Baskin Engineering Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Baskin Engineering Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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% 5% 2 4 3 3 3 3 5% 8% 56%	123925
	Night	9% 6% 2 5% 4 3 4 3 6% 8% 51%	124108
Transparency	Day	26% 6% 10% 20% 23% 14%	123686
	Night	31% 7% 10% 19% 21% 12%	123935
Seeing	Day	9% 5% 18% 32% 28% 8%	16396
	Night	11% 3 11% 31% 33% 10%	25780

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	12% 5% 2 5% 2 3 3 3 5% 7% 53%	3961
	11th to 20th	Day	12% 5% 2 5% 4 4 3 3 4 5% 53%	4100
	21st to end	Day	15% 7% 3 6% 5% 4 4 3 5% 6% 42%	4510
February	1st to 10th	Day	20% 5% 2 5% 4 3 3 3 5% 6% 44%	1800
	11th to 20th	Day	13% 5% 2 6% 5% 4 4 4 6% 7% 44%	1866
	21st to end	Day	13% 5% 3 6% 4 3 5% 4 5% 6% 46%	1650
March	1st to 10th	Day	12% 8% 3 8% 5% 5% 6% 5% 7% 8% 33%	4868
	11th to 20th	Day	13% 5% 2 4 3 4 4 3 6% 8% 49%	5000
	21st to end	Day	14% 9% 3 6% 5% 4 6% 4 7% 10% 34%	5568
April	1st to 10th	Day	11% 6% 2 4 5% 4 3 4 5% 8% 48%	2400
	11th to 20th	Day	7% 6% 2 5% 3 3 4 4 6% 8% 52%	2369
	21st to end	Day	5% 5% 2 6% 4 4 4 4 6% 8% 52%	2400
May	1st to 10th	Day	4 3 1 3 2 2 2 2 4 8% 69%	5525
	11th to 20th	Day	2 4 2 4 3 3 3 3 5% 9% 63%	5899
	21st to end	Day	2 4 1 4 3 2 3 2 5% 8% 65%	6902
June	1st to 10th	Day	4 6% 2 5% 4 3 4 4 6% 9% 55%	2800
	11th to 20th	Day	2 4 1 3 3 2 2 3 6% 8% 65%	2800
	21st to end	Day	2 3 1 4 3 3 3 3 5% 9% 65%	2800
July	1st to 10th	Day	1 3 1 3 3 3 2 3 4 8% 70%	4505
	11th to 20th	Day	1 3 2 5% 3 3 3 3 6% 8% 63%	4240
	21st to end	Day	4 1 3 2 2 3 4 6% 9% 64%	4456
August	1st to 10th	Day	1 3 1 5% 4 4 5% 5% 7% 10% 56%	4091
	11th to 20th	Day	2 1 2 2 3 3 3 6% 10% 67%	3791
	21st to end	Day	3 2 5% 3 2 3 3 6% 9% 64%	4180
September	1st to 10th	Day	2 3 1 2 2 2 2 3 4 7% 72%	2280
	11th to 20th	Day	3 4 2 4 3 3 4 3 5% 10% 58%	2280
	21st to end	Day	3 3 1 3 2 2 3 3 5% 6% 72%	2019
October	1st to 10th	Day	2 2 1 3 3 2 3 3 4 8% 69%	3180
	11th to 20th	Day	3 2 1 4 3 3 4 3 4 7% 66%	3161
	21st to end	Day	4 3 1 3 2 3 4 3 5% 9% 62%	3448
November	1st to 10th	Day	8% 5% 1 4 3 4 4 3 6% 7% 56%	1944
	11th to 20th	Day	9% 6% 2 5% 3 3 4 3 6% 7% 53%	1800
	21st to end	Day	12% 4 2 4 3 2 3 3 4 8% 55%	1680
December	1st to 10th	Day	9% 4 1 4 3 3 4 3 5% 9% 53%	2620
	11th to 20th	Day	22% 7% 2 5% 3 3 4 3 6% 7% 37%	3028
	21st to end	Day	26% 8% 2 5% 3 3 3 3 5% 7% 34%	4004
January	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 8% 55%	5982
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 7% 50%	5880
	21st to end	Night	14% 7% 2 6% 3 4 4 3 5% 7% 45%	6468
February	1st to 10th	Night	19% 5% 2 4 4 2 3 3 4 5% 48%	2600
	11th to 20th	Night	13% 6% 2 4 3 4 4 5% 6% 7% 45%	2420
	21st to end	Night	11% 6% 3 4 2 3 3 3 6% 7% 52%	1980
March	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 5% 6% 8% 34%	5011
