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 2024-01-31.

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% 7% 3 6% 5% 3 5% 5% 5% 8% 41%	74989
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	75156
Transparency	Day	41% 6% 12% 24% 15% 3	74694
	Night	41% 5% 11% 23% 17% 3	74972
Seeing	Day	16% 8% 23% 25% 19% 9%	14385
	Night	17% 7% 25% 31% 17% 2	24335

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	19% 9% 5% 7% 5% 2 4 4 4 5% 36%	1517
	11th to 20th	Day	19% 11% 4 8% 6% 3 4 4 4 8% 29%	1520
	21st to end	Day	16% 10% 4 7% 6% 3 4 4 5% 6% 34%	1853
February	1st to 10th	Day	19% 10% 5% 6% 5% 3 4 4 4 7% 34%	1800
	11th to 20th	Day	18% 8% 4 8% 5% 3 4 4 5% 6% 34%	1748
	21st to end	Day	16% 9% 4 10% 5% 3 5% 5% 5% 6% 32%	1480
March	1st to 10th	Day	16% 9% 3 5% 5% 3 4 4 4 5% 42%	1800
	11th to 20th	Day	20% 10% 3 7% 5% 2 5% 4 4 6% 34%	1849
	21st to end	Day	19% 10% 4 7% 5% 3 5% 4 5% 6% 32%	2366
April	1st to 10th	Day	12% 7% 3 5% 4 3 5% 5% 5% 9% 43%	2160
	11th to 20th	Day	16% 7% 3 6% 6% 3 4 5% 5% 7% 40%	2134
	21st to end	Day	18% 6% 3 5% 5% 3 5% 4 5% 8% 39%	2196
May	1st to 10th	Day	17% 6% 2 6% 6% 3 5% 4 6% 8% 36%	2340
	11th to 20th	Day	17% 7% 3 6% 5% 3 5% 4 6% 7% 37%	2301
	21st to end	Day	8% 6% 3 6% 6% 4 6% 6% 8% 10% 36%	2602
June	1st to 10th	Day	11% 4 2 5% 5% 3 4 5% 6% 10% 45%	2520
	11th to 20th	Day	5% 3 2 5% 5% 4 6% 6% 6% 9% 48%	2520
	21st to end	Day	7% 5% 2 5% 5% 2 5% 5% 8% 11% 45%	2520
July	1st to 10th	Day	11% 5% 3 5% 5% 4 5% 6% 8% 10% 39%	2520
	11th to 20th	Day	6% 4 2 4 5% 3 5% 6% 8% 11% 46%	2520
	21st to end	Day	7% 4 2 5% 6% 4 6% 6% 7% 11% 42%	2694
August	1st to 10th	Day	7% 5% 2 5% 5% 4 5% 6% 8% 9% 44%	2234
	11th to 20th	Day	9% 5% 2 4 6% 3 5% 7% 6% 11% 42%	2125
	21st to end	Day	6% 4 2 5% 5% 3 5% 5% 6% 9% 48%	2376
September	1st to 10th	Day	10% 4 3 5% 6% 4 4 5% 6% 7% 47%	2160
	11th to 20th	Day	8% 5% 2 5% 6% 3 5% 4 5% 8% 49%	2003
	21st to end	Day	11% 3 2 5% 5% 3 3 4 4 6% 54%	1980
October	1st to 10th	Day	11% 4 2 4 4 2 4 4 5% 6% 56%	1980
	11th to 20th	Day	13% 5% 2 5% 5% 2 4 4 5% 7% 48%	1846
	21st to end	Day	18% 7% 2 5% 5% 2 3 4 4 5% 44%	1807
November	1st to 10th	Day	9% 5% 2 4 4 2 4 4 5% 7% 54%	1672
	11th to 20th	Day	17% 6% 3 4 4 3 3 4 4 6% 46%	1710
	21st to end	Day	19% 7% 3 5% 4 2 4 4 4 6% 41%	1710
December	1st to 10th	Day	19% 10% 4 6% 5% 3 4 3 3 5% 38%	1710
	11th to 20th	Day	16% 11% 6% 8% 6% 3 4 4 5% 6% 32%	1933
	21st to end	Day	22% 15% 5% 8% 5% 3 4 3 4 5% 24%	2783
January	1st to 10th	Night	17% 9% 4 7% 5% 3 3 4 5% 6% 37%	2648
	11th to 20th	Night	20% 10% 5% 8% 6% 3 4 4 4 6% 32%	2660
	21st to end	Night	16% 10% 4 7% 6% 4 4 4 5% 6% 33%	2726
February	1st to 10th	Night	21% 10% 4 7% 5% 3 4 4 6% 5% 31%	2160
	11th to 20th	Night	17% 7% 4 7% 5% 3 5% 3 4 6% 38%	2098
	21st to end	Night	21% 9% 4 7% 4 3 4 4 6% 5% 35%	1776
