Beaglehaven Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Beaglehaven Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	14% 6% 2 6% 4 4 4 4 6% 8% 42%	114120
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	115176
Transparency	Day	23% 43% 7% 15% 10% 2	117760
	Night	23% 41% 7% 15% 11% 3	119675
Seeing	Day	19% 13% 27% 23% 13% 4	15627
	Night	18% 12% 29% 28% 11% 1	25266

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	15% 4 2 4 3 4 4 4 4 6% 50%	3757
	11th to 20th	Day	13% 9% 3 6% 4 3 4 3 4 4 46%	4060
	21st to end	Day	15% 9% 2 6% 4 4 3 3 5% 6% 43%	4466
February	1st to 10th	Day	13% 8% 2 6% 6% 4 5% 4 6% 7% 38%	1800
	11th to 20th	Day	18% 8% 2 5% 5% 4 4 4 4 6% 40%	1788
	21st to end	Day	17% 8% 3 6% 4 4 5% 3 5% 5% 41%	1650
March	1st to 10th	Day	18% 6% 1 5% 4 5% 5% 3 4 7% 41%	4903
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 49%	4980
	21st to end	Day	15% 6% 3 6% 5% 4 5% 4 5% 7% 40%	5525
April	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 6% 7% 42%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 5% 6% 46%	2374
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 6% 42%	2400
May	1st to 10th	Day	22% 8% 2 7% 4 4 5% 4 6% 8% 30%	4194
	11th to 20th	Day	23% 6% 2 6% 4 4 4 4 7% 9% 30%	4387
	21st to end	Day	11% 5% 2 5% 4 4 4 4 7% 9% 45%	4893
June	1st to 10th	Day	10% 4 2 5% 5% 5% 5% 4 7% 9% 43%	2660
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 8% 10% 41%	2660
	21st to end	Day	9% 4 2 5% 5% 5% 5% 4 7% 10% 45%	2660
July	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 6% 8% 11% 42%	4440
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 5% 8% 10% 40%	4258
	21st to end	Day	10% 7% 3 7% 6% 5% 6% 6% 7% 8% 37%	4466
August	1st to 10th	Day	14% 5% 2 7% 6% 6% 5% 5% 8% 9% 34%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	3898
	21st to end	Day	10% 5% 2 5% 4 3 5% 4 5% 8% 48%	4180
September	1st to 10th	Day	14% 5% 2 4 4 4 4 4 5% 7% 46%	2280
	11th to 20th	Day	12% 6% 2 6% 3 3 4 3 5% 8% 48%	2280
	21st to end	Day	20% 6% 2 6% 4 3 4 4 5% 7% 38%	1987
October	1st to 10th	Day	16% 6% 1 4 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Day	16% 4 2 6% 3 3 3 3 5% 6% 49%	1900
	21st to end	Day	13% 5% 3 5% 3 4 4 3 4 6% 50%	2090
November	1st to 10th	Day	12% 5% 2 5% 4 4 3 3 5% 7% 50%	1875
	11th to 20th	Day	15% 5% 2 4 3 3 3 4 6% 6% 50%	1755
	21st to end	Day	15% 6% 2 5% 3 4 5% 4 5% 7% 46%	1600
December	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 7% 48%	2600
	11th to 20th	Day	18% 8% 2 5% 3 4 4 3 5% 6% 42%	2990
	21st to end	Day	18% 8% 3 6% 5% 4 4 4 6% 8% 35%	3960
January	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 5% 6% 49%	6186
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 6% 51%	5900
	21st to end	Night	15% 8% 2 6% 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	15% 8% 3 5% 4 4 4 4 6% 5% 42%	2600
