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 2024-09-14.

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 7% 4 4 4 4 6% 8% 41%	100491
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 42%	97440
Transparency	Day	42% 5% 11% 25% 15% 2	100282
	Night	42% 5% 11% 24% 16% 3	97320
Seeing	Day	16% 8% 23% 25% 19% 8%	15309
	Night	17% 7% 26% 31% 17% 2	25069

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% 13% 7% 9% 5% 4 4 3 5% 6% 32%	2527
	11th to 20th	Day	18% 9% 3 9% 8% 7% 5% 3 4 7% 27%	2530
	21st to end	Day	26% 15% 6% 8% 4 3 3 2 4 5% 23%	3102
February	1st to 10th	Day	18% 11% 4 7% 4 4 3 3 4 7% 35%	1900
	11th to 20th	Day	17% 9% 3 8% 4 5% 4 3 5% 6% 36%	1848
	21st to end	Day	16% 9% 4 10% 4 5% 4 4 5% 6% 33%	1570
March	1st to 10th	Day	22% 13% 3 6% 4 4 3 2 3 4 35%	3120
	11th to 20th	Day	13% 8% 2 6% 3 4 3 3 5% 7% 46%	3251
	21st to end	Day	13% 9% 3 7% 5% 5% 5% 4 6% 7% 39%	4155
April	1st to 10th	Day	13% 8% 2 6% 3 4 5% 4 5% 8% 42%	2280
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 5% 7% 41%	2254
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 8% 40%	2318
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 8% 9% 30%	4082
	11th to 20th	Day	18% 7% 3 7% 4 4 4 3 6% 7% 37%	4081
	21st to end	Day	10% 7% 3 7% 4 5% 5% 4 6% 8% 41%	4603
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	2660
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 6% 8% 48%	2660
	21st to end	Day	7% 5% 2 6% 5% 3 5% 5% 8% 10% 45%	2660
July	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 3 7% 10% 41%	4440
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 7% 11% 48%	4440
	21st to end	Day	9% 7% 3 10% 7% 7% 6% 5% 8% 9% 29%	4725
August	1st to 10th	Day	5% 4 2 5% 3 4 4 4 7% 9% 54%	3927
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 6% 10% 43%	3729
	21st to end	Day	5% 4 2 5% 4 4 4 3 6% 9% 55%	4180
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 4 4 6% 7% 48%	2280
	11th to 20th	Day	9% 5% 2 6% 5% 4 4 4 5% 7% 48%	2038
	21st to end	Day	11% 4 2 6% 4 4 3 3 4 6% 54%	1980
October	1st to 10th	Day	11% 4 2 4 3 3 4 4 5% 6% 56%	1980
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 5% 7% 48%	1846
	21st to end	Day	18% 8% 2 6% 4 3 4 3 4 5% 44%	1807
November	1st to 10th	Day	9% 5% 2 5% 3 4 4 3 5% 7% 54%	1672
	11th to 20th	Day	17% 7% 3 5% 4 4 3 3 4 6% 45%	1710
	21st to end	Day	19% 8% 3 6% 4 3 3 3 5% 6% 41%	1710
December	1st to 10th	Day	19% 11% 3 7% 4 4 3 2 3 5% 38%	1710
	11th to 20th	Day	16% 11% 5% 9% 5% 4 4 3 5% 5% 32%	1933
	21st to end	Day	22% 16% 5% 9% 4 4 4 2 4 5% 24%	2783
January	1st to 10th	Night	18% 11% 4 7% 4 4 4 4 5% 6% 33%	4428
	11th to 20th	Night	16% 12% 4 9% 5% 6% 6% 3 3 5% 30%	4440
	21st to end	Night	25% 14% 5% 6% 4 4 3 2 4 5% 27%	4559
February	1st to 10th	Night	21% 11% 4 7% 4 3 4 3 6% 5% 32%	2280
	11th to 20th	Night	16% 8% 3 8% 4 5% 4 3 4 5% 39%	2218
	21st to end	Night	20% 9% 3 7% 4 4 3 4 6% 5% 36%	1884
March	1st to 10th	Night	25% 10% 3 7% 4 3 3 3 4 5% 34%	3733
	11th to 20th	Night	12% 6% 3 6% 3 3 3 3 5% 6% 49%	3709
	21st to end	Night	16% 8% 3 8% 5% 3 4 4 4 6% 39%	3503
April	1st to 10th	Night	13% 6% 2 6% 5% 4 4 3 4 7% 46%	1900
	11th to 20th	Night	16% 7% 2 6% 4 3 3 4 4 7% 44%	1875
