Mt Bachelor Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt Bachelor Star Party clear sky chart from 2005-11-06 to 2025-07-06.

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	4 9% 4 10% 6% 4 4 3 5% 6% 45%	124207
	Night	3 9% 4 10% 7% 5% 5% 4 5% 6% 42%	123402
Transparency	Day	34% 10% 10% 19% 17% 9%	123951
	Night	35% 12% 11% 18% 16% 8%	123245
Seeing	Day	13% 20% 28% 24% 12% 3	15499
	Night	10% 15% 25% 31% 17% 2	24561

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	5% 18% 7% 17% 10% 6% 5% 4 5% 5% 18%	3639
	11th to 20th	Day	5% 11% 4 8% 9% 5% 4 3 3 4 43%	3640
	21st to end	Day	3 14% 7% 13% 6% 5% 4 2 4 5% 39%	4390
February	1st to 10th	Day	5% 13% 7% 15% 8% 6% 7% 5% 4 6% 24%	1800
	11th to 20th	Day	5% 14% 6% 13% 9% 6% 5% 4 5% 6% 28%	1755
	21st to end	Day	2 13% 7% 17% 11% 7% 6% 4 5% 5% 24%	1640
March	1st to 10th	Day	3 12% 7% 14% 9% 6% 5% 4 6% 7% 28%	4636
	11th to 20th	Day	3 15% 7% 16% 8% 5% 4 2 3 5% 32%	5000
	21st to end	Day	3 15% 8% 21% 10% 6% 5% 4 5% 6% 17%	5695
April	1st to 10th	Day	7% 15% 8% 13% 8% 6% 6% 5% 5% 5% 23%	2400
	11th to 20th	Day	6% 14% 6% 10% 6% 5% 5% 4 6% 5% 32%	2369
	21st to end	Day	3 13% 4 10% 7% 5% 5% 4 6% 8% 34%	2480
May	1st to 10th	Day	6% 9% 3 8% 5% 4 4 4 6% 5% 45%	6262
	11th to 20th	Day	4 9% 3 10% 6% 4 4 3 6% 6% 44%	6277
	21st to end	Day	5% 8% 2 6% 5% 4 4 4 8% 9% 44%	6902
June	1st to 10th	Day	9% 11% 4 8% 4 5% 4 5% 5% 7% 40%	2800
	11th to 20th	Day	5% 9% 3 7% 6% 4 5% 4 5% 8% 45%	2800
	21st to end	Day	5% 4 2 5% 4 3 3 3 5% 6% 60%	2800
July	1st to 10th	Day	1 1 1 2 1 2 2 1 2 4 82%	4491
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 78%	4420
	21st to end	Day	2 2 1 2 1 2 2 2 3 4 80%	4790
August	1st to 10th	Day	4 3 1 3 2 2 3 2 5% 6% 67%	4134
	11th to 20th	Day	2 2 1 4 3 3 4 3 5% 7% 67%	4026
	21st to end	Day	6% 3 1 3 2 3 3 2 3 5% 69%	4180
September	1st to 10th	Day	2 3 1 3 2 3 3 2 4 5% 73%	2280
	11th to 20th	Day	4 6% 2 7% 5% 4 5% 3 5% 6% 52%	2280
	21st to end	Day	5% 7% 2 5% 4 4 4 3 5% 6% 53%	2023
October	1st to 10th	Day	3 6% 2 5% 4 4 3 2 4 6% 61%	3180
	11th to 20th	Day	3 7% 3 9% 7% 5% 5% 4 6% 8% 43%	3161
	21st to end	Day	4 9% 5% 15% 7% 5% 5% 4 5% 7% 33%	3336
November	1st to 10th	Day	5% 12% 5% 13% 9% 6% 6% 4 6% 6% 28%	1764
	11th to 20th	Day	3 11% 5% 16% 8% 7% 6% 5% 5% 7% 26%	1605
	21st to end	Day	2 12% 6% 13% 9% 7% 6% 4 6% 6% 29%	1600
December	1st to 10th	Day	3 6% 3 9% 7% 5% 5% 4 5% 7% 46%	2620
	11th to 20th	Day	6% 16% 8% 17% 8% 6% 5% 4 6% 6% 18%	3028
	21st to end	Day	3 14% 8% 22% 11% 6% 5% 4 5% 5% 17%	4004
January	1st to 10th	Night	4 14% 7% 17% 11% 7% 6% 5% 6% 6% 18%	6306
	11th to 20th	Night	3 10% 4 10% 8% 7% 4 3 4 5% 42%	6320
	21st to end	Night	3 14% 6% 12% 7% 5% 4 3 4 4 38%	6578
February	1st to 10th	Night	4 11% 5% 14% 10% 7% 7% 5% 5% 7% 26%	2600
	11th to 20th	Night	3 11% 5% 12% 9% 7% 7% 5% 6% 6% 29%	2532
	21st to end	Night	3 8% 6% 15% 11% 7% 6% 5% 6% 6% 26%	1990
