Bowman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bowman Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	17% 7% 2 6% 4 4 4 4 6% 7% 40%	126379
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 42%	122990
Transparency	Day	41% 6% 13% 22% 16% 3	129906
	Night	42% 5% 12% 22% 17% 3	127550
Seeing	Day	18% 12% 30% 25% 12% 3	16189
	Night	20% 12% 31% 27% 10% 1	25247

By Month

Percentage Cloud Cover NE (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	13% 6% 2 4 4 4 4 4 5% 6% 48%	3597
	11th to 20th	Day	13% 9% 3 6% 5% 4 4 3 5% 5% 42%	3570
	21st to end	Day	19% 7% 3 7% 4 4 4 3 4 4 40%	4218
February	1st to 10th	Day	17% 8% 3 6% 4 4 5% 4 4 7% 39%	1833
	11th to 20th	Day	15% 7% 2 7% 4 4 4 3 4 6% 44%	1948
	21st to end	Day	19% 9% 2 5% 3 4 3 4 5% 6% 39%	1650
March	1st to 10th	Day	22% 6% 2 5% 3 3 4 3 4 6% 42%	4476
	11th to 20th	Day	13% 7% 2 5% 4 4 4 4 6% 7% 42%	4698
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 6% 41%	5970
April	1st to 10th	Day	19% 8% 3 5% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	17% 7% 3 5% 4 4 4 3 6% 7% 40%	2374
	21st to end	Day	18% 6% 2 5% 4 4 4 3 6% 6% 42%	2455
May	1st to 10th	Day	29% 8% 2 5% 3 3 4 3 5% 8% 28%	5886
	11th to 20th	Day	21% 9% 3 6% 3 4 4 3 5% 6% 36%	6219
	21st to end	Day	21% 6% 2 5% 4 4 5% 4 6% 8% 36%	6949
June	1st to 10th	Day	17% 6% 1 6% 5% 4 5% 4 7% 9% 37%	2800
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 4 6% 9% 38%	2800
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 6% 9% 40%	2800
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 10% 36%	6360
	11th to 20th	Day	11% 7% 2 8% 5% 5% 5% 5% 8% 10% 34%	6360
	21st to end	Day	14% 6% 2 6% 4 5% 5% 4 6% 8% 39%	6926
August	1st to 10th	Day	19% 7% 2 6% 5% 4 5% 4 7% 8% 33%	4329
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 4 6% 9% 40%	3850
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 46%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 5% 7% 52%	2294
	21st to end	Day	18% 6% 2 7% 5% 5% 3 3 6% 6% 40%	2200
October	1st to 10th	Day	21% 7% 2 5% 3 3 3 3 5% 6% 43%	2047
	11th to 20th	Day	15% 5% 2 6% 3 4 3 3 5% 7% 48%	1900
	21st to end	Day	21% 5% 2 5% 3 4 4 4 5% 6% 42%	1911
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 4 4 6% 51%	1761
	11th to 20th	Day	18% 7% 2 4 2 3 4 3 4 7% 48%	1800
	21st to end	Day	16% 6% 2 5% 3 3 4 4 5% 6% 45%	1740
December	1st to 10th	Day	16% 6% 3 5% 4 4 4 3 6% 7% 43%	2600
	11th to 20th	Day	23% 6% 2 5% 3 3 2 2 4 5% 46%	2986
	21st to end	Day	18% 9% 3 6% 5% 4 4 3 4 6% 39%	3960
January	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 5% 7% 49%	6349
	11th to 20th	Night	14% 8% 3 6% 3 3 3 3 4 6% 47%	6315
	21st to end	Night	18% 9% 2 6% 4 4 4 3 5% 6% 39%	6722
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 4 5% 42%	2565
	11th to 20th	Night	18% 7% 3 6% 4 4 3 4 5% 6% 42%	2338
	21st to end	Night	19% 7% 2 6% 4 5% 3 3 5% 5% 41%	1980