	21st to end	Night	17% 7% 3 6% 5% 3 5% 4 5% 7% 38%	1860
May	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 6% 8% 35%	2813
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 5% 6% 43%	2812
	21st to end	Night	8% 5% 3 6% 4 5% 5% 4 7% 10% 42%	2940
June	1st to 10th	Night	10% 5% 2 5% 3 4 5% 3 7% 8% 47%	1520
	11th to 20th	Night	5% 4 1 5% 4 4 4 4 6% 9% 53%	1520
	21st to end	Night	4 3 1 4 3 4 4 4 7% 12% 53%	1520
July	1st to 10th	Night	9% 5% 2 5% 4 4 3 4 5% 9% 49%	2530
	11th to 20th	Night	5% 3 2 5% 4 4 4 3 6% 8% 57%	2530
	21st to end	Night	8% 7% 3 8% 6% 5% 7% 6% 8% 9% 33%	2833
August	1st to 10th	Night	5% 3 2 5% 3 4 4 3 5% 8% 60%	2999
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 8% 52%	3100
	21st to end	Night	4 3 1 4 3 3 3 4 6% 10% 60%	3476
September	1st to 10th	Night	10% 4 2 5% 4 3 3 4 4 6% 54%	1900
	11th to 20th	Night	8% 5% 2 5% 4 3 4 4 6% 7% 52%	1980
	21st to end	Night	11% 5% 2 4 4 3 4 3 5% 8% 52%	1980
October	1st to 10th	Night	9% 5% 2 6% 3 3 3 3 4 6% 58%	1980
	11th to 20th	Night	14% 5% 2 6% 4 4 4 2 4 5% 51%	2111
	21st to end	Night	18% 8% 2 5% 3 3 4 2 4 5% 45%	2547
November	1st to 10th	Night	11% 6% 3 5% 3 3 3 3 4 6% 54%	2406
	11th to 20th	Night	14% 5% 2 6% 4 3 3 3 4 6% 49%	2470
	21st to end	Night	19% 7% 2 5% 4 3 3 3 5% 5% 44%	2470
December	1st to 10th	Night	20% 9% 3 7% 4 3 4 3 5% 6% 37%	2470
	11th to 20th	Night	17% 10% 3 7% 4 4 5% 4 5% 5% 37%	3260
	21st to end	Night	21% 15% 5% 8% 5% 5% 5% 3 4 5% 24%	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	53% 4 12% 22% 8% 1	2528
	11th to 20th	Day	59% 3 10% 22% 5%	2530
	21st to end	Day	65% 3 8% 18% 5% 1	3102
February	1st to 10th	Day	51% 4 10% 22% 11% 2	1900
	11th to 20th	Day	49% 5% 10% 22% 13% 2	1858
	21st to end	Day	51% 5% 9% 19% 13% 3	1570
March	1st to 10th	Day	55% 3 6% 19% 14% 4	3120
	11th to 20th	Day	38% 4 10% 28% 16% 3	3251
	21st to end	Day	43% 5% 12% 24% 14% 1	4155
April	1st to 10th	Day	39% 6% 14% 25% 16% 1	2280
	11th to 20th	Day	43% 5% 10% 23% 16% 3	2254
	21st to end	Day	43% 5% 12% 26% 12% 2	2318
May	1st to 10th	Day	46% 8% 15% 22% 9% 1	4120
	11th to 20th	Day	45% 6% 10% 25% 12% 1	4085
	21st to end	Day	39% 7% 12% 20% 21% 2	4616
June	1st to 10th	Day	35% 6% 12% 25% 19% 3	2660
	11th to 20th	Day	30% 8% 12% 26% 21% 3	2660
	21st to end	Day	31% 8% 15% 24% 17% 6%	2660
July	1st to 10th	Day	38% 5% 12% 27% 15% 3	4371
	11th to 20th	Day	30% 5% 12% 35% 16% 2	4365
	21st to end	Day	49% 5% 16% 20% 9% 1	4575
August	1st to 10th	Day	26% 4 15% 37% 17% 1	3914
	11th to 20th	Day	36% 4 13% 27% 18% 1	3765
	21st to end	Day	27% 3 13% 37% 17% 3	4179
September	1st to 10th	Day	34% 7% 9% 25% 20% 5%	2280
	11th to 20th	Day	34% 6% 9% 26% 23% 3	2038
	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	50% 5% 11% 23% 10% 1	4428
	11th to 20th	Night	57% 4 9% 20% 9% 1	4440
	21st to end	Night	60% 3 8% 18% 10% 1	4559
February	1st to 10th	Night	53% 4 10% 22% 9% 1	2280
	11th to 20th	Night	47% 5% 8% 23% 15% 2	2219
	21st to end	Night	49% 4 11% 20% 13% 4	1884
March	1st to 10th	Night	53% 5% 7% 18% 15% 3	3733
	11th to 20th	Night	37% 5% 12% 24% 17% 6%	3709
	21st to end	Night	46% 5% 11% 25% 12% 1	3503
