% 9% 29% 34% 13% 4	16451
	Night	11% 3 15% 36% 29% 7%	25900

By Month

Percentage Cloud Cover SW (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	11% 7% 2 4 2 2 2 3 5% 8% 54%	3639
	11th to 20th	Day	11% 5% 3 5% 4 4 3 3 3 4 56%	3898
	21st to end	Day	13% 7% 2 5% 4 4 4 3 5% 6% 47%	4510
February	1st to 10th	Day	17% 7% 2 4 2 3 3 4 4 5% 49%	1885
	11th to 20th	Day	13% 8% 2 5% 3 3 3 4 5% 6% 48%	1950
	21st to end	Day	16% 10% 3 6% 4 3 4 2 4 4 44%	1650
March	1st to 10th	Day	14% 13% 2 7% 4 4 4 4 5% 7% 35%	4518
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 4 7% 47%	4906
	21st to end	Day	17% 8% 2 5% 4 4 4 4 5% 7% 40%	5867
April	1st to 10th	Day	12% 7% 2 4 3 3 3 3 5% 6% 53%	2400
	11th to 20th	Day	10% 7% 1 5% 4 3 3 3 5% 7% 52%	2369
	21st to end	Day	9% 6% 1 3 4 3 4 3 4 6% 56%	2400
May	1st to 10th	Day	5% 4 1 3 2 2 2 2 3 4 73%	5589
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 76%	5898
	21st to end	Day	3 2 1 2 1 2 2 2 3 4 79%	6902
June	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 6% 69%	2800
	11th to 20th	Day	1 1 1 2 1 2 2 1 2 4 84%	2800
	21st to end	Day	1 2 2 1 1 1 1 2 3 84%	2800
July	1st to 10th	Day	1 1 1 1 2 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	6320
	21st to end	Day	1 1 1 1 1 2 2 3 3 85%	6540
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 89%	5523
	11th to 20th	Day	1 1 1 1 2 93%	5364
	21st to end	Day	1 1 1 1 1 1 2 1 2 3 84%	5984
September	1st to 10th	Day	2 1 1 1 2 2 1 1 2 87%	2400
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 4 80%	2348
	21st to end	Day	3 2 1 2 1 1 2 2 4 4 80%	2200
October	1st to 10th	Day	3 3 1 2 2 2 2 3 4 4 76%	3323
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 3 78%	3161
	21st to end	Day	4 2 1 4 3 3 4 3 4 7% 65%	3417
November	1st to 10th	Day	9% 5% 1 3 3 3 3 3 5% 6% 58%	1764
	11th to 20th	Day	7% 4 1 3 3 2 3 3 5% 6% 62%	1800
	21st to end	Day	11% 5% 2 4 3 3 2 3 3 6% 59%	1800
December	1st to 10th	Day	8% 3 1 3 2 3 4 3 6% 6% 60%	2724
	11th to 20th	Day	20% 7% 2 6% 4 3 3 3 4 7% 41%	3028
	21st to end	Day	17% 7% 3 6% 3 4 4 3 6% 8% 40%	4004
January	1st to 10th	Night	11% 5% 2 4 2 3 3 3 6% 7% 54%	6306
	11th to 20th	Night	14% 5% 2 3 2 3 3 3 4 5% 56%	6056
	21st to end	Night	14% 6% 2 5% 3 4 3 3 4 7% 50%	6468
February	1st to 10th	Night	16% 7% 2 4 3 2 3 3 4 6% 50%	2512
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 4 7% 53%	2338
	21st to end	Night	14% 8% 2 4 3 2 2 2 4 6% 51%	1980
March	1st to 10th	Night	16% 9% 2 5% 4 4 4 4 6% 6% 41%	5376
	11th to 20th	Night	13% 7% 2 5% 4 3 3 3 4 6% 51%	5091
	21st to end	Night	20% 7% 2 4 3 2 2 3 4 7% 47%	5106
April	1st to 10th	Night	12% 5% 1 3 3 2 3 3 5% 6% 59%	2000
	11th to 20th	Night	8% 4 1 4 3 3 4 3 4 4 61%	1979
