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 2025-01-07.

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	6% 4 1 3 2 2 3 3 4 7% 64%	108513
	Night	7% 5% 2 5% 3 3 3 3 4 8% 57%	109273
Transparency	Day	20% 4 7% 21% 25% 23%	108272
	Night	24% 4 8% 20% 22% 21%	109106
Seeing	Day	7% 8% 23% 35% 22% 5%	15757
	Night	8% 3 14% 32% 34% 9%	25404

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	9% 4 2 5% 4 4 4 3 4 7% 56%	2784
	11th to 20th	Day	14% 6% 2 5% 4 4 3 4 6% 8% 45%	2860
	21st to end	Day	14% 6% 2 6% 4 4 5% 5% 6% 9% 39%	3146
February	1st to 10th	Day	14% 4 2 5% 3 3 2 3 4 5% 56%	1770
	11th to 20th	Day	11% 4 2 5% 3 3 4 2 5% 7% 53%	1850
	21st to end	Day	13% 5% 2 4 4 3 3 3 5% 6% 52%	1570
March	1st to 10th	Day	13% 9% 3 6% 4 3 4 3 6% 8% 42%	3252
	11th to 20th	Day	7% 4 1 4 3 3 3 2 4 6% 62%	3490
	21st to end	Day	10% 7% 2 6% 4 4 5% 4 6% 9% 43%	3979
April	1st to 10th	Day	9% 5% 1 4 3 3 3 3 5% 7% 57%	2280
	11th to 20th	Day	7% 6% 2 4 3 3 3 2 6% 7% 58%	2249
	21st to end	Day	8% 4 1 4 2 2 3 3 4 6% 63%	2280
May	1st to 10th	Day	6% 3 1 2 1 2 1 1 3 4 76%	3800
	11th to 20th	Day	2 7% 3 6% 3 3 4 3 6% 10% 55%	3902
	21st to end	Day	3 4 1 3 2 3 2 2 4 7% 68%	4813
June	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 9% 63%	2660
	11th to 20th	Day	1 3 1 3 2 1 2 2 3 7% 73%	2660
	21st to end	Day	1 2 1 2 2 2 2 3 4 7% 74%	2660
July	1st to 10th	Day	1 2 1 1 1 2 3 7% 81%	4405
	11th to 20th	Day	2 2 1 3 2 2 2 2 4 6% 74%	4110
	21st to end	Day	1 1 2 2 2 2 3 5% 9% 74%	4455
August	1st to 10th	Day	1 2 1 2 2 2 2 3 4 8% 73%	3920
	11th to 20th	Day	1 2 1 2 1 1 1 1 2 5% 85%	3724
	21st to end	Day	1 2 1 2 1 1 1 2 3 6% 80%	4180
September	1st to 10th	Day	1 2 1 2 1 1 2 2 4 7% 76%	2280
	11th to 20th	Day	3 3 1 3 2 1 2 3 3 8% 73%	2280
	21st to end	Day	2 3 1 2 1 2 2 2 5% 6% 73%	2083
October	1st to 10th	Day	2 1 1 3 1 2 2 2 4 4 77%	3180
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 5% 76%	3161
	21st to end	Day	4 3 1 4 2 2 3 3 4 6% 67%	3448
November	1st to 10th	Day	6% 4 1 3 3 2 2 2 4 5% 67%	1894
	11th to 20th	Day	7% 3 1 3 2 2 3 3 5% 6% 66%	1800
	21st to end	Day	9% 4 2 4 4 3 3 3 4 5% 60%	1800
December	1st to 10th	Day	6% 5% 2 4 4 3 4 3 5% 7% 57%	2756
	11th to 20th	Day	18% 5% 2 4 2 2 4 3 5% 7% 47%	3028
	21st to end	Day	15% 8% 2 5% 3 4 4 4 6% 9% 40%	4004
January	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 4 6% 57%	4298
	11th to 20th	Night	14% 8% 2 5% 4 3 4 3 5% 7% 44%	4110
	21st to end	Night	18% 8% 2 5% 4 4 4 3 5% 6% 42%	4521
February	1st to 10th	Night	13% 5% 1 4 3 3 2 3 3 5% 56%	2409
	11th to 20th	Night	11% 5% 2 5% 2 3 3 3 4 7% 57%	2218
	21st to end	Night	12% 6% 2 4 4 3 2 2 4 6% 55%	1884
March	1st to 10th	Night	15% 10% 3 6% 4 3 5% 3 6% 7% 39%	3617
	11th to 20th	Night	7% 4 1 4 3 2 3 3 5% 6% 62%	3490