March	1st to 10th	Night	20% 8% 2 5% 3 3 3 3 4 6% 43%	5364
	11th to 20th	Night	15% 9% 2 5% 4 4 4 3 4 6% 44%	5259
	21st to end	Night	19% 9% 2 6% 4 4 4 3 5% 5% 39%	4962
April	1st to 10th	Night	19% 8% 3 6% 4 4 5% 3 5% 6% 37%	2000
	11th to 20th	Night	20% 6% 2 7% 4 3 3 3 5% 7% 40%	1975
	21st to end	Night	19% 7% 3 6% 4 3 3 3 4 6% 42%	1944
May	1st to 10th	Night	28% 10% 3 7% 4 3 4 3 5% 7% 27%	4054
	11th to 20th	Night	22% 10% 3 6% 4 4 4 3 5% 6% 35%	3717
	21st to end	Night	23% 8% 2 5% 4 3 4 3 5% 7% 37%	3926
June	1st to 10th	Night	18% 6% 2 5% 5% 3 5% 4 5% 7% 41%	1600
	11th to 20th	Night	14% 8% 3 5% 4 4 4 4 6% 8% 40%	1600
	21st to end	Night	12% 5% 1 6% 4 3 5% 4 7% 9% 43%	1600
July	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	3600
	11th to 20th	Night	17% 8% 3 8% 5% 5% 4 4 7% 9% 33%	3600
	21st to end	Night	15% 6% 2 5% 3 4 4 3 6% 8% 44%	3926
August	1st to 10th	Night	23% 6% 2 6% 5% 4 4 4 6% 7% 32%	2769
	11th to 20th	Night	15% 6% 2 7% 5% 5% 4 4 6% 7% 40%	3197
	21st to end	Night	10% 4 2 4 4 4 4 4 5% 8% 52%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 5% 6% 45%	2000
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 6% 52%	2105
	21st to end	Night	18% 7% 3 6% 3 4 3 2 4 6% 44%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 4 5% 42%	2196
	11th to 20th	Night	15% 5% 2 5% 4 4 2 3 5% 6% 50%	2280
	21st to end	Night	21% 7% 2 6% 3 3 3 3 4 6% 42%	2685
November	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 51%	2537
	11th to 20th	Night	20% 6% 2 4 3 3 3 3 4 5% 48%	2600
	21st to end	Night	16% 5% 2 4 4 3 5% 3 6% 7% 46%	2655
December	1st to 10th	Night	16% 6% 2 5% 4 3 3 2 4 5% 49%	4580
	11th to 20th	Night	26% 7% 2 4 3 3 3 3 4 4 43%	5208
	21st to end	Night	19% 8% 3 6% 4 4 3 3 5% 5% 41%	6996

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	34% 4 9% 31% 20% 3	3598
	11th to 20th	Day	42% 4 10% 23% 15% 6%	3600
	21st to end	Day	48% 3 10% 24% 13% 1	4218
February	1st to 10th	Day	44% 6% 13% 22% 14% 2	1833
	11th to 20th	Day	41% 4 9% 25% 17% 2	1957
	21st to end	Day	45% 4 11% 23% 15% 3	1650
March	1st to 10th	Day	43% 3 12% 20% 13% 9%	4479
	11th to 20th	Day	37% 7% 14% 28% 11% 3	4698
	21st to end	Day	43% 5% 10% 13% 23% 6%	5970
April	1st to 10th	Day	45% 5% 10% 20% 16% 4	2400
	11th to 20th	Day	41% 5% 12% 21% 18% 3	2374
	21st to end	Day	41% 5% 11% 22% 17% 4	2455
May	1st to 10th	Day	53% 6% 16% 14% 10% 1	5897
	11th to 20th	Day	48% 5% 13% 19% 14% 2	6223
	21st to end	Day	44% 6% 12% 23% 14% 1	6949
June	1st to 10th	Day	40% 8% 13% 22% 15% 1	2800
	11th to 20th	Day	41% 8% 14% 24% 13% 2	2800
	21st to end	Day	38% 8% 15% 23% 15% 1	2800
July	1st to 10th	Day	41% 10% 12% 21% 15% 1	6290
	11th to 20th	Day	39% 8% 19% 23% 10% 1	6285
	21st to end	Day	40% 7% 12% 20% 20% 1	6728
August	1st to 10th	Day	46% 7% 16% 23% 9% 1	4315
	11th to 20th	Day	37% 6% 14% 31% 11% 1	3886
