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 2025-04-11.

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	15% 11% 4 9% 5% 5% 4 3 5% 6% 32%	113165
	Night	15% 14% 5% 12% 6% 5% 5% 3 5% 6% 26%	115178
Transparency	Day	31% 43% 7% 12% 6% 1	116728
	Night	36% 42% 7% 10% 6% 1	119753
Seeing	Day	19% 11% 27% 26% 13% 3	15684
	Night	21% 13% 27% 25% 12% 2	25115

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	12% 19% 12% 17% 12% 6% 5% 3 3 3 8%	3186
	11th to 20th	Day	11% 21% 11% 27% 10% 5% 4 3 2 2 4	3589
	21st to end	Day	15% 17% 9% 19% 8% 7% 5% 3 4 4 10%	4442
February	1st to 10th	Day	14% 20% 7% 17% 9% 5% 4 3 3 5% 13%	1800
	11th to 20th	Day	12% 16% 6% 13% 9% 6% 5% 3 5% 7% 18%	1753
	21st to end	Day	13% 17% 6% 12% 6% 6% 5% 4 5% 5% 21%	1618
March	1st to 10th	Day	16% 12% 4 10% 5% 5% 4 4 5% 5% 31%	4748
	11th to 20th	Day	12% 14% 6% 11% 6% 6% 5% 4 5% 6% 25%	4980
	21st to end	Day	18% 12% 5% 9% 5% 5% 4 3 5% 5% 29%	5465
April	1st to 10th	Day	19% 12% 3 7% 5% 4 4 4 4 6% 32%	2318
	11th to 20th	Day	19% 9% 3 6% 4 4 3 3 5% 5% 40%	2442
	21st to end	Day	19% 8% 3 7% 4 4 3 3 6% 6% 37%	2470
May	1st to 10th	Day	15% 8% 2 6% 3 4 4 3 5% 7% 42%	4108
	11th to 20th	Day	13% 8% 2 6% 4 4 4 4 6% 7% 44%	4254
	21st to end	Day	14% 6% 2 6% 4 3 4 3 4 7% 46%	4837
June	1st to 10th	Day	13% 7% 3 6% 4 4 4 5% 6% 8% 41%	2849
	11th to 20th	Day	11% 5% 2 5% 5% 4 5% 4 5% 9% 44%	2850
	21st to end	Day	14% 7% 2 6% 4 4 4 3 6% 7% 41%	2850
July	1st to 10th	Day	12% 7% 2 7% 5% 4 5% 4 6% 8% 39%	4760
	11th to 20th	Day	10% 5% 2 5% 3 4 3 4 5% 8% 52%	4670
	21st to end	Day	9% 5% 2 5% 3 3 3 4 6% 8% 52%	4529
August	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 9% 44%	4104
	11th to 20th	Day	13% 7% 2 6% 3 4 4 4 6% 8% 42%	4044
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 46%	4532
September	1st to 10th	Day	11% 7% 2 7% 4 4 5% 3 6% 8% 43%	2240
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 5% 8% 46%	2090
	21st to end	Day	17% 7% 3 6% 5% 4 5% 3 5% 8% 36%	2090
October	1st to 10th	Day	17% 9% 4 7% 4 4 4 4 6% 6% 34%	1937
	11th to 20th	Day	24% 13% 4 10% 4 4 4 3 5% 6% 22%	1900
	21st to end	Day	27% 15% 4 9% 4 5% 4 3 4 6% 18%	1990
November	1st to 10th	Day	19% 15% 5% 10% 6% 5% 4 4 5% 6% 21%	1726
	11th to 20th	Day	19% 18% 5% 11% 6% 5% 5% 4 5% 6% 17%	1600
	21st to end	Day	22% 24% 6% 13% 6% 4 4 3 4 4 10%	1600
December	1st to 10th	Day	17% 29% 8% 13% 7% 5% 4 3 4 3 7%	2600
	11th to 20th	Day	21% 23% 6% 17% 6% 5% 3 3 3 4 9%	2718
	21st to end	Day	26% 22% 8% 15% 5% 7% 3 2 3 4 5%	3476
January	1st to 10th	Night	12% 20% 10% 20% 10% 5% 6% 3 3 3 7%	6765
	11th to 20th	Night	13% 19% 8% 24% 8% 6% 4 3 4 4 8%	6316
	21st to end	Night	12% 18% 9% 19% 7% 7% 5% 3 4 4 12%	6512
February	1st to 10th	Night	11% 21% 6% 18% 8% 4 4 3 5% 5% 14%	2600
	11th to 20th	Night	12% 18% 6% 15% 6% 5% 5% 4 5% 5% 18%	2533
