Eagle Rock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eagle Rock Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	6% 9% 3 7% 4 4 4 3 5% 6% 49%	123927
	Night	7% 10% 3 7% 4 4 4 3 5% 6% 45%	124053
Transparency	Day	36% 4 9% 23% 19% 10%	123677
	Night	40% 4 10% 22% 16% 8%	123889
Seeing	Day	13% 12% 27% 28% 16% 4	16253
	Night	14% 8% 23% 35% 17% 3	25842

By Month

Percentage Cloud Cover SW (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	10% 23% 8% 9% 5% 4 4 3 4 5% 23%	3528
	11th to 20th	Day	8% 15% 7% 14% 7% 4 4 2 4 4 31%	3799
	21st to end	Day	9% 16% 5% 10% 4 4 3 3 3 4 39%	4510
February	1st to 10th	Day	11% 17% 6% 12% 6% 4 5% 4 5% 5% 25%	1800
	11th to 20th	Day	11% 16% 5% 10% 7% 5% 5% 3 5% 6% 28%	1765
	21st to end	Day	7% 16% 5% 11% 7% 5% 5% 3 5% 6% 30%	1650
March	1st to 10th	Day	7% 15% 5% 11% 5% 4 4 4 5% 6% 34%	4771
	11th to 20th	Day	6% 14% 4 8% 6% 5% 4 3 5% 5% 41%	5000
	21st to end	Day	7% 15% 6% 11% 6% 6% 6% 5% 7% 6% 25%	5486
April	1st to 10th	Day	9% 13% 3 7% 4 5% 5% 4 6% 7% 36%	2269
	11th to 20th	Day	7% 11% 3 8% 5% 4 4 4 5% 8% 42%	2567
	21st to end	Day	9% 11% 3 7% 5% 5% 5% 5% 6% 7% 38%	2600
May	1st to 10th	Day	7% 9% 3 5% 4 4 4 3 5% 6% 50%	5880
	11th to 20th	Day	7% 7% 2 7% 5% 4 5% 4 6% 8% 45%	5841
	21st to end	Day	6% 6% 2 5% 3 3 4 3 5% 6% 57%	6869
June	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 6% 8% 49%	2913
	11th to 20th	Day	7% 5% 2 6% 4 4 4 4 5% 8% 53%	3000
	21st to end	Day	2 2 1 3 2 3 2 3 5% 7% 69%	3000
July	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 77%	4838
	11th to 20th	Day	1 2 1 3 3 3 3 3 5% 6% 71%	4506
	21st to end	Day	3 2 1 4 3 3 3 3 4 7% 67%	4455
August	1st to 10th	Day	4 4 1 5% 3 4 4 4 5% 8% 59%	4092
	11th to 20th	Day	2 2 1 3 3 3 4 3 4 7% 70%	4028
	21st to end	Day	2 2 1 3 2 2 3 3 4 6% 72%	4521
September	1st to 10th	Day	4 3 1 3 2 2 2 3 4 5% 70%	2205
	11th to 20th	Day	6% 4 1 4 3 3 3 3 5% 6% 61%	2090
	21st to end	Day	7% 4 2 3 2 2 3 2 4 7% 63%	2090
October	1st to 10th	Day	7% 4 1 5% 2 3 2 2 3 4 67%	3306
	11th to 20th	Day	5% 5% 2 6% 3 3 3 2 4 5% 61%	3150
	21st to end	Day	7% 9% 3 7% 4 4 5% 4 6% 6% 45%	3292
November	1st to 10th	Day	12% 13% 4 6% 4 3 4 3 5% 6% 41%	1764
	11th to 20th	Day	12% 12% 3 7% 4 4 4 4 6% 8% 38%	1670
	21st to end	Day	9% 13% 4 10% 4 4 3 3 4 5% 41%	1600
December	1st to 10th	Day	8% 9% 4 9% 4 3 2 2 4 4 51%	2620
	11th to 20th	Day	11% 19% 8% 12% 6% 5% 5% 4 4 6% 21%	2954
	21st to end	Day	12% 17% 5% 12% 5% 4 4 4 5% 6% 26%	3498
January	1st to 10th	Night	9% 17% 7% 12% 5% 4 4 3 4 6% 28%	6447
	11th to 20th	Night	12% 18% 6% 13% 6% 4 3 3 4 5% 27%	6164
	21st to end	Night	9% 17% 4 7% 4 3 3 2 3 4 43%	6468
February	1st to 10th	Night	12% 15% 6% 10% 6% 4 4 4 6% 6% 29%	2600
	11th to 20th	Night	12% 17% 5% 8% 4 4 3 3 5% 5% 34%	2517
	21st to end	Night	9% 14% 6% 11% 6% 5% 5% 4 6% 6% 29%	1980
March	1st to 10th	Night	7% 14% 4 8% 5% 5% 5% 4 5% 6% 37%	5122
	11th to 20th	Night	5% 9% 3 8% 6% 4 5% 4 6% 8% 42%	5000
	21st to end	Night	6% 10% 4 8% 6% 5% 5% 5% 7% 9% 35%	5489
April	1st to 10th	Night	9% 10% 4 8% 5% 4 5% 4 6% 7% 39%	2127
	11th to 20th	Night	8% 12% 3 7% 5% 5% 4 4 5% 7% 41%	1781
	21st to end	Night	9% 9% 3 6% 5% 4 4 3 6% 6% 45%	1800
May	1st to 10th	Night	7% 10% 3 7% 4 3 4 3 5% 7% 47%	4100
	11th to 20th	Night	6% 6% 2 7% 4 5% 4 4 6% 8% 47%	4074
	21st to end	Night	5% 5% 2 5% 4 4 4 4 7% 9% 51%	4026
June	1st to 10th	Night	6% 5% 2 5% 4 4 5% 5% 7% 10% 48%	1481
	11th to 20th	Night	7% 5% 2 4 3 4 4 3 6% 7% 55%	1400
	21st to end	Night	3 3 1 4 2 3 2 2 5% 7% 69%	1400
July	1st to 10th	Night	3 1 1 3 2 2 3 3 4 6% 71%	2323
	11th to 20th	Night	1 1 1 2 3 3 4 3 5% 8% 70%	2671
	21st to end	Night	1 2 1 3 3 4 4 4 6% 7% 66%	3109
August	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 8% 61%	2847
	11th to 20th	Night	2 2 1 3 3 3 4 4 5% 7% 65%	2811
	21st to end	Night	3 3 1 3 2 2 3 2 3 4 73%	3146
September	1st to 10th	Night	4 3 1 4 2 3 3 3 4 5% 69%	1971
	11th to 20th	Night	6% 4 1 4 4 3 4 4 5% 7% 59%	2090
	21st to end	Night	7% 4 1 3 2 3 3 3 4 5% 64%	2090
October	1st to 10th	Night	6% 3 1 3 2 2 2 2 4 5% 69%	3667
	11th to 20th	Night	9% 7% 2 6% 3 3 4 3 4 5% 55%	3778
	21st to end	Night	7% 9% 3 7% 4 4 4 5% 5% 7% 46%	4262
November	1st to 10th	Night	11% 9% 3 6% 4 3 4 3 5% 6% 45%	2535
	11th to 20th	Night	10% 13% 4 7% 5% 4 4 3 5% 6% 40%	2707
	21st to end	Night	8% 12% 4 9% 5% 4 4 3 5% 6% 39%	2800
December	1st to 10th	Night	8% 10% 4 7% 5% 3 3 3 4 5% 48%	4570
	11th to 20th	Night	10% 18% 5% 10% 5% 4 4 3 5% 7% 29%	5242
	21st to end	Night	12% 18% 6% 11% 5% 5% 5% 3 4 5% 28%	7458