	21st to end	Night	15% 8% 2 5% 3 3 3 3 5% 6% 48%	3661
April	1st to 10th	Night	8% 4 2 5% 3 2 3 3 4 7% 57%	1900
	11th to 20th	Night	6% 6% 2 4 3 2 2 2 3 7% 63%	1879
	21st to end	Night	6% 8% 1 3 2 2 3 3 4 8% 62%	1900
May	1st to 10th	Night	2 4 1 3 2 2 2 2 3 7% 73%	3180
	11th to 20th	Night	2 14% 6% 9% 4 2 2 2 4 8% 46%	2987
	21st to end	Night	2 10% 4 11% 5% 3 3 2 4 6% 50%	2762
June	1st to 10th	Night	3 6% 3 6% 4 3 4 3 5% 8% 57%	1520
	11th to 20th	Night	2 6% 1 4 3 3 2 3 4 6% 66%	1520
	21st to end	Night	1 4 3 4 2 2 2 2 4 11% 66%	1520
July	1st to 10th	Night	2 2 3 2 1 2 2 3 9% 74%	2540
	11th to 20th	Night	1 2 1 7% 5% 4 3 3 4 10% 61%	2856
	21st to end	Night	1 1 4 8% 5% 4 3 3 5% 20% 46%	3109
August	1st to 10th	Night	1 2 1 3 3 3 3 3 6% 14% 61%	3023
	11th to 20th	Night	2 2 2 2 2 3 3 13% 71%	3126
	21st to end	Night	2 2 4 3 3 3 3 5% 14% 62%	3498
September	1st to 10th	Night	2 3 1 3 1 1 2 2 3 7% 73%	1900
	11th to 20th	Night	2 4 1 4 3 3 2 2 3 9% 67%	1900
	21st to end	Night	1 4 1 2 2 2 2 2 3 6% 75%	2091
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 6% 76%	3800
	11th to 20th	Night	2 2 2 4 4 3 4 3 4 10% 62%	3777
	21st to end	Night	4 4 2 4 2 2 2 2 3 6% 68%	4121
November	1st to 10th	Night	5% 4 2 3 3 2 2 2 4 7% 65%	2404
	11th to 20th	Night	4 4 2 4 2 3 3 3 5% 6% 65%	2600
	21st to end	Night	6% 6% 2 4 3 3 5% 3 5% 6% 59%	2600
December	1st to 10th	Night	7% 4 1 4 3 3 4 3 5% 6% 60%	4424
	11th to 20th	Night	16% 6% 1 4 4 4 4 4 6% 7% 44%	5176
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	6952

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% 6% 9% 35% 17% 4	2784
	11th to 20th	Day	37% 6% 10% 19% 16% 11%	2860
	21st to end	Day	40% 7% 10% 20% 14% 9%	3146
February	1st to 10th	Day	32% 3 6% 20% 21% 18%	1770
	11th to 20th	Day	31% 5% 8% 23% 22% 11%	1860
	21st to end	Day	33% 5% 10% 21% 23% 8%	1570
March	1st to 10th	Day	37% 9% 12% 20% 17% 6%	3252
	11th to 20th	Day	23% 6% 10% 32% 17% 12%	3490
	21st to end	Day	33% 10% 14% 25% 14% 4	3979
April	1st to 10th	Day	28% 5% 11% 26% 21% 8%	2280
	11th to 20th	Day	25% 4 12% 26% 24% 8%	2252
	21st to end	Day	21% 5% 12% 26% 28% 8%	2280
May	1st to 10th	Day	14% 2 7% 27% 35% 14%	3800
	11th to 20th	Day	18% 2 9% 30% 29% 11%	3905
	21st to end	Day	13% 3 13% 29% 36% 6%	4814
June	1st to 10th	Day	16% 6% 9% 28% 29% 12%	2660
	11th to 20th	Day	10% 2 8% 19% 35% 25%	2660
	21st to end	Day	10% 2 5% 20% 28% 35%	2660
July	1st to 10th	Day	5% 1 2 12% 15% 65%	4336
	11th to 20th	Day	11% 2 4 11% 17% 54%	4041
	21st to end	Day	7% 2 4 14% 28% 44%	4315
August	1st to 10th	Day	10% 1 4 17% 32% 37%	3905
	11th to 20th	Day	5% 1 2 12% 30% 51%	3760
	21st to end	Day	7% 1 3 16% 27% 46%	4179
September	1st to 10th	Day	9% 1 4 16% 28% 42%	2280
	11th to 20th	Day	11% 2 5% 23% 36% 23%	2280
	21st to end	Day	11% 2 7% 18% 34% 28%	2083
