Blue Heron Cove Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Heron Cove Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	17% 11% 4 9% 6% 5% 5% 4 5% 6% 28%	131118
	Night	17% 11% 5% 11% 6% 5% 4 3 5% 6% 28%	130055
Transparency	Day	51% 5% 12% 20% 10% 1	134582
	Night	52% 5% 12% 19% 10% 1	134678
Seeing	Day	27% 14% 28% 19% 9% 3	16318
	Night	24% 14% 27% 25% 9% 1	25536

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	11% 14% 8% 21% 11% 6% 4 3 4 3 15%	4661
	11th to 20th	Day	10% 17% 9% 18% 10% 9% 4 3 4 4 13%	4650
	21st to end	Day	12% 14% 6% 13% 8% 6% 5% 4 4 6% 22%	5382
February	1st to 10th	Day	13% 15% 5% 13% 9% 6% 5% 4 5% 5% 20%	1890
	11th to 20th	Day	12% 15% 7% 16% 8% 4 5% 4 5% 4 21%	1966
	21st to end	Day	18% 15% 6% 11% 7% 4 4 3 5% 5% 22%	1730
March	1st to 10th	Day	20% 15% 7% 13% 5% 4 4 2 4 3 23%	4496
	11th to 20th	Day	12% 14% 6% 10% 5% 4 4 3 5% 5% 31%	4829
	21st to end	Day	20% 13% 4 10% 5% 4 4 3 4 4 31%	5892
April	1st to 10th	Day	19% 15% 3 9% 4 4 4 3 4 5% 30%	2400
	11th to 20th	Day	18% 11% 4 8% 5% 4 4 4 4 6% 32%	2374
	21st to end	Day	20% 12% 4 8% 4 5% 4 4 4 5% 30%	2464
May	1st to 10th	Day	33% 11% 3 7% 4 4 4 3 4 5% 22%	6158
	11th to 20th	Day	19% 11% 3 8% 5% 5% 5% 4 6% 6% 28%	6318
	21st to end	Day	22% 9% 3 7% 5% 5% 5% 4 6% 7% 28%	6949
June	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 9% 29%	2800
	11th to 20th	Day	19% 7% 3 7% 5% 4 5% 5% 7% 9% 30%	2800
	21st to end	Day	19% 7% 3 8% 5% 5% 5% 5% 6% 8% 30%	2800
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	6360
	11th to 20th	Day	12% 6% 2 7% 5% 6% 6% 5% 7% 10% 34%	6360
	21st to end	Day	15% 5% 2 6% 5% 5% 6% 4 6% 7% 39%	6926
August	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 4 7% 8% 34%	4498
	11th to 20th	Day	17% 8% 2 7% 5% 6% 6% 5% 7% 9% 27%	3922
	21st to end	Day	17% 5% 2 6% 4 5% 6% 4 7% 8% 36%	4312
September	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 4 6% 7% 41%	2400
	11th to 20th	Day	15% 5% 3 5% 4 4 4 4 6% 7% 43%	2340
	21st to end	Day	19% 7% 3 6% 5% 5% 4 4 6% 6% 36%	2200
October	1st to 10th	Day	23% 10% 3 8% 4 5% 4 3 4 5% 32%	2035
	11th to 20th	Day	18% 10% 4 8% 4 4 5% 3 5% 7% 33%	2000
	21st to end	Day	24% 14% 3 7% 4 4 5% 4 5% 5% 26%	2016
November	1st to 10th	Day	14% 13% 3 6% 5% 5% 4 4 6% 6% 32%	1851
	11th to 20th	Day	15% 13% 6% 9% 4 4 4 3 5% 7% 29%	1865
	21st to end	Day	18% 16% 6% 9% 5% 5% 4 3 4 6% 24%	1680
December	1st to 10th	Day	17% 22% 6% 18% 5% 4 4 3 4 4 13%	2680
	11th to 20th	Day	20% 18% 7% 12% 6% 3 3 3 4 5% 19%	3066
	21st to end	Day	18% 17% 6% 14% 5% 4 4 3 4 5% 20%	4048
January	1st to 10th	Night	12% 13% 9% 19% 10% 5% 4 3 4 4 16%	8234
	11th to 20th	Night	10% 13% 10% 21% 12% 7% 4 3 4 4 11%	8235
	21st to end	Night	11% 12% 6% 12% 7% 9% 7% 5% 5% 6% 21%	8782
February	1st to 10th	Night	12% 13% 5% 14% 8% 6% 6% 3 6% 5% 21%	2730
	11th to 20th	Night	12% 11% 5% 14% 8% 6% 5% 5% 5% 6% 22%	2538