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 5% 6% 41%	2497
	21st to end	Night	16% 8% 2 6% 4 4 4 3 5% 6% 42%	1980
March	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 4 5% 47%	4946
	11th to 20th	Night	11% 6% 2 4 3 4 4 4 5% 7% 49%	4980
	21st to end	Night	12% 7% 2 6% 4 4 5% 4 6% 6% 44%	5405
April	1st to 10th	Night	14% 8% 2 5% 5% 4 5% 4 6% 6% 40%	2000
	11th to 20th	Night	13% 6% 2 6% 3 4 4 3 6% 7% 44%	1975
	21st to end	Night	15% 5% 2 5% 4 4 5% 4 6% 7% 44%	2000
May	1st to 10th	Night	23% 8% 2 6% 4 5% 5% 4 5% 7% 31%	2950
	11th to 20th	Night	23% 9% 3 7% 5% 4 4 3 4 6% 33%	2722
	21st to end	Night	10% 5% 2 5% 4 5% 5% 5% 8% 9% 43%	2778
June	1st to 10th	Night	10% 5% 2 4 4 5% 5% 4 6% 9% 45%	1520
	11th to 20th	Night	12% 5% 2 5% 5% 4 6% 4 7% 8% 42%	1520
	21st to end	Night	8% 4 2 5% 5% 5% 5% 4 7% 10% 44%	1520
July	1st to 10th	Night	6% 5% 3 7% 6% 6% 6% 4 8% 10% 40%	2520
	11th to 20th	Night	7% 6% 2 7% 6% 6% 6% 5% 7% 11% 38%	2698
	21st to end	Night	10% 7% 3 7% 5% 6% 5% 4 7% 9% 37%	3087
August	1st to 10th	Night	16% 6% 2 8% 5% 5% 6% 5% 7% 10% 31%	2825
	11th to 20th	Night	9% 6% 3 8% 5% 6% 5% 4 6% 9% 38%	2899
	21st to end	Night	11% 4 2 5% 3 4 4 3 5% 9% 48%	3476
September	1st to 10th	Night	14% 5% 2 6% 4 4 4 3 5% 7% 44%	1900
	11th to 20th	Night	10% 6% 2 5% 3 4 3 2 5% 7% 51%	1900
	21st to end	Night	21% 7% 3 6% 4 4 4 3 4 6% 40%	2188
October	1st to 10th	Night	20% 5% 2 6% 3 3 3 3 4 6% 45%	2280
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 4 5% 53%	2280
	21st to end	Night	15% 5% 2 5% 3 3 3 2 4 6% 51%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 54%	2421
	11th to 20th	Night	14% 6% 2 4 3 4 3 3 4 6% 49%	2644
	21st to end	Night	13% 4 2 5% 4 4 4 4 6% 7% 47%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	4580
	11th to 20th	Night	20% 6% 2 5% 4 4 4 4 5% 6% 40%	5205
	21st to end	Night	17% 7% 2 6% 5% 5% 5% 4 7% 8% 34%	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	25% 27% 5% 21% 16% 6%	3758
	11th to 20th	Day	30% 27% 5% 16% 14% 8%	4060
	21st to end	Day	30% 30% 7% 20% 11% 2	4466
February	1st to 10th	Day	19% 63% 6% 8% 4 1	1800
	11th to 20th	Day	19% 61% 3 9% 7% 1	1797
	21st to end	Day	18% 62% 4 9% 6% 1	1650
March	1st to 10th	Day	32% 31% 6% 14% 11% 7%	4903
	11th to 20th	Day	23% 30% 7% 21% 15% 4	4980
	21st to end	Day	30% 31% 7% 13% 14% 5%	5525
April	1st to 10th	Day	19% 63% 4 7% 7% 1	2400
	11th to 20th	Day	15% 63% 5% 9% 7% 1	2374
	21st to end	Day	17% 62% 4 10% 7%	2400
May	1st to 10th	Day	34% 42% 6% 13% 4	4232
	11th to 20th	Day	31% 43% 9% 15% 2	4391
	21st to end	Day	18% 44% 10% 15% 11% 2	4907
June	1st to 10th	Day	14% 66% 4 10% 5%	2660
	11th to 20th	Day	12% 67% 5% 10% 6%	2660
	21st to end	Day	11% 65% 6% 10% 7%	2660
July	1st to 10th	Day	21% 41% 15% 14% 6% 4	4370
	11th to 20th	Day	25% 38% 11% 20% 6%	4184
	21st to end	Day	29% 38% 6% 15% 11% 1	4324
August	1st to 10th	Day	30% 40% 10% 16% 3	4091
