Big Sky Observatory Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Sky Observatory Site clear sky chart from 2005-11-06 to 2024-11-04.

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	8% 8% 3 7% 5% 5% 5% 4 6% 8% 42%	105537
	Night	6% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	102061
Transparency	Day	39% 6% 14% 26% 12% 3	105283
	Night	40% 7% 14% 24% 13% 2	101961
Seeing	Day	12% 9% 26% 29% 18% 6%	15376
	Night	11% 7% 30% 35% 15% 2	24464

By Month

Percentage Cloud Cover NW (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	3 11% 5% 10% 5% 6% 5% 5% 7% 9% 34%	2219
	11th to 20th	Day	4 8% 4 12% 7% 5% 6% 5% 7% 6% 35%	2267
	21st to end	Day	3 8% 3 10% 7% 6% 5% 4 6% 10% 39%	2794
February	1st to 10th	Day	5% 12% 4 11% 7% 7% 6% 5% 6% 7% 28%	1558
	11th to 20th	Day	5% 13% 5% 9% 8% 6% 5% 5% 7% 8% 31%	1665
	21st to end	Day	4 14% 5% 10% 8% 6% 5% 5% 5% 7% 31%	1517
March	1st to 10th	Day	2 11% 3 12% 8% 6% 5% 4 5% 8% 35%	3138
	11th to 20th	Day	6% 8% 2 7% 5% 4 4 3 6% 8% 47%	3488
	21st to end	Day	10% 18% 6% 13% 4 4 4 3 5% 6% 26%	3825
April	1st to 10th	Day	9% 14% 4 8% 7% 5% 5% 4 5% 8% 31%	2340
	11th to 20th	Day	10% 13% 4 8% 5% 4 5% 5% 7% 7% 32%	2440
	21st to end	Day	11% 10% 3 7% 6% 5% 4 4 7% 8% 35%	2470
May	1st to 10th	Day	18% 13% 4 7% 5% 3 4 3 5% 6% 31%	4218
	11th to 20th	Day	10% 7% 2 8% 5% 6% 7% 5% 8% 10% 33%	4694
	21st to end	Day	15% 9% 3 9% 6% 6% 6% 5% 7% 8% 26%	5161
June	1st to 10th	Day	14% 7% 2 6% 6% 5% 6% 5% 7% 8% 34%	2850
	11th to 20th	Day	18% 8% 2 7% 6% 5% 6% 4 6% 8% 30%	2850
	21st to end	Day	8% 5% 2 4 4 4 5% 5% 7% 8% 47%	2850
July	1st to 10th	Day	6% 4 2 5% 3 4 4 4 6% 8% 55%	4740
	11th to 20th	Day	4 3 1 4 3 3 4 4 6% 9% 57%	4740
	21st to end	Day	3 2 1 3 3 3 3 3 6% 8% 65%	4995
August	1st to 10th	Day	8% 5% 2 5% 4 4 5% 5% 8% 10% 45%	4358
	11th to 20th	Day	6% 4 2 5% 3 4 4 4 7% 10% 50%	4028
	21st to end	Day	8% 3 2 4 3 3 4 4 6% 8% 57%	4521
September	1st to 10th	Day	9% 5% 2 5% 3 4 4 3 5% 6% 54%	2345
	11th to 20th	Day	13% 6% 2 6% 4 4 3 2 5% 7% 47%	2090
	21st to end	Day	9% 7% 2 6% 3 4 4 3 6% 8% 48%	2090
October	1st to 10th	Day	8% 7% 2 7% 4 5% 5% 5% 7% 9% 42%	3330
	11th to 20th	Day	6% 7% 2 5% 3 4 4 4 7% 10% 49%	2976
	21st to end	Day	7% 6% 2 6% 5% 4 5% 5% 6% 8% 45%	3101
November	1st to 10th	Day	9% 14% 3 7% 5% 4 6% 4 6% 6% 34%	1583
