Bowen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bowen Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	5% 4 1 3 2 2 3 2 4 7% 65%	140004
	Night	7% 5% 2 5% 3 3 3 3 4 9% 56%	139998
Transparency	Day	19% 4 7% 20% 27% 23%	135028
	Night	24% 4 8% 19% 23% 22%	135955
Seeing	Day	7% 8% 23% 35% 21% 5%	17522
	Night	8% 3 14% 33% 33% 9%	27191

By Month

Percentage Cloud Cover SW (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	11% 4 2 4 3 3 3 3 4 7% 56%	5097
	11th to 20th	Day	8% 3 1 3 2 3 2 3 4 6% 65%	5340
	21st to end	Day	9% 5% 2 5% 3 3 4 4 6% 8% 50%	5843
February	1st to 10th	Day	14% 5% 2 5% 3 3 2 3 4 5% 55%	1957
	11th to 20th	Day	12% 4 2 5% 3 3 4 2 5% 7% 51%	2050
	21st to end	Day	12% 4 2 4 4 3 4 3 5% 6% 53%	1730
March	1st to 10th	Day	12% 7% 2 5% 3 4 4 4 6% 8% 44%	4787
	11th to 20th	Day	10% 5% 1 4 3 3 3 2 4 6% 58%	5110
	21st to end	Day	11% 7% 2 5% 4 4 5% 4 7% 9% 43%	5831
April	1st to 10th	Day	9% 5% 1 4 3 3 3 3 5% 7% 57%	2520
	11th to 20th	Day	6% 7% 2 4 3 3 3 2 6% 7% 58%	2489
	21st to end	Day	8% 4 1 4 3 2 3 3 5% 6% 61%	2520
May	1st to 10th	Day	4 3 1 2 1 2 1 1 3 5% 77%	5453
	11th to 20th	Day	1 6% 2 5% 3 2 3 3 5% 8% 61%	5594
	21st to end	Day	2 3 1 3 2 2 2 2 4 8% 72%	6902
June	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 9% 63%	2800
	11th to 20th	Day	1 3 1 3 2 1 2 2 3 7% 75%	2800
	21st to end	Day	1 2 1 2 2 2 2 2 4 7% 75%	2800
July	1st to 10th	Day	1 2 1 1 1 1 3 6% 84%	6304
	11th to 20th	Day	1 2 1 3 2 2 2 2 4 8% 74%	5880
	21st to end	Day	1 1 3 2 2 2 3 5% 10% 71%	6402
August	1st to 10th	Day	1 1 1 1 1 1 2 2 4 7% 78%	5546
	11th to 20th	Day	1 1 1 1 1 1 1 2 5% 86%	5364
	21st to end	Day	1 1 1 2 1 1 2 2 3 7% 79%	5984
September	1st to 10th	Day	1 2 1 2 1 1 2 2 4 7% 76%	2400
	11th to 20th	Day	3 3 1 3 2 1 2 3 3 8% 72%	2400
	21st to end	Day	3 3 1 2 1 2 2 2 5% 7% 72%	2192
October	1st to 10th	Day	2 1 1 3 1 2 2 2 4 5% 77%	3280
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 76%	3261
	21st to end	Day	3 3 1 4 2 2 3 3 4 6% 67%	3558
November	1st to 10th	Day	6% 4 1 3 3 2 3 2 4 5% 67%	1990
	11th to 20th	Day	8% 3 1 3 2 2 3 3 5% 5% 64%	1890
	21st to end	Day	8% 4 2 4 3 3 3 3 4 5% 60%	1890
December	1st to 10th	Day	6% 5% 2 4 4 3 4 3 5% 7% 58%	2840
	11th to 20th	Day	18% 5% 2 4 2 2 4 3 5% 7% 48%	3108
	21st to end	Day	16% 8% 2 5% 3 4 4 4 6% 9% 40%	4092
January	1st to 10th	Night	13% 7% 2 4 3 3 2 2 3 5% 56%	7814
	11th to 20th	Night	7% 4 1 4 2 2 3 2 5% 6% 62%	7650
	21st to end	Night	12% 7% 2 6% 4 4 4 3 5% 7% 46%	8371
February	1st to 10th	Night	13% 5% 2 4 3 3 2 3 3 5% 56%	2662
	11th to 20th	Night	12% 5% 2 4 2 3 3 3 5% 7% 54%	2458
	21st to end	Night	11% 6% 2 4 4 4 3 2 4 6% 55%	2076
March	1st to 10th	Night	13% 10% 3 6% 3 3 4 3 5% 7% 42%	5321
	11th to 20th	Night	9% 6% 1 4 3 2 3 3 4 6% 57%	5110
