3 Herons Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3 Herons Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 12% 4 10% 6% 5% 5% 4 5% 6% 26%	132027
	Night	16% 14% 5% 13% 7% 6% 4 3 4 5% 24%	131117
Transparency	Day	58% 5% 11% 18% 8% 1	135495
	Night	62% 4 10% 16% 8% 1	135736
Seeing	Day	28% 16% 29% 18% 8% 2	16633
	Night	26% 14% 25% 26% 9% 1	25638

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% 16% 9% 20% 11% 8% 5% 3 3 3 10%	4661
	11th to 20th	Day	8% 14% 7% 22% 15% 7% 5% 3 4 3 12%	4650
	21st to end	Day	8% 16% 8% 12% 10% 12% 6% 4 5% 5% 14%	5124
February	1st to 10th	Day	12% 15% 5% 18% 10% 6% 4 3 5% 4 18%	1822
	11th to 20th	Day	10% 16% 7% 17% 11% 6% 4 4 4 4 17%	2023
	21st to end	Day	11% 17% 7% 14% 9% 6% 5% 3 4 4 20%	1730
March	1st to 10th	Day	17% 16% 7% 14% 7% 4 4 3 3 4 19%	4576
	11th to 20th	Day	12% 17% 5% 11% 6% 5% 5% 3 5% 6% 25%	4752
	21st to end	Day	19% 13% 5% 12% 6% 4 4 3 4 5% 26%	6102
April	1st to 10th	Day	18% 17% 5% 10% 5% 4 3 2 4 5% 26%	2520
	11th to 20th	Day	18% 15% 4 8% 5% 4 4 3 4 5% 31%	2494
	21st to end	Day	23% 12% 3 8% 5% 4 4 3 5% 5% 28%	2715
May	1st to 10th	Day	30% 14% 4 8% 5% 4 4 3 4 4 19%	6173
	11th to 20th	Day	18% 10% 4 9% 6% 5% 6% 4 6% 7% 26%	6458
	21st to end	Day	21% 10% 3 7% 5% 5% 5% 4 6% 7% 28%	7103
June	1st to 10th	Day	26% 7% 3 6% 5% 4 5% 4 6% 6% 27%	2800
	11th to 20th	Day	18% 7% 2 6% 5% 5% 5% 4 7% 8% 33%	2800
	21st to end	Day	22% 7% 3 7% 5% 5% 4 5% 6% 8% 28%	2800
July	1st to 10th	Day	13% 6% 3 7% 6% 6% 7% 5% 8% 10% 28%	6360
	11th to 20th	Day	12% 6% 2 7% 6% 6% 6% 5% 7% 9% 33%	6360
	21st to end	Day	14% 7% 2 6% 4 4 5% 5% 6% 8% 38%	6926
August	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 5% 6% 7% 30%	4563
	11th to 20th	Day	17% 8% 4 7% 5% 5% 5% 4 6% 8% 30%	4060
	21st to end	Day	17% 6% 2 6% 5% 4 5% 4 5% 9% 38%	4312
September	1st to 10th	Day	17% 6% 2 5% 4 3 4 3 6% 6% 43%	2400
	11th to 20th	Day	17% 7% 1 5% 3 4 4 3 4 7% 46%	2273
	21st to end	Day	17% 8% 3 6% 5% 4 5% 3 6% 7% 36%	2200
October	1st to 10th	Day	22% 10% 3 7% 4 4 4 3 4 7% 31%	2132
	11th to 20th	Day	18% 12% 4 7% 5% 4 4 4 5% 5% 30%	1927
	21st to end	Day	25% 20% 4 7% 5% 5% 3 2 3 4 23%	1980
November	1st to 10th	Day	16% 20% 4 8% 5% 5% 3 3 3 5% 27%	1851
	11th to 20th	Day	16% 18% 7% 10% 6% 4 4 2 4 4 23%	1890
	21st to end	Day	17% 19% 6% 14% 4 5% 4 4 4 5% 18%	1696
December	1st to 10th	Day	12% 19% 8% 24% 8% 6% 4 2 4 3 10%	2680
	11th to 20th	Day	15% 14% 8% 16% 6% 5% 4 3 4 4 19%	3066
	21st to end	Day	15% 15% 7% 16% 7% 5% 6% 3 3 4 18%	4048
January	1st to 10th	Night	9% 18% 8% 23% 11% 7% 4 3 3 3 12%	8234
	11th to 20th	Night	8% 17% 8% 23% 13% 7% 5% 3 4 3 10%	8235
	21st to end	Night	10% 13% 5% 14% 11% 13% 8% 5% 4 3 14%	9057
February	1st to 10th	Night	13% 16% 6% 18% 9% 7% 4 3 4 4 16%	2795
	11th to 20th	Night	12% 16% 6% 16% 11% 6% 5% 3 4 3 19%	2481