October	1st to 10th	Day	12% 2 5% 13% 23% 44%	3180
	11th to 20th	Day	14% 2 5% 21% 37% 21%	3161
	21st to end	Day	19% 3 4 13% 33% 28%	3448
November	1st to 10th	Day	20% 4 7% 20% 29% 20%	1894
	11th to 20th	Day	21% 3 9% 20% 27% 20%	1800
	21st to end	Day	28% 4 6% 17% 24% 21%	1800
December	1st to 10th	Day	28% 3 5% 14% 29% 20%	2756
	11th to 20th	Day	37% 5% 9% 18% 21% 10%	3028
	21st to end	Day	37% 8% 11% 25% 13% 5%	4004
January	1st to 10th	Night	29% 5% 11% 27% 22% 5%	4298
	11th to 20th	Night	37% 6% 9% 24% 12% 11%	4110
	21st to end	Night	42% 5% 10% 17% 15% 10%	4521
February	1st to 10th	Night	31% 4 7% 18% 23% 17%	2409
	11th to 20th	Night	28% 4 8% 23% 24% 14%	2219
	21st to end	Night	31% 5% 8% 23% 24% 9%	1884
March	1st to 10th	Night	40% 9% 10% 20% 16% 5%	3617
	11th to 20th	Night	23% 6% 11% 26% 24% 10%	3490
	21st to end	Night	35% 7% 14% 22% 18% 4	3661
April	1st to 10th	Night	27% 5% 13% 26% 19% 11%	1900
	11th to 20th	Night	24% 3 11% 30% 24% 9%	1879
	21st to end	Night	23% 4 10% 27% 28% 8%	1900
May	1st to 10th	Night	13% 4 9% 26% 27% 21%	3180
	11th to 20th	Night	31% 4 11% 26% 15% 12%	2989
	21st to end	Night	24% 6% 9% 25% 29% 7%	2762
June	1st to 10th	Night	20% 5% 11% 23% 27% 13%	1520
	11th to 20th	Night	16% 3 6% 22% 30% 24%	1520
	21st to end	Night	13% 1 6% 21% 26% 34%	1520
July	1st to 10th	Night	7% 1 1 14% 12% 65%	2500
	11th to 20th	Night	11% 1 11% 16% 13% 48%	2803
	21st to end	Night	16% 3 8% 15% 27% 31%	3011
August	1st to 10th	Night	12% 1 4 20% 28% 36%	3014
	11th to 20th	Night	6% 1 3 14% 21% 54%	3156
	21st to end	Night	9% 2 5% 16% 25% 43%	3498
September	1st to 10th	Night	12% 1 3 16% 22% 45%	1900
	11th to 20th	Night	16% 2 5% 21% 27% 28%	1900
	21st to end	Night	10% 2 6% 18% 34% 30%	2091
October	1st to 10th	Night	11% 2 4 14% 22% 48%	3800
	11th to 20th	Night	16% 4 7% 17% 33% 23%	3777
	21st to end	Night	18% 2 4 14% 34% 28%	4121
November	1st to 10th	Night	19% 3 8% 21% 26% 23%	2404
	11th to 20th	Night	20% 3 10% 19% 26% 21%	2600
	21st to end	Night	26% 5% 9% 16% 21% 22%	2600
December	1st to 10th	Night	26% 3 4 15% 30% 21%	4424
	11th to 20th	Night	39% 5% 10% 17% 19% 11%	5176
	21st to end	Night	43% 9% 10% 21% 13% 4	6952

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% 10% 26% 28% 12% 1	311
	11th to 20th	Day	13% 9% 28% 29% 19% 1	370
	21st to end	Day	11% 12% 26% 33% 16% 2	407
February	1st to 10th	Day	15% 10% 27% 29% 16% 4	399
	11th to 20th	Day	12% 12% 30% 26% 17% 2	455
	21st to end	Day	10% 14% 31% 32% 11% 2	382
March	1st to 10th	Day	15% 16% 32% 30% 7%	460
	11th to 20th	Day	13% 12% 29% 33% 11% 1	460
	21st to end	Day	13% 18% 30% 27% 10% 1	506
April	1st to 10th	Day	14% 11% 31% 28% 13% 3	460
	11th to 20th	Day	12% 14% 29% 32% 12% 2	452
	21st to end	Day	6% 12% 33% 30% 15% 4	460
May	1st to 10th	Day	9% 9% 32% 34% 14% 3	463