	21st to end	Night	14% 13% 6% 12% 7% 5% 4 3 5% 7% 23%	2076
March	1st to 10th	Night	19% 12% 5% 12% 6% 6% 4 4 5% 6% 22%	5389
	11th to 20th	Night	16% 12% 5% 8% 5% 4 4 3 6% 6% 32%	5127
	21st to end	Night	20% 10% 5% 13% 5% 4 4 3 4 4 27%	5038
April	1st to 10th	Night	20% 12% 4 7% 4 4 4 3 5% 7% 29%	2000
	11th to 20th	Night	17% 10% 3 7% 6% 3 5% 3 6% 6% 34%	1975
	21st to end	Night	20% 9% 3 7% 5% 3 4 4 4 5% 36%	1935
May	1st to 10th	Night	32% 12% 3 7% 4 4 3 2 4 5% 22%	3824
	11th to 20th	Night	17% 13% 3 9% 5% 4 4 3 5% 6% 31%	3575
	21st to end	Night	20% 11% 4 8% 4 5% 5% 3 5% 6% 29%	3926
June	1st to 10th	Night	18% 7% 2 6% 5% 5% 5% 3 5% 7% 37%	1600
	11th to 20th	Night	20% 6% 2 5% 3 4 4 4 5% 9% 40%	1600
	21st to end	Night	17% 6% 2 6% 4 4 5% 4 6% 7% 39%	1600
July	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	3600
	11th to 20th	Night	20% 7% 3 6% 4 4 4 4 5% 6% 38%	3600
	21st to end	Night	11% 6% 2 5% 4 5% 4 3 5% 7% 49%	3926
August	1st to 10th	Night	23% 8% 2 6% 4 4 4 3 5% 6% 35%	2646
	11th to 20th	Night	20% 9% 2 6% 3 3 3 3 5% 5% 41%	3094
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 8% 44%	3586
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 7% 44%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 4 7% 50%	2059
	21st to end	Night	18% 6% 2 6% 4 4 5% 4 5% 7% 40%	2200
October	1st to 10th	Night	20% 9% 3 8% 5% 4 4 3 5% 6% 34%	2362
	11th to 20th	Night	18% 9% 3 8% 4 5% 3 3 5% 7% 35%	2400
	21st to end	Night	22% 12% 3 7% 5% 4 5% 4 6% 6% 27%	2822
November	1st to 10th	Night	14% 10% 4 8% 5% 4 4 3 5% 5% 36%	2667
	11th to 20th	Night	17% 12% 5% 9% 5% 4 4 4 5% 6% 30%	2751
	21st to end	Night	15% 14% 5% 10% 5% 5% 4 3 6% 6% 27%	2940
December	1st to 10th	Night	15% 18% 7% 17% 6% 4 4 3 4 5% 16%	4720
	11th to 20th	Night	22% 16% 6% 12% 6% 4 4 3 4 4 19%	5348
	21st to end	Night	23% 16% 5% 11% 5% 4 3 3 4 5% 22%	7150

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	65% 7% 10% 13% 5% 1	4662
	11th to 20th	Day	67% 6% 8% 9% 8% 2	4680
	21st to end	Day	55% 6% 11% 20% 7% 1	5382
February	1st to 10th	Day	63% 3 11% 17% 6% 1	1890
	11th to 20th	Day	64% 4 8% 15% 8% 1	1975
	21st to end	Day	62% 3 11% 15% 8%	1730
March	1st to 10th	Day	60% 5% 10% 13% 10% 3	4499
	11th to 20th	Day	53% 4 19% 13% 8% 3	4829
	21st to end	Day	54% 4 6% 15% 18% 2	5892
April	1st to 10th	Day	55% 4 7% 20% 12% 2	2400
	11th to 20th	Day	52% 4 10% 21% 12% 1	2374
	21st to end	Day	54% 5% 8% 18% 12% 3	2464
May	1st to 10th	Day	64% 4 7% 16% 8% 1	6169
	11th to 20th	Day	53% 5% 12% 20% 9% 1	6322
	21st to end	Day	52% 6% 12% 22% 8% 1	6949
June	1st to 10th	Day	50% 6% 12% 21% 11%	2800
	11th to 20th	Day	47% 5% 15% 22% 10% 1	2800
	21st to end	Day	47% 6% 14% 23% 10% 1	2800
July	1st to 10th	Day	40% 7% 15% 25% 12%	6290
	11th to 20th	Day	39% 7% 15% 26% 12%	6285
	21st to end	Day	39% 6% 13% 32% 10%	6728
August	1st to 10th	Day	44% 6% 16% 21% 12% 1	4484