April	1st to 10th	Night	38% 5% 11% 26% 17% 3	1900
	11th to 20th	Night	40% 4 9% 23% 19% 5%	1875
	21st to end	Night	43% 6% 10% 23% 15% 2	1860
May	1st to 10th	Night	45% 7% 16% 23% 8% 1	2840
	11th to 20th	Night	41% 7% 12% 25% 14% 1	2813
	21st to end	Night	34% 8% 15% 24% 17% 3	2948
June	1st to 10th	Night	33% 6% 12% 25% 21% 4	1520
	11th to 20th	Night	26% 6% 14% 22% 26% 5%	1520
	21st to end	Night	23% 7% 17% 26% 19% 8%	1520
July	1st to 10th	Night	34% 5% 11% 27% 19% 4	2490
	11th to 20th	Night	27% 4 12% 36% 19% 3	2482
	21st to end	Night	45% 5% 15% 22% 11% 1	2743
August	1st to 10th	Night	24% 3 14% 43% 15% 1	2988
	11th to 20th	Night	30% 4 13% 29% 22% 1	3130
	21st to end	Night	22% 4 12% 35% 24% 3	3476
September	1st to 10th	Night	30% 6% 8% 27% 23% 6%	1900
	11th to 20th	Night	31% 5% 9% 26% 24% 5%	1980
	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% 14% 4	297
February	1st to 10th	Day	26% 14% 37% 20% 2 1	270
	11th to 20th	Day	26% 17% 40% 13% 3 1	270
	21st to end	Day	29% 18% 31% 19% 4	225
March	1st to 10th	Day	23% 16% 40% 17% 4	270
	11th to 20th	Day	23% 18% 37% 16% 6% 1	322
	21st to end	Day	26% 15% 32% 20% 7% 1	506
April	1st to 10th	Day	18% 15% 33% 25% 8% 1	460
	11th to 20th	Day	22% 14% 25% 23% 15% 1	452
	21st to end	Day	19% 12% 34% 23% 9% 3	460
May	1st to 10th	Day	17% 14% 27% 29% 10% 4	463
	11th to 20th	Day	22% 5% 23% 28% 16% 8%	487
	21st to end	Day	9% 5% 26% 27% 21% 12%	604
June	1st to 10th	Day	13% 5% 18% 26% 29% 10%	680
	11th to 20th	Day	6% 3 20% 33% 27% 12%	680
	21st to end	Day	9% 4 21% 33% 22% 11%	680
July	1st to 10th	Day	11% 3 13% 24% 27% 23%	671
	11th to 20th	Day	7% 4 12% 31% 31% 15%	668
	21st to end	Day	9% 2 12% 31% 32% 14%	687
August	1st to 10th	Day	9% 3 11% 28% 31% 17%	486
	11th to 20th	Day	14% 2 13% 28% 32% 12%	479
	21st to end	Day	8% 1 11% 23% 39% 18%	539
September	1st to 10th	Day	9% 3 15% 25% 33% 15%	490
	11th to 20th	Day	10% 2 10% 28% 33% 17%	463
	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	1007
February	1st to 10th	Night	26% 11% 35% 25% 3	760
	11th to 20th	Night	21% 15% 37% 25% 2	743
	21st to end	Night	25% 14% 39% 21% 2	628
March	1st to 10th	Night	23% 11% 33% 28% 5%	760
	11th to 20th	Night	20% 11% 33% 28% 7%	760
	21st to end	Night	23% 10% 28% 32% 6%	651
April	1st to 10th	Night	15% 10% 32% 34% 8% 1	570
	11th to 20th	Night	21% 7% 31% 28% 12% 1	563
	21st to end	Night	21% 7% 30% 32% 9% 2	570
May	1st to 10th	Night	17% 7% 29% 33% 14% 1	570
	11th to 20th	Night	14% 6% 21% 36% 20% 3	561
	21st to end	Night	7% 2 20% 37% 29% 5%	617
June	1st to 10th	Night	12% 3 15% 40% 27% 3	570
	11th to 20th	Night	7% 1 16% 44% 30% 2	570
	21st to end	Night	4 2 19% 40% 32% 4	570
July	1st to 10th	Night	9% 1 11% 32% 38% 8%	555
	11th to 20th	Night	4 3 10% 36% 42% 5%	552
	21st to end	Night	8% 2 10% 36% 37% 7%	624
August	1st to 10th	Night	8% 2 6% 37% 41% 6%	567
	11th to 20th	Night	7% 2 10% 39% 37% 6%	564
	21st to end	Night	6% 1 6% 37% 42% 7%	627
September	1st to 10th	Night	12% 2 10% 32% 38% 6%	570
	11th to 20th	Night	10% 2 12% 33% 37% 6%	692
	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

Page generated at 2024-09-11 09:00:05. Version ac07

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