	21st to end	Day	31% 6% 13% 28% 20% 1	4312
September	1st to 10th	Day	37% 6% 14% 23% 18% 2	2400
	11th to 20th	Day	32% 5% 12% 25% 24% 2	2294
	21st to end	Day	44% 5% 11% 19% 18% 3	2200
October	1st to 10th	Day	35% 5% 11% 20% 24% 6%	3385
	11th to 20th	Day	27% 4 11% 23% 31% 5%	3140
	21st to end	Day	29% 4 14% 25% 23% 3	3123
November	1st to 10th	Day	34% 5% 13% 23% 23% 2	1761
	11th to 20th	Day	38% 4 10% 23% 22% 4	1800
	21st to end	Day	39% 4 15% 28% 12% 2	1740
December	1st to 10th	Day	40% 4 16% 28% 12% 1	2600
	11th to 20th	Day	42% 3 14% 22% 17% 3	2986
	21st to end	Day	46% 6% 8% 22% 15% 3	3960
January	1st to 10th	Night	36% 2 10% 25% 24% 4	6349
	11th to 20th	Night	40% 3 14% 21% 16% 6%	6360
	21st to end	Night	46% 3 10% 20% 16% 5%	6722
February	1st to 10th	Night	45% 4 12% 24% 13% 2	2565
	11th to 20th	Night	44% 5% 9% 20% 17% 5%	2340
	21st to end	Night	45% 4 7% 23% 17% 4	1980
March	1st to 10th	Night	44% 4 11% 20% 13% 9%	5374
	11th to 20th	Night	41% 5% 14% 24% 12% 4	5260
	21st to end	Night	47% 4 7% 14% 24% 5%	4962
April	1st to 10th	Night	47% 4 10% 19% 16% 4	2000
	11th to 20th	Night	44% 5% 10% 19% 21% 2	1975
	21st to end	Night	42% 6% 9% 21% 18% 4	1944
May	1st to 10th	Night	57% 5% 13% 13% 10% 1	4060
	11th to 20th	Night	51% 4 15% 14% 14% 2	3718
	21st to end	Night	48% 5% 13% 21% 10% 2	3927
June	1st to 10th	Night	42% 7% 10% 20% 20% 1	1600
	11th to 20th	Night	40% 6% 15% 26% 12% 1	1600
	21st to end	Night	37% 8% 15% 24% 14% 2	1600
July	1st to 10th	Night	45% 8% 13% 21% 12% 2	3560
	11th to 20th	Night	46% 7% 16% 20% 10% 1	3552
	21st to end	Night	38% 5% 15% 23% 18% 1	3809
August	1st to 10th	Night	51% 7% 15% 17% 10% 1	2759
	11th to 20th	Night	42% 7% 13% 29% 9% 1	3227
	21st to end	Night	30% 6% 12% 32% 18% 1	3586
September	1st to 10th	Night	39% 6% 10% 25% 18% 1	2000
	11th to 20th	Night	34% 4 11% 25% 23% 3	2105
	21st to end	Night	42% 4 10% 21% 20% 2	2200
October	1st to 10th	Night	40% 5% 11% 23% 19% 3	3638
	11th to 20th	Night	24% 4 10% 28% 30% 4	3779
	21st to end	Night	31% 4 13% 29% 21% 2	4423
November	1st to 10th	Night	35% 5% 11% 24% 21% 4	2537
	11th to 20th	Night	40% 3 9% 24% 20% 5%	2600
	21st to end	Night	37% 4 13% 30% 14% 2	2655
December	1st to 10th	Night	39% 4 12% 30% 13% 2	4580
	11th to 20th	Night	46% 3 10% 21% 16% 3	5208
	21st to end	Night	45% 5% 10% 19% 17% 4	6996

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	21% 30% 29% 17% 4 1	200
	11th to 20th	Day	16% 30% 37% 15% 3	200
	21st to end	Day	18% 17% 43% 16% 5% 1	277
February	1st to 10th	Day	24% 17% 40% 17% 2	333
	11th to 20th	Day	22% 20% 41% 16% 1	496
	21st to end	Day	22% 22% 33% 19% 3	414
March	1st to 10th	Day	17% 24% 36% 20% 4	500
	11th to 20th	Day	18% 21% 36% 20% 4 1	500
	21st to end	Day	23% 18% 36% 17% 4 1	550
April	1st to 10th	Day	23% 21% 34% 15% 6% 1	500
	11th to 20th	Day	21% 17% 34% 18% 10% 1	492
	21st to end	Day	23% 15% 34% 20% 6% 1	500