	11th to 20th	Day	46% 7% 17% 24% 5%	3960
	21st to end	Day	39% 7% 14% 31% 9% 1	4312
September	1st to 10th	Day	40% 6% 12% 30% 12% 1	2400
	11th to 20th	Day	37% 4 13% 28% 17% 1	2340
	21st to end	Day	46% 4 11% 25% 12% 2	2200
October	1st to 10th	Day	45% 6% 10% 25% 13% 1	3308
	11th to 20th	Day	41% 4 14% 25% 14% 2	3240
	21st to end	Day	40% 6% 15% 27% 11% 1	3228
November	1st to 10th	Day	46% 5% 14% 23% 12% 1	1851
	11th to 20th	Day	52% 4 12% 18% 13% 1	1865
	21st to end	Day	59% 4 15% 16% 6% 1	1680
December	1st to 10th	Day	66% 7% 12% 12% 3	2680
	11th to 20th	Day	62% 5% 12% 16% 5%	3066
	21st to end	Day	64% 5% 7% 14% 9% 1	4048
January	1st to 10th	Night	64% 6% 12% 11% 7%	8234
	11th to 20th	Night	66% 7% 9% 9% 7% 1	8280
	21st to end	Night	52% 6% 11% 18% 10% 2	8782
February	1st to 10th	Night	59% 3 12% 17% 7% 1	2730
	11th to 20th	Night	57% 4 11% 19% 8% 1	2540
	21st to end	Night	57% 4 12% 18% 9% 1	2076
March	1st to 10th	Night	57% 5% 11% 15% 10% 1	5399
	11th to 20th	Night	50% 4 17% 18% 8% 3	5128
	21st to end	Night	55% 4 6% 18% 14% 2	5038
April	1st to 10th	Night	53% 3 10% 22% 10% 1	2000
	11th to 20th	Night	50% 4 11% 21% 13% 1	1975
	21st to end	Night	50% 3 10% 19% 15% 3	1935
May	1st to 10th	Night	62% 5% 9% 13% 9% 1	3830
	11th to 20th	Night	52% 6% 12% 22% 7% 1	3576
	21st to end	Night	54% 5% 11% 18% 11% 1	3927
June	1st to 10th	Night	44% 5% 13% 25% 12% 2	1600
	11th to 20th	Night	42% 4 16% 25% 12% 1	1600
	21st to end	Night	41% 7% 15% 26% 12%	1600
July	1st to 10th	Night	41% 7% 16% 22% 14% 1	3560
	11th to 20th	Night	44% 7% 13% 20% 16%	3552
	21st to end	Night	32% 6% 13% 35% 13% 1	3809
August	1st to 10th	Night	47% 7% 13% 23% 9%	2636
	11th to 20th	Night	44% 5% 15% 27% 9%	3122
	21st to end	Night	33% 5% 16% 35% 10%	3586
September	1st to 10th	Night	37% 5% 14% 30% 13% 1	2000
	11th to 20th	Night	35% 4 12% 29% 19% 1	2059
	21st to end	Night	42% 4 12% 26% 14% 2	2200
October	1st to 10th	Night	47% 6% 12% 22% 12% 1	3869
	11th to 20th	Night	38% 4 15% 23% 15% 5%	3899
	21st to end	Night	44% 5% 14% 27% 9% 1	4560
November	1st to 10th	Night	46% 4 12% 23% 13% 2	2667
	11th to 20th	Night	51% 5% 10% 21% 12% 2	2751
	21st to end	Night	52% 4 16% 20% 7% 1	2940
December	1st to 10th	Night	59% 8% 16% 14% 4	4720
	11th to 20th	Night	62% 5% 10% 15% 8%	5348
	21st to end	Night	63% 4 8% 12% 11% 2	7150

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	32% 17% 33% 13% 4 1	220
	11th to 20th	Day	39% 23% 28% 8% 2	220
	21st to end	Day	31% 23% 30% 11% 3	289
February	1st to 10th	Day	36% 24% 30% 9% 2	330
	11th to 20th	Day	37% 25% 28% 9% 2	454
	21st to end	Day	33% 23% 29% 13% 2	446
March	1st to 10th	Day	31% 19% 33% 13% 3 1	508
	11th to 20th	Day	34% 21% 27% 14% 3 1	500
	21st to end	Day	35% 20% 27% 12% 5% 1	550
April	1st to 10th	Day	38% 19% 23% 13% 5% 1	500
	11th to 20th	Day	35% 20% 21% 16% 7% 1	492
	21st to end	Day	32% 16% 30% 14% 5% 2	500
