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 2024-11-03.

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 9% 6% 5% 4 3 5% 6% 46%	101533
	Night	4 9% 4 10% 7% 6% 5% 4 5% 6% 41%	97617
Transparency	Day	34% 9% 11% 19% 17% 10%	101276
	Night	36% 12% 12% 18% 15% 7%	97463
Seeing	Day	12% 20% 28% 24% 12% 3	14701
	Night	9% 15% 25% 31% 17% 2	23716

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	4 13% 7% 18% 11% 8% 5% 4 5% 5% 19%	2539
	11th to 20th	Day	7% 16% 6% 12% 12% 7% 4 3 4 4 25%	2540
	21st to end	Day	4 19% 9% 16% 8% 6% 5% 3 4 5% 22%	3054
February	1st to 10th	Day	5% 13% 6% 14% 8% 6% 7% 5% 4 6% 25%	1710
	11th to 20th	Day	5% 14% 5% 12% 9% 6% 5% 4 5% 6% 28%	1665
	21st to end	Day	2 13% 7% 16% 11% 7% 6% 4 5% 5% 23%	1560
March	1st to 10th	Day	2 13% 7% 17% 10% 7% 4 4 5% 4 26%	3217
	11th to 20th	Day	2 12% 5% 10% 6% 4 4 2 3 6% 45%	3490
	21st to end	Day	4 16% 8% 17% 10% 7% 5% 4 5% 5% 19%	3969
April	1st to 10th	Day	7% 15% 8% 12% 8% 6% 6% 5% 5% 5% 24%	2280
	11th to 20th	Day	6% 15% 6% 11% 7% 5% 5% 4 6% 6% 29%	2249
	21st to end	Day	3 14% 5% 11% 7% 5% 5% 4 6% 8% 33%	2356
May	1st to 10th	Day	7% 12% 3 10% 6% 4 4 4 6% 5% 38%	4375
	11th to 20th	Day	4 7% 2 6% 4 3 3 3 5% 6% 57%	4377
	21st to end	Day	6% 10% 3 8% 6% 4 5% 4 7% 9% 37%	4813
June	1st to 10th	Day	10% 11% 4 9% 5% 5% 4 5% 5% 6% 38%	2660
	11th to 20th	Day	5% 9% 3 7% 6% 3 5% 4 5% 8% 44%	2660
	21st to end	Day	5% 4 2 5% 4 3 3 3 4 6% 61%	2660
July	1st to 10th	Day	1 1 1 2 1 2 2 1 2 4 82%	4420
	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	5% 10% 3 7% 5% 5% 4 3 4 4 50%	1900
	11th to 20th	Day	4 8% 3 9% 6% 6% 5% 4 5% 6% 43%	1900
	21st to end	Day	6% 8% 5% 10% 6% 5% 5% 4 5% 6% 39%	2027
November	1st to 10th	Day	5% 12% 5% 13% 9% 6% 6% 4 6% 6% 28%	1692
	11th to 20th	Day	3 10% 5% 14% 8% 7% 6% 6% 6% 8% 26%	1525
	21st to end	Day	2 12% 5% 13% 10% 7% 6% 4 6% 6% 29%	1520
December	1st to 10th	Day	5% 9% 4 12% 8% 7% 6% 5% 6% 5% 32%	1520
	11th to 20th	Day	6% 14% 6% 13% 8% 6% 6% 5% 7% 7% 22%	1928
	21st to end	Day	3 13% 8% 18% 10% 6% 6% 3 5% 5% 23%	2794
January	1st to 10th	Night	4 14% 7% 14% 11% 9% 6% 5% 6% 7% 17%	4406
	11th to 20th	Night	4 14% 6% 14% 12% 10% 5% 4 5% 5% 22%	4420
	21st to end	Night	5% 20% 7% 14% 9% 6% 5% 4 5% 4 20%	4605
February	1st to 10th	Night	4 11% 5% 13% 10% 7% 7% 5% 5% 7% 26%	2470
	11th to 20th	Night	3 11% 5% 11% 9% 7% 7% 5% 6% 6% 29%	2402
	21st to end	Night	3 9% 6% 15% 11% 7% 6% 5% 6% 6% 25%	1894
March	1st to 10th	Night	3 8% 4 14% 11% 9% 6% 6% 7% 7% 25%	3653
	11th to 20th	Night	1 7% 4 12% 8% 6% 5% 3 4 5% 44%	3490
	21st to end	Night	3 12% 6% 17% 11% 8% 7% 5% 6% 6% 18%	3684
April	1st to 10th	Night	4 11% 5% 13% 8% 6% 7% 5% 6% 7% 26%	1900
	11th to 20th	Night	3 10% 5% 11% 9% 7% 6% 5% 6% 7% 32%	1879
	21st to end	Night	3 11% 5% 11% 7% 5% 6% 5% 6% 7% 34%	1820
May	1st to 10th	Night	7% 13% 5% 8% 5% 4 4 4 4 6% 41%	2583
	11th to 20th	Night	3 5% 2 4 4 4 3 4 6% 6% 58%	2516
	21st to end	Night	3 7% 4 9% 8% 4 4 4 4 8% 47%	2762
June	1st to 10th	Night	9% 11% 4 7% 5% 5% 3 4 5% 8% 39%	1520
	11th to 20th	Night	4 7% 3 6% 4 4 5% 5% 6% 8% 49%	1520
	21st to end	Night	4 4 2 3 3 3 3 3 5% 5% 64%	1520
July	1st to 10th	Night	1 2 1 1 1 1 1 2 3 4 84%	2540
	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	4 7% 3 7% 6% 5% 3 3 5% 6% 52%	2280
	11th to 20th	Night	5% 7% 3 7% 7% 5% 5% 3 6% 5% 48%	2280
	21st to end	Night	6% 9% 3 10% 7% 6% 5% 5% 6% 4 39%	2570
November	1st to 10th	Night	4 10% 5% 14% 10% 7% 6% 4 5% 6% 29%	2431
	11th to 20th	Night	3 10% 4 12% 10% 8% 7% 6% 7% 7% 26%	2650
	21st to end	Night	2 7% 5% 12% 10% 7% 6% 5% 7% 7% 31%	2660
December	1st to 10th	Night	5% 9% 4 12% 8% 7% 7% 5% 6% 7% 29%	2660
	11th to 20th	Night	5% 15% 5% 15% 10% 7% 6% 4 5% 7% 21%	3276
	21st to end	Night	3 13% 6% 14% 10% 7% 6% 5% 5% 5% 26%	4862

