Blue Heron Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Heron Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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% 11% 4 9% 5% 5% 4 3 5% 7% 33%	143885
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 27%	138129
Transparency	Day	34% 35% 8% 15% 7% 1	147405
	Night	38% 36% 7% 12% 7% 1	142672
Seeing	Day	20% 12% 28% 25% 12% 3	17561
	Night	21% 13% 27% 25% 12% 2	26718

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	13% 19% 11% 19% 10% 5% 5% 3 3 3 9%	4136
	11th to 20th	Day	11% 23% 11% 26% 10% 6% 3 2 2 2 4	4679
	21st to end	Day	11% 15% 9% 19% 10% 8% 5% 3 5% 4 10%	5756
February	1st to 10th	Day	13% 20% 7% 16% 9% 5% 4 3 3 5% 14%	1890
	11th to 20th	Day	12% 16% 6% 13% 9% 6% 6% 3 5% 7% 17%	1843
	21st to end	Day	13% 17% 6% 12% 7% 6% 5% 4 5% 5% 21%	1697
March	1st to 10th	Day	16% 12% 4 10% 5% 5% 4 4 5% 5% 31%	4854
	11th to 20th	Day	12% 14% 6% 11% 6% 6% 5% 4 5% 6% 25%	5090
	21st to end	Day	18% 12% 5% 9% 5% 5% 4 3 4 5% 29%	5586
April	1st to 10th	Day	19% 12% 4 7% 5% 4 4 4 4 6% 32%	2434
	11th to 20th	Day	19% 9% 3 5% 4 4 3 3 5% 5% 40%	2702
	21st to end	Day	18% 8% 3 7% 4 3 4 4 6% 7% 36%	2730
May	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 6% 8% 41%	7642
	11th to 20th	Day	12% 8% 3 7% 4 4 4 4 5% 7% 42%	7939
	21st to end	Day	17% 7% 2 6% 4 3 4 3 4 6% 43%	9060
June	1st to 10th	Day	12% 7% 3 6% 4 4 4 4 6% 7% 42%	3149
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 4 6% 9% 43%	3150
	21st to end	Day	14% 7% 2 6% 4 4 4 4 6% 7% 41%	3150
July	1st to 10th	Day	11% 7% 3 7% 5% 4 5% 4 6% 9% 39%	8879
	11th to 20th	Day	9% 5% 2 5% 3 4 3 3 5% 9% 53%	8706
	21st to end	Day	8% 5% 2 5% 4 4 4 4 6% 8% 51%	8458
August	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 9% 45%	4364
	11th to 20th	Day	13% 7% 2 6% 3 4 4 4 6% 8% 43%	4226
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 46%	4675
September	1st to 10th	Day	11% 6% 2 7% 5% 4 5% 3 6% 8% 42%	2358
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 5% 8% 47%	2200
	21st to end	Day	17% 8% 3 6% 5% 4 5% 3 4 8% 36%	2200
October	1st to 10th	Day	16% 9% 4 7% 4 4 4 3 5% 6% 36%	2039
	11th to 20th	Day	24% 13% 4 9% 4 4 4 3 5% 6% 24%	2000
	21st to end	Day	26% 15% 4 9% 5% 5% 5% 3 4 6% 19%	2095
November	1st to 10th	Day	20% 15% 6% 10% 6% 5% 4 4 5% 6% 21%	1814
	11th to 20th	Day	18% 18% 5% 11% 6% 5% 5% 4 5% 6% 17%	1680
	21st to end	Day	21% 25% 6% 13% 6% 4 4 3 4 4 10%	1680
December	1st to 10th	Day	16% 29% 8% 13% 6% 5% 4 3 4 3 7%	2680
	11th to 20th	Day	21% 23% 6% 17% 6% 5% 3 3 3 4 8%	2791
	21st to end	Day	26% 22% 8% 15% 5% 7% 3 2 3 4 5%	3553
January	1st to 10th	Night	12% 19% 10% 20% 9% 5% 6% 4 3 4 8%	8774
	11th to 20th	Night	11% 23% 9% 22% 9% 6% 4 2 3 3 7%	8236
	21st to end	Night	10% 16% 8% 19% 10% 8% 5% 3 4 4 14%	8423
February	1st to 10th	Night	11% 21% 6% 17% 8% 4 4 3 5% 5% 15%	2730
