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 2024-02-20.

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% 5% 8% 6% 3 5% 4 5% 6% 31%	78117
	Night	16% 13% 6% 10% 7% 4 5% 4 4 6% 25%	81076
Transparency	Day	36% 37% 8% 12% 6% 1	77823
	Night	43% 35% 7% 9% 5% 1	80906
Seeing	Day	19% 10% 27% 27% 14% 3	14174
	Night	21% 12% 27% 26% 12% 2	23624

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	16% 22% 15% 16% 10% 3 5% 3 2 2 7%	2222
	11th to 20th	Day	12% 20% 11% 22% 15% 3 5% 3 2 2 4	2530
	21st to end	Day	20% 19% 9% 13% 9% 5% 5% 4 4 4 8%	3100
February	1st to 10th	Day	14% 18% 8% 15% 11% 4 5% 4 3 5% 13%	1710
	11th to 20th	Day	12% 14% 7% 13% 9% 4 6% 4 5% 7% 19%	1654
	21st to end	Day	14% 15% 7% 11% 7% 5% 5% 5% 4 5% 22%	1451
March	1st to 10th	Day	13% 10% 4 8% 6% 4 4 5% 5% 6% 34%	1900
	11th to 20th	Day	12% 10% 5% 10% 6% 4 5% 4 6% 5% 32%	1980
	21st to end	Day	17% 7% 4 7% 6% 3 5% 4 4 5% 36%	2178
April	1st to 10th	Day	20% 11% 4 6% 5% 4 4 5% 4 6% 32%	2085
	11th to 20th	Day	18% 9% 3 5% 5% 3 4 3 5% 5% 40%	2312
	21st to end	Day	19% 7% 4 6% 4 3 4 4 5% 6% 37%	2340
May	1st to 10th	Day	17% 7% 3 5% 4 3 5% 5% 5% 8% 39%	2340
	11th to 20th	Day	15% 7% 3 5% 5% 3 4 4 5% 5% 44%	2390
	21st to end	Day	11% 5% 3 5% 4 3 4 5% 5% 8% 46%	2725
June	1st to 10th	Day	12% 6% 3 6% 5% 3 5% 5% 6% 8% 42%	2699
	11th to 20th	Day	11% 5% 3 5% 5% 3 5% 5% 5% 9% 44%	2700
	21st to end	Day	13% 7% 2 6% 5% 3 5% 4 6% 8% 43%	2700
July	1st to 10th	Day	8% 5% 2 4 5% 2 4 5% 6% 9% 51%	2700
	11th to 20th	Day	10% 5% 3 5% 4 3 4 5% 5% 8% 47%	2652
	21st to end	Day	10% 4 2 5% 5% 3 4 5% 7% 9% 47%	2582
August	1st to 10th	Day	12% 5% 2 3 5% 2 3 4 7% 8% 49%	2327
	11th to 20th	Day	9% 4 2 5% 5% 3 5% 6% 6% 9% 46%	2303
	21st to end	Day	13% 7% 2 5% 4 3 5% 5% 6% 9% 42%	2574
September	1st to 10th	Day	11% 6% 2 6% 6% 3 4 4 6% 9% 42%	2123
	11th to 20th	Day	13% 5% 2 6% 4 3 4 6% 5% 8% 44%	1980
	21st to end	Day	17% 7% 3 5% 6% 3 4 5% 5% 8% 36%	1980
October	1st to 10th	Day	18% 9% 4 6% 5% 4 4 4 5% 6% 33%	1836
	11th to 20th	Day	26% 13% 4 9% 5% 3 4 4 5% 6% 21%	1800
	21st to end	Day	28% 14% 5% 8% 5% 3 4 4 4 6% 18%	1886
November	1st to 10th	Day	19% 14% 6% 9% 7% 3 5% 5% 5% 6% 21%	1638
	11th to 20th	Day	19% 17% 5% 9% 8% 4 5% 5% 5% 6% 16%	1520
	21st to end	Day	23% 22% 7% 11% 7% 3 4 4 4 4 10%	1520
December	1st to 10th	Day	17% 28% 9% 13% 8% 5% 5% 3 3 2 7%	1520
	11th to 20th	Day	20% 23% 8% 14% 9% 3 4 3 3 4 9%	1729
	21st to end	Day	27% 22% 8% 15% 7% 3 4 3 3 3 4	2431
January	1st to 10th	Night	16% 22% 11% 17% 11% 4 4 3 3 3 7%	4719
	11th to 20th	Night	15% 17% 9% 22% 13% 4 5% 4 3 3 6%	4440
	21st to end	Night	17% 18% 8% 16% 8% 4 6% 4 4 4 11%	4554
February	1st to 10th	Night	12% 19% 8% 15% 11% 4 5% 4 5% 5% 14%	2470
	11th to 20th	Night	13% 15% 8% 13% 8% 3 5% 5% 5% 5% 19%	2397
	21st to end	Night	16% 14% 7% 10% 7% 4 6% 4 3 5% 24%	1803