	11th to 20th	Day	20% 40% 8% 21% 10%	3937
	21st to end	Day	19% 39% 8% 19% 14% 1	4180
September	1st to 10th	Day	15% 61% 7% 9% 7% 1	2280
	11th to 20th	Day	14% 60% 4 11% 9% 2	2280
	21st to end	Day	22% 60% 3 9% 5% 1	1987
October	1st to 10th	Day	24% 37% 4 10% 22% 4	3160
	11th to 20th	Day	12% 37% 5% 19% 19% 8%	3140
	21st to end	Day	9% 36% 8% 21% 21% 5%	3428
November	1st to 10th	Day	17% 57% 6% 10% 8% 3	1875
	11th to 20th	Day	13% 57% 4 11% 13% 2	1755
	21st to end	Day	16% 57% 5% 14% 7% 1	1600
December	1st to 10th	Day	22% 37% 10% 19% 9% 4	2600
	11th to 20th	Day	31% 32% 9% 19% 8% 1	2986
	21st to end	Day	33% 30% 12% 14% 10% 1	3960
January	1st to 10th	Night	25% 28% 5% 19% 19% 3	6186
	11th to 20th	Night	25% 26% 5% 20% 17% 7%	5900
	21st to end	Night	27% 29% 6% 21% 14% 2	6490
February	1st to 10th	Night	17% 63% 6% 10% 3 1	2600
	11th to 20th	Night	17% 61% 3 9% 8% 2	2499
	21st to end	Night	17% 62% 4 9% 6% 3	1980
March	1st to 10th	Night	30% 31% 5% 17% 9% 8%	4946
	11th to 20th	Night	21% 31% 9% 21% 14% 5%	4980
	21st to end	Night	28% 30% 7% 15% 17% 3	5405
April	1st to 10th	Night	17% 63% 4 8% 7% 1	2000
	11th to 20th	Night	15% 63% 5% 9% 6% 1	1975
	21st to end	Night	16% 62% 4 10% 7% 1	2000
May	1st to 10th	Night	37% 41% 6% 10% 5% 1	2977
	11th to 20th	Night	34% 43% 7% 13% 3	2723
	21st to end	Night	20% 44% 11% 17% 7% 2	2785
June	1st to 10th	Night	14% 66% 4 9% 7%	1520
	11th to 20th	Night	12% 67% 5% 10% 5%	1520
	21st to end	Night	12% 64% 6% 10% 7% 1	1520
July	1st to 10th	Night	24% 40% 14% 10% 9% 3	2480
	11th to 20th	Night	26% 39% 13% 15% 6%	2649
	21st to end	Night	31% 39% 8% 11% 10% 1	2988
August	1st to 10th	Night	32% 40% 12% 12% 3 1	2814
	11th to 20th	Night	25% 40% 8% 19% 8%	2926
	21st to end	Night	18% 38% 8% 23% 12%	3476
September	1st to 10th	Night	15% 61% 7% 10% 6% 2	1900
	11th to 20th	Night	14% 60% 5% 11% 9% 2	1900
	21st to end	Night	21% 61% 4 9% 5%	2188
October	1st to 10th	Night	26% 38% 7% 13% 14% 3	3800
	11th to 20th	Night	11% 37% 7% 13% 24% 7%	3779
	21st to end	Night	11% 36% 6% 23% 18% 5%	4120
November	1st to 10th	Night	15% 56% 7% 11% 9% 3	2421
	11th to 20th	Night	14% 57% 3 9% 13% 3	2644
	21st to end	Night	14% 57% 6% 13% 8% 2	2800
December	1st to 10th	Night	20% 36% 10% 19% 13% 1	4580
	11th to 20th	Night	32% 32% 8% 13% 11% 3	5208
	21st to end	Night	34% 31% 11% 13% 11%	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	33% 25% 28% 12% 1 1	235
	11th to 20th	Day	35% 23% 29% 11% 2	300
	21st to end	Day	23% 28% 34% 11% 3 1	330
February	1st to 10th	Day	25% 26% 34% 11% 4	300
	11th to 20th	Day	36% 25% 33% 4 2	335
	21st to end	Day	28% 20% 32% 18% 2	414
March	1st to 10th	Day	24% 24% 31% 18% 4	500
	11th to 20th	Day	27% 22% 29% 16% 5% 2	500
	21st to end	Day	24% 20% 33% 18% 5% 1	550
April	1st to 10th	Day	26% 22% 29% 16% 5% 2	500
	11th to 20th	Day	19% 21% 28% 17% 12% 3	492
	21st to end	Day	26% 14% 32% 18% 9% 2	500