	11th to 20th	Night	13% 18% 6% 15% 7% 5% 5% 4 5% 5% 19%	2663
	21st to end	Night	15% 15% 5% 12% 6% 5% 6% 3 4 6% 23%	2107
March	1st to 10th	Night	14% 11% 4 10% 6% 5% 4 3 5% 6% 32%	5206
	11th to 20th	Night	12% 11% 4 9% 5% 5% 5% 4 6% 7% 31%	5089
	21st to end	Night	20% 11% 4 9% 5% 5% 4 3 4 5% 30%	5588
April	1st to 10th	Night	19% 11% 3 8% 4 4 4 3 5% 6% 32%	2204
	11th to 20th	Night	18% 9% 3 5% 4 4 5% 3 6% 7% 36%	1867
	21st to end	Night	19% 9% 4 8% 5% 4 5% 3 5% 6% 33%	1890
May	1st to 10th	Night	13% 7% 3 8% 5% 6% 5% 4 6% 7% 37%	5257
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 42%	4919
	21st to end	Night	17% 6% 2 7% 4 4 4 3 4 6% 42%	5098
June	1st to 10th	Night	14% 7% 2 5% 6% 5% 4 3 5% 8% 41%	1470
	11th to 20th	Night	10% 6% 2 5% 5% 4 6% 5% 6% 10% 41%	1470
	21st to end	Night	12% 6% 3 7% 4 5% 5% 5% 5% 9% 39%	1470
July	1st to 10th	Night	10% 7% 3 7% 5% 4 4 4 5% 8% 43%	4110
	11th to 20th	Night	10% 4 2 5% 4 3 5% 4 6% 8% 50%	4280
	21st to end	Night	9% 6% 2 5% 4 4 5% 3 6% 9% 48%	5622
August	1st to 10th	Night	10% 7% 3 6% 4 4 5% 4 6% 7% 45%	3005
	11th to 20th	Night	11% 6% 3 6% 4 5% 5% 5% 5% 7% 43%	2911
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 8% 47%	3223
September	1st to 10th	Night	11% 4 2 6% 4 3 5% 4 6% 8% 46%	2018
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 6% 6% 49%	2200
	21st to end	Night	17% 8% 3 7% 5% 5% 4 3 5% 7% 38%	2200
October	1st to 10th	Night	18% 7% 3 8% 4 5% 4 3 5% 7% 36%	2360
	11th to 20th	Night	23% 12% 4 9% 5% 5% 4 3 5% 6% 24%	2400
	21st to end	Night	25% 12% 5% 9% 5% 5% 5% 3 5% 6% 20%	2740
November	1st to 10th	Night	20% 14% 4 9% 6% 5% 4 4 4 6% 24%	2703
	11th to 20th	Night	17% 19% 5% 12% 6% 4 4 4 5% 6% 17%	2940
	21st to end	Night	23% 22% 7% 11% 6% 5% 4 3 4 4 11%	2940
December	1st to 10th	Night	17% 25% 7% 15% 7% 5% 4 3 4 4 8%	4720
	11th to 20th	Night	17% 24% 8% 17% 7% 6% 4 3 4 4 7%	5629
	21st to end	Night	23% 23% 7% 15% 6% 7% 4 2 3 3 6%	7667

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	63% 24% 5% 6% 2	4137
	11th to 20th	Day	74% 20% 2 3 1	4710
	21st to end	Day	60% 27% 5% 6% 3	5756
February	1st to 10th	Day	29% 61% 3 5% 2	1890
	11th to 20th	Day	28% 59% 6% 4 3	1852
	21st to end	Day	27% 60% 4 6% 3	1697
March	1st to 10th	Day	41% 31% 10% 8% 10% 1	4857
	11th to 20th	Day	43% 31% 8% 12% 4 1	5090
	21st to end	Day	41% 31% 10% 12% 4 1	5586
April	1st to 10th	Day	24% 59% 4 9% 4	2434
	11th to 20th	Day	21% 59% 3 10% 6% 1	2702
	21st to end	Day	19% 59% 6% 10% 5% 1	2730
May	1st to 10th	Day	32% 25% 10% 20% 12% 2	7651
	11th to 20th	Day	32% 25% 9% 18% 14% 1	7943
	21st to end	Day	34% 23% 8% 20% 13% 2	9061
June	1st to 10th	Day	17% 60% 3 11% 8% 1	3149
	11th to 20th	Day	18% 60% 4 11% 6% 1	3150
	21st to end	Day	18% 58% 5% 11% 7% 1	3150
July	1st to 10th	Day	34% 23% 13% 20% 10% 1	8806
	11th to 20th	Day	23% 20% 11% 35% 11%	8625
