BoonHill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BoonHill Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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% 11% 5% 11% 6% 5% 4 4 5% 6% 30%	126551
	Night	13% 13% 5% 12% 6% 5% 4 3 4 5% 29%	124346
Transparency	Day	53% 5% 10% 19% 11% 1	130051
	Night	56% 5% 10% 18% 9% 1	128907
Seeing	Day	21% 15% 30% 22% 9% 3	17270
	Night	20% 11% 31% 28% 9% 1	27253

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	9% 19% 11% 30% 9% 5% 3 2 2 2 9%	3617
	11th to 20th	Day	11% 14% 8% 23% 16% 10% 7% 2 2 2 4	3620
	21st to end	Day	13% 17% 9% 18% 10% 5% 6% 5% 3 3 12%	3982
February	1st to 10th	Day	9% 17% 7% 14% 15% 8% 6% 3 3 5% 13%	1744
	11th to 20th	Day	10% 16% 7% 18% 12% 9% 4 4 4 3 14%	1948
	21st to end	Day	10% 19% 9% 15% 7% 5% 4 3 4 4 19%	1650
March	1st to 10th	Day	10% 12% 4 11% 9% 5% 5% 4 5% 6% 29%	4480
	11th to 20th	Day	11% 9% 5% 15% 6% 5% 4 4 6% 8% 28%	4836
	21st to end	Day	20% 13% 7% 16% 6% 4 3 3 4 4 22%	5685
April	1st to 10th	Day	16% 18% 5% 9% 4 4 4 3 5% 6% 27%	2400
	11th to 20th	Day	18% 14% 4 8% 4 3 4 3 4 5% 32%	2458
	21st to end	Day	15% 12% 4 9% 6% 5% 4 4 5% 7% 30%	2600
May	1st to 10th	Day	15% 10% 3 7% 4 4 4 3 5% 7% 39%	5862
	11th to 20th	Day	10% 10% 3 8% 5% 5% 5% 4 5% 6% 39%	6035
	21st to end	Day	13% 9% 3 7% 4 4 5% 4 6% 7% 38%	6935
June	1st to 10th	Day	11% 7% 2 7% 5% 4 5% 4 6% 7% 41%	2800
	11th to 20th	Day	13% 8% 2 7% 5% 5% 4 3 7% 8% 38%	2800
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	2800
July	1st to 10th	Day	12% 7% 2 6% 3 4 5% 4 7% 10% 38%	6360
	11th to 20th	Day	8% 6% 2 5% 4 4 4 4 7% 9% 46%	6360
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 4 7% 10% 43%	6924
August	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 8% 38%	4365
	11th to 20th	Day	13% 6% 3 6% 4 4 4 4 6% 7% 43%	4174
	21st to end	Day	11% 6% 2 8% 5% 5% 5% 4 5% 8% 41%	4428
September	1st to 10th	Day	14% 5% 3 6% 6% 5% 6% 4 5% 7% 39%	2375
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2200
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 5% 8% 37%	2200
October	1st to 10th	Day	16% 9% 2 7% 4 5% 3 3 4 7% 40%	2200
	11th to 20th	Day	20% 16% 3 10% 6% 5% 4 4 5% 6% 22%	1906
	21st to end	Day	21% 16% 4 10% 6% 5% 4 4 5% 6% 20%	1980
November	1st to 10th	Day	14% 17% 5% 8% 5% 4 4 4 6% 7% 28%	1852
	11th to 20th	Day	13% 21% 8% 13% 6% 5% 3 3 4 6% 17%	1890
	21st to end	Day	16% 23% 9% 15% 5% 4 3 2 2 5% 15%	1864
December	1st to 10th	Day	10% 23% 14% 23% 6% 4 4 2 3 3 8%	2379
	11th to 20th	Day	11% 22% 14% 19% 10% 5% 3 2 3 3 7%	2860
	21st to end	Day	18% 24% 9% 17% 7% 5% 4 2 3 3 8%	3982
January	1st to 10th	Night	10% 17% 9% 26% 9% 5% 4 2 2 3 11%	6348
	11th to 20th	Night	9% 15% 7% 22% 13% 14% 6% 2 3 3 6%	6360
	21st to end	Night	10% 18% 9% 22% 8% 7% 7% 3 4 3 9%	6996
February	1st to 10th	Night	10% 15% 6% 16% 13% 9% 5% 4 3 4 15%	2654