March	1st to 10th	Night	14% 11% 4 7% 8% 4 5% 4 4 5% 33%	2047
	11th to 20th	Night	14% 11% 5% 7% 6% 4 5% 4 5% 5% 33%	1979
	21st to end	Night	16% 8% 3 8% 6% 4 4 5% 4 6% 36%	2178
April	1st to 10th	Night	20% 10% 4 6% 5% 3 4 4 5% 5% 33%	1891
	11th to 20th	Night	18% 8% 3 5% 4 2 4 4 6% 7% 38%	1597
	21st to end	Night	19% 9% 3 7% 5% 3 5% 4 5% 7% 33%	1620
May	1st to 10th	Night	14% 8% 3 6% 6% 4 6% 4 5% 7% 38%	1620
	11th to 20th	Night	14% 6% 4 5% 5% 3 5% 5% 5% 7% 42%	1501
	21st to end	Night	10% 6% 2 6% 5% 3 6% 5% 5% 7% 45%	1547
June	1st to 10th	Night	14% 6% 3 5% 7% 3 6% 3 5% 8% 41%	1260
	11th to 20th	Night	10% 6% 2 4 5% 3 5% 6% 6% 11% 43%	1260
	21st to end	Night	11% 6% 2 6% 5% 3 5% 6% 5% 9% 41%	1260
July	1st to 10th	Night	7% 5% 2 3 5% 3 4 5% 5% 9% 53%	1260
	11th to 20th	Night	8% 5% 2 5% 5% 3 6% 6% 6% 9% 45%	1304
	21st to end	Night	8% 4 2 5% 4 3 5% 6% 6% 8% 50%	1733
August	1st to 10th	Night	9% 5% 3 4 4 3 4 5% 6% 7% 51%	1611
	11th to 20th	Night	9% 4 3 4 4 3 5% 6% 6% 8% 48%	1598
	21st to end	Night	12% 6% 2 6% 4 3 5% 5% 5% 7% 44%	1782
September	1st to 10th	Night	12% 4 2 5% 5% 3 5% 5% 6% 8% 46%	1816
	11th to 20th	Night	12% 5% 2 5% 4 4 4 5% 6% 7% 46%	1980
	21st to end	Night	17% 7% 3 6% 5% 4 5% 4 4 7% 37%	1980
October	1st to 10th	Night	20% 8% 3 7% 5% 4 4 4 5% 6% 33%	2124
	11th to 20th	Night	25% 12% 5% 8% 5% 4 5% 4 5% 6% 22%	2160
	21st to end	Night	28% 12% 5% 9% 6% 4 5% 4 4 5% 18%	2466
November	1st to 10th	Night	21% 13% 4 8% 7% 3 5% 5% 4 6% 25%	2439
	11th to 20th	Night	19% 18% 6% 11% 7% 3 4 5% 5% 5% 17%	2660
	21st to end	Night	24% 20% 7% 10% 7% 4 4 4 4 5% 11%	2660
December	1st to 10th	Night	21% 25% 10% 13% 9% 3 4 3 3 3 7%	2660
	11th to 20th	Night	18% 23% 7% 14% 8% 4 4 4 4 4 10%	3464
	21st to end	Night	24% 22% 9% 14% 7% 4 4 3 2 3 7%	5236

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	66% 28% 4 2	2223
	11th to 20th	Day	72% 25% 2 1	2530
	21st to end	Day	64% 27% 5% 3 1	3100
February	1st to 10th	Day	51% 37% 5% 7% 1	1710
	11th to 20th	Day	48% 38% 6% 6% 2	1663
	21st to end	Day	44% 39% 6% 9% 2	1451
March	1st to 10th	Day	34% 38% 9% 13% 4 1	1903
	11th to 20th	Day	35% 38% 9% 12% 6% 1	1980
	21st to end	Day	33% 38% 8% 12% 7% 1	2178
April	1st to 10th	Day	33% 39% 8% 13% 7% 1	2085
	11th to 20th	Day	30% 37% 6% 15% 11% 1	2312
	21st to end	Day	32% 38% 8% 12% 8% 2	2340
May	1st to 10th	Day	27% 38% 7% 16% 10% 2	2340
	11th to 20th	Day	28% 38% 6% 15% 13% 1	2394
	21st to end	Day	23% 38% 10% 15% 12% 2	2725
June	1st to 10th	Day	26% 39% 9% 16% 9%	2699
	11th to 20th	Day	22% 37% 8% 19% 12%	2700
	21st to end	Day	26% 38% 11% 17% 7% 1	2700
July	1st to 10th	Day	20% 40% 12% 17% 10% 1	2626
	11th to 20th	Day	21% 40% 11% 19% 8% 1	2571
	21st to end	Day	22% 42% 10% 16% 10%	2439
August	1st to 10th	Day	24% 38% 11% 18% 7% 1	2314