May	1st to 10th	Day	22% 17% 30% 19% 9% 3	503
	11th to 20th	Day	21% 12% 29% 22% 13% 3	527
	21st to end	Day	17% 8% 26% 26% 19% 5%	549
June	1st to 10th	Day	14% 8% 28% 24% 20% 5%	490
	11th to 20th	Day	13% 8% 26% 32% 17% 5%	490
	21st to end	Day	10% 6% 29% 32% 18% 5%	490
July	1st to 10th	Day	11% 4 17% 32% 26% 10%	485
	11th to 20th	Day	6% 3 20% 32% 31% 7%	484
	21st to end	Day	12% 3 22% 32% 24% 7%	532
August	1st to 10th	Day	12% 6% 20% 27% 25% 12%	486
	11th to 20th	Day	10% 5% 19% 33% 22% 10%	479
	21st to end	Day	9% 5% 20% 31% 22% 13%	539
September	1st to 10th	Day	17% 7% 21% 22% 22% 11%	490
	11th to 20th	Day	13% 4 20% 30% 20% 11%	490
	21st to end	Day	27% 9% 24% 24% 11% 4	490
October	1st to 10th	Day	22% 8% 20% 26% 18% 7%	490
	11th to 20th	Day	16% 15% 27% 25% 14% 3	490
	21st to end	Day	17% 12% 33% 22% 12% 3	539
November	1st to 10th	Day	16% 11% 34% 25% 12% 2	453
	11th to 20th	Day	20% 15% 35% 21% 9% 1	355
	21st to end	Day	24% 21% 32% 15% 9% 1	200
December	1st to 10th	Day	22% 22% 37% 15% 4 1	200
	11th to 20th	Day	28% 20% 26% 21% 6%	200
	21st to end	Day	26% 20% 30% 18% 6%	220
January	1st to 10th	Night	22% 19% 40% 16% 3	800
	11th to 20th	Night	23% 17% 33% 24% 3	800
	21st to end	Night	20% 18% 40% 21% 1	880
February	1st to 10th	Night	19% 17% 37% 25% 2	800
	11th to 20th	Night	26% 22% 36% 15% 1	783
	21st to end	Night	24% 18% 37% 18% 3	660
March	1st to 10th	Night	21% 18% 37% 21% 3	800
	11th to 20th	Night	23% 20% 33% 19% 5%	800
	21st to end	Night	19% 19% 38% 20% 4 1	880
April	1st to 10th	Night	19% 22% 34% 21% 4	800
	11th to 20th	Night	16% 16% 35% 26% 6% 1	791
	21st to end	Night	19% 13% 34% 27% 7%	800
May	1st to 10th	Night	21% 9% 28% 32% 10%	634
	11th to 20th	Night	21% 7% 28% 34% 10%	489
	21st to end	Night	13% 5% 25% 38% 17% 1	423
June	1st to 10th	Night	10% 4 26% 37% 22%	380
	11th to 20th	Night	11% 6% 23% 42% 17% 1	380
	21st to end	Night	9% 3 26% 45% 17% 1	380
July	1st to 10th	Night	11% 2 18% 41% 24% 3	370
	11th to 20th	Night	6% 1 14% 39% 37% 2	467
	21st to end	Night	10% 2 17% 42% 26% 3	624
August	1st to 10th	Night	13% 1 12% 43% 28% 3	567
	11th to 20th	Night	12% 4 14% 42% 26% 3	626
	21st to end	Night	12% 3 16% 39% 26% 4	836
September	1st to 10th	Night	15% 3 21% 31% 26% 3	760
	11th to 20th	Night	10% 4 18% 36% 29% 3	760
	21st to end	Night	24% 4 22% 30% 20% 1	760
October	1st to 10th	Night	22% 6% 21% 29% 19% 3	760
	11th to 20th	Night	16% 12% 28% 29% 14% 1	760
	21st to end	Night	18% 11% 32% 27% 12%	836
November	1st to 10th	Night	13% 11% 32% 33% 10% 1	780
	11th to 20th	Night	18% 16% 32% 28% 6% 1	800
	21st to end	Night	17% 16% 33% 28% 7%	800
December	1st to 10th	Night	22% 16% 33% 22% 6%	800
	11th to 20th	Night	19% 14% 38% 25% 4	800
	21st to end	Night	20% 15% 35% 28% 3	880

Page generated at 2025-05-23 09:20:59. Version ac07

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