March	1st to 10th	Night	3 8% 4 12% 9% 8% 7% 6% 7% 8% 28%	5254
	11th to 20th	Night	1 9% 6% 15% 13% 6% 5% 3 4 5% 32%	5000
	21st to end	Night	2 11% 6% 21% 11% 7% 6% 5% 6% 7% 18%	5280
April	1st to 10th	Night	4 11% 5% 13% 8% 6% 7% 5% 7% 7% 26%	2000
	11th to 20th	Night	3 9% 4 10% 9% 7% 6% 5% 6% 7% 34%	1979
	21st to end	Night	3 11% 5% 11% 7% 5% 6% 4 6% 7% 35%	1916
May	1st to 10th	Night	6% 9% 4 7% 4 4 3 3 3 5% 52%	3697
	11th to 20th	Night	3 7% 3 7% 6% 5% 4 4 6% 6% 48%	3616
	21st to end	Night	4 6% 3 7% 6% 3 4 3 4 8% 52%	3972
June	1st to 10th	Night	9% 11% 4 7% 4 4 3 3 5% 8% 42%	1600
	11th to 20th	Night	4 7% 3 6% 4 4 5% 5% 6% 8% 48%	1600
	21st to end	Night	4 4 2 4 3 3 3 3 5% 5% 64%	1600
July	1st to 10th	Night	1 2 1 1 1 1 1 2 3 4 83%	2580
	11th to 20th	Night	2 2 1 2 1 1 2 2 2 4 81%	2540
	21st to end	Night	1 2 1 2 1 2 2 2 3 4 81%	2761
August	1st to 10th	Night	2 3 1 4 2 2 2 2 4 5% 72%	2801
	11th to 20th	Night	2 3 2 3 2 2 2 2 3 5% 72%	2812
	21st to end	Night	5% 3 1 3 2 2 2 2 3 4 73%	3497
September	1st to 10th	Night	2 2 1 2 2 2 3 2 3 6% 76%	1900
	11th to 20th	Night	3 4 2 7% 5% 4 4 3 5% 6% 57%	1900
	21st to end	Night	4 5% 2 5% 4 4 4 4 5% 7% 57%	2153
October	1st to 10th	Night	2 5% 2 6% 4 3 2 2 3 5% 65%	3800
	11th to 20th	Night	4 6% 2 8% 6% 5% 5% 4 6% 7% 47%	3777
	21st to end	Night	4 8% 4 16% 9% 6% 5% 4 6% 6% 33%	4228
November	1st to 10th	Night	4 10% 5% 14% 9% 6% 6% 4 5% 7% 29%	2535
	11th to 20th	Night	3 11% 5% 12% 11% 8% 7% 6% 7% 6% 25%	2790
	21st to end	Night	2 7% 5% 12% 10% 7% 6% 5% 7% 7% 32%	2800
December	1st to 10th	Night	3 6% 3 8% 6% 5% 4 4 5% 7% 49%	4560
	11th to 20th	Night	5% 18% 7% 17% 10% 6% 6% 4 5% 7% 17%	5176
	21st to end	Night	5% 16% 7% 16% 10% 6% 5% 4 5% 5% 22%	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	54% 17% 10% 10% 6% 3	3639
	11th to 20th	Day	40% 10% 9% 16% 20% 6%	3640
	21st to end	Day	40% 13% 7% 12% 18% 11%	4390
February	1st to 10th	Day	52% 12% 13% 15% 7% 1	1800
	11th to 20th	Day	49% 13% 11% 14% 10% 3	1764
	21st to end	Day	53% 13% 13% 13% 6% 2	1640
March	1st to 10th	Day	39% 16% 11% 20% 11% 3	4636
	11th to 20th	Day	47% 12% 8% 22% 10%	5000
	21st to end	Day	48% 21% 13% 14% 4	5695
April	1st to 10th	Day	54% 11% 12% 14% 7% 1	2400
	11th to 20th	Day	47% 9% 13% 16% 12% 2	2372
	21st to end	Day	41% 10% 15% 22% 10% 2	2480
May	1st to 10th	Day	31% 10% 10% 24% 21% 4	6262
	11th to 20th	Day	31% 11% 10% 22% 23% 2	6280
	21st to end	Day	28% 9% 20% 33% 9% 1	6904
June	1st to 10th	Day	40% 8% 13% 27% 11% 1	2800
	11th to 20th	Day	33% 8% 14% 27% 14% 4	2800
	21st to end	Day	22% 6% 12% 30% 23% 8%	2800
July	1st to 10th	Day	9% 3 5% 13% 20% 50%	4422
	11th to 20th	Day	11% 2 6% 9% 21% 51%	4345
	21st to end	Day	9% 2 5% 16% 38% 29%	4639
August	1st to 10th	Day	17% 4 6% 18% 33% 22%	4118
	11th to 20th	Day	16% 4 7% 27% 34% 12%	4065
	21st to end	Day	18% 4 6% 26% 26% 21%	4179