	21st to end	Day	24% 21% 11% 29% 14% 1	8273
August	1st to 10th	Day	22% 36% 10% 20% 10% 1	4351
	11th to 20th	Day	28% 38% 10% 18% 6%	4265
	21st to end	Day	21% 38% 10% 25% 6%	4674
September	1st to 10th	Day	18% 59% 7% 11% 5%	2358
	11th to 20th	Day	13% 58% 5% 17% 7%	2200
	21st to end	Day	18% 58% 7% 12% 5%	2200
October	1st to 10th	Day	23% 36% 8% 21% 12%	3312
	11th to 20th	Day	31% 37% 8% 19% 6%	3240
	21st to end	Day	36% 37% 9% 14% 4	3358
November	1st to 10th	Day	32% 54% 5% 6% 2	1814
	11th to 20th	Day	31% 56% 7% 5% 1	1680
	21st to end	Day	34% 56% 5% 4 1	1680
December	1st to 10th	Day	52% 37% 7% 3 1	2680
	11th to 20th	Day	56% 33% 6% 4 1	2791
	21st to end	Day	64% 28% 3 4 1	3553
January	1st to 10th	Night	61% 26% 6% 6% 2	8774
	11th to 20th	Night	66% 23% 3 6% 2	8280
	21st to end	Night	57% 26% 5% 7% 5%	8423
February	1st to 10th	Night	28% 61% 3 6% 3	2730
	11th to 20th	Night	25% 58% 4 10% 3	2665
	21st to end	Night	25% 61% 4 6% 4	2107
March	1st to 10th	Night	37% 32% 9% 11% 8% 2	5216
	11th to 20th	Night	36% 30% 13% 14% 5% 1	5090
	21st to end	Night	42% 30% 7% 11% 9% 1	5588
April	1st to 10th	Night	26% 59% 3 8% 4	2204
	11th to 20th	Night	21% 60% 3 9% 6% 1	1867
	21st to end	Night	22% 60% 4 8% 6% 1	1890
May	1st to 10th	Night	35% 25% 6% 17% 13% 4	5264
	11th to 20th	Night	34% 25% 6% 18% 16% 1	4920
	21st to end	Night	35% 23% 6% 20% 14% 2	5099
June	1st to 10th	Night	14% 59% 3 11% 11% 2	1470
	11th to 20th	Night	16% 59% 5% 12% 7% 1	1470
	21st to end	Night	17% 58% 5% 12% 7% 1	1470
July	1st to 10th	Night	31% 23% 13% 20% 12% 2	4075
	11th to 20th	Night	25% 22% 7% 33% 13%	4238
	21st to end	Night	29% 21% 7% 25% 17% 1	5494
August	1st to 10th	Night	24% 36% 9% 20% 11% 1	2994
	11th to 20th	Night	30% 38% 12% 13% 7%	2938
	21st to end	Night	22% 38% 8% 24% 8%	3223
September	1st to 10th	Night	15% 58% 7% 14% 6%	2018
	11th to 20th	Night	13% 58% 6% 14% 10%	2200
	21st to end	Night	19% 58% 6% 11% 6%	2200
October	1st to 10th	Night	20% 37% 6% 24% 13%	3854
	11th to 20th	Night	29% 37% 10% 17% 6%	3899
	21st to end	Night	31% 38% 12% 15% 4	4414
November	1st to 10th	Night	31% 55% 5% 7% 2	2703
	11th to 20th	Night	29% 55% 8% 5% 2	2940
	21st to end	Night	36% 56% 5% 3 1	2940
December	1st to 10th	Night	48% 38% 9% 3 1	4720
	11th to 20th	Night	53% 34% 6% 6% 2	5628
	21st to end	Night	61% 29% 6% 4 1	7667

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	24% 24% 34% 14% 4 1	228
	11th to 20th	Day	24% 21% 34% 14% 6%	330
	21st to end	Day	28% 15% 33% 19% 5%	363
February	1st to 10th	Day	25% 25% 30% 17% 3	330
	11th to 20th	Day	25% 20% 35% 17% 3	330
	21st to end	Day	26% 28% 34% 11%	273
March	1st to 10th	Day	22% 19% 33% 22% 5%	451
	11th to 20th	Day	24% 17% 33% 21% 5% 1	540
	21st to end	Day	29% 14% 30% 21% 6% 1	594
April	1st to 10th	Day	23% 14% 34% 22% 7% 1	540
	11th to 20th	Day	27% 15% 27% 17% 11% 4	532
	21st to end	Day	24% 14% 31% 23% 8% 1	540