	21st to end	Night	13% 15% 6% 14% 8% 6% 5% 3 4 4 23%	2076
March	1st to 10th	Night	16% 17% 7% 14% 7% 7% 3 2 4 4 20%	5484
	11th to 20th	Night	13% 16% 6% 11% 6% 5% 4 3 5% 5% 26%	5422
	21st to end	Night	16% 18% 6% 13% 7% 4 3 2 3 3 24%	5072
April	1st to 10th	Night	20% 15% 5% 10% 4 4 4 3 4 5% 26%	2100
	11th to 20th	Night	22% 13% 4 10% 5% 4 4 2 3 4 29%	2075
	21st to end	Night	21% 12% 3 6% 5% 4 4 3 4 5% 32%	1903
May	1st to 10th	Night	28% 16% 4 8% 5% 4 3 3 4 6% 21%	4018
	11th to 20th	Night	19% 12% 4 7% 5% 4 4 3 5% 7% 29%	3655
	21st to end	Night	19% 11% 3 8% 5% 4 4 3 5% 7% 30%	4014
June	1st to 10th	Night	27% 8% 3 7% 4 4 4 4 4 7% 29%	1600
	11th to 20th	Night	18% 7% 3 7% 4 4 5% 4 6% 8% 34%	1600
	21st to end	Night	21% 9% 3 6% 4 4 5% 5% 6% 7% 31%	1600
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 6% 5% 7% 8% 32%	3600
	11th to 20th	Night	16% 8% 3 8% 5% 5% 4 3 5% 8% 36%	3600
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 5% 8% 38%	3926
August	1st to 10th	Night	21% 9% 3 6% 4 3 4 3 5% 7% 33%	2586
	11th to 20th	Night	16% 8% 3 9% 5% 4 4 3 5% 5% 37%	2939
	21st to end	Night	16% 8% 3 6% 3 4 4 4 5% 7% 40%	3586
September	1st to 10th	Night	16% 6% 2 6% 4 4 5% 4 4 6% 43%	2000
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 5% 7% 46%	2125
	21st to end	Night	19% 8% 2 5% 3 4 4 3 5% 7% 41%	2200
October	1st to 10th	Night	22% 9% 2 7% 4 5% 4 3 4 6% 34%	2265
	11th to 20th	Night	21% 12% 3 7% 5% 4 5% 3 3 6% 30%	2471
	21st to end	Night	25% 16% 4 8% 5% 4 3 3 4 5% 23%	2860
November	1st to 10th	Night	19% 17% 4 8% 4 4 4 3 4 5% 28%	2667
	11th to 20th	Night	16% 17% 7% 10% 5% 3 3 3 4 5% 25%	2730
	21st to end	Night	16% 19% 6% 14% 5% 5% 3 3 4 4 19%	2923
December	1st to 10th	Night	13% 19% 10% 23% 7% 5% 4 3 3 3 9%	4720
	11th to 20th	Night	17% 21% 6% 16% 6% 5% 4 3 3 3 16%	5348
	21st to end	Night	15% 17% 6% 16% 7% 6% 6% 2 3 3 18%	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	79% 4 8% 8% 1	4662
	11th to 20th	Day	76% 5% 5% 8% 4 1	4680
	21st to end	Day	70% 4 8% 11% 6%	5124
February	1st to 10th	Day	70% 4 6% 14% 6% 1	1822
	11th to 20th	Day	70% 4 7% 13% 5%	2032
	21st to end	Day	67% 4 7% 12% 8% 2	1730
March	1st to 10th	Day	70% 4 7% 8% 8% 3	4579
	11th to 20th	Day	59% 4 13% 13% 9% 2	4752
	21st to end	Day	61% 4 6% 13% 15% 1	6102
April	1st to 10th	Day	62% 3 7% 18% 9% 1	2520
	11th to 20th	Day	56% 5% 6% 18% 13% 1	2494
	21st to end	Day	58% 4 9% 18% 9% 2	2715
May	1st to 10th	Day	68% 4 7% 15% 5% 1	6184
	11th to 20th	Day	56% 6% 13% 17% 7% 1	6462
	21st to end	Day	53% 6% 11% 22% 8%	7103
June	1st to 10th	Day	55% 5% 11% 19% 10%	2800
	11th to 20th	Day	47% 6% 11% 23% 11% 1	2800
	21st to end	Day	54% 5% 11% 21% 8% 1	2800
July	1st to 10th	Day	47% 6% 16% 23% 9%	6290
	11th to 20th	Day	43% 7% 14% 27% 9%	6285
	21st to end	Day	42% 5% 12% 33% 7%	6728
August	1st to 10th	Day	51% 5% 16% 21% 7%	4549
