Borman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Borman Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	13% 8% 3 7% 4 4 4 4 6% 8% 40%	137158
	Night	14% 8% 3 7% 4 4 4 3 5% 7% 42%	135053
Transparency	Day	43% 4 11% 25% 16% 2	140772
	Night	42% 4 10% 24% 18% 3	139515
Seeing	Day	16% 9% 24% 25% 18% 8%	17638
	Night	17% 8% 26% 31% 16% 2	27462

By Month

Percentage Cloud Cover SE (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	14% 10% 6% 11% 5% 6% 5% 4 4 5% 31%	4691
	11th to 20th	Day	11% 10% 4 10% 6% 5% 4 3 4 7% 36%	4710
	21st to end	Day	19% 13% 5% 9% 5% 4 3 3 4 5% 29%	5761
February	1st to 10th	Day	17% 11% 4 7% 4 4 3 3 4 7% 36%	2100
	11th to 20th	Day	18% 9% 4 9% 4 4 3 3 5% 6% 35%	2048
	21st to end	Day	15% 9% 4 10% 4 5% 4 4 5% 6% 34%	1730
March	1st to 10th	Day	18% 11% 3 7% 4 4 3 2 3 4 41%	4580
	11th to 20th	Day	12% 10% 2 6% 3 4 3 3 5% 7% 45%	4756
	21st to end	Day	13% 8% 3 7% 4 4 4 4 6% 7% 39%	6089
April	1st to 10th	Day	13% 7% 2 6% 3 4 5% 4 5% 8% 41%	2520
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 5% 7% 41%	2494
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 8% 40%	2562
May	1st to 10th	Day	16% 7% 2 8% 5% 5% 6% 4 7% 9% 30%	7616
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 4 6% 7% 39%	7626
	21st to end	Day	17% 9% 3 8% 4 5% 5% 4 6% 8% 30%	8630
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	2940
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 4 6% 9% 47%	2871
	21st to end	Day	7% 5% 2 6% 5% 3 5% 5% 8% 10% 46%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 6% 10% 44%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 12% 40%	6360
	21st to end	Day	8% 6% 3 9% 6% 6% 6% 5% 8% 11% 32%	6794
August	1st to 10th	Day	5% 4 2 5% 3 4 4 4 7% 9% 54%	4051
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	3849
	21st to end	Day	4 4 2 5% 4 4 4 3 6% 9% 56%	4312
September	1st to 10th	Day	10% 4 2 6% 5% 5% 4 4 6% 7% 48%	2400
	11th to 20th	Day	8% 4 2 6% 5% 4 4 4 5% 8% 50%	2225
	21st to end	Day	12% 4 2 6% 4 4 4 3 4 5% 52%	2200
October	1st to 10th	Day	10% 3 2 4 3 3 4 3 5% 6% 57%	2200
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 7% 50%	2050
	21st to end	Day	18% 7% 2 6% 4 3 4 3 4 5% 45%	2007
November	1st to 10th	Day	9% 6% 2 6% 3 4 4 4 5% 6% 51%	1852
	11th to 20th	Day	17% 7% 3 5% 4 4 3 3 4 6% 45%	1890
	21st to end	Day	19% 9% 3 6% 4 3 3 3 4 6% 39%	1890
December	1st to 10th	Day	14% 10% 3 7% 3 3 3 2 3 5% 47%	3020
	11th to 20th	Day	15% 10% 5% 10% 5% 4 4 3 5% 6% 32%	3104
	21st to end	Day	26% 15% 5% 9% 5% 5% 4 2 4 4 22%	4070
January	1st to 10th	Night	16% 12% 5% 8% 3 4 4 3 4 6% 35%	8232
	11th to 20th	Night	13% 10% 4 9% 5% 5% 5% 3 3 5% 39%	8280
	21st to end	Night	19% 10% 4 6% 4 4 4 2 4 5% 38%	8429
February	1st to 10th	Night	20% 11% 4 8% 5% 4 4 3 6% 5% 31%	2520
	11th to 20th	Night	18% 8% 3 8% 4 5% 4 3 4 5% 38%	2458