September	1st to 10th	Day	14% 4 7% 24% 33% 19%	2280
	11th to 20th	Day	28% 7% 10% 22% 23% 9%	2280
	21st to end	Day	27% 6% 12% 24% 20% 10%	2023
October	1st to 10th	Day	24% 5% 8% 19% 34% 11%	3180
	11th to 20th	Day	33% 11% 14% 22% 18% 3	3161
	21st to end	Day	39% 15% 9% 18% 18% 2	3336
November	1st to 10th	Day	47% 11% 12% 16% 13% 1	1764
	11th to 20th	Day	51% 14% 12% 13% 8% 2	1605
	21st to end	Day	48% 10% 13% 11% 16% 2	1600
December	1st to 10th	Day	31% 10% 8% 18% 22% 10%	2620
	11th to 20th	Day	58% 14% 10% 13% 5%	3028
	21st to end	Day	53% 21% 9% 13% 4 1	4004
January	1st to 10th	Night	49% 21% 11% 11% 6% 2	6306
	11th to 20th	Night	38% 11% 8% 16% 18% 8%	6320
	21st to end	Night	44% 10% 8% 8% 16% 15%	6578
February	1st to 10th	Night	50% 14% 12% 13% 9% 1	2600
	11th to 20th	Night	45% 14% 15% 14% 7% 4	2534
	21st to end	Night	48% 15% 12% 13% 9% 2	1990
March	1st to 10th	Night	38% 15% 13% 20% 11% 4	5254
	11th to 20th	Night	36% 20% 12% 22% 9%	5000
	21st to end	Night	46% 23% 12% 16% 3	5280
April	1st to 10th	Night	48% 14% 17% 14% 7% 1	2000
	11th to 20th	Night	41% 13% 15% 18% 11% 2	1979
	21st to end	Night	42% 12% 15% 19% 10% 1	1916
May	1st to 10th	Night	30% 10% 10% 24% 23% 2	3694
	11th to 20th	Night	28% 11% 14% 28% 20% 1	3616
	21st to end	Night	26% 12% 21% 34% 6% 1	3972
June	1st to 10th	Night	38% 9% 12% 30% 10% 1	1600
	11th to 20th	Night	27% 9% 17% 30% 14% 3	1600
	21st to end	Night	20% 6% 12% 32% 24% 6%	1600
July	1st to 10th	Night	8% 3 6% 14% 18% 51%	2540
	11th to 20th	Night	11% 1 6% 13% 21% 49%	2492
	21st to end	Night	9% 2 4 16% 44% 26%	2674
August	1st to 10th	Night	16% 2 5% 19% 39% 19%	2793
	11th to 20th	Night	15% 3 4 29% 37% 11%	2839
	21st to end	Night	15% 3 7% 29% 27% 19%	3497
September	1st to 10th	Night	11% 4 7% 25% 37% 17%	1900
	11th to 20th	Night	23% 8% 10% 21% 27% 10%	1900
	21st to end	Night	23% 7% 12% 24% 22% 12%	2153
October	1st to 10th	Night	22% 5% 7% 20% 35% 11%	3800
	11th to 20th	Night	31% 9% 17% 24% 16% 4	3777
	21st to end	Night	36% 20% 8% 16% 17% 3	4228
November	1st to 10th	Night	46% 13% 13% 13% 12% 3	2535
	11th to 20th	Night	49% 15% 13% 13% 9% 2	2790
	21st to end	Night	43% 12% 12% 13% 14% 6%	2800
December	1st to 10th	Night	29% 10% 8% 15% 26% 11%	4560
	11th to 20th	Night	57% 16% 11% 10% 6%	5176
	21st to end	Night	48% 20% 12% 14% 5% 1	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	26% 30% 22% 15% 6% 2	200
	11th to 20th	Day	28% 15% 24% 24% 8% 3	200
	21st to end	Day	15% 27% 23% 21% 12% 2	385
February	1st to 10th	Day	21% 28% 25% 18% 8%	400
	11th to 20th	Day	18% 36% 24% 16% 6% 1	395
	21st to end	Day	20% 28% 26% 19% 7%	409
March	1st to 10th	Day	17% 30% 29% 19% 5%	500
	11th to 20th	Day	20% 24% 26% 21% 8% 1	500
	21st to end	Day	22% 29% 30% 15% 5%	550
April	1st to 10th	Day	22% 23% 29% 14% 9% 2	500
	11th to 20th	Day	18% 29% 28% 17% 6% 2	492
	21st to end	Day	11% 30% 30% 19% 8% 2	500