	11th to 20th	Day	5% 10% 3 7% 6% 4 4 4 6% 8% 41%	1512
	21st to end	Day	5% 11% 4 9% 4 3 4 4 8% 10% 38%	1330
December	1st to 10th	Day	4 10% 4 10% 6% 6% 5% 4 5% 9% 37%	1330
	11th to 20th	Day	4 9% 5% 12% 8% 7% 6% 5% 6% 6% 32%	1682
	21st to end	Day	3 8% 3 7% 5% 4 5% 5% 6% 9% 44%	2442
January	1st to 10th	Night	4 12% 5% 10% 6% 6% 7% 5% 8% 9% 28%	4726
	11th to 20th	Night	4 9% 5% 13% 6% 5% 6% 5% 6% 8% 34%	4663
	21st to end	Night	4 10% 4 8% 6% 6% 5% 4 6% 8% 40%	4862
February	1st to 10th	Night	5% 11% 4 12% 7% 6% 5% 4 7% 8% 32%	2621
	11th to 20th	Night	5% 12% 5% 10% 6% 6% 6% 5% 6% 8% 29%	2402
	21st to end	Night	4 11% 5% 12% 8% 6% 4 4 5% 7% 34%	1932
March	1st to 10th	Night	2 9% 4 14% 7% 7% 6% 4 8% 8% 31%	3736
	11th to 20th	Night	4 9% 3 6% 5% 4 4 4 6% 9% 45%	3491
	21st to end	Night	12% 20% 6% 11% 6% 4 4 4 5% 7% 21%	3828
April	1st to 10th	Night	9% 11% 4 10% 5% 6% 5% 4 6% 7% 33%	1837
	11th to 20th	Night	9% 11% 4 9% 6% 6% 5% 5% 5% 6% 33%	1691
	21st to end	Night	11% 9% 3 8% 5% 6% 5% 4 6% 8% 34%	1710
May	1st to 10th	Night	17% 14% 4 9% 5% 5% 5% 4 5% 5% 30%	2747
	11th to 20th	Night	9% 6% 2 7% 5% 6% 5% 4 8% 9% 38%	2199
	21st to end	Night	14% 7% 3 9% 6% 7% 6% 6% 6% 8% 29%	2414
June	1st to 10th	Night	14% 7% 3 6% 5% 5% 4 4 7% 7% 39%	1330
	11th to 20th	Night	18% 7% 3 7% 6% 4 5% 4 5% 8% 34%	1330
	21st to end	Night	7% 3 1 5% 4 5% 5% 4 8% 9% 48%	1330
July	1st to 10th	Night	4 4 2 6% 5% 5% 4 5% 7% 9% 49%	2220
	11th to 20th	Night	5% 3 1 4 3 4 5% 5% 8% 10% 50%	2220
	21st to end	Night	4 2 1 4 4 3 3 4 6% 7% 62%	2555
August	1st to 10th	Night	7% 5% 2 7% 4 5% 5% 4 6% 8% 47%	2560
	11th to 20th	Night	5% 4 2 6% 5% 4 6% 4 8% 10% 45%	2811
	21st to end	Night	6% 3 1 4 3 3 3 4 6% 8% 60%	3146
September	1st to 10th	Night	10% 4 2 6% 4 4 3 3 5% 7% 52%	1833
	11th to 20th	Night	12% 6% 2 5% 3 2 4 2 5% 7% 53%	2090
	21st to end	Night	8% 7% 2 5% 4 4 4 4 6% 7% 49%	2090
October	1st to 10th	Night	6% 5% 2 5% 4 3 4 4 5% 8% 55%	3645
	11th to 20th	Night	6% 6% 2 5% 4 4 4 4 6% 8% 50%	3948
	21st to end	Night	5% 6% 2 6% 5% 4 5% 5% 7% 8% 47%	4457
November	1st to 10th	Night	9% 12% 3 7% 6% 5% 5% 5% 7% 8% 35%	2549
	11th to 20th	Night	4 9% 3 8% 5% 4 5% 5% 7% 8% 41%	2664
	21st to end	Night	5% 11% 3 7% 5% 4 4 4 7% 10% 40%	2850
December	1st to 10th	Night	4 11% 3 9% 6% 7% 6% 5% 6% 8% 36%	2850