	11th to 20th	Night	11% 14% 6% 16% 11% 6% 6% 4 5% 5% 16%	2338
	21st to end	Night	13% 14% 5% 14% 8% 5% 5% 3 5% 6% 21%	1980
March	1st to 10th	Night	14% 9% 5% 10% 6% 5% 4 4 5% 5% 33%	5373
	11th to 20th	Night	10% 12% 5% 9% 5% 4 4 4 6% 8% 34%	5123
	21st to end	Night	15% 14% 5% 12% 6% 4 4 3 4 6% 27%	5287
April	1st to 10th	Night	15% 12% 5% 9% 5% 4 4 4 5% 5% 30%	2000
	11th to 20th	Night	19% 12% 4 8% 3 3 5% 3 4 6% 34%	1890
	21st to end	Night	16% 10% 4 8% 5% 4 3 3 5% 5% 37%	1800
May	1st to 10th	Night	14% 9% 3 6% 4 4 4 3 5% 8% 38%	4033
	11th to 20th	Night	12% 10% 3 8% 5% 5% 5% 3 5% 7% 38%	3863
	21st to end	Night	19% 6% 2 6% 3 3 3 3 5% 6% 43%	3942
June	1st to 10th	Night	13% 8% 2 6% 5% 3 4 4 6% 8% 41%	1600
	11th to 20th	Night	10% 6% 2 8% 5% 4 4 5% 6% 9% 41%	1600
	21st to end	Night	11% 7% 2 6% 4 4 5% 4 5% 7% 44%	1600
July	1st to 10th	Night	11% 8% 2 5% 4 4 5% 4 6% 8% 42%	3618
	11th to 20th	Night	7% 6% 2 5% 3 4 4 4 5% 6% 52%	3617
	21st to end	Night	6% 7% 2 5% 4 4 4 4 5% 7% 50%	3947
August	1st to 10th	Night	11% 5% 3 7% 5% 5% 5% 4 7% 8% 42%	2785
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 6% 51%	2875
	21st to end	Night	8% 7% 2 6% 4 5% 4 4 5% 6% 48%	3454
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 6% 50%	2004
	11th to 20th	Night	11% 5% 2 5% 3 3 4 4 6% 7% 50%	2200
	21st to end	Night	14% 8% 3 7% 4 4 5% 3 5% 7% 39%	2200
October	1st to 10th	Night	15% 9% 3 7% 4 4 3 2 5% 6% 43%	2200
	11th to 20th	Night	18% 14% 4 8% 5% 4 4 4 5% 5% 29%	2490
	21st to end	Night	22% 14% 4 8% 5% 4 4 3 5% 5% 27%	2860
November	1st to 10th	Night	11% 17% 5% 9% 6% 4 4 2 4 6% 31%	2666
	11th to 20th	Night	13% 19% 6% 14% 6% 5% 5% 3 4 5% 20%	2730
	21st to end	Night	15% 20% 8% 14% 5% 5% 4 3 4 5% 17%	2754
December	1st to 10th	Night	9% 19% 9% 21% 7% 5% 5% 3 4 4 13%	4832
	11th to 20th	Night	11% 25% 9% 17% 10% 5% 4 3 3 3 10%	5331
	21st to end	Night	19% 25% 8% 14% 5% 4 3 2 3 4 12%	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	84% 4 5% 6% 1	3618
	11th to 20th	Day	89% 3 3 4 1	3620
	21st to end	Day	79% 5% 8% 7% 1	3982
February	1st to 10th	Day	74% 5% 7% 11% 3	1744
	11th to 20th	Day	74% 5% 7% 9% 4	1958
	21st to end	Day	68% 5% 10% 14% 3 1	1650
March	1st to 10th	Day	55% 5% 14% 15% 10% 1	4480
	11th to 20th	Day	54% 4 15% 16% 9% 1	4836
	21st to end	Day	66% 4 9% 13% 6% 1	5687
April	1st to 10th	Day	59% 5% 10% 17% 11%	2400
	11th to 20th	Day	54% 5% 8% 18% 14% 2	2458
	21st to end	Day	52% 7% 11% 14% 14% 2	2600
May	1st to 10th	Day	44% 5% 9% 22% 17% 3	5897
	11th to 20th	Day	43% 6% 9% 22% 17% 2	6039
	21st to end	Day	44% 6% 9% 20% 18% 3	6949
June	1st to 10th	Day	40% 7% 10% 25% 17% 2	2800
	11th to 20th	Day	42% 5% 12% 22% 18% 2	2800
	21st to end	Day	42% 6% 13% 23% 15% 1	2800
July	1st to 10th	Day	39% 5% 12% 27% 16% 1	6291
	11th to 20th	Day	33% 6% 11% 33% 17% 1	6285
	21st to end	Day	35% 6% 13% 28% 17% 1	6728