	21st to end	Night	15% 9% 2 5% 3 3 2 2 4 6% 49%	5373
April	1st to 10th	Night	8% 5% 2 5% 3 2 4 3 4 7% 57%	2100
	11th to 20th	Night	6% 6% 2 4 3 2 2 3 3 7% 61%	2079
	21st to end	Night	6% 8% 1 3 3 2 3 3 4 8% 59%	2100
May	1st to 10th	Night	2 4 2 6% 3 2 2 2 3 8% 67%	4570
	11th to 20th	Night	2 11% 5% 8% 4 2 2 2 4 10% 51%	4297
	21st to end	Night	2 7% 3 10% 5% 4 3 2 4 10% 51%	3972
June	1st to 10th	Night	3 6% 3 6% 4 3 4 3 5% 9% 55%	1600
	11th to 20th	Night	2 5% 1 4 3 3 2 3 4 7% 66%	1600
	21st to end	Night	1 3 3 3 1 2 2 2 4 11% 66%	1600
July	1st to 10th	Night	1 1 4 2 3 2 2 4 14% 66%	3640
	11th to 20th	Night	4 2 8% 6% 5% 4 3 6% 15% 48%	4096
	21st to end	Night	8% 7% 12% 5% 3 3 3 5% 20% 34%	4472
August	1st to 10th	Night	2 1 3 3 3 3 3 6% 14% 63%	4275
	11th to 20th	Night	1 3 2 3 2 3 4 17% 65%	4506
	21st to end	Night	1 2 4 3 3 3 3 5% 18% 59%	5015
September	1st to 10th	Night	2 3 1 4 2 2 2 3 4 8% 70%	2000
	11th to 20th	Night	3 4 1 4 3 3 2 2 3 9% 66%	2000
	21st to end	Night	2 4 1 2 2 2 2 2 3 6% 73%	2202
October	1st to 10th	Night	2 3 1 2 2 2 2 2 3 6% 75%	3920
	11th to 20th	Night	3 2 2 4 4 3 3 3 4 10% 63%	3897
	21st to end	Night	4 4 2 4 3 2 2 2 4 6% 68%	4253
November	1st to 10th	Night	5% 4 2 3 3 2 2 2 4 7% 65%	2527
	11th to 20th	Night	5% 4 2 4 2 3 3 3 5% 6% 63%	2730
	21st to end	Night	6% 6% 1 4 2 3 5% 3 5% 6% 59%	2730
December	1st to 10th	Night	7% 3 1 4 3 3 4 3 5% 6% 61%	4560
	11th to 20th	Night	15% 5% 1 4 4 4 4 4 6% 7% 45%	5316
	21st to end	Night	13% 9% 3 8% 5% 5% 5% 4 6% 8% 35%	7106

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	28% 5% 9% 29% 18% 10%	5098
	11th to 20th	Day	22% 3 6% 12% 20% 37%	5340
	21st to end	Day	31% 5% 7% 16% 28% 13%	5843
February	1st to 10th	Day	32% 4 6% 19% 21% 18%	1957
	11th to 20th	Day	32% 5% 9% 22% 22% 10%	2060
	21st to end	Day	33% 5% 10% 21% 23% 8%	1730
March	1st to 10th	Day	34% 7% 11% 18% 25% 5%	4787
	11th to 20th	Day	29% 4 9% 32% 16% 9%	5110
	21st to end	Day	33% 10% 13% 25% 16% 3	5831
April	1st to 10th	Day	28% 5% 11% 26% 22% 8%	2520
	11th to 20th	Day	25% 4 11% 28% 24% 7%	2492
	21st to end	Day	22% 6% 12% 26% 26% 8%	2520
May	1st to 10th	Day	12% 3 7% 30% 37% 12%	5453
	11th to 20th	Day	15% 4 8% 32% 33% 9%	5597
	21st to end	Day	11% 3 10% 27% 41% 8%	6904
June	1st to 10th	Day	16% 6% 8% 28% 29% 12%	2800
	11th to 20th	Day	10% 2 8% 18% 36% 26%	2800
	21st to end	Day	9% 2 5% 19% 29% 36%	2800
July	1st to 10th	Day	4 2 9% 22% 62%	6236
	11th to 20th	Day	9% 2 5% 8% 23% 55%	5811
	21st to end	Day	7% 3 3 20% 31% 36%	6217
August	1st to 10th	Day	10% 1 4 16% 32% 37%	4030
	11th to 20th	Day	5% 1 2 11% 30% 50%	3880
	21st to end	Day	7% 1 3 16% 28% 46%	4311
September	1st to 10th	Day	8% 2 4 16% 29% 40%	2400
	11th to 20th	Day	12% 2 5% 23% 37% 22%	2400
	21st to end	Day	12% 2 7% 18% 33% 27%	2192