March	1st to 10th	Night	18% 6% 2 6% 5% 2 4 5% 4 5% 43%	2160
	11th to 20th	Night	17% 8% 4 7% 5% 3 5% 5% 5% 6% 37%	2111
	21st to end	Night	21% 7% 3 6% 6% 2 5% 5% 5% 7% 34%	2003
April	1st to 10th	Night	13% 5% 2 5% 5% 4 3 5% 4 7% 47%	1800
	11th to 20th	Night	17% 6% 2 5% 5% 2 3 4 4 7% 44%	1775
	21st to end	Night	18% 6% 3 6% 5% 2 5% 5% 5% 7% 39%	1763
May	1st to 10th	Night	15% 6% 3 5% 5% 3 4 4 6% 8% 40%	1620
	11th to 20th	Night	11% 7% 3 7% 5% 4 5% 5% 5% 8% 40%	1592
	21st to end	Night	6% 4 3 5% 5% 5% 6% 5% 8% 10% 41%	1670
June	1st to 10th	Night	9% 5% 2 3 4 4 5% 5% 7% 9% 48%	1440
	11th to 20th	Night	5% 3 1 4 5% 3 4 6% 6% 9% 54%	1440
	21st to end	Night	5% 3 2 4 4 3 4 6% 7% 11% 53%	1440
July	1st to 10th	Night	7% 4 2 5% 4 3 4 5% 7% 11% 47%	1440
	11th to 20th	Night	5% 3 2 4 5% 3 5% 4 6% 9% 54%	1440
	21st to end	Night	7% 3 2 4 5% 2 6% 5% 6% 10% 49%	1621
August	1st to 10th	Night	7% 4 2 5% 4 3 5% 4 6% 9% 52%	1701
	11th to 20th	Night	5% 4 2 4 5% 4 5% 6% 6% 10% 48%	1776
	21st to end	Night	6% 4 2 4 4 2 4 4 6% 10% 55%	1980
September	1st to 10th	Night	10% 4 2 4 4 3 4 5% 5% 6% 54%	1800
	11th to 20th	Night	7% 5% 2 5% 5% 3 4 5% 6% 7% 53%	1938
	21st to end	Night	11% 5% 3 3 4 2 3 4 5% 8% 53%	1980
October	1st to 10th	Night	9% 4 2 5% 4 2 3 3 4 5% 59%	1980
	11th to 20th	Night	14% 5% 2 4 5% 3 3 3 4 5% 52%	2111
	21st to end	Night	18% 7% 2 4 4 3 4 3 4 5% 45%	2547
November	1st to 10th	Night	11% 5% 3 4 3 3 3 3 4 5% 55%	2406
	11th to 20th	Night	14% 5% 3 5% 5% 3 3 4 4 6% 49%	2470
	21st to end	Night	19% 6% 2 5% 5% 2 3 4 4 5% 45%	2470
December	1st to 10th	Night	20% 8% 3 6% 4 2 4 4 5% 6% 37%	2470
	11th to 20th	Night	17% 9% 3 7% 5% 3 5% 4 5% 5% 37%	3260
	21st to end	Night	21% 15% 5% 7% 6% 4 5% 4 4 5% 25%	4884

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% 4 10% 23% 11% 1	1518
	11th to 20th	Day	56% 4 11% 21% 8%	1520
	21st to end	Day	51% 4 11% 25% 8% 2	1853
February	1st to 10th	Day	52% 4 10% 22% 11% 1	1800
	11th to 20th	Day	51% 5% 10% 22% 11% 2	1758
	21st to end	Day	52% 5% 9% 19% 11% 3	1480
March	1st to 10th	Day	46% 4 8% 23% 18% 1	1800
	11th to 20th	Day	52% 5% 10% 21% 9% 2	1849
	21st to end	Day	53% 5% 11% 22% 8% 1	2366
April	1st to 10th	Day	38% 6% 14% 25% 16%	2160
	11th to 20th	Day	44% 5% 11% 22% 16% 3	2134
	21st to end	Day	44% 5% 12% 26% 12% 2	2196
May	1st to 10th	Day	45% 7% 14% 24% 10% 1	2340
	11th to 20th	Day	45% 6% 13% 20% 14% 1	2305
	21st to end	Day	41% 8% 17% 20% 12% 2	2602
June	1st to 10th	Day	34% 6% 12% 26% 20% 3	2520
	11th to 20th	Day	30% 8% 13% 27% 19% 3	2520
	21st to end	Day	31% 8% 15% 23% 17% 6%	2520
July	1st to 10th	Day	37% 8% 15% 22% 14% 4	2451
	11th to 20th	Day	30% 8% 16% 27% 16% 4	2445
	21st to end	Day	33% 8% 17% 26% 14% 2	2544
August	1st to 10th	Day	33% 8% 15% 28% 15% 1	2221
	11th to 20th	Day	34% 8% 14% 26% 15% 2	2125
	21st to end	Day	32% 6% 14% 25% 18% 6%	2376