August	1st to 10th	Day	39% 7% 12% 27% 14% 1	4350
	11th to 20th	Day	39% 5% 12% 27% 16% 1	4213
	21st to end	Day	40% 5% 13% 28% 13% 1	4427
September	1st to 10th	Day	43% 6% 12% 23% 13% 1	2375
	11th to 20th	Day	36% 7% 13% 28% 15% 1	2200
	21st to end	Day	45% 6% 13% 24% 12%	2200
October	1st to 10th	Day	32% 4 8% 34% 20% 1	3590
	11th to 20th	Day	54% 5% 10% 19% 12%	3070
	21st to end	Day	61% 5% 13% 16% 5%	3180
November	1st to 10th	Day	55% 7% 11% 18% 9%	1852
	11th to 20th	Day	69% 5% 11% 11% 4	1890
	21st to end	Day	74% 4 11% 8% 3	1864
December	1st to 10th	Day	84% 4 5% 5% 1	2379
	11th to 20th	Day	84% 4 6% 5% 1	2857
	21st to end	Day	81% 5% 4 6% 3	3982
January	1st to 10th	Night	79% 5% 6% 7% 3	6348
	11th to 20th	Night	85% 4 5% 4 2	6360
	21st to end	Night	80% 5% 6% 7% 2	6996
February	1st to 10th	Night	73% 5% 6% 11% 4	2654
	11th to 20th	Night	69% 5% 10% 12% 4	2339
	21st to end	Night	64% 5% 12% 14% 4 1	1980
March	1st to 10th	Night	51% 6% 16% 19% 8%	5373
	11th to 20th	Night	48% 4 19% 19% 8% 1	5123
	21st to end	Night	57% 6% 9% 18% 9% 1	5287
April	1st to 10th	Night	53% 6% 9% 19% 12% 1	2000
	11th to 20th	Night	52% 4 9% 19% 14% 2	1890
	21st to end	Night	49% 4 10% 20% 14% 3	1800
May	1st to 10th	Night	43% 5% 12% 18% 18% 4	4060
	11th to 20th	Night	45% 7% 8% 25% 15% 1	3864
	21st to end	Night	42% 6% 9% 23% 16% 4	3950
June	1st to 10th	Night	39% 6% 11% 26% 17% 1	1600
	11th to 20th	Night	39% 6% 12% 22% 18% 2	1600
	21st to end	Night	38% 8% 11% 27% 15% 2	1600
July	1st to 10th	Night	40% 4 13% 25% 14% 4	3580
	11th to 20th	Night	30% 6% 13% 31% 18% 2	3572
	21st to end	Night	32% 6% 15% 27% 18% 1	3832
August	1st to 10th	Night	39% 6% 15% 26% 13% 1	2776
	11th to 20th	Night	34% 4 14% 31% 15% 1	2902
	21st to end	Night	36% 5% 13% 24% 20% 1	3454
September	1st to 10th	Night	34% 6% 14% 28% 17% 2	2004
	11th to 20th	Night	32% 7% 14% 32% 15% 1	2200
	21st to end	Night	44% 6% 12% 26% 11% 1	2200
October	1st to 10th	Night	36% 5% 9% 36% 13% 1	3590
	11th to 20th	Night	47% 4 10% 23% 14%	4052
	21st to end	Night	52% 4 12% 24% 7%	4610
November	1st to 10th	Night	55% 6% 12% 18% 9% 1	2666
	11th to 20th	Night	67% 5% 11% 11% 5% 1	2730
	21st to end	Night	71% 5% 12% 10% 2 1	2754
December	1st to 10th	Night	76% 5% 10% 8% 1	4832
	11th to 20th	Night	80% 4 7% 7% 1	5333
	21st to end	Night	76% 5% 7% 7% 5%	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	20% 20% 34% 19% 6%	300
	11th to 20th	Day	27% 20% 38% 12% 2	300
	21st to end	Day	25% 21% 32% 18% 3	330
February	1st to 10th	Day	23% 28% 31% 13% 3 1	300
	11th to 20th	Day	23% 27% 36% 13% 1	300
	21st to end	Day	28% 31% 35% 6%	249
March	1st to 10th	Day	24% 28% 36% 11% 2	300
	11th to 20th	Day	27% 16% 29% 21% 6% 1	434
	21st to end	Day	31% 18% 26% 20% 6%	550
April	1st to 10th	Day	29% 17% 35% 14% 4 1	500
	11th to 20th	Day	32% 18% 29% 13% 6% 3	492
	21st to end	Day	25% 15% 29% 22% 7% 2	500