	21st to end	Night	7% 5% 1 3 3 2 2 3 5% 6% 65%	2000
May	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 77%	4388
	11th to 20th	Night	2 1 1 2 1 2 2 2 3 5% 77%	3995
	21st to end	Night	2 1 1 1 1 1 1 1 2 4 85%	3972
June	1st to 10th	Night	3 3 1 2 2 2 2 2 4 6% 72%	1600
	11th to 20th	Night	1 1 1 1 1 2 2 2 4 86%	1600
	21st to end	Night	1 1 1 1 1 1 1 1 4 87%	1600
July	1st to 10th	Night	1 1 1 1 95%	3640
	11th to 20th	Night	2 2 1 2 1 1 1 1 2 2 85%	3640
	21st to end	Night	1 1 1 1 1 1 1 2 3 86%	4323
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 87%	4299
	11th to 20th	Night	1 1 1 1 1 2 93%	4506
	21st to end	Night	2 1 1 1 1 2 2 2 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 2 90%	2000
	11th to 20th	Night	3 2 1 2 1 2 1 2 3 3 82%	2050
	21st to end	Night	2 1 1 1 1 1 2 2 3 4 82%	2200
October	1st to 10th	Night	2 1 1 2 1 2 2 2 3 3 82%	3708
	11th to 20th	Night	2 1 1 2 1 2 2 2 3 4 81%	3777
	21st to end	Night	5% 2 1 3 1 2 2 2 3 5% 75%	4149
November	1st to 10th	Night	9% 4 1 3 2 2 2 2 4 6% 64%	2535
	11th to 20th	Night	7% 3 1 4 2 2 3 3 5% 7% 64%	2600
	21st to end	Night	10% 5% 2 3 3 2 3 3 5% 6% 58%	2600
December	1st to 10th	Night	8% 3 1 4 3 3 3 2 4 6% 62%	4454
	11th to 20th	Night	20% 7% 2 4 3 2 3 4 6% 9% 40%	5176
	21st to end	Night	13% 7% 2 8% 5% 5% 6% 5% 7% 7% 35%	6952

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	29% 3 17% 26% 19% 6%	3639
	11th to 20th	Day	33% 4 6% 14% 22% 21%	3898
	21st to end	Day	37% 4 9% 16% 22% 11%	4510
February	1st to 10th	Day	38% 4 7% 24% 19% 9%	1885
	11th to 20th	Day	37% 4 8% 22% 21% 7%	1960
	21st to end	Day	44% 4 7% 23% 19% 3	1650
March	1st to 10th	Day	46% 7% 8% 15% 18% 6%	4518
	11th to 20th	Day	36% 6% 13% 28% 9% 8%	4906
	21st to end	Day	41% 8% 12% 22% 15% 3	5867
April	1st to 10th	Day	32% 4 12% 27% 21% 5%	2400
	11th to 20th	Day	32% 5% 10% 26% 22% 5%	2372
	21st to end	Day	28% 5% 9% 32% 23% 3	2400
May	1st to 10th	Day	17% 4 6% 29% 36% 8%	5589
	11th to 20th	Day	13% 3 5% 35% 38% 6%	5901
	21st to end	Day	12% 2 3 28% 50% 5%	6904
June	1st to 10th	Day	17% 5% 9% 23% 33% 13%	2800
	11th to 20th	Day	8% 3 4 18% 41% 26%	2800
	21st to end	Day	9% 2 2 9% 36% 43%	2800
July	1st to 10th	Day	3 1 3 19% 74%	6251
	11th to 20th	Day	5% 1 2 4 19% 69%	6245
	21st to end	Day	6% 2 2 9% 38% 44%	6351
August	1st to 10th	Day	8% 1 1 6% 32% 51%	4018
	11th to 20th	Day	4 1 1 5% 27% 61%	3880
	21st to end	Day	6% 1 2 7% 31% 52%	4311
September	1st to 10th	Day	7% 2 2 9% 32% 49%	2400
	11th to 20th	Day	11% 3 2 21% 38% 25%	2348
	21st to end	Day	10% 2 3 18% 38% 29%	2200
October	1st to 10th	Day	14% 2 4 10% 21% 49%	3323
	11th to 20th	Day	13% 2 3 19% 40% 23%	3161
	21st to end	Day	19% 3 4 16% 39% 19%	3417