	11th to 20th	Day	20% 38% 12% 19% 10%	2303
	21st to end	Day	26% 39% 9% 17% 9% 1	2574
September	1st to 10th	Day	25% 38% 9% 19% 8% 1	2123
	11th to 20th	Day	27% 39% 12% 17% 5%	1980
	21st to end	Day	32% 39% 9% 14% 5%	1980
October	1st to 10th	Day	33% 39% 8% 14% 6%	1836
	11th to 20th	Day	44% 38% 7% 7% 4	1800
	21st to end	Day	45% 39% 7% 7% 2	1886
November	1st to 10th	Day	37% 42% 8% 11% 3	1638
	11th to 20th	Day	42% 41% 8% 5% 4	1520
	21st to end	Day	49% 42% 6% 3	1520
December	1st to 10th	Day	54% 41% 3 2	1520
	11th to 20th	Day	56% 34% 6% 3	1729
	21st to end	Day	67% 26% 5% 1	2431
January	1st to 10th	Night	66% 26% 6% 3	4719
	11th to 20th	Night	68% 26% 4 2	4440
	21st to end	Night	63% 26% 7% 4	4554
February	1st to 10th	Night	49% 36% 6% 7% 2	2470
	11th to 20th	Night	50% 37% 6% 5% 1	2399
	21st to end	Night	42% 38% 10% 8% 2	1803
March	1st to 10th	Night	36% 37% 10% 12% 5%	2057
	11th to 20th	Night	35% 37% 8% 12% 7% 1	1980
	21st to end	Night	32% 37% 8% 14% 7% 1	2178
April	1st to 10th	Night	31% 36% 8% 16% 8% 1	1891
	11th to 20th	Night	29% 36% 6% 15% 12% 1	1597
	21st to end	Night	33% 37% 6% 11% 11% 2	1620
May	1st to 10th	Night	31% 37% 7% 12% 10% 2	1620
	11th to 20th	Night	26% 38% 5% 16% 12% 2	1502
	21st to end	Night	21% 39% 10% 15% 12% 3	1548
June	1st to 10th	Night	32% 38% 6% 13% 10% 1	1260
	11th to 20th	Night	24% 37% 8% 17% 13%	1260
	21st to end	Night	28% 39% 10% 15% 7% 1	1260
July	1st to 10th	Night	19% 39% 8% 19% 14% 2	1225
	11th to 20th	Night	22% 40% 8% 18% 10% 2	1262
	21st to end	Night	21% 42% 9% 19% 9%	1637
August	1st to 10th	Night	24% 39% 11% 18% 8% 1	1600
	11th to 20th	Night	20% 38% 13% 16% 12% 1	1598
	21st to end	Night	25% 38% 11% 17% 9% 2	1782
September	1st to 10th	Night	23% 38% 10% 18% 10% 2	1816
	11th to 20th	Night	26% 38% 12% 18% 6%	1980
	21st to end	Night	32% 38% 9% 15% 6%	1980
October	1st to 10th	Night	34% 37% 7% 15% 7%	2124
	11th to 20th	Night	44% 38% 7% 8% 2	2160
	21st to end	Night	47% 39% 6% 7% 3	2466
November	1st to 10th	Night	34% 43% 8% 12% 3	2439
	11th to 20th	Night	43% 40% 8% 5% 2 1	2660
	21st to end	Night	48% 40% 8% 4	2660
December	1st to 10th	Night	54% 41% 5% 1	2660
	11th to 20th	Night	55% 35% 7% 3	3463
	21st to end	Night	66% 27% 5% 1	5236

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% 21% 33% 17% 4 1	186
	11th to 20th	Day	22% 17% 36% 17% 7%	270
	21st to end	Day	29% 10% 34% 21% 6%	297
February	1st to 10th	Day	25% 24% 29% 19% 3	270
	11th to 20th	Day	20% 20% 39% 19% 3	267
	21st to end	Day	26% 27% 38% 8% 1	198
March	1st to 10th	Day	19% 17% 33% 24% 6%	343
	11th to 20th	Day	20% 14% 36% 22% 6% 1	420
	21st to end	Day	26% 10% 31% 25% 7% 1	462
April	1st to 10th	Day	21% 11% 36% 24% 8% 1	420
	11th to 20th	Day	25% 13% 26% 18% 12% 5%	412
	21st to end	Day	24% 11% 30% 24% 9% 2	420
May	1st to 10th	Day	20% 6% 32% 28% 12% 2	423