	11th to 20th	Day	6% 9% 28% 35% 18% 4	488
	21st to end	Day	7% 12% 25% 32% 19% 5%	529
June	1st to 10th	Day	6% 4 19% 41% 24% 5%	490
	11th to 20th	Day	2 5% 24% 37% 26% 6%	490
	21st to end	Day	2 2 19% 40% 28% 9%	490
July	1st to 10th	Day	1 1 14% 43% 33% 8%	486
	11th to 20th	Day	3 8% 40% 35% 14%	484
	21st to end	Day	1 8% 45% 34% 12%	531
August	1st to 10th	Day	3 2 16% 42% 30% 7%	487
	11th to 20th	Day	3 1 11% 40% 35% 10%	477
	21st to end	Day	2 1 13% 43% 32% 9%	539
September	1st to 10th	Day	2 2 13% 38% 37% 8%	490
	11th to 20th	Day	4 5% 19% 40% 27% 5%	490
	21st to end	Day	2 6% 15% 40% 31% 6%	490
October	1st to 10th	Day	2 6% 23% 39% 25% 5%	490
	11th to 20th	Day	4 7% 22% 38% 26% 4	490
	21st to end	Day	4 5% 23% 38% 25% 4	539
November	1st to 10th	Day	5% 8% 24% 33% 25% 4	430
	11th to 20th	Day	8% 7% 23% 36% 21% 4	300
	21st to end	Day	9% 7% 25% 31% 24% 4	300
December	1st to 10th	Day	12% 9% 26% 35% 15% 3	242
	11th to 20th	Day	13% 10% 21% 34% 19% 4	200
	21st to end	Day	19% 12% 28% 27% 11% 3	220
January	1st to 10th	Night	16% 4 18% 36% 20% 5%	784
	11th to 20th	Night	13% 2 17% 40% 23% 6%	760
	21st to end	Night	15% 3 15% 42% 21% 4	836
February	1st to 10th	Night	15% 6% 12% 36% 26% 5%	760
	11th to 20th	Night	12% 6% 19% 35% 24% 5%	744
	21st to end	Night	14% 7% 18% 39% 20% 3	628
March	1st to 10th	Night	15% 6% 19% 37% 20% 3	760
	11th to 20th	Night	12% 3 18% 37% 24% 6%	760
	21st to end	Night	13% 5% 21% 33% 23% 4	836
April	1st to 10th	Night	11% 4 20% 36% 27% 3	760
	11th to 20th	Night	8% 6% 21% 34% 25% 6%	751
	21st to end	Night	8% 4 23% 30% 25% 9%	760
May	1st to 10th	Night	8% 4 18% 31% 32% 8%	760
	11th to 20th	Night	6% 3 17% 32% 34% 8%	663
	21st to end	Night	6% 4 21% 30% 31% 8%	411
June	1st to 10th	Night	4 2 15% 27% 44% 8%	380
	11th to 20th	Night	3 1 12% 28% 46% 9%	380
	21st to end	Night	3 2 8% 23% 48% 17%	380
July	1st to 10th	Night	2 7% 14% 58% 19%	370
	11th to 20th	Night	2 3 11% 57% 28%	549
	21st to end	Night	2 4 14% 53% 27%	624
August	1st to 10th	Night	3 9% 22% 49% 17%	664
	11th to 20th	Night	1 1 5% 20% 57% 16%	752
	21st to end	Night	1 1 6% 22% 54% 17%	836
September	1st to 10th	Night	3 5% 21% 56% 15%	760
	11th to 20th	Night	4 1 7% 31% 43% 13%	760
	21st to end	Night	2 1 9% 27% 48% 13%	760
October	1st to 10th	Night	5% 1 9% 34% 43% 9%	760
	11th to 20th	Night	5% 2 11% 33% 39% 10%	760
	21st to end	Night	6% 3 11% 37% 33% 10%	836
November	1st to 10th	Night	6% 3 14% 36% 34% 7%	780
	11th to 20th	Night	6% 2 15% 41% 30% 7%	800
	21st to end	Night	7% 4 17% 40% 29% 4	800
December	1st to 10th	Night	13% 4 15% 39% 26% 4	800
	11th to 20th	Night	16% 3 18% 39% 22% 3	800
	21st to end	Night	13% 7% 21% 39% 18% 2	880

Page generated at 2025-01-04 00:23:58. Version ac07

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