	11th to 20th	Night	3 9% 4 10% 7% 5% 5% 5% 7% 10% 35%	3510
	21st to end	Night	4 9% 3 8% 5% 4 5% 4 6% 7% 44%	5214

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	43% 7% 15% 24% 8% 2	2219
	11th to 20th	Day	47% 6% 11% 16% 17% 4	2288
	21st to end	Day	38% 7% 16% 30% 8% 1	2794
February	1st to 10th	Day	52% 8% 12% 17% 9% 2	1558
	11th to 20th	Day	47% 8% 15% 19% 8% 2	1674
	21st to end	Day	51% 6% 12% 17% 12% 1	1517
March	1st to 10th	Day	46% 6% 16% 19% 12% 1	3138
	11th to 20th	Day	34% 7% 18% 27% 13% 1	3489
	21st to end	Day	58% 6% 13% 18% 4	3825
April	1st to 10th	Day	50% 6% 15% 21% 8% 1	2340
	11th to 20th	Day	48% 7% 12% 21% 10% 2	2440
	21st to end	Day	46% 7% 14% 21% 11% 1	2470
May	1st to 10th	Day	53% 7% 9% 25% 6%	4218
	11th to 20th	Day	41% 9% 15% 24% 10% 1	4698
	21st to end	Day	51% 8% 15% 17% 8% 1	5162
June	1st to 10th	Day	44% 8% 16% 20% 10% 1	2850
	11th to 20th	Day	49% 8% 14% 19% 9% 2	2850
	21st to end	Day	32% 8% 18% 28% 12% 2	2850
July	1st to 10th	Day	27% 5% 14% 31% 18% 5%	4666
	11th to 20th	Day	23% 5% 17% 34% 17% 3	4659
	21st to end	Day	18% 5% 9% 28% 25% 15%	4837
August	1st to 10th	Day	33% 5% 13% 35% 12% 2	4341
	11th to 20th	Day	29% 5% 21% 34% 9% 2	4067
	21st to end	Day	27% 3 13% 40% 15% 2	4520
September	1st to 10th	Day	31% 6% 13% 26% 18% 8%	2345
	11th to 20th	Day	37% 5% 13% 28% 13% 3	2090
	21st to end	Day	34% 6% 13% 31% 14% 3	2090
October	1st to 10th	Day	37% 4 11% 36% 11% 1	3330
	11th to 20th	Day	31% 4 21% 27% 16% 3	2976
	21st to end	Day	37% 5% 11% 32% 12% 3	3101
November	1st to 10th	Day	48% 7% 12% 21% 11% 1	1583
	11th to 20th	Day	40% 7% 18% 24% 10% 1	1512
	21st to end	Day	40% 7% 19% 22% 11% 3	1330
December	1st to 10th	Day	45% 8% 13% 19% 11% 3	1330
	11th to 20th	Day	49% 7% 15% 21% 6% 1	1684
	21st to end	Day	34% 7% 14% 22% 19% 3	2442
January	1st to 10th	Night	48% 8% 15% 21% 7% 2	4726
	11th to 20th	Night	49% 5% 13% 15% 15% 3	4704
	21st to end	Night	40% 7% 16% 25% 11% 1	4862
February	1st to 10th	Night	49% 7% 13% 17% 12% 2	2621
	11th to 20th	Night	49% 8% 12% 19% 10% 1	2403
	21st to end	Night	49% 6% 12% 21% 10% 2	1932
March	1st to 10th	Night	46% 8% 13% 22% 10% 1	3736
	11th to 20th	Night	34% 8% 21% 23% 14% 1	3491
	21st to end	Night	61% 7% 12% 15% 4 1	3828
April	1st to 10th	Night	49% 7% 11% 21% 11% 1	1837
	11th to 20th	Night	49% 7% 13% 21% 9% 1	1691