October	1st to 10th	Day	12% 2 5% 14% 24% 43%	3280
	11th to 20th	Day	14% 2 5% 21% 37% 21%	3261
	21st to end	Day	19% 3 4 14% 33% 28%	3558
November	1st to 10th	Day	20% 4 7% 20% 29% 19%	1990
	11th to 20th	Day	24% 3 9% 20% 26% 19%	1890
	21st to end	Day	27% 4 6% 18% 24% 21%	1890
December	1st to 10th	Day	28% 3 5% 15% 29% 20%	2840
	11th to 20th	Day	36% 5% 9% 18% 21% 11%	3108
	21st to end	Day	38% 8% 11% 25% 13% 5%	4092
January	1st to 10th	Night	32% 4 10% 23% 21% 10%	7814
	11th to 20th	Night	23% 4 5% 13% 19% 35%	7650
	21st to end	Night	37% 4 7% 15% 22% 15%	8371
February	1st to 10th	Night	32% 3 7% 18% 22% 17%	2662
	11th to 20th	Night	30% 4 8% 23% 22% 13%	2459
	21st to end	Night	31% 5% 7% 23% 25% 9%	2076
March	1st to 10th	Night	38% 7% 9% 17% 22% 6%	5321
	11th to 20th	Night	28% 5% 12% 26% 19% 9%	5110
	21st to end	Night	36% 6% 11% 26% 18% 4	5373
April	1st to 10th	Night	27% 5% 13% 25% 20% 10%	2100
	11th to 20th	Night	25% 3 11% 29% 24% 9%	2079
	21st to end	Night	24% 4 10% 27% 27% 8%	2100
May	1st to 10th	Night	14% 7% 9% 25% 30% 15%	4567
	11th to 20th	Night	25% 5% 9% 25% 23% 12%	4299
	21st to end	Night	17% 8% 8% 23% 33% 10%	3972
June	1st to 10th	Night	21% 5% 11% 23% 27% 12%	1600
	11th to 20th	Night	15% 3 5% 21% 30% 26%	1600
	21st to end	Night	12% 1 6% 20% 26% 35%	1600
July	1st to 10th	Night	6% 1 6% 10% 17% 61%	3600
	11th to 20th	Night	12% 2 15% 11% 14% 46%	4043
	21st to end	Night	26% 5% 9% 13% 24% 23%	4345
August	1st to 10th	Night	11% 1 4 19% 28% 37%	3109
	11th to 20th	Night	6% 1 4 14% 21% 54%	3256
	21st to end	Night	9% 2 5% 16% 25% 43%	3608
September	1st to 10th	Night	12% 2 4 16% 23% 43%	2000
	11th to 20th	Night	16% 2 5% 21% 28% 28%	2000
	21st to end	Night	12% 2 6% 18% 33% 29%	2202
October	1st to 10th	Night	11% 2 4 15% 22% 47%	3920
	11th to 20th	Night	15% 4 6% 17% 33% 23%	3897
	21st to end	Night	17% 2 4 14% 33% 28%	4253
November	1st to 10th	Night	19% 3 7% 21% 26% 23%	2527
	11th to 20th	Night	22% 3 10% 19% 26% 20%	2730
	21st to end	Night	26% 5% 9% 17% 22% 22%	2730
December	1st to 10th	Night	25% 3 4 16% 31% 21%	4560
	11th to 20th	Night	38% 5% 10% 17% 19% 12%	5316
	21st to end	Night	44% 9% 10% 21% 13% 5%	7106

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	22% 11% 27% 26% 11% 1	348
	11th to 20th	Day	12% 9% 29% 30% 19% 1	430
	21st to end	Day	10% 12% 27% 31% 17% 3	473
February	1st to 10th	Day	15% 11% 27% 29% 14% 3	466
	11th to 20th	Day	15% 13% 31% 23% 15% 2	535
	21st to end	Day	9% 13% 34% 31% 10% 2	446
March	1st to 10th	Day	16% 16% 33% 29% 6%	540
	11th to 20th	Day	14% 12% 28% 33% 11% 1	540
	21st to end	Day	13% 17% 30% 28% 11% 1	594
April	1st to 10th	Day	13% 11% 33% 28% 13% 3	540
	11th to 20th	Day	10% 14% 29% 31% 13% 2	532
	21st to end	Day	7% 14% 31% 30% 14% 4	540
May	1st to 10th	Day	9% 9% 31% 34% 14% 3	508