	21st to end	Night	14% 15% 5% 11% 6% 5% 6% 3 4 6% 24%	2010
March	1st to 10th	Night	15% 11% 4 10% 6% 5% 4 3 5% 6% 32%	5092
	11th to 20th	Night	11% 11% 4 9% 5% 5% 5% 4 6% 7% 32%	4979
	21st to end	Night	20% 11% 4 9% 5% 5% 4 3 4 5% 30%	5467
April	1st to 10th	Night	19% 11% 3 8% 4 4 4 3 5% 5% 33%	2099
	11th to 20th	Night	19% 9% 3 5% 4 3 5% 3 6% 7% 37%	1693
	21st to end	Night	20% 9% 3 8% 4 4 5% 3 5% 6% 33%	1710
May	1st to 10th	Night	14% 7% 3 7% 5% 6% 5% 4 5% 6% 38%	2833
	11th to 20th	Night	12% 6% 3 7% 5% 4 5% 3 5% 8% 41%	2637
	21st to end	Night	12% 6% 2 8% 4 5% 5% 4 5% 7% 44%	2722
June	1st to 10th	Night	14% 8% 2 5% 7% 5% 5% 3 5% 7% 40%	1330
	11th to 20th	Night	10% 6% 2 5% 4 3 6% 4 6% 10% 43%	1330
	21st to end	Night	12% 7% 3 6% 4 5% 5% 5% 5% 8% 40%	1330
July	1st to 10th	Night	10% 8% 3 6% 5% 3 4 3 5% 8% 45%	2210
	11th to 20th	Night	11% 5% 2 5% 3 3 4 4 5% 6% 50%	2296
	21st to end	Night	8% 6% 2 5% 4 4 4 3 5% 8% 51%	3023
August	1st to 10th	Night	10% 7% 3 5% 4 4 4 4 6% 7% 45%	2825
	11th to 20th	Night	11% 6% 3 6% 4 5% 5% 5% 5% 7% 42%	2785
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 8% 47%	3124
September	1st to 10th	Night	11% 4 2 6% 4 3 5% 4 6% 8% 47%	1917
	11th to 20th	Night	11% 5% 2 6% 4 5% 4 3 6% 7% 48%	2090
	21st to end	Night	17% 8% 3 7% 5% 5% 4 3 5% 7% 37%	2090
October	1st to 10th	Night	19% 8% 3 8% 4 5% 4 3 5% 6% 34%	2242
	11th to 20th	Night	23% 12% 4 9% 5% 5% 4 3 5% 6% 23%	2280
	21st to end	Night	26% 12% 5% 9% 5% 5% 5% 3 5% 5% 19%	2603
November	1st to 10th	Night	21% 14% 3 9% 6% 5% 4 4 4 6% 25%	2571
	11th to 20th	Night	18% 19% 5% 12% 6% 4 4 4 5% 5% 17%	2800
	21st to end	Night	23% 22% 7% 11% 6% 5% 4 3 4 5% 11%	2800
December	1st to 10th	Night	18% 25% 6% 14% 7% 5% 4 3 4 4 8%	4580
	11th to 20th	Night	17% 24% 8% 17% 7% 6% 4 3 4 4 7%	5482
	21st to end	Night	23% 24% 7% 15% 6% 7% 4 2 3 3 6%	7502

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	59% 30% 4 4 2	3187
	11th to 20th	Day	68% 26% 2 3 1	3620
	21st to end	Day	56% 32% 5% 5% 2	4442
February	1st to 10th	Day	28% 64% 3 5% 1	1800
	11th to 20th	Day	25% 61% 6% 4 3	1762
	21st to end	Day	25% 62% 4 6% 3	1618
March	1st to 10th	Day	41% 31% 9% 8% 10% 1	4751
	11th to 20th	Day	43% 32% 8% 12% 4	4980
	21st to end	Day	41% 32% 10% 12% 4 1	5465
April	1st to 10th	Day	23% 62% 4 8% 4	2318
	11th to 20th	Day	18% 65% 2 9% 4 1	2442
	21st to end	Day	16% 65% 5% 8% 5% 1	2470
May	1st to 10th	Day	25% 42% 7% 12% 13% 1	4120
	11th to 20th	Day	21% 44% 12% 16% 6% 1	4258
	21st to end	Day	24% 41% 8% 10% 14% 3	4837
June	1st to 10th	Day	14% 66% 3 10% 6% 1	2849
	11th to 20th	Day	15% 66% 3 9% 6% 1	2850
	21st to end	Day	15% 64% 4 9% 7% 1	2850
July	1st to 10th	Day	31% 38% 10% 13% 7% 1	4686
	11th to 20th	Day	19% 36% 11% 25% 9%	4589
	21st to end	Day	18% 36% 6% 24% 14% 1	4386
August	1st to 10th	Day	21% 38% 10% 19% 10% 1	4091