September	1st to 10th	Day	35% 7% 10% 26% 19% 4	2160
	11th to 20th	Day	33% 6% 9% 26% 23% 3	2003
	21st to end	Day	32% 5% 7% 26% 27% 4	1980
October	1st to 10th	Day	29% 5% 8% 27% 26% 5%	1980
	11th to 20th	Day	36% 4 9% 25% 22% 5%	1846
	21st to end	Day	42% 4 9% 24% 18% 3	1807
November	1st to 10th	Day	31% 4 10% 31% 18% 6%	1672
	11th to 20th	Day	41% 4 7% 25% 19% 5%	1710
	21st to end	Day	46% 3 9% 24% 16% 2	1710
December	1st to 10th	Day	51% 3 11% 22% 12% 1	1710
	11th to 20th	Day	53% 5% 8% 24% 9% 1	1930
	21st to end	Day	62% 5% 11% 15% 8%	2783
January	1st to 10th	Night	48% 4 10% 22% 14% 2	2648
	11th to 20th	Night	55% 4 10% 21% 9% 1	2660
	21st to end	Night	51% 4 11% 21% 11% 2	2726
February	1st to 10th	Night	55% 4 11% 21% 8% 1	2160
	11th to 20th	Night	48% 4 8% 23% 14% 2	2099
	21st to end	Night	50% 4 11% 20% 12% 3	1776
March	1st to 10th	Night	43% 5% 8% 21% 18% 4	2160
	11th to 20th	Night	49% 5% 12% 19% 13% 3	2111
	21st to end	Night	50% 5% 13% 20% 10% 2	2003
April	1st to 10th	Night	37% 5% 11% 27% 17% 3	1800
	11th to 20th	Night	40% 4 9% 22% 19% 6%	1775
	21st to end	Night	43% 7% 10% 23% 15% 2	1763
May	1st to 10th	Night	42% 6% 12% 27% 13% 1	1620
	11th to 20th	Night	42% 8% 11% 20% 18% 1	1593
	21st to end	Night	34% 8% 16% 22% 15% 4	1671
June	1st to 10th	Night	32% 6% 12% 25% 22% 3	1440
	11th to 20th	Night	26% 6% 15% 22% 25% 5%	1440
	21st to end	Night	23% 7% 17% 27% 18% 8%	1440
July	1st to 10th	Night	30% 7% 16% 26% 15% 5%	1400
	11th to 20th	Night	26% 6% 15% 29% 19% 5%	1392
	21st to end	Night	30% 7% 13% 31% 18% 2	1531
August	1st to 10th	Night	28% 6% 18% 25% 20% 2	1690
	11th to 20th	Night	30% 7% 13% 28% 20% 2	1776
	21st to end	Night	25% 7% 14% 27% 21% 6%	1980
September	1st to 10th	Night	30% 6% 8% 28% 21% 6%	1800
	11th to 20th	Night	30% 5% 9% 26% 24% 5%	1938
	21st to end	Night	31% 5% 9% 25% 24% 7%	1980
October	1st to 10th	Night	28% 4 10% 22% 28% 7%	1980
	11th to 20th	Night	36% 4 6% 25% 23% 7%	2111
	21st to end	Night	42% 4 7% 24% 17% 5%	2547
November	1st to 10th	Night	32% 4 9% 24% 24% 6%	2406
	11th to 20th	Night	37% 4 7% 24% 22% 7%	2470
	21st to end	Night	43% 3 9% 25% 18% 2	2470
December	1st to 10th	Night	48% 4 11% 23% 12% 1	2470
	11th to 20th	Night	48% 5% 10% 21% 14% 1	3262
	21st to end	Night	62% 4 9% 15% 8% 1	4884

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	24% 16% 31% 21% 7% 1	270
	11th to 20th	Day	26% 20% 31% 19% 4	270
	21st to end	Day	24% 21% 37% 13% 4	294
February	1st to 10th	Day	28% 13% 40% 16% 2 2	240
	11th to 20th	Day	28% 18% 39% 13% 3	240
	21st to end	Day	29% 17% 31% 20% 4	198
March	1st to 10th	Day	22% 16% 41% 18% 3	240
	11th to 20th	Day	25% 16% 36% 16% 7% 1	289
	21st to end	Day	27% 15% 31% 19% 7% 1	462
April	1st to 10th	Day	16% 13% 35% 26% 9% 1	420
	11th to 20th	Day	22% 14% 24% 23% 16% 1	412