	21st to end	Night	19% 8% 3 7% 4 4 3 4 6% 5% 38%	2076
March	1st to 10th	Night	22% 10% 3 6% 4 4 4 3 4 5% 35%	5493
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 5% 6% 49%	5424
	21st to end	Night	16% 8% 3 9% 5% 4 5% 3 4 6% 36%	5124
April	1st to 10th	Night	14% 5% 2 5% 5% 4 4 3 4 7% 46%	2100
	11th to 20th	Night	16% 7% 2 6% 4 3 3 4 5% 7% 44%	2075
	21st to end	Night	16% 7% 3 6% 5% 4 5% 4 5% 7% 38%	2056
May	1st to 10th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 9% 32%	5237
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	5234
	21st to end	Night	16% 9% 3 8% 5% 5% 5% 4 6% 8% 32%	5518
June	1st to 10th	Night	10% 5% 2 5% 3 4 6% 3 7% 9% 46%	1680
	11th to 20th	Night	6% 4 1 5% 4 4 4 4 6% 9% 52%	1640
	21st to end	Night	4 3 1 4 3 3 4 4 8% 12% 53%	1600
July	1st to 10th	Night	8% 6% 3 6% 4 4 4 4 5% 8% 48%	3618
	11th to 20th	Night	6% 4 2 6% 4 5% 5% 4 7% 10% 48%	3617
	21st to end	Night	6% 5% 3 7% 5% 5% 6% 6% 8% 12% 37%	4069
August	1st to 10th	Night	5% 3 2 5% 3 3 4 3 5% 8% 60%	3094
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 8% 52%	3200
	21st to end	Night	4 3 1 4 3 3 4 4 6% 10% 60%	3586
September	1st to 10th	Night	9% 4 2 5% 4 4 3 4 5% 6% 54%	2000
	11th to 20th	Night	7% 5% 2 5% 4 3 4 4 6% 7% 53%	2154
	21st to end	Night	12% 6% 2 4 4 3 4 3 5% 7% 50%	2200
October	1st to 10th	Night	8% 4 2 6% 3 3 3 3 4 6% 60%	2200
	11th to 20th	Night	13% 5% 1 5% 3 4 4 2 4 5% 54%	2347
	21st to end	Night	18% 8% 2 5% 3 3 4 2 4 5% 45%	2831
November	1st to 10th	Night	11% 6% 3 5% 3 4 3 3 4 6% 53%	2666
	11th to 20th	Night	14% 6% 2 6% 4 3 3 3 4 6% 49%	2730
	21st to end	Night	19% 8% 3 6% 4 3 3 3 5% 5% 43%	2730
December	1st to 10th	Night	18% 9% 3 7% 3 3 3 3 4 5% 43%	4380
	11th to 20th	Night	18% 9% 3 6% 4 4 4 4 5% 5% 39%	5305
	21st to end	Night	27% 14% 4 8% 4 4 4 3 3 4 25%	7150

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	56% 2 12% 23% 6%	4692
	11th to 20th	Day	51% 2 7% 27% 13% 1	4710
	21st to end	Day	59% 2 10% 17% 8% 4	5761
February	1st to 10th	Day	51% 3 11% 23% 11% 1	2100
	11th to 20th	Day	50% 4 9% 21% 13% 2	2058
	21st to end	Day	50% 4 9% 20% 13% 3	1730
March	1st to 10th	Day	49% 2 5% 20% 21% 3	4580
	11th to 20th	Day	40% 3 9% 28% 18% 2	4756
	21st to end	Day	44% 3 12% 27% 13% 1	6091
April	1st to 10th	Day	40% 5% 14% 24% 16% 1	2520
	11th to 20th	Day	43% 4 11% 24% 16% 3	2494
	21st to end	Day	43% 4 12% 27% 12% 2	2562
May	1st to 10th	Day	50% 5% 12% 22% 11%	7651
	11th to 20th	Day	45% 4 12% 23% 15% 1	7630
	21st to end	Day	52% 4 10% 19% 14% 1	8644
June	1st to 10th	Day	36% 5% 12% 25% 20% 3	2940
	11th to 20th	Day	31% 7% 13% 26% 20% 3	2871
	21st to end	Day	31% 7% 14% 24% 17% 5%	2800
July	1st to 10th	Day	37% 4 10% 29% 19% 2	6291
	11th to 20th	Day	36% 5% 17% 29% 12% 1	6285
	21st to end	Day	45% 4 19% 23% 8% 1	6599
August	1st to 10th	Day	27% 4 15% 37% 16% 1	4038
	11th to 20th	Day	35% 4 14% 28% 17% 1	3885