May	1st to 10th	Day	30% 12% 30% 20% 7% 2	503
	11th to 20th	Day	19% 12% 34% 26% 8% 2	527
	21st to end	Day	19% 6% 26% 28% 16% 4	573
June	1st to 10th	Day	16% 8% 27% 31% 16% 2	530
	11th to 20th	Day	16% 8% 28% 32% 13% 2	530
	21st to end	Day	13% 7% 25% 33% 18% 4	530
July	1st to 10th	Day	12% 5% 21% 37% 19% 6%	525
	11th to 20th	Day	10% 3 22% 36% 25% 5%	524
	21st to end	Day	13% 6% 21% 38% 20% 2	576
August	1st to 10th	Day	11% 5% 23% 34% 20% 6%	526
	11th to 20th	Day	15% 4 15% 38% 23% 5%	519
	21st to end	Day	10% 4 28% 33% 17% 8%	583
September	1st to 10th	Day	18% 5% 26% 26% 19% 6%	530
	11th to 20th	Day	13% 4 26% 28% 18% 10%	530
	21st to end	Day	22% 7% 27% 27% 14% 4	530
October	1st to 10th	Day	25% 7% 24% 24% 15% 6%	502
	11th to 20th	Day	15% 13% 30% 26% 13% 2	490
	21st to end	Day	23% 12% 30% 23% 10% 2	444
November	1st to 10th	Day	15% 13% 32% 27% 10% 3	395
	11th to 20th	Day	20% 19% 35% 20% 6% 1	400
	21st to end	Day	19% 17% 35% 21% 6% 1	340
December	1st to 10th	Day	19% 22% 35% 18% 7% 1	200
	11th to 20th	Day	26% 26% 32% 14% 3	200
	21st to end	Day	25% 17% 35% 14% 9%	220
January	1st to 10th	Night	20% 19% 39% 19% 3	800
	11th to 20th	Night	23% 18% 34% 22% 3	800
	21st to end	Night	18% 19% 41% 20% 2	880
February	1st to 10th	Night	22% 15% 41% 21% 2	800
	11th to 20th	Night	23% 21% 37% 17% 1	783
	21st to end	Night	23% 17% 36% 20% 3	660
March	1st to 10th	Night	22% 15% 38% 21% 4	800
	11th to 20th	Night	22% 15% 37% 22% 4 1	800
	21st to end	Night	21% 16% 37% 21% 4	880
April	1st to 10th	Night	23% 15% 36% 21% 5%	800
	11th to 20th	Night	20% 14% 34% 26% 6%	791
	21st to end	Night	22% 11% 35% 26% 6% 1	744
May	1st to 10th	Night	25% 10% 31% 27% 7% 1	600
	11th to 20th	Night	22% 11% 34% 26% 7%	458
	21st to end	Night	22% 5% 19% 40% 13% 1	433
June	1st to 10th	Night	18% 5% 28% 36% 13% 1	400
	11th to 20th	Night	17% 3 24% 42% 15%	400
	21st to end	Night	10% 3 23% 51% 14% 1	400
July	1st to 10th	Night	12% 3 19% 44% 21% 1	390
	11th to 20th	Night	13% 4 18% 42% 21% 2	388
	21st to end	Night	13% 5% 16% 46% 18% 1	438
August	1st to 10th	Night	12% 3 20% 44% 20% 2	503
	11th to 20th	Night	14% 4 17% 37% 26% 2	790
	21st to end	Night	14% 5% 19% 35% 25% 3	880
September	1st to 10th	Night	16% 4 26% 33% 19% 3	800
	11th to 20th	Night	15% 4 22% 34% 21% 4	800
	21st to end	Night	21% 6% 22% 33% 16% 2	800
October	1st to 10th	Night	23% 8% 27% 26% 14% 2	773
	11th to 20th	Night	17% 10% 32% 29% 12% 1	760
	21st to end	Night	25% 11% 33% 23% 7%	836
November	1st to 10th	Night	14% 13% 35% 27% 10% 1	780
	11th to 20th	Night	22% 14% 33% 23% 7%	800
	21st to end	Night	18% 18% 33% 25% 6% 1	800
December	1st to 10th	Night	24% 17% 35% 19% 5% 1	800
	11th to 20th	Night	24% 17% 34% 21% 4	800
	21st to end	Night	24% 15% 37% 21% 4	880

Page generated at 2025-10-01 01:39:07. Version ac07

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