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 6% 7% 48%	5000
	21st to end	Night	16% 9% 3 5% 4 3 4 3 5% 7% 41%	5406
April	1st to 10th	Night	11% 6% 2 4 4 3 4 4 6% 8% 49%	2000
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 6% 8% 51%	1979
	21st to end	Night	5% 5% 2 5% 3 3 4 3 6% 9% 55%	2000
May	1st to 10th	Night	2 5% 2 4 3 2 2 2 5% 6% 65%	4454
	11th to 20th	Night	2 10% 3 7% 4 3 4 4 8% 8% 49%	4010
	21st to end	Night	2 4 1 4 3 3 4 3 8% 10% 59%	3972
June	1st to 10th	Night	3 4 2 6% 4 4 2 4 6% 8% 57%	1600
	11th to 20th	Night	1 5% 2 5% 4 4 4 3 7% 7% 60%	1600
	21st to end	Night	2 3 2 5% 3 4 3 3 6% 10% 58%	1600
July	1st to 10th	Night	1 3 2 4 5% 3 3 3 5% 8% 64%	2672
	11th to 20th	Night	1 4 3 8% 4 5% 4 3 7% 11% 49%	2950
	21st to end	Night	8% 4 9% 5% 4 5% 4 8% 11% 43%	3109
August	1st to 10th	Night	1 5% 3 8% 6% 6% 5% 4 9% 13% 39%	2848
	11th to 20th	Night	1 3 1 4 4 5% 6% 4 9% 13% 51%	3056
	21st to end	Night	1 4 3 8% 3 3 3 2 6% 8% 57%	3498
September	1st to 10th	Night	3 1 3 3 3 4 4 6% 9% 64%	1900
	11th to 20th	Night	3 5% 2 6% 3 4 3 3 6% 7% 58%	1900
	21st to end	Night	2 5% 2 3 2 1 2 2 4 8% 68%	2157
October	1st to 10th	Night	2 3 1 4 3 3 3 3 5% 8% 66%	3800
	11th to 20th	Night	3 5% 2 5% 4 4 4 3 7% 7% 57%	3777
	21st to end	Night	5% 3 1 4 3 2 3 3 5% 7% 64%	4121
November	1st to 10th	Night	8% 5% 1 4 3 3 3 3 5% 7% 59%	2354
	11th to 20th	Night	9% 5% 2 4 4 3 3 4 6% 10% 53%	2600
	21st to end	Night	9% 5% 1 5% 4 3 4 2 5% 6% 57%	2716
December	1st to 10th	Night	9% 4 1 3 3 3 4 3 5% 7% 58%	4560
	11th to 20th	Night	22% 7% 2 4 3 3 4 4 8% 9% 34%	5176
	21st to end	Night	26% 8% 2 6% 4 4 4 2 4 5% 34%	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% 5% 20% 24% 17% 4	3961
	11th to 20th	Day	33% 6% 9% 15% 18% 19%	4100
	21st to end	Day	41% 6% 8% 13% 18% 14%	4510
February	1st to 10th	Day	38% 5% 8% 19% 19% 10%	1800
	11th to 20th	Day	36% 7% 12% 19% 18% 8%	1875
	21st to end	Day	36% 7% 9% 24% 17% 7%	1650
March	1st to 10th	Day	40% 11% 9% 18% 19% 2	4868
	11th to 20th	Day	31% 4 17% 29% 11% 8%	5000
	21st to end	Day	39% 12% 18% 20% 10% 2	5568
April	1st to 10th	Day	32% 8% 14% 22% 19% 4	2400
	11th to 20th	Day	28% 7% 15% 25% 18% 6%	2372
	21st to end	Day	28% 7% 15% 27% 19% 4	2400
May	1st to 10th	Day	16% 5% 10% 29% 28% 12%	5525
	11th to 20th	Day	17% 8% 13% 33% 24% 5%	5902
	21st to end	Day	18% 8% 11% 30% 31% 2	6904
June	1st to 10th	Day	27% 8% 13% 25% 23% 5%	2800
	11th to 20th	Day	20% 4 12% 23% 29% 12%	2800
	21st to end	Day	21% 3 8% 16% 29% 21%	2800
July	1st to 10th	Day	16% 5% 3 10% 16% 50%	4437
	11th to 20th	Day	21% 7% 5% 10% 15% 43%	4171
	21st to end	Day	20% 5% 4 12% 32% 27%	4316
August	1st to 10th	Day	24% 7% 5% 14% 26% 24%	4076
	11th to 20th	Day	15% 9% 5% 13% 40% 17%	3828
	21st to end	Day	20% 6% 6% 14% 33% 21%	4179
September	1st to 10th	Day	18% 3 4 15% 27% 34%	2280
	11th to 20th	Day	23% 5% 11% 21% 28% 12%	2280