November	1st to 10th	Day	27% 4 7% 21% 23% 17%	1764
	11th to 20th	Day	23% 4 8% 24% 27% 14%	1800
	21st to end	Day	30% 2 7% 16% 28% 17%	1800
December	1st to 10th	Day	25% 3 6% 25% 28% 14%	2724
	11th to 20th	Day	44% 4 9% 14% 22% 7%	3028
	21st to end	Day	42% 5% 13% 25% 10% 5%	4004
January	1st to 10th	Night	29% 4 16% 26% 19% 7%	6306
	11th to 20th	Night	31% 4 7% 18% 27% 14%	6056
	21st to end	Night	35% 4 8% 18% 21% 14%	6468
February	1st to 10th	Night	37% 3 8% 22% 20% 11%	2512
	11th to 20th	Night	32% 4 10% 25% 22% 8%	2339
	21st to end	Night	37% 2 10% 27% 17% 7%	1980
March	1st to 10th	Night	41% 5% 9% 18% 20% 7%	5376
	11th to 20th	Night	35% 4 12% 30% 11% 7%	5091
	21st to end	Night	38% 5% 19% 20% 16% 3	5106
April	1st to 10th	Night	28% 4 17% 28% 19% 6%	2000
	11th to 20th	Night	26% 5% 12% 30% 20% 8%	1979
	21st to end	Night	21% 5% 11% 35% 24% 3	2000
May	1st to 10th	Night	13% 3 9% 36% 31% 8%	4385
	11th to 20th	Night	10% 3 6% 35% 39% 7%	3996
	21st to end	Night	7% 2 5% 32% 50% 5%	3972
June	1st to 10th	Night	15% 4 9% 25% 33% 14%	1600
	11th to 20th	Night	6% 2 4 23% 38% 27%	1600
	21st to end	Night	6% 1 2 11% 34% 45%	1600
July	1st to 10th	Night	2 1 1 3 17% 78%	3600
	11th to 20th	Night	8% 1 1 4 20% 66%	3592
	21st to end	Night	6% 1 1 11% 35% 46%	4200
August	1st to 10th	Night	9% 2 1 9% 27% 52%	3121
	11th to 20th	Night	4 1 1 5% 21% 67%	3256
	21st to end	Night	7% 1 1 6% 32% 53%	3608
September	1st to 10th	Night	5% 2 1 10% 30% 52%	2000
	11th to 20th	Night	10% 2 3 17% 44% 24%	2050
	21st to end	Night	9% 2 2 20% 40% 27%	2200
October	1st to 10th	Night	10% 2 3 11% 26% 48%	3708
	11th to 20th	Night	10% 2 4 21% 41% 22%	3777
	21st to end	Night	14% 2 3 20% 36% 24%	4149
November	1st to 10th	Night	23% 3 9% 23% 25% 18%	2535
	11th to 20th	Night	21% 3 9% 22% 29% 16%	2600
	21st to end	Night	27% 2 7% 20% 29% 14%	2600
December	1st to 10th	Night	25% 2 6% 25% 25% 16%	4454
	11th to 20th	Night	40% 4 11% 17% 22% 6%	5176
	21st to end	Night	44% 6% 11% 23% 10% 5%	6952

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	31% 8% 23% 23% 12% 5%	200
	11th to 20th	Day	20% 7% 25% 28% 15% 5%	315
	21st to end	Day	21% 7% 27% 26% 14% 5%	440
February	1st to 10th	Day	26% 13% 26% 21% 12% 3	443
	11th to 20th	Day	21% 13% 33% 22% 9% 2	495
	21st to end	Day	18% 13% 35% 25% 8% 1	414
March	1st to 10th	Day	26% 10% 33% 23% 7% 1	500
	11th to 20th	Day	25% 13% 28% 23% 9% 2	500
	21st to end	Day	23% 15% 29% 26% 6% 1	550
April	1st to 10th	Day	18% 15% 32% 26% 9%	500
	11th to 20th	Day	18% 18% 31% 25% 7% 1	492
	21st to end	Day	11% 16% 31% 28% 12% 3	500
May	1st to 10th	Day	12% 16% 34% 28% 7% 3	503
	11th to 20th	Day	12% 13% 31% 30% 11% 3	528