	21st to end	Day	26% 3 12% 37% 18% 3	4311
September	1st to 10th	Day	34% 7% 9% 26% 20% 5%	2400
	11th to 20th	Day	32% 5% 9% 27% 24% 3	2225
	21st to end	Day	35% 4 7% 24% 27% 4	2200
October	1st to 10th	Day	19% 3 5% 25% 43% 6%	3590
	11th to 20th	Day	24% 2 9% 26% 34% 6%	3316
	21st to end	Day	33% 2 6% 26% 28% 5%	3219
November	1st to 10th	Day	34% 4 10% 30% 17% 6%	1852
	11th to 20th	Day	41% 4 7% 25% 18% 5%	1890
	21st to end	Day	48% 3 8% 23% 16% 2	1890
December	1st to 10th	Day	44% 2 12% 27% 16%	3020
	11th to 20th	Day	54% 3 10% 21% 11% 1	3101
	21st to end	Day	66% 3 11% 13% 6%	4070
January	1st to 10th	Night	51% 3 13% 25% 8% 1	8233
	11th to 20th	Night	50% 2 5% 21% 18% 3	8280
	21st to end	Night	50% 2 9% 21% 17% 1	8429
February	1st to 10th	Night	54% 3 11% 21% 9% 1	2520
	11th to 20th	Night	49% 4 8% 24% 14% 2	2459
	21st to end	Night	48% 3 10% 22% 13% 3	2076
March	1st to 10th	Night	52% 3 6% 18% 18% 3	5493
	11th to 20th	Night	37% 3 9% 24% 21% 6%	5424
	21st to end	Night	49% 4 10% 24% 12% 2	5124
April	1st to 10th	Night	38% 4 12% 27% 16% 3	2100
	11th to 20th	Night	40% 4 11% 22% 19% 5%	2075
	21st to end	Night	43% 6% 11% 23% 15% 2	2056
May	1st to 10th	Night	49% 4 14% 24% 9%	5264
	11th to 20th	Night	45% 5% 11% 22% 16% 1	5235
	21st to end	Night	49% 5% 13% 20% 11% 2	5528
June	1st to 10th	Night	34% 6% 11% 24% 21% 4	1680
	11th to 20th	Night	28% 6% 14% 22% 26% 5%	1640
	21st to end	Night	23% 7% 17% 26% 20% 7%	1600
July	1st to 10th	Night	36% 4 9% 27% 21% 3	3580
	11th to 20th	Night	33% 5% 18% 29% 13% 2	3572
	21st to end	Night	39% 4 21% 24% 11% 1	3952
August	1st to 10th	Night	24% 3 13% 43% 15% 1	3083
	11th to 20th	Night	30% 4 13% 29% 22% 1	3230
	21st to end	Night	22% 4 12% 34% 25% 3	3586
September	1st to 10th	Night	30% 6% 7% 27% 23% 6%	2000
	11th to 20th	Night	30% 4 9% 26% 25% 5%	2154
	21st to end	Night	34% 4 9% 23% 24% 7%	2200
October	1st to 10th	Night	19% 2 5% 15% 51% 7%	3590
	11th to 20th	Night	24% 2 5% 26% 37% 6%	3820
	21st to end	Night	32% 2 7% 23% 27% 9%	4569
November	1st to 10th	Night	34% 3 9% 24% 24% 5%	2666
	11th to 20th	Night	37% 3 7% 25% 22% 6%	2730
	21st to end	Night	44% 3 9% 25% 17% 2	2730
December	1st to 10th	Night	45% 3 10% 25% 17% 1	4380
	11th to 20th	Night	47% 3 11% 22% 15% 1	5307
	21st to end	Night	64% 3 9% 17% 6% 1	7150

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	23% 17% 32% 20% 7% 1	330
	11th to 20th	Day	25% 23% 31% 18% 4	330
	21st to end	Day	23% 21% 39% 13% 4	363
February	1st to 10th	Day	25% 14% 38% 21% 2 1	330
	11th to 20th	Day	27% 20% 38% 12% 2 1	330
	21st to end	Day	26% 19% 33% 19% 3	273
March	1st to 10th	Day	25% 16% 38% 16% 4	330
	11th to 20th	Day	25% 18% 38% 14% 5% 1	388
	21st to end	Day	25% 17% 32% 19% 6% 1	594
April	1st to 10th	Day	19% 14% 35% 24% 7% 1	540
	11th to 20th	Day	22% 16% 26% 22% 13% 1	532
	21st to end	Day	18% 13% 34% 24% 8% 3	540