	21st to end	Day	16% 4 7% 22% 35% 17%	2019
October	1st to 10th	Day	15% 5% 6% 12% 27% 35%	3180
	11th to 20th	Day	19% 6% 8% 24% 26% 17%	3161
	21st to end	Day	21% 3 7% 24% 30% 15%	3448
November	1st to 10th	Day	27% 6% 9% 25% 23% 10%	1944
	11th to 20th	Day	29% 5% 12% 22% 23% 8%	1800
	21st to end	Day	28% 5% 11% 19% 27% 11%	1680
December	1st to 10th	Day	27% 4 7% 22% 33% 7%	2620
	11th to 20th	Day	44% 5% 12% 17% 15% 7%	3028
	21st to end	Day	49% 5% 13% 20% 11% 3	4004
January	1st to 10th	Night	27% 5% 21% 26% 16% 5%	5982
	11th to 20th	Night	34% 7% 11% 17% 20% 12%	5880
	21st to end	Night	39% 6% 9% 13% 19% 14%	6468
February	1st to 10th	Night	39% 4 9% 17% 19% 13%	2600
	11th to 20th	Night	35% 6% 12% 19% 20% 7%	2422
	21st to end	Night	31% 6% 12% 29% 16% 7%	1980
March	1st to 10th	Night	44% 8% 11% 17% 17% 3	5011
	11th to 20th	Night	34% 6% 13% 23% 16% 8%	5000
	21st to end	Night	41% 9% 14% 22% 12% 2	5406
April	1st to 10th	Night	32% 8% 15% 23% 17% 5%	2000
	11th to 20th	Night	32% 6% 12% 28% 15% 7%	1979
	21st to end	Night	25% 8% 14% 29% 19% 5%	2000
May	1st to 10th	Night	20% 6% 13% 25% 26% 11%	4451
	11th to 20th	Night	29% 12% 13% 21% 19% 6%	4011
	21st to end	Night	19% 11% 13% 23% 32% 2	3972
June	1st to 10th	Night	27% 8% 12% 24% 22% 7%	1600
	11th to 20th	Night	25% 6% 8% 16% 29% 16%	1600
	21st to end	Night	25% 7% 7% 12% 26% 23%	1600
July	1st to 10th	Night	21% 7% 2 7% 16% 47%	2631
	11th to 20th	Night	29% 12% 4 8% 14% 33%	2896
	21st to end	Night	36% 6% 4 10% 20% 25%	3011
August	1st to 10th	Night	37% 13% 4 11% 20% 15%	2839
	11th to 20th	Night	23% 14% 3 11% 31% 19%	3085
	21st to end	Night	27% 9% 6% 11% 33% 15%	3498
September	1st to 10th	Night	21% 4 4 11% 28% 31%	1900
	11th to 20th	Night	26% 5% 7% 21% 28% 13%	1900
	21st to end	Night	20% 4 5% 18% 33% 20%	2157
October	1st to 10th	Night	18% 5% 5% 11% 23% 37%	3800
	11th to 20th	Night	25% 10% 6% 22% 20% 18%	3777
	21st to end	Night	23% 4 6% 22% 29% 16%	4121
November	1st to 10th	Night	26% 5% 9% 24% 23% 13%	2354
	11th to 20th	Night	28% 5% 12% 23% 23% 9%	2600
	21st to end	Night	29% 5% 8% 20% 23% 15%	2716
December	1st to 10th	Night	27% 4 8% 24% 27% 11%	4560
	11th to 20th	Night	45% 5% 12% 16% 18% 5%	5176
	21st to end	Night	52% 6% 13% 18% 10% 3	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	25% 7% 17% 27% 18% 6%	340
	11th to 20th	Day	15% 4 20% 28% 28% 5%	400
	21st to end	Day	13% 5% 20% 31% 24% 7%	440
February	1st to 10th	Day	22% 7% 21% 22% 21% 9%	400
	11th to 20th	Day	14% 12% 23% 29% 18% 4	452
	21st to end	Day	11% 9% 23% 35% 19% 4	414
March	1st to 10th	Day	20% 11% 22% 29% 15% 2	500
	11th to 20th	Day	20% 7% 22% 26% 19% 6%	500
	21st to end	Day	13% 9% 28% 30% 16% 4	550
April	1st to 10th	Day	12% 7% 27% 31% 18% 5%	500
	11th to 20th	Day	9% 10% 26% 31% 18% 6%	492
	21st to end	Day	5% 8% 28% 30% 22% 7%	500