May	1st to 10th	Day	20% 10% 32% 27% 9% 2	543
	11th to 20th	Day	17% 9% 27% 29% 14% 4	567
	21st to end	Day	16% 7% 25% 31% 16% 4	617
June	1st to 10th	Day	12% 7% 28% 38% 13% 2	779
	11th to 20th	Day	13% 8% 30% 33% 14% 2	780
	21st to end	Day	17% 7% 27% 34% 13% 2	780
July	1st to 10th	Day	9% 5% 26% 36% 22% 2	771
	11th to 20th	Day	12% 7% 25% 32% 18% 5%	768
	21st to end	Day	10% 6% 27% 32% 20% 4	661
August	1st to 10th	Day	13% 7% 23% 30% 22% 5%	566
	11th to 20th	Day	13% 6% 19% 34% 21% 7%	535
	21st to end	Day	14% 8% 26% 25% 22% 5%	583
September	1st to 10th	Day	17% 8% 23% 29% 18% 6%	530
	11th to 20th	Day	10% 5% 22% 30% 24% 9%	530
	21st to end	Day	19% 12% 21% 25% 18% 4	530
October	1st to 10th	Day	22% 14% 25% 23% 14% 2	530
	11th to 20th	Day	25% 20% 30% 18% 6% 1	530
	21st to end	Day	30% 19% 29% 19% 3 1	478
November	1st to 10th	Day	30% 18% 31% 17% 4	310
	11th to 20th	Day	25% 24% 32% 16% 2	220
	21st to end	Day	29% 23% 36% 11%	220
December	1st to 10th	Day	33% 21% 28% 12% 5%	220
	11th to 20th	Day	35% 23% 28% 13% 2	220
	21st to end	Day	37% 19% 35% 6% 2	242
January	1st to 10th	Night	24% 21% 32% 18% 4 1	1050
	11th to 20th	Night	27% 19% 30% 18% 5%	1050
	21st to end	Night	21% 15% 31% 23% 8% 1	933
February	1st to 10th	Night	19% 22% 34% 20% 5%	840
	11th to 20th	Night	21% 14% 35% 22% 8%	823
	21st to end	Night	22% 16% 35% 22% 4	692
March	1st to 10th	Night	18% 16% 34% 25% 6% 1	840
	11th to 20th	Night	20% 16% 34% 22% 8% 1	840
	21st to end	Night	22% 10% 29% 29% 10% 1	924
April	1st to 10th	Night	25% 13% 29% 27% 5% 1	824
	11th to 20th	Night	22% 12% 28% 26% 11% 2	623
	21st to end	Night	24% 12% 24% 26% 13% 1	630
May	1st to 10th	Night	17% 6% 25% 36% 15% 1	630
	11th to 20th	Night	17% 7% 22% 34% 20% 1	514
	21st to end	Night	16% 4 23% 36% 18% 3	455
June	1st to 10th	Night	16% 7% 24% 36% 15% 2	420
	11th to 20th	Night	13% 6% 22% 39% 18% 1	420
	21st to end	Night	13% 6% 22% 35% 22% 1	420
July	1st to 10th	Night	8% 4 17% 48% 21% 2	410
	11th to 20th	Night	11% 3 22% 36% 25% 3	461
	21st to end	Night	9% 4 19% 41% 25% 2	690
August	1st to 10th	Night	12% 3 19% 37% 26% 4	627
	11th to 20th	Night	12% 6% 16% 31% 29% 7%	606
	21st to end	Night	14% 4 21% 32% 25% 4	660
September	1st to 10th	Night	13% 7% 18% 33% 22% 7%	779
	11th to 20th	Night	12% 5% 20% 31% 24% 7%	800
	21st to end	Night	20% 8% 21% 25% 21% 6%	800
October	1st to 10th	Night	21% 9% 26% 25% 17% 2	800
	11th to 20th	Night	27% 16% 25% 23% 8% 2	800
	21st to end	Night	31% 14% 29% 21% 4 1	880
November	1st to 10th	Night	28% 17% 25% 22% 7% 1	820
	11th to 20th	Night	26% 22% 26% 18% 7%	840
	21st to end	Night	33% 19% 28% 16% 3	840
December	1st to 10th	Night	31% 23% 28% 12% 6%	840
	11th to 20th	Night	28% 23% 33% 13% 3	982
	21st to end	Night	34% 17% 31% 14% 4	1155

Page generated at 2026-08-12 03:37:20. Version ac07

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