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	51% 21% 13% 11% 4 1	2539
	11th to 20th	Day	56% 13% 9% 14% 4 3	2540
	21st to end	Day	53% 16% 10% 13% 6% 2	3054
February	1st to 10th	Day	52% 12% 14% 16% 7%	1710
	11th to 20th	Day	49% 13% 11% 14% 10% 2	1674
	21st to end	Day	54% 13% 13% 13% 6% 1	1560
March	1st to 10th	Day	40% 18% 12% 19% 10% 1	3217
	11th to 20th	Day	36% 10% 11% 30% 13%	3490
	21st to end	Day	48% 19% 14% 14% 5%	3969
April	1st to 10th	Day	55% 11% 12% 13% 7% 2	2280
	11th to 20th	Day	48% 10% 14% 14% 11% 2	2252
	21st to end	Day	42% 10% 16% 21% 9% 2	2356
May	1st to 10th	Day	35% 13% 14% 22% 15% 1	4375
	11th to 20th	Day	26% 6% 10% 26% 29% 3	4380
	21st to end	Day	34% 11% 22% 27% 6%	4814
June	1st to 10th	Day	42% 9% 13% 25% 10% 1	2660
	11th to 20th	Day	34% 8% 15% 28% 12% 3	2660
	21st to end	Day	22% 5% 12% 30% 23% 8%	2660
July	1st to 10th	Day	9% 3 6% 12% 20% 50%	4351
	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	36% 6% 9% 17% 24% 9%	1900
	11th to 20th	Day	37% 8% 12% 19% 19% 4	1900
	21st to end	Day	39% 11% 12% 16% 19% 3	2027
November	1st to 10th	Day	48% 11% 13% 15% 13% 1	1692
	11th to 20th	Day	51% 13% 12% 14% 8% 2	1525
	21st to end	Day	48% 10% 14% 11% 15% 3	1520
December	1st to 10th	Day	44% 13% 12% 19% 10% 2	1520
	11th to 20th	Day	52% 14% 14% 13% 6% 1	1928
	21st to end	Day	52% 17% 11% 15% 4 1	2794
January	1st to 10th	Night	52% 20% 13% 11% 4 1	4406
	11th to 20th	Night	53% 15% 11% 12% 4 4	4420
	21st to end	Night	58% 12% 11% 10% 7% 2	4605
February	1st to 10th	Night	49% 14% 13% 14% 9% 1	2470
	11th to 20th	Night	45% 14% 16% 15% 7% 3	2404
	21st to end	Night	50% 15% 12% 14% 8% 2	1894
March	1st to 10th	Night	39% 17% 14% 22% 7% 1	3653
	11th to 20th	Night	30% 13% 13% 30% 13%	3490
	21st to end	Night	49% 19% 15% 14% 3	3684
April	1st to 10th	Night	49% 13% 17% 14% 7% 1	1900
	11th to 20th	Night	42% 14% 16% 18% 9% 2	1879
	21st to end	Night	43% 13% 16% 18% 9% 2	1820
May	1st to 10th	Night	35% 14% 14% 27% 9%	2583
	11th to 20th	Night	22% 8% 14% 32% 23% 1	2516
	21st to end	Night	29% 16% 26% 25% 4	2762
June	1st to 10th	Night	40% 10% 12% 28% 9% 1	1520
	11th to 20th	Night	28% 9% 17% 31% 13% 3	1520
	21st to end	Night	19% 6% 12% 32% 24% 7%	1520
July	1st to 10th	Night	7% 3 6% 14% 18% 52%	2500
	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	31% 8% 9% 19% 25% 9%	2280
	11th to 20th	Night	33% 9% 14% 18% 19% 6%	2280
	21st to end	Night	38% 14% 12% 12% 19% 5%	2570
November	1st to 10th	Night	47% 13% 13% 12% 12% 3	2431
	11th to 20th	Night	49% 13% 13% 13% 9% 2	2650
	21st to end	Night	43% 13% 13% 13% 13% 6%	2660
December	1st to 10th	Night	44% 14% 13% 16% 11% 2	2660
	11th to 20th	Night	54% 14% 11% 11% 9% 1	3276
	21st to end	Night	45% 17% 15% 16% 6% 1	4862