	21st to end	Night	46% 8% 12% 23% 9% 2	1710
May	1st to 10th	Night	55% 6% 10% 22% 6% 1	2747
	11th to 20th	Night	38% 9% 15% 30% 7% 1	2199
	21st to end	Night	48% 11% 16% 17% 7% 1	2414
June	1st to 10th	Night	42% 8% 14% 24% 11% 1	1330
	11th to 20th	Night	47% 6% 16% 20% 9% 2	1330
	21st to end	Night	29% 9% 19% 26% 15% 2	1330
July	1st to 10th	Night	30% 6% 18% 24% 18% 4	2185
	11th to 20th	Night	28% 5% 16% 31% 15% 4	2178
	21st to end	Night	21% 4 9% 30% 23% 14%	2474
August	1st to 10th	Night	35% 5% 14% 27% 16% 3	2552
	11th to 20th	Night	33% 4 20% 29% 12% 2	2838
	21st to end	Night	23% 3 13% 40% 17% 4	3146
September	1st to 10th	Night	30% 6% 13% 27% 18% 6%	1833
	11th to 20th	Night	32% 5% 14% 27% 17% 5%	2090
	21st to end	Night	33% 6% 15% 28% 15% 4	2090
October	1st to 10th	Night	29% 4 11% 32% 21% 3	3645
	11th to 20th	Night	32% 5% 17% 25% 18% 3	3948
	21st to end	Night	34% 5% 12% 33% 16%	4457
November	1st to 10th	Night	46% 8% 12% 22% 11% 1	2549
	11th to 20th	Night	38% 8% 16% 21% 16% 1	2664
	21st to end	Night	38% 7% 18% 21% 12% 3	2850
December	1st to 10th	Night	44% 8% 15% 20% 10% 3	2850
	11th to 20th	Night	41% 9% 22% 21% 6% 1	3507
	21st to end	Night	37% 6% 13% 25% 13% 5%	5214

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	6% 10% 29% 31% 13% 12%	180
	11th to 20th	Day	13% 4 34% 27% 14% 7%	180
	21st to end	Day	3 8% 34% 35% 17% 3	198
February	1st to 10th	Day	11% 10% 37% 29% 10% 4	198
	11th to 20th	Day	7% 13% 30% 31% 15% 4	270
	21st to end	Day	12% 12% 30% 27% 16% 3	329
March	1st to 10th	Day	10% 16% 33% 27% 10% 3	460
	11th to 20th	Day	9% 12% 31% 31% 14% 3	460
	21st to end	Day	12% 15% 32% 24% 12% 5%	506
April	1st to 10th	Day	15% 15% 32% 27% 10% 2	460
	11th to 20th	Day	13% 14% 27% 27% 15% 3	452
	21st to end	Day	16% 13% 30% 25% 13% 3	460
May	1st to 10th	Day	16% 10% 23% 29% 15% 7%	496
	11th to 20th	Day	17% 7% 21% 28% 22% 6%	675
	21st to end	Day	18% 8% 17% 29% 19% 8%	734
June	1st to 10th	Day	14% 9% 29% 28% 17% 3	680
	11th to 20th	Day	19% 10% 26% 27% 16% 2	680
	21st to end	Day	11% 6% 23% 32% 22% 7%	680
July	1st to 10th	Day	8% 3 20% 35% 27% 6%	671
	11th to 20th	Day	9% 5% 22% 34% 22% 7%	668
	21st to end	Day	5% 5% 25% 34% 25% 7%	658
August	1st to 10th	Day	9% 5% 23% 34% 22% 8%	487
	11th to 20th	Day	8% 6% 20% 33% 25% 9%	477
	21st to end	Day	9% 8% 25% 30% 18% 10%	539