	21st to end	Day	12% 9% 28% 35% 11% 4	573
June	1st to 10th	Day	5% 10% 28% 36% 16% 5%	530
	11th to 20th	Day	4 10% 35% 34% 12% 5%	530
	21st to end	Day	3 7% 31% 39% 13% 7%	530
July	1st to 10th	Day	2 3 28% 47% 14% 7%	526
	11th to 20th	Day	2 1 19% 52% 14% 12%	524
	21st to end	Day	1 2 23% 52% 14% 8%	575
August	1st to 10th	Day	4 4 33% 40% 14% 6%	527
	11th to 20th	Day	2 5% 25% 47% 15% 7%	517
	21st to end	Day	2 2 26% 49% 17% 5%	583
September	1st to 10th	Day	4 4 30% 46% 15% 3	530
	11th to 20th	Day	5% 8% 32% 38% 15% 2	530
	21st to end	Day	4 7% 31% 43% 14% 2	530
October	1st to 10th	Day	4 9% 32% 37% 16% 2	503
	11th to 20th	Day	4 10% 32% 38% 14% 2	490
	21st to end	Day	7% 8% 27% 38% 17% 3	520
November	1st to 10th	Day	12% 6% 27% 32% 19% 4	300
	11th to 20th	Day	7% 10% 32% 29% 16% 6%	300
	21st to end	Day	16% 8% 24% 30% 17% 5%	300
December	1st to 10th	Day	15% 3 28% 29% 20% 5%	233
	11th to 20th	Day	17% 6% 25% 32% 15% 6%	200
	21st to end	Day	23% 6% 29% 23% 12% 7%	220
January	1st to 10th	Night	19% 4 20% 32% 18% 8%	800
	11th to 20th	Night	13% 4 23% 35% 19% 6%	800
	21st to end	Night	19% 4 19% 34% 19% 5%	880
February	1st to 10th	Night	22% 5% 16% 34% 19% 5%	800
	11th to 20th	Night	18% 6% 17% 30% 23% 7%	784
	21st to end	Night	20% 4 18% 38% 16% 3	660
March	1st to 10th	Night	22% 5% 17% 36% 16% 4	800
	11th to 20th	Night	18% 6% 14% 31% 26% 5%	800
	21st to end	Night	19% 3 16% 38% 20% 3	880
April	1st to 10th	Night	14% 5% 18% 39% 21% 3	800
	11th to 20th	Night	11% 6% 19% 36% 24% 4	791
	21st to end	Night	9% 3 17% 39% 26% 6%	800
May	1st to 10th	Night	7% 3 19% 39% 26% 6%	732
	11th to 20th	Night	6% 3 18% 37% 34% 3	565
	21st to end	Night	6% 1 16% 37% 34% 6%	433
June	1st to 10th	Night	5% 2 10% 37% 40% 7%	400
	11th to 20th	Night	1 1 16% 40% 40% 4	400
	21st to end	Night	2 13% 33% 47% 5%	400
July	1st to 10th	Night	12% 35% 46% 7%	390
	11th to 20th	Night	3 1 6% 37% 44% 9%	388
	21st to end	Night	2 1 9% 29% 46% 14%	586
August	1st to 10th	Night	2 1 12% 33% 43% 8%	711
	11th to 20th	Night	1 10% 33% 48% 8%	792
	21st to end	Night	2 8% 30% 50% 10%	880
September	1st to 10th	Night	1 1 9% 38% 41% 11%	800
	11th to 20th	Night	3 1 10% 38% 39% 9%	800
	21st to end	Night	3 1 12% 39% 39% 7%	800
October	1st to 10th	Night	5% 1 12% 47% 29% 6%	772
	11th to 20th	Night	6% 2 13% 40% 31% 8%	760
	21st to end	Night	5% 2 16% 31% 31% 14%	836
November	1st to 10th	Night	11% 2 12% 36% 27% 12%	780
	11th to 20th	Night	9% 3 19% 35% 27% 7%	800
	21st to end	Night	12% 3 18% 37% 23% 7%	800
December	1st to 10th	Night	16% 3 17% 38% 21% 5%	800
	11th to 20th	Night	21% 2 20% 35% 17% 5%	800
	21st to end	Night	21% 5% 20% 33% 16% 4	880

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