May	1st to 10th	Day	36% 13% 28% 14% 6% 3	503
	11th to 20th	Day	21% 15% 34% 21% 8% 2	527
	21st to end	Day	22% 10% 29% 24% 12% 3	573
June	1st to 10th	Day	23% 13% 27% 24% 12% 2	530
	11th to 20th	Day	21% 12% 32% 23% 10% 2	530
	21st to end	Day	22% 15% 30% 23% 8% 2	530
July	1st to 10th	Day	16% 9% 30% 26% 17% 3	525
	11th to 20th	Day	18% 5% 31% 29% 14% 3	524
	21st to end	Day	18% 9% 36% 24% 11% 2	576
August	1st to 10th	Day	13% 9% 25% 28% 18% 7%	526
	11th to 20th	Day	20% 8% 24% 27% 14% 8%	519
	21st to end	Day	16% 10% 29% 26% 13% 6%	583
September	1st to 10th	Day	18% 8% 23% 24% 18% 8%	530
	11th to 20th	Day	14% 6% 25% 25% 18% 12%	530
	21st to end	Day	21% 7% 27% 25% 14% 5%	530
October	1st to 10th	Day	28% 11% 24% 18% 14% 5%	530
	11th to 20th	Day	23% 18% 27% 22% 9% 1	530
	21st to end	Day	32% 13% 27% 21% 6%	481
November	1st to 10th	Day	31% 15% 25% 18% 10% 1	425
	11th to 20th	Day	30% 21% 26% 17% 4 1	405
	21st to end	Day	42% 15% 28% 13% 2	220
December	1st to 10th	Day	35% 20% 30% 11% 4	220
	11th to 20th	Day	40% 17% 31% 7% 5%	220
	21st to end	Day	38% 14% 29% 17% 2 1	242
January	1st to 10th	Night	28% 22% 31% 17% 2	840
	11th to 20th	Night	29% 22% 28% 18% 3	840
	21st to end	Night	23% 21% 31% 22% 2	924
February	1st to 10th	Night	26% 20% 33% 19% 2	840
	11th to 20th	Night	27% 22% 35% 14% 2	823
	21st to end	Night	29% 19% 33% 17% 3	692
March	1st to 10th	Night	26% 19% 33% 19% 3	809
	11th to 20th	Night	29% 16% 32% 19% 3	800
	21st to end	Night	26% 18% 30% 21% 5% 1	880
April	1st to 10th	Night	29% 15% 34% 19% 3	800
	11th to 20th	Night	26% 15% 32% 22% 6% 1	791
	21st to end	Night	27% 15% 27% 23% 8%	735
May	1st to 10th	Night	33% 10% 26% 20% 11% 1	500
	11th to 20th	Night	21% 14% 28% 29% 8% 1	394
	21st to end	Night	19% 6% 26% 34% 14% 1	433
June	1st to 10th	Night	19% 9% 24% 33% 14% 2	400
	11th to 20th	Night	20% 9% 22% 35% 15%	400
	21st to end	Night	18% 6% 26% 37% 12% 1	400
July	1st to 10th	Night	13% 4 22% 41% 20% 1	390
	11th to 20th	Night	16% 3 18% 42% 20% 1	388
	21st to end	Night	12% 3 26% 42% 14% 1	438
August	1st to 10th	Night	14% 3 19% 40% 22% 1	432
	11th to 20th	Night	18% 4 18% 35% 22% 3	723
	21st to end	Night	16% 5% 20% 37% 19% 2	880
September	1st to 10th	Night	15% 7% 21% 36% 21% 1	800
	11th to 20th	Night	16% 5% 20% 31% 24% 4	800
	21st to end	Night	20% 7% 24% 31% 17% 2	800
October	1st to 10th	Night	25% 11% 22% 24% 16% 2	800
	11th to 20th	Night	23% 13% 26% 28% 10% 1	800
	21st to end	Night	28% 14% 31% 22% 5%	880
November	1st to 10th	Night	22% 15% 26% 28% 8%	820
	11th to 20th	Night	26% 18% 25% 23% 6% 1	840
	21st to end	Night	27% 19% 26% 22% 5% 1	840
December	1st to 10th	Night	29% 19% 32% 18% 3	840
	11th to 20th	Night	33% 19% 27% 16% 4	840
	21st to end	Night	33% 22% 26% 16% 3	924

Page generated at 2026-02-28 01:40:22. Version ac07

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