September	1st to 10th	Day	10% 8% 28% 26% 20% 8%	490
	11th to 20th	Day	16% 7% 25% 29% 17% 6%	490
	21st to end	Day	16% 15% 29% 22% 14% 4	490
October	1st to 10th	Day	17% 8% 22% 24% 22% 7%	395
	11th to 20th	Day	13% 13% 28% 23% 16% 7%	300
	21st to end	Day	14% 15% 28% 28% 12% 3	330
November	1st to 10th	Day	20% 8% 29% 26% 13% 4	279
	11th to 20th	Day	11% 14% 34% 28% 11% 2	266
	21st to end	Day	5% 17% 28% 28% 13% 9%	180
December	1st to 10th	Day	8% 12% 29% 32% 14% 6%	180
	11th to 20th	Day	7% 11% 42% 23% 13% 3	180
	21st to end	Day	10% 8% 22% 34% 19% 8%	198
January	1st to 10th	Night	9% 12% 34% 31% 13% 2	950
	11th to 20th	Night	9% 8% 36% 35% 11% 2	927
	21st to end	Night	9% 8% 36% 32% 13% 1	836
February	1st to 10th	Night	10% 11% 28% 32% 16% 3	760
	11th to 20th	Night	10% 6% 31% 38% 13% 2	744
	21st to end	Night	9% 7% 26% 33% 21% 3	628
March	1st to 10th	Night	8% 7% 36% 34% 13% 2	760
	11th to 20th	Night	10% 8% 37% 31% 12% 2	760
	21st to end	Night	13% 9% 29% 34% 12% 3	836
April	1st to 10th	Night	14% 8% 34% 31% 11% 2	697
	11th to 20th	Night	15% 7% 33% 34% 10% 1	563
	21st to end	Night	15% 9% 29% 34% 12% 1	570
May	1st to 10th	Night	16% 7% 25% 34% 18% 1	542
	11th to 20th	Night	12% 7% 21% 37% 21% 1	374
	21st to end	Night	19% 5% 21% 32% 21% 1	411
June	1st to 10th	Night	16% 6% 26% 36% 16% 1	380
	11th to 20th	Night	22% 6% 26% 32% 14%	380
	21st to end	Night	9% 3 24% 42% 21% 2	380
July	1st to 10th	Night	6% 3 19% 43% 28% 1	370
	11th to 20th	Night	8% 2 22% 44% 23% 1	368
	21st to end	Night	5% 2 24% 44% 24% 2	416
August	1st to 10th	Night	10% 2 15% 42% 29% 2	396
	11th to 20th	Night	7% 2 21% 46% 23% 2	564
	21st to end	Night	9% 5% 23% 41% 20% 1	627
September	1st to 10th	Night	11% 3 23% 40% 21% 2	651
	11th to 20th	Night	16% 3 20% 37% 22% 2	760
	21st to end	Night	11% 7% 29% 35% 14% 4	760
October	1st to 10th	Night	14% 6% 24% 34% 19% 2	760
	11th to 20th	Night	12% 6% 34% 30% 15% 3	760
	21st to end	Night	11% 7% 34% 33% 14% 1	836
November	1st to 10th	Night	15% 9% 37% 30% 8% 1	853
	11th to 20th	Night	10% 11% 41% 28% 9% 2	950
	21st to end	Night	10% 10% 35% 32% 11% 2	950
December	1st to 10th	Night	9% 9% 30% 35% 14% 3	950
	11th to 20th	Night	8% 9% 36% 37% 9% 2	950
	21st to end	Night	12% 8% 32% 34% 12% 2	1045

Page generated at 2024-11-01 20:47:48. Version ac07

Copyright © 2024 Attilla Danko.