May	1st to 10th	Day	19% 13% 28% 28% 8% 3	543
	11th to 20th	Day	21% 7% 25% 27% 14% 7%	567
	21st to end	Day	13% 5% 27% 26% 19% 10%	700
June	1st to 10th	Day	14% 4 19% 26% 28% 9%	780
	11th to 20th	Day	7% 3 20% 34% 25% 11%	756
	21st to end	Day	9% 4 20% 33% 23% 11%	730
July	1st to 10th	Day	11% 3 12% 23% 27% 23%	721
	11th to 20th	Day	8% 4 12% 31% 31% 15%	718
	21st to end	Day	9% 2 11% 30% 34% 14%	739
August	1st to 10th	Day	9% 3 11% 28% 32% 17%	526
	11th to 20th	Day	13% 2 12% 27% 32% 13%	519
	21st to end	Day	7% 1 11% 24% 39% 17%	583
September	1st to 10th	Day	9% 3 16% 26% 33% 14%	530
	11th to 20th	Day	8% 2 10% 29% 33% 18%	530
	21st to end	Day	15% 5% 21% 22% 25% 12%	530
October	1st to 10th	Day	10% 3 20% 35% 25% 8%	530
	11th to 20th	Day	13% 9% 32% 27% 14% 5%	380
	21st to end	Day	25% 13% 27% 26% 9%	363
November	1st to 10th	Day	15% 8% 34% 27% 11% 4	330
	11th to 20th	Day	16% 19% 31% 22% 11% 1	330
	21st to end	Day	27% 8% 32% 24% 8% 2	330
December	1st to 10th	Day	22% 18% 34% 19% 6% 2	330
	11th to 20th	Day	26% 15% 30% 21% 7% 1	330
	21st to end	Day	29% 19% 31% 16% 5%	363
January	1st to 10th	Night	22% 13% 34% 27% 4	1050
	11th to 20th	Night	25% 15% 35% 21% 4	1050
	21st to end	Night	20% 13% 38% 26% 2	1113
February	1st to 10th	Night	25% 11% 36% 24% 3	840
	11th to 20th	Night	23% 15% 35% 25% 2	823
	21st to end	Night	24% 13% 39% 22% 2	692
March	1st to 10th	Night	25% 12% 31% 28% 4	840
	11th to 20th	Night	21% 12% 33% 27% 7%	840
	21st to end	Night	23% 11% 29% 31% 6%	719
April	1st to 10th	Night	16% 10% 34% 32% 7% 1	630
	11th to 20th	Night	20% 8% 31% 28% 11% 1	623
	21st to end	Night	20% 6% 31% 32% 9% 1	630
May	1st to 10th	Night	17% 7% 29% 33% 13% 1	630
	11th to 20th	Night	14% 6% 22% 35% 19% 2	621
	21st to end	Night	10% 2 21% 37% 27% 4	683
June	1st to 10th	Night	13% 3 15% 40% 26% 3	630
	11th to 20th	Night	8% 1 16% 42% 30% 2	615
	21st to end	Night	5% 2 18% 39% 33% 4	600
July	1st to 10th	Night	10% 1 10% 32% 39% 8%	585
	11th to 20th	Night	5% 3 9% 36% 42% 5%	582
	21st to end	Night	8% 2 10% 35% 38% 7%	657
August	1st to 10th	Night	8% 2 7% 37% 41% 6%	597
	11th to 20th	Night	7% 2 9% 38% 39% 6%	594
	21st to end	Night	6% 1 7% 38% 41% 7%	660
September	1st to 10th	Night	11% 2 10% 34% 38% 6%	600
	11th to 20th	Night	8% 2 12% 34% 38% 5%	754
	21st to end	Night	15% 3 18% 31% 29% 5%	800
October	1st to 10th	Night	10% 4 19% 35% 27% 5%	800
	11th to 20th	Night	16% 6% 27% 33% 16% 2	800
	21st to end	Night	22% 8% 31% 28% 10% 1	880
November	1st to 10th	Night	14% 7% 31% 34% 13% 1	820
	11th to 20th	Night	17% 10% 33% 29% 11%	840
	21st to end	Night	23% 10% 27% 30% 9% 1	840
December	1st to 10th	Night	24% 8% 31% 31% 5%	840
	11th to 20th	Night	23% 8% 33% 29% 5% 1	1029
	21st to end	Night	29% 13% 32% 22% 4	1155

Page generated at 2026-06-13 01:31:43. Version ac07

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