	11th to 20th	Day	28% 39% 10% 17% 6%	4083
	21st to end	Day	21% 39% 9% 25% 7%	4531
September	1st to 10th	Day	16% 62% 6% 10% 5%	2240
	11th to 20th	Day	13% 61% 4 15% 7%	2090
	21st to end	Day	16% 61% 7% 11% 4	2090
October	1st to 10th	Day	23% 37% 7% 20% 12%	3210
	11th to 20th	Day	31% 38% 8% 18% 6%	3140
	21st to end	Day	36% 38% 9% 13% 3	3253
November	1st to 10th	Day	31% 57% 5% 5% 2	1726
	11th to 20th	Day	29% 58% 6% 5% 1	1600
	21st to end	Day	32% 59% 5% 3 1	1600
December	1st to 10th	Day	51% 38% 7% 3 1	2600
	11th to 20th	Day	55% 34% 6% 4 1	2718
	21st to end	Day	63% 28% 3 4 1	3476
January	1st to 10th	Night	58% 32% 5% 4 1	6765
	11th to 20th	Night	59% 30% 4 5% 2	6360
	21st to end	Night	54% 31% 6% 5% 4 1	6512
February	1st to 10th	Night	27% 63% 3 5% 2	2600
	11th to 20th	Night	22% 61% 4 9% 3	2535
	21st to end	Night	22% 63% 4 6% 4	2010
March	1st to 10th	Night	37% 32% 10% 11% 9% 2	5102
	11th to 20th	Night	35% 31% 13% 14% 5% 1	4980
	21st to end	Night	42% 31% 7% 11% 9% 1	5467
April	1st to 10th	Night	24% 62% 2 7% 4	2099
	11th to 20th	Night	18% 66% 2 7% 6% 1	1693
	21st to end	Night	20% 66% 3 5% 5% 1	1710
May	1st to 10th	Night	25% 43% 8% 11% 12% 1	2840
	11th to 20th	Night	24% 43% 6% 14% 11% 1	2638
	21st to end	Night	25% 42% 7% 13% 11% 2	2723
June	1st to 10th	Night	12% 65% 2 10% 9% 1	1330
	11th to 20th	Night	12% 65% 5% 10% 7% 1	1330
	21st to end	Night	13% 64% 4 11% 7% 1	1330
July	1st to 10th	Night	27% 37% 9% 14% 10% 4	2175
	11th to 20th	Night	20% 37% 6% 27% 9%	2254
	21st to end	Night	22% 36% 6% 15% 20% 1	2927
August	1st to 10th	Night	24% 38% 9% 19% 10% 1	2814
	11th to 20th	Night	29% 39% 12% 12% 7%	2812
	21st to end	Night	22% 39% 7% 24% 9%	3124
September	1st to 10th	Night	14% 61% 6% 13% 6%	1917
	11th to 20th	Night	13% 61% 5% 12% 9%	2090
	21st to end	Night	18% 61% 6% 10% 5%	2090
October	1st to 10th	Night	20% 38% 6% 23% 12%	3736
	11th to 20th	Night	29% 38% 10% 17% 6%	3779
	21st to end	Night	31% 39% 12% 14% 3	4277
November	1st to 10th	Night	29% 57% 5% 7% 1	2571
	11th to 20th	Night	28% 58% 8% 4 2	2800
	21st to end	Night	34% 58% 5% 2 1	2800
December	1st to 10th	Night	47% 39% 10% 3 1	4580
	11th to 20th	Night	53% 35% 5% 6% 2	5481
	21st to end	Night	61% 29% 6% 4	7502

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	22% 22% 35% 16% 4 1	207
	11th to 20th	Day	22% 20% 35% 16% 7%	300
	21st to end	Day	29% 13% 33% 19% 6%	330
February	1st to 10th	Day	26% 25% 29% 17% 3	300
	11th to 20th	Day	24% 21% 35% 17% 3	300
	21st to end	Day	26% 28% 35% 10%	249
March	1st to 10th	Day	21% 17% 33% 24% 6%	415
	11th to 20th	Day	21% 16% 34% 22% 6% 1	500
	21st to end	Day	27% 12% 31% 23% 6% 1	550
April	1st to 10th	Day	22% 13% 35% 23% 7% 1	500
	11th to 20th	Day	26% 15% 26% 17% 11% 4	452
	21st to end	Day	24% 13% 29% 24% 9% 2	460
May	1st to 10th	Day	20% 6% 31% 30% 11% 2	463