	21st to end	Day	20% 11% 33% 24% 9% 4	420
May	1st to 10th	Day	18% 13% 28% 28% 9% 4	423
	11th to 20th	Day	22% 4 23% 29% 15% 7%	447
	21st to end	Day	10% 4 24% 27% 22% 12%	556
June	1st to 10th	Day	13% 4 17% 26% 29% 11%	630
	11th to 20th	Day	6% 3 19% 33% 27% 12%	630
	21st to end	Day	9% 4 21% 32% 22% 11%	630
July	1st to 10th	Day	11% 3 12% 22% 28% 25%	621
	11th to 20th	Day	7% 4 11% 30% 32% 16%	618
	21st to end	Day	9% 2 11% 30% 33% 15%	635
August	1st to 10th	Day	9% 3 10% 28% 33% 18%	446
	11th to 20th	Day	15% 2 10% 27% 33% 13%	439
	21st to end	Day	8% 1 11% 23% 39% 18%	495
September	1st to 10th	Day	9% 3 15% 24% 35% 15%	450
	11th to 20th	Day	9% 2 10% 28% 33% 18%	450
	21st to end	Day	14% 6% 20% 22% 26% 13%	450
October	1st to 10th	Day	11% 3 21% 35% 22% 8%	450
	11th to 20th	Day	14% 10% 34% 26% 12% 4	316
	21st to end	Day	25% 13% 26% 26% 9%	297
November	1st to 10th	Day	14% 6% 34% 28% 13% 5%	270
	11th to 20th	Day	16% 19% 28% 24% 12% 1	270
	21st to end	Day	25% 7% 33% 25% 9% 2	270
December	1st to 10th	Day	20% 19% 31% 21% 8% 2	270
	11th to 20th	Day	26% 14% 28% 24% 7% 1	270
	21st to end	Day	26% 22% 30% 16% 6%	297
January	1st to 10th	Night	21% 13% 34% 27% 4	950
	11th to 20th	Night	26% 14% 35% 21% 4	950
	21st to end	Night	21% 13% 37% 26% 2	1006
February	1st to 10th	Night	27% 11% 34% 25% 3	720
	11th to 20th	Night	22% 15% 37% 24% 2	703
	21st to end	Night	25% 13% 38% 21% 2	592
March	1st to 10th	Night	23% 11% 33% 28% 4	720
	11th to 20th	Night	22% 11% 32% 27% 8%	720
	21st to end	Night	24% 9% 27% 33% 6%	617
April	1st to 10th	Night	15% 9% 32% 35% 8% 1	540
	11th to 20th	Night	20% 7% 30% 28% 13% 1	533
	21st to end	Night	21% 6% 29% 33% 9% 2	540
May	1st to 10th	Night	17% 6% 30% 33% 13% 1	540
	11th to 20th	Night	15% 6% 21% 36% 20% 3	531
	21st to end	Night	7% 2 19% 36% 30% 5%	584
June	1st to 10th	Night	12% 3 15% 39% 28% 3	540
	11th to 20th	Night	7% 1 16% 43% 31% 2	540
	21st to end	Night	4 2 18% 40% 32% 4	540
July	1st to 10th	Night	9% 1 10% 32% 40% 8%	525
	11th to 20th	Night	4 3 9% 34% 43% 6%	522
	21st to end	Night	7% 2 10% 36% 37% 8%	591
August	1st to 10th	Night	8% 2 5% 37% 42% 6%	537
	11th to 20th	Night	7% 1 9% 38% 38% 7%	534
	21st to end	Night	7% 1 6% 37% 42% 7%	594
September	1st to 10th	Night	12% 1 9% 33% 39% 6%	540
	11th to 20th	Night	9% 2 13% 33% 38% 6%	678
	21st to end	Night	13% 3 19% 31% 29% 5%	720
October	1st to 10th	Night	11% 4 20% 34% 26% 5%	720
	11th to 20th	Night	17% 6% 28% 32% 15% 2	720
	21st to end	Night	21% 8% 31% 29% 10% 1	792
November	1st to 10th	Night	13% 6% 29% 35% 15% 1	740
	11th to 20th	Night	16% 10% 32% 30% 12%	760
	21st to end	Night	22% 9% 27% 31% 10% 1	760
December	1st to 10th	Night	24% 8% 31% 31% 6%	760
	11th to 20th	Night	23% 8% 32% 30% 6% 1	931
	21st to end	Night	27% 14% 32% 23% 4	1045

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