	11th to 20th	Day	50% 5% 17% 23% 5%	4099
	21st to end	Day	43% 5% 16% 21% 13% 1	4312
September	1st to 10th	Day	41% 4 12% 31% 12% 1	2400
	11th to 20th	Day	40% 4 10% 31% 15% 1	2273
	21st to end	Day	46% 5% 14% 23% 11% 1	2200
October	1st to 10th	Day	47% 5% 12% 26% 10%	3470
	11th to 20th	Day	53% 4 13% 21% 9%	3117
	21st to end	Day	53% 3 13% 23% 7% 1	3180
November	1st to 10th	Day	61% 4 10% 18% 7%	1851
	11th to 20th	Day	65% 4 9% 12% 8% 2	1890
	21st to end	Day	69% 4 12% 11% 4	1696
December	1st to 10th	Day	79% 3 8% 7% 2	2680
	11th to 20th	Day	67% 5% 7% 17% 4	3066
	21st to end	Day	70% 4 6% 11% 7% 1	4048
January	1st to 10th	Night	78% 4 9% 7% 2	8234
	11th to 20th	Night	79% 3 7% 7% 4	8280
	21st to end	Night	72% 4 7% 9% 7%	9057
February	1st to 10th	Night	71% 4 7% 12% 5%	2795
	11th to 20th	Night	70% 3 7% 13% 6%	2483
	21st to end	Night	64% 4 8% 15% 8% 1	2076
March	1st to 10th	Night	68% 4 6% 12% 9% 1	5494
	11th to 20th	Night	60% 4 12% 14% 8% 1	5423
	21st to end	Night	65% 4 4 15% 11% 1	5072
April	1st to 10th	Night	61% 3 8% 17% 10% 1	2100
	11th to 20th	Night	59% 3 7% 19% 11% 1	2075
	21st to end	Night	55% 4 8% 16% 14% 3	1903
May	1st to 10th	Night	66% 4 9% 15% 6% 1	4024
	11th to 20th	Night	55% 6% 12% 19% 7% 1	3656
	21st to end	Night	53% 5% 11% 19% 11% 1	4015
June	1st to 10th	Night	55% 5% 10% 20% 10% 1	1600
	11th to 20th	Night	46% 7% 13% 22% 10% 2	1600
	21st to end	Night	51% 6% 13% 20% 10%	1600
July	1st to 10th	Night	46% 6% 19% 22% 8%	3560
	11th to 20th	Night	46% 5% 13% 27% 9%	3552
	21st to end	Night	44% 5% 14% 27% 9%	3809
August	1st to 10th	Night	51% 4 15% 23% 7%	2576
	11th to 20th	Night	49% 4 17% 20% 9% 1	2966
	21st to end	Night	43% 4 15% 23% 13% 1	3586
September	1st to 10th	Night	41% 5% 14% 29% 11% 1	2000
	11th to 20th	Night	38% 4 14% 31% 12% 1	2125
	21st to end	Night	44% 4 12% 24% 14% 2	2200
October	1st to 10th	Night	54% 4 10% 22% 9%	3707
	11th to 20th	Night	53% 3 10% 21% 12%	4020
	21st to end	Night	55% 4 13% 19% 8% 1	4610
November	1st to 10th	Night	59% 4 11% 18% 7% 1	2667
	11th to 20th	Night	63% 4 9% 15% 8% 2	2730
	21st to end	Night	69% 3 11% 14% 3	2923
December	1st to 10th	Night	80% 4 8% 7% 1	4720
	11th to 20th	Night	74% 4 5% 10% 6%	5348
	21st to end	Night	72% 3 5% 11% 7% 1	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	25% 24% 31% 17% 3	220
	11th to 20th	Day	31% 25% 31% 9% 3	220
	21st to end	Day	28% 21% 36% 12% 3	242
February	1st to 10th	Day	33% 29% 29% 7% 2	294
	11th to 20th	Day	29% 26% 32% 11% 2	511
	21st to end	Day	32% 20% 33% 14% 2	446
March	1st to 10th	Day	34% 19% 29% 15% 3	540
	11th to 20th	Day	31% 25% 28% 12% 4	540
	21st to end	Day	36% 22% 25% 10% 7% 1	594
April	1st to 10th	Day	39% 15% 25% 14% 5% 1	540
	11th to 20th	Day	37% 18% 25% 14% 5% 2	532
	21st to end	Day	33% 19% 28% 12% 7% 1	540