May	1st to 10th	Day	17% 22% 30% 20% 8% 3	503
	11th to 20th	Day	20% 22% 24% 18% 12% 3	528
	21st to end	Day	13% 17% 31% 22% 13% 4	573
June	1st to 10th	Day	11% 20% 26% 25% 13% 4	530
	11th to 20th	Day	13% 24% 32% 20% 9% 2	530
	21st to end	Day	6% 16% 28% 29% 18% 4	530
July	1st to 10th	Day	3 10% 38% 32% 14% 4	507
	11th to 20th	Day	3 12% 38% 27% 16% 5%	484
	21st to end	Day	2 11% 33% 32% 16% 6%	531
August	1st to 10th	Day	3 8% 32% 33% 17% 7%	487
	11th to 20th	Day	2 10% 26% 34% 20% 9%	477
	21st to end	Day	6% 9% 29% 33% 16% 6%	539
September	1st to 10th	Day	3 9% 30% 34% 18% 5%	490
	11th to 20th	Day	7% 15% 26% 35% 15% 2	490
	21st to end	Day	10% 18% 25% 30% 16% 1	490
October	1st to 10th	Day	11% 19% 27% 22% 20% 2	490
	11th to 20th	Day	13% 20% 22% 24% 19% 2	490
	21st to end	Day	15% 24% 27% 16% 15% 3	476
November	1st to 10th	Day	17% 21% 28% 21% 12% 1	300
	11th to 20th	Day	17% 25% 28% 19% 8% 3	203
	21st to end	Day	16% 25% 22% 25% 11% 3	200
December	1st to 10th	Day	14% 23% 21% 24% 12% 8%	200
	11th to 20th	Day	20% 27% 29% 18% 7% 1	200
	21st to end	Day	19% 28% 28% 16% 8% 1	220
January	1st to 10th	Night	19% 21% 30% 18% 11% 2	800
	11th to 20th	Night	15% 20% 26% 22% 16% 2	800
	21st to end	Night	12% 17% 27% 28% 12% 4	880
February	1st to 10th	Night	13% 21% 28% 28% 9% 1	800
	11th to 20th	Night	11% 22% 28% 27% 11% 1	784
	21st to end	Night	14% 20% 32% 27% 7%	660
March	1st to 10th	Night	13% 24% 27% 28% 9% 1	800
	11th to 20th	Night	9% 24% 26% 28% 12% 1	800
	21st to end	Night	12% 24% 30% 24% 10%	880
April	1st to 10th	Night	12% 20% 30% 27% 11% 1	800
	11th to 20th	Night	8% 21% 32% 24% 13% 3	791
	21st to end	Night	8% 17% 30% 31% 13% 1	716
May	1st to 10th	Night	9% 16% 26% 29% 19% 1	431
	11th to 20th	Night	10% 13% 20% 34% 22% 1	394
	21st to end	Night	9% 7% 24% 36% 21% 2	433
June	1st to 10th	Night	11% 10% 22% 36% 19% 2	400
	11th to 20th	Night	7% 10% 29% 39% 15% 1	400
	21st to end	Night	6% 4 20% 51% 20%	400
July	1st to 10th	Night	3 3 16% 52% 25% 1	380
	11th to 20th	Night	2 1 17% 58% 22% 1	368
	21st to end	Night	2 2 11% 55% 30% 1	416
August	1st to 10th	Night	2 1 10% 44% 39% 4	533
	11th to 20th	Night	2 1 10% 43% 38% 5%	564
	21st to end	Night	3 3 22% 42% 28% 2	835
September	1st to 10th	Night	2 4 20% 41% 30% 3	760
	11th to 20th	Night	5% 6% 17% 36% 32% 4	760
	21st to end	Night	6% 8% 23% 34% 26% 3	760
October	1st to 10th	Night	6% 7% 30% 30% 24% 3	760
	11th to 20th	Night	9% 14% 21% 31% 23% 4	760
	21st to end	Night	12% 17% 26% 25% 15% 5%	836
November	1st to 10th	Night	10% 19% 30% 27% 12% 1	780
	11th to 20th	Night	15% 22% 25% 25% 12% 1	800
	21st to end	Night	10% 22% 23% 30% 13% 3	800
December	1st to 10th	Night	12% 15% 27% 29% 15% 2	800
	11th to 20th	Night	14% 22% 26% 26% 11% 1	800
	21st to end	Night	14% 30% 26% 21% 8% 1	880

Page generated at 2025-07-02 22:30:12. Version ac07

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