	11th to 20th	Day	17% 6% 26% 30% 16% 4	447
	21st to end	Day	13% 5% 22% 36% 19% 5%	485
June	1st to 10th	Day	11% 3 28% 40% 14% 3	629
	11th to 20th	Day	12% 5% 26% 37% 17% 3	630
	21st to end	Day	15% 5% 24% 38% 16% 3	630
July	1st to 10th	Day	8% 4 21% 38% 26% 3	621
	11th to 20th	Day	12% 4 22% 34% 21% 6%	618
	21st to end	Day	10% 4 23% 36% 22% 5%	523
August	1st to 10th	Day	14% 5% 18% 31% 25% 6%	446
	11th to 20th	Day	12% 3 14% 38% 25% 8%	439
	21st to end	Day	16% 7% 24% 26% 22% 5%	495
September	1st to 10th	Day	15% 6% 22% 30% 20% 7%	450
	11th to 20th	Day	11% 6% 24% 28% 24% 6%	450
	21st to end	Day	18% 13% 20% 26% 19% 5%	450
October	1st to 10th	Day	23% 13% 24% 22% 15% 2	450
	11th to 20th	Day	26% 18% 30% 19% 6% 1	450
	21st to end	Day	31% 17% 29% 19% 3 1	401
November	1st to 10th	Day	26% 18% 33% 19% 4	254
	11th to 20th	Day	23% 24% 32% 18% 2 1	180
	21st to end	Day	26% 23% 37% 13% 1	180
December	1st to 10th	Day	31% 22% 31% 12% 5%	180
	11th to 20th	Day	29% 24% 31% 13% 2	180
	21st to end	Day	34% 22% 36% 6% 2 1	198
January	1st to 10th	Night	24% 20% 31% 19% 5% 1	950
	11th to 20th	Night	28% 18% 29% 19% 6% 1	950
	21st to end	Night	20% 14% 31% 24% 9% 1	845
February	1st to 10th	Night	19% 21% 34% 20% 5%	760
	11th to 20th	Night	20% 13% 36% 23% 8%	742
	21st to end	Night	23% 16% 34% 23% 4	592
March	1st to 10th	Night	17% 15% 33% 27% 7% 1	720
	11th to 20th	Night	18% 15% 34% 23% 9% 1	720
	21st to end	Night	20% 8% 29% 31% 11% 1	792
April	1st to 10th	Night	25% 11% 29% 29% 6% 1	707
	11th to 20th	Night	21% 11% 27% 26% 12% 2	533
	21st to end	Night	23% 11% 24% 27% 15% 1	540
May	1st to 10th	Night	17% 4 24% 37% 17% 1	540
	11th to 20th	Night	17% 5% 21% 34% 21% 1	440
	21st to end	Night	14% 3 21% 39% 20% 3	389
June	1st to 10th	Night	17% 3 24% 37% 16% 3	360
	11th to 20th	Night	13% 4 20% 41% 20% 2	360
	21st to end	Night	12% 5% 21% 35% 26% 1	360
July	1st to 10th	Night	8% 5% 13% 49% 23% 3	350
	11th to 20th	Night	11% 2 21% 35% 29% 3	392
	21st to end	Night	9% 4 17% 43% 26% 2	591
August	1st to 10th	Night	11% 3 17% 37% 28% 4	537
	11th to 20th	Night	11% 6% 14% 31% 31% 7%	534
	21st to end	Night	14% 3 22% 30% 25% 4	594
September	1st to 10th	Night	13% 6% 18% 33% 23% 7%	701
	11th to 20th	Night	13% 6% 22% 31% 22% 6%	720
	21st to end	Night	20% 9% 21% 24% 20% 6%	720
October	1st to 10th	Night	23% 8% 26% 24% 18% 2	720
	11th to 20th	Night	28% 15% 24% 22% 8% 2	720
	21st to end	Night	33% 14% 28% 21% 4 1	792
November	1st to 10th	Night	27% 17% 26% 22% 7% 1	740
	11th to 20th	Night	26% 22% 27% 19% 7%	760
	21st to end	Night	32% 18% 28% 18% 4	760
December	1st to 10th	Night	32% 22% 28% 12% 6%	760
	11th to 20th	Night	27% 23% 33% 14% 3	888
	21st to end	Night	34% 17% 31% 15% 4	1045

Page generated at 2024-02-17 06:28:42. Version ac07

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