May	1st to 10th	Day	36% 17% 27% 13% 6% 1	543
	11th to 20th	Day	26% 17% 32% 17% 7% 2	567
	21st to end	Day	25% 19% 29% 20% 6% 1	617
June	1st to 10th	Day	29% 13% 33% 16% 7% 1	530
	11th to 20th	Day	22% 13% 32% 22% 8% 2	530
	21st to end	Day	26% 16% 32% 18% 7% 1	530
July	1st to 10th	Day	19% 10% 33% 25% 11% 2	525
	11th to 20th	Day	17% 9% 38% 25% 9% 3	524
	21st to end	Day	17% 11% 35% 26% 9% 2	576
August	1st to 10th	Day	18% 9% 27% 29% 13% 3	526
	11th to 20th	Day	19% 8% 29% 23% 14% 5%	519
	21st to end	Day	20% 14% 26% 23% 12% 4	583
September	1st to 10th	Day	19% 12% 27% 21% 15% 6%	530
	11th to 20th	Day	18% 11% 22% 22% 18% 8%	530
	21st to end	Day	19% 11% 30% 23% 12% 5%	530
October	1st to 10th	Day	27% 11% 26% 20% 14% 3	530
	11th to 20th	Day	30% 16% 24% 19% 9% 1	489
	21st to end	Day	38% 16% 25% 17% 4	462
November	1st to 10th	Day	34% 16% 21% 21% 7% 1	425
	11th to 20th	Day	30% 21% 26% 15% 7% 1	430
	21st to end	Day	38% 17% 29% 15% 1	236
December	1st to 10th	Day	29% 20% 33% 14% 3	220
	11th to 20th	Day	34% 22% 32% 10% 2	220
	21st to end	Day	31% 21% 33% 13% 3	242
January	1st to 10th	Night	26% 22% 29% 20% 3	840
	11th to 20th	Night	29% 19% 28% 20% 4	840
	21st to end	Night	21% 17% 33% 24% 4 1	924
February	1st to 10th	Night	23% 19% 33% 22% 3	840
	11th to 20th	Night	25% 22% 30% 21% 2	823
	21st to end	Night	23% 20% 31% 23% 3	692
March	1st to 10th	Night	29% 15% 29% 22% 4	840
	11th to 20th	Night	30% 19% 25% 20% 6%	840
	21st to end	Night	30% 17% 26% 20% 7% 1	924
April	1st to 10th	Night	33% 15% 26% 21% 5%	840
	11th to 20th	Night	31% 16% 24% 21% 8%	831
	21st to end	Night	30% 15% 25% 23% 7%	643
May	1st to 10th	Night	31% 11% 26% 22% 9% 1	582
	11th to 20th	Night	23% 9% 29% 30% 8% 2	414
	21st to end	Night	20% 10% 24% 35% 11%	455
June	1st to 10th	Night	30% 6% 23% 30% 11% 1	400
	11th to 20th	Night	23% 7% 21% 33% 16% 1	400
	21st to end	Night	24% 7% 20% 38% 12% 1	400
July	1st to 10th	Night	15% 3 23% 39% 18% 1	390
	11th to 20th	Night	12% 3 20% 44% 19% 1	388
	21st to end	Night	16% 4 18% 45% 16%	438
August	1st to 10th	Night	18% 4 14% 43% 21% 1	398
	11th to 20th	Night	18% 4 20% 32% 24% 2	632
	21st to end	Night	20% 5% 17% 35% 21% 2	880
September	1st to 10th	Night	19% 6% 20% 35% 18% 2	800
	11th to 20th	Night	20% 5% 17% 31% 24% 4	800
	21st to end	Night	23% 8% 23% 28% 16% 2	800
October	1st to 10th	Night	27% 11% 17% 28% 15% 2	800
	11th to 20th	Night	29% 11% 21% 27% 11% 1	800
	21st to end	Night	33% 13% 26% 23% 5%	880
November	1st to 10th	Night	31% 16% 23% 22% 7% 1	820
	11th to 20th	Night	29% 15% 26% 20% 8% 1	840
	21st to end	Night	31% 18% 26% 18% 6% 1	840
December	1st to 10th	Night	31% 19% 26% 20% 4	840
	11th to 20th	Night	29% 21% 26% 19% 4	840
	21st to end	Night	31% 21% 24% 20% 3	924

Page generated at 2026-05-29 01:53:26. Version ac07

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