May	1st to 10th	Day	8% 7% 28% 31% 21% 6%	503
	11th to 20th	Day	9% 6% 23% 30% 26% 6%	528
	21st to end	Day	8% 7% 21% 30% 27% 7%	573
June	1st to 10th	Day	6% 4 17% 40% 25% 7%	530
	11th to 20th	Day	4 5% 22% 36% 28% 4	530
	21st to end	Day	2 3 14% 40% 33% 8%	530
July	1st to 10th	Day	5% 1 11% 38% 36% 9%	526
	11th to 20th	Day	4 1 6% 38% 40% 12%	524
	21st to end	Day	2 1 7% 38% 41% 12%	532
August	1st to 10th	Day	5% 11% 41% 36% 7%	487
	11th to 20th	Day	3 9% 42% 38% 8%	477
	21st to end	Day	4 1 8% 35% 39% 12%	539
September	1st to 10th	Day	2 2 10% 32% 40% 13%	490
	11th to 20th	Day	5% 3 16% 36% 30% 10%	490
	21st to end	Day	3 3 11% 30% 40% 12%	490
October	1st to 10th	Day	2 3 19% 28% 34% 14%	490
	11th to 20th	Day	6% 3 16% 28% 32% 15%	490
	21st to end	Day	9% 4 17% 28% 29% 14%	539
November	1st to 10th	Day	8% 6% 16% 25% 31% 15%	480
	11th to 20th	Day	10% 5% 16% 31% 27% 11%	300
	21st to end	Day	11% 3 15% 33% 30% 8%	240
December	1st to 10th	Day	16% 7% 15% 24% 29% 10%	200
	11th to 20th	Day	17% 6% 18% 24% 26% 10%	200
	21st to end	Day	17% 11% 20% 27% 17% 7%	220
January	1st to 10th	Night	18% 5% 13% 33% 25% 6%	800
	11th to 20th	Night	15% 3 13% 33% 27% 8%	800
	21st to end	Night	16% 5% 10% 35% 30% 5%	880
February	1st to 10th	Night	21% 6% 13% 29% 25% 7%	800
	11th to 20th	Night	16% 6% 16% 34% 23% 5%	784
	21st to end	Night	13% 7% 16% 37% 24% 3	660
March	1st to 10th	Night	19% 8% 14% 32% 23% 4	800
	11th to 20th	Night	18% 5% 14% 32% 27% 6%	800
	21st to end	Night	16% 5% 18% 35% 24% 4	880
April	1st to 10th	Night	13% 5% 16% 37% 26% 4	800
	11th to 20th	Night	12% 8% 16% 34% 25% 6%	791
	21st to end	Night	8% 4 20% 33% 26% 10%	800
May	1st to 10th	Night	8% 3 17% 34% 31% 7%	759
	11th to 20th	Night	7% 2 10% 36% 35% 10%	567
	21st to end	Night	8% 4 14% 34% 32% 8%	433
June	1st to 10th	Night	6% 3 10% 38% 36% 8%	400
	11th to 20th	Night	5% 2 10% 38% 41% 5%	400
	21st to end	Night	4 1 7% 31% 48% 11%	400
July	1st to 10th	Night	6% 1 3 25% 53% 13%	415
	11th to 20th	Night	8% 1 1 20% 46% 25%	582
	21st to end	Night	6% 1 2 25% 48% 18%	624
August	1st to 10th	Night	8% 1 6% 26% 43% 14%	567
	11th to 20th	Night	5% 1 4 28% 48% 13%	706
	21st to end	Night	6% 1 4 24% 46% 18%	836
September	1st to 10th	Night	3 5% 25% 48% 18%	760
	11th to 20th	Night	6% 2 6% 29% 43% 14%	760
	21st to end	Night	5% 2 7% 24% 45% 18%	760
October	1st to 10th	Night	6% 2 8% 31% 38% 16%	760
	11th to 20th	Night	8% 2 7% 28% 39% 15%	760
	21st to end	Night	8% 1 9% 29% 34% 18%	836
November	1st to 10th	Night	9% 2 12% 29% 36% 12%	780
	11th to 20th	Night	12% 2 9% 35% 35% 8%	800
	21st to end	Night	10% 2 13% 38% 30% 8%	800
December	1st to 10th	Night	16% 4 12% 33% 28% 8%	800
	11th to 20th	Night	21% 4 13% 34% 23% 4	800
	21st to end	Night	20% 6% 17% 34% 19% 4	880

Page generated at 2025-07-17 22:10:00. Version ac07

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