	11th to 20th	Day	6% 10% 28% 35% 17% 4	528
	21st to end	Day	6% 12% 26% 32% 19% 5%	573
June	1st to 10th	Day	5% 5% 20% 41% 24% 5%	530
	11th to 20th	Day	2 6% 25% 37% 25% 5%	530
	21st to end	Day	3 2 19% 40% 27% 9%	530
July	1st to 10th	Day	1 1 15% 43% 32% 8%	526
	11th to 20th	Day	3 7% 41% 34% 15%	524
	21st to end	Day	1 1 9% 45% 33% 12%	575
August	1st to 10th	Day	3 2 17% 41% 29% 7%	527
	11th to 20th	Day	3 1 12% 39% 34% 10%	517
	21st to end	Day	2 1 13% 43% 32% 9%	583
September	1st to 10th	Day	2 2 15% 37% 36% 8%	530
	11th to 20th	Day	5% 5% 19% 39% 26% 5%	530
	21st to end	Day	3 7% 16% 38% 30% 6%	530
October	1st to 10th	Day	3 7% 24% 38% 24% 5%	530
	11th to 20th	Day	4 8% 22% 37% 25% 4	530
	21st to end	Day	4 5% 24% 39% 25% 3	583
November	1st to 10th	Day	5% 8% 23% 34% 26% 4	466
	11th to 20th	Day	12% 7% 23% 34% 20% 4	330
	21st to end	Day	8% 7% 25% 33% 23% 3	330
December	1st to 10th	Day	11% 9% 27% 37% 14% 3	266
	11th to 20th	Day	12% 9% 21% 35% 19% 4	220
	21st to end	Day	21% 12% 27% 27% 10% 3	242
January	1st to 10th	Night	17% 5% 20% 35% 19% 4	840
	11th to 20th	Night	11% 2 19% 40% 22% 5%	840
	21st to end	Night	14% 3 16% 42% 21% 4	924
February	1st to 10th	Night	15% 5% 14% 37% 23% 5%	840
	11th to 20th	Night	14% 6% 19% 35% 22% 4	824
	21st to end	Night	13% 7% 19% 41% 19% 3	692
March	1st to 10th	Night	14% 6% 20% 38% 18% 3	840
	11th to 20th	Night	12% 3 17% 38% 24% 6%	840
	21st to end	Night	13% 5% 20% 35% 23% 3	924
April	1st to 10th	Night	11% 4 20% 37% 26% 3	840
	11th to 20th	Night	8% 6% 20% 35% 25% 6%	831
	21st to end	Night	8% 5% 23% 30% 24% 9%	840
May	1st to 10th	Night	7% 3 18% 32% 31% 8%	804
	11th to 20th	Night	6% 4 16% 33% 33% 8%	698
	21st to end	Night	6% 4 20% 30% 32% 8%	433
June	1st to 10th	Night	4 2 15% 26% 44% 9%	400
	11th to 20th	Night	3 1 11% 30% 46% 9%	400
	21st to end	Night	3 2 8% 24% 48% 17%	400
July	1st to 10th	Night	2 7% 14% 59% 18%	390
	11th to 20th	Night	2 3 11% 55% 30%	579
	21st to end	Night	2 1 4 14% 52% 26%	657
August	1st to 10th	Night	3 8% 23% 48% 17%	699
	11th to 20th	Night	1 1 5% 20% 57% 16%	792
	21st to end	Night	1 1 5% 23% 54% 17%	880
September	1st to 10th	Night	3 1 5% 22% 55% 15%	800
	11th to 20th	Night	4 1 7% 31% 43% 13%	800
	21st to end	Night	3 1 9% 28% 47% 12%	800
October	1st to 10th	Night	5% 1 9% 34% 42% 9%	800
	11th to 20th	Night	5% 2 11% 34% 39% 10%	800
	21st to end	Night	6% 3 11% 39% 32% 10%	880
November	1st to 10th	Night	6% 3 14% 36% 35% 7%	820
	11th to 20th	Night	7% 3 16% 40% 29% 7%	840
	21st to end	Night	7% 4 17% 41% 28% 3	840
December	1st to 10th	Night	13% 4 16% 40% 25% 4	840
	11th to 20th	Night	15% 3 17% 40% 22% 3	840
	21st to end	Night	15% 7% 21% 38% 17% 2	924

Page generated at 2026-04-28 23:21:42. Version ac07

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