	11th to 20th	Day	17% 6% 26% 31% 16% 4	487
	21st to end	Day	13% 5% 23% 36% 18% 5%	529
June	1st to 10th	Day	12% 5% 27% 39% 14% 3	679
	11th to 20th	Day	12% 6% 28% 36% 16% 3	680
	21st to end	Day	16% 6% 24% 36% 14% 3	680
July	1st to 10th	Day	9% 4 24% 36% 24% 3	671
	11th to 20th	Day	12% 5% 23% 34% 20% 6%	668
	21st to end	Day	10% 5% 24% 34% 22% 5%	569
August	1st to 10th	Day	13% 7% 20% 30% 24% 6%	486
	11th to 20th	Day	13% 5% 15% 36% 23% 8%	479
	21st to end	Day	15% 7% 25% 26% 22% 5%	539
September	1st to 10th	Day	17% 7% 23% 29% 18% 6%	490
	11th to 20th	Day	10% 6% 22% 29% 24% 8%	490
	21st to end	Day	18% 13% 20% 26% 19% 4	490
October	1st to 10th	Day	23% 14% 24% 22% 14% 2	490
	11th to 20th	Day	26% 19% 30% 19% 6% 1	490
	21st to end	Day	31% 18% 28% 18% 3 1	439
November	1st to 10th	Day	28% 17% 32% 18% 4	282
	11th to 20th	Day	25% 23% 33% 17% 2 1	200
	21st to end	Day	27% 25% 36% 13% 1	200
December	1st to 10th	Day	31% 23% 29% 12% 6%	200
	11th to 20th	Day	32% 24% 30% 13% 2	200
	21st to end	Day	35% 20% 35% 7% 2	220
January	1st to 10th	Night	23% 21% 32% 19% 5% 1	1000
	11th to 20th	Night	27% 18% 30% 19% 6% 1	1000
	21st to end	Night	21% 15% 31% 24% 8% 1	889
February	1st to 10th	Night	19% 22% 34% 20% 5%	800
	11th to 20th	Night	21% 14% 36% 21% 8%	783
	21st to end	Night	21% 16% 35% 23% 4	660
March	1st to 10th	Night	18% 16% 34% 26% 6% 1	800
	11th to 20th	Night	18% 16% 35% 22% 8% 1	800
	21st to end	Night	21% 9% 29% 30% 10% 1	880
April	1st to 10th	Night	25% 12% 29% 28% 5% 1	785
	11th to 20th	Night	22% 11% 27% 26% 12% 2	565
	21st to end	Night	24% 11% 24% 26% 15% 1	570
May	1st to 10th	Night	16% 4 25% 37% 16% 1	570
	11th to 20th	Night	16% 5% 21% 36% 21% 1	464
	21st to end	Night	14% 4 21% 38% 20% 3	411
June	1st to 10th	Night	16% 6% 23% 37% 15% 2	380
	11th to 20th	Night	13% 5% 21% 41% 19% 2	380
	21st to end	Night	13% 6% 21% 35% 24% 1	380
July	1st to 10th	Night	8% 4 14% 48% 22% 2	370
	11th to 20th	Night	11% 2 21% 36% 27% 3	415
	21st to end	Night	9% 4 17% 42% 26% 2	624
August	1st to 10th	Night	12% 3 17% 37% 27% 4	567
	11th to 20th	Night	11% 6% 15% 30% 30% 7%	564
	21st to end	Night	14% 3 22% 31% 25% 4	627
September	1st to 10th	Night	13% 6% 19% 33% 22% 7%	740
	11th to 20th	Night	13% 5% 21% 31% 24% 7%	760
	21st to end	Night	20% 9% 21% 24% 20% 6%	760
October	1st to 10th	Night	22% 10% 26% 24% 17% 2	760
	11th to 20th	Night	28% 15% 24% 23% 8% 2	760
	21st to end	Night	32% 15% 29% 20% 4 1	836
November	1st to 10th	Night	27% 17% 26% 22% 7% 1	780
	11th to 20th	Night	26% 22% 26% 18% 7%	800
	21st to end	Night	32% 19% 28% 17% 4	800
December	1st to 10th	Night	32% 22% 28% 12% 6%	800
	11th to 20th	Night	27% 23% 33% 13% 4	935
	21st to end	Night	33% 17% 31% 15% 4	1100

Page generated at 2025-04-08 09:17:27. Version ac07

Copyright © 2025 Attilla Danko.