May	1st to 10th	Day	24% 20% 30% 17% 6% 2	503
	11th to 20th	Day	21% 14% 29% 23% 9% 3	597
	21st to end	Day	17% 14% 30% 26% 10% 2	789
June	1st to 10th	Day	14% 15% 36% 23% 9% 2	730
	11th to 20th	Day	19% 14% 33% 22% 9% 2	730
	21st to end	Day	18% 12% 33% 25% 9% 2	730
July	1st to 10th	Day	11% 8% 31% 31% 15% 4	721
	11th to 20th	Day	13% 9% 29% 32% 13% 5%	718
	21st to end	Day	10% 7% 33% 31% 13% 6%	794
August	1st to 10th	Day	11% 6% 27% 29% 20% 7%	606
	11th to 20th	Day	15% 6% 23% 29% 17% 11%	519
	21st to end	Day	16% 7% 25% 29% 17% 6%	583
September	1st to 10th	Day	18% 8% 23% 28% 16% 7%	530
	11th to 20th	Day	10% 6% 23% 32% 19% 9%	530
	21st to end	Day	21% 10% 23% 26% 15% 5%	530
October	1st to 10th	Day	22% 14% 28% 22% 12% 2	530
	11th to 20th	Day	31% 15% 31% 19% 3	406
	21st to end	Day	32% 20% 31% 13% 4 1	363
November	1st to 10th	Day	32% 15% 35% 12% 7%	330
	11th to 20th	Day	30% 24% 32% 11% 3	330
	21st to end	Day	31% 22% 34% 9% 3	330
December	1st to 10th	Day	30% 21% 29% 16% 3 2	233
	11th to 20th	Day	25% 26% 35% 12% 2 1	253
	21st to end	Day	32% 24% 30% 12% 2 1	330
January	1st to 10th	Night	22% 16% 35% 21% 6% 1	1000
	11th to 20th	Night	23% 17% 34% 20% 6%	1000
	21st to end	Night	19% 13% 38% 25% 5% 1	1100
February	1st to 10th	Night	20% 14% 37% 24% 4 1	879
	11th to 20th	Night	20% 18% 35% 23% 5%	783
	21st to end	Night	21% 18% 37% 20% 3	660
March	1st to 10th	Night	18% 15% 36% 23% 8% 1	800
	11th to 20th	Night	19% 15% 38% 23% 5%	800
	21st to end	Night	22% 12% 34% 26% 5% 1	803
April	1st to 10th	Night	23% 12% 37% 22% 5% 1	600
	11th to 20th	Night	26% 14% 31% 21% 7%	593
	21st to end	Night	20% 12% 32% 29% 7% 1	600
May	1st to 10th	Night	19% 10% 35% 29% 7% 1	600
	11th to 20th	Night	19% 9% 33% 29% 9% 1	591
	21st to end	Night	16% 7% 29% 35% 12% 1	650
June	1st to 10th	Night	16% 7% 31% 36% 9% 1	600
	11th to 20th	Night	13% 8% 29% 35% 14% 1	600
	21st to end	Night	16% 6% 27% 38% 13% 1	600
July	1st to 10th	Night	12% 3 22% 40% 20% 2	585
	11th to 20th	Night	8% 4 25% 41% 20% 2	582
	21st to end	Night	11% 4 23% 40% 21% 1	657
August	1st to 10th	Night	10% 3 20% 46% 20% 1	597
	11th to 20th	Night	11% 4 20% 38% 23% 5%	594
	21st to end	Night	14% 5% 21% 38% 21% 1	660
September	1st to 10th	Night	16% 3 22% 36% 19% 4	604
	11th to 20th	Night	13% 4 23% 37% 20% 3	800
	21st to end	Night	18% 10% 24% 31% 15% 3	800
October	1st to 10th	Night	20% 9% 27% 31% 12% 1	800
	11th to 20th	Night	27% 12% 29% 26% 6% 1	800
	21st to end	Night	32% 12% 27% 22% 6%	880
November	1st to 10th	Night	24% 13% 32% 25% 6% 1	820
	11th to 20th	Night	25% 15% 37% 19% 4	840
	21st to end	Night	26% 15% 34% 22% 3	864
December	1st to 10th	Night	24% 15% 35% 22% 4	1011
	11th to 20th	Night	25% 17% 36% 18% 4 1	1000
	21st to end	Night	28% 15% 34% 21% 2	1100

Page generated at 2025-11-30 01:00:55. Version ac07

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