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	22% 32% 24% 14% 7% 2	180
	11th to 20th	Day	30% 14% 23% 23% 8% 2	180
	21st to end	Day	15% 28% 24% 20% 12% 2	354
February	1st to 10th	Day	18% 28% 27% 19% 9%	370
	11th to 20th	Day	16% 38% 23% 16% 6% 1	365
	21st to end	Day	18% 30% 27% 19% 7%	377
March	1st to 10th	Day	17% 30% 29% 19% 5%	460
	11th to 20th	Day	17% 23% 28% 22% 8% 1	460
	21st to end	Day	20% 29% 31% 16% 5%	506
April	1st to 10th	Day	22% 23% 29% 14% 9% 2	460
	11th to 20th	Day	18% 29% 27% 17% 6% 2	452
	21st to end	Day	11% 31% 29% 19% 8% 2	460
May	1st to 10th	Day	16% 22% 31% 21% 8% 3	463
	11th to 20th	Day	18% 20% 26% 19% 13% 3	488
	21st to end	Day	14% 16% 31% 22% 13% 4	529
June	1st to 10th	Day	11% 22% 25% 25% 13% 4	490
	11th to 20th	Day	14% 23% 31% 21% 9% 2	490
	21st to end	Day	5% 15% 28% 30% 18% 4	490
July	1st to 10th	Day	3 10% 37% 32% 14% 4	486
	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% 23% 27% 16% 15% 3	476
November	1st to 10th	Day	16% 20% 30% 21% 12% 1	276
	11th to 20th	Day	10% 26% 31% 21% 9% 3	183
	21st to end	Day	14% 26% 22% 26% 10% 2	180
December	1st to 10th	Day	13% 25% 22% 24% 10% 6%	180
	11th to 20th	Day	15% 28% 30% 19% 7% 1	180
	21st to end	Day	14% 28% 29% 18% 9% 2	198
January	1st to 10th	Night	17% 21% 30% 19% 11% 2	760
	11th to 20th	Night	15% 21% 25% 22% 16% 2	760
	21st to end	Night	12% 17% 27% 28% 12% 4	836
February	1st to 10th	Night	12% 21% 28% 29% 9% 1	760
	11th to 20th	Night	10% 21% 28% 28% 12% 1	744
	21st to end	Night	13% 21% 32% 26% 7%	628
March	1st to 10th	Night	12% 24% 27% 27% 9% 1	760
	11th to 20th	Night	6% 24% 27% 30% 12% 1	760
	21st to end	Night	10% 24% 31% 24% 11%	836
April	1st to 10th	Night	11% 21% 29% 26% 12% 1	760
	11th to 20th	Night	7% 21% 32% 24% 13% 3	751
	21st to end	Night	8% 17% 28% 32% 13% 1	680
May	1st to 10th	Night	9% 16% 25% 30% 19% 1	410
	11th to 20th	Night	9% 11% 20% 36% 23% 1	374
	21st to end	Night	9% 7% 25% 35% 22% 2	411
June	1st to 10th	Night	12% 10% 22% 36% 19% 2	380
	11th to 20th	Night	7% 10% 27% 39% 16% 1	380
	21st to end	Night	6% 3 18% 52% 21%	380
July	1st to 10th	Night	3 2 15% 53% 26% 1	370
	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% 28% 11% 1	748
	11th to 20th	Night	12% 23% 26% 26% 13% 1	760
	21st to end	Night	8% 22% 24% 30% 13% 3	760
December	1st to 10th	Night	12% 16% 27% 29% 14% 2	760
	11th to 20th	Night	12% 23% 26% 27% 11% 1	760
	21st to end	Night	11% 31% 27% 22% 8% 1	836

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