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% 4 12% 16% 5% 1	3528
	11th to 20th	Day	57% 4 14% 14% 7% 3	3799
	21st to end	Day	51% 3 12% 22% 11% 1	4510
February	1st to 10th	Day	62% 4 11% 17% 5% 1	1800
	11th to 20th	Day	57% 4 13% 19% 6% 1	1774
	21st to end	Day	55% 7% 10% 18% 8% 1	1650
March	1st to 10th	Day	50% 4 10% 24% 8% 4	4771
	11th to 20th	Day	45% 4 8% 31% 12% 1	5000
	21st to end	Day	57% 6% 14% 19% 4	5486
April	1st to 10th	Day	47% 5% 14% 19% 13% 2	2269
	11th to 20th	Day	41% 5% 9% 27% 15% 3	2570
	21st to end	Day	44% 5% 10% 26% 14% 1	2600
May	1st to 10th	Day	33% 5% 7% 28% 22% 4	5880
	11th to 20th	Day	36% 6% 6% 28% 19% 5%	5844
	21st to end	Day	29% 4 6% 37% 22% 3	6870
June	1st to 10th	Day	33% 4 11% 32% 19% 1	2913
	11th to 20th	Day	30% 4 8% 24% 24% 11%	3000
	21st to end	Day	16% 3 3 24% 37% 17%	3000
July	1st to 10th	Day	11% 2 3 11% 29% 44%	4765
	11th to 20th	Day	14% 3 3 10% 26% 44%	4431
	21st to end	Day	19% 3 3 12% 33% 30%	4315
August	1st to 10th	Day	24% 3 3 15% 29% 25%	4078
	11th to 20th	Day	16% 3 3 20% 38% 21%	4067
	21st to end	Day	14% 3 4 21% 30% 28%	4520
September	1st to 10th	Day	18% 3 5% 18% 33% 23%	2205
	11th to 20th	Day	24% 3 7% 25% 27% 13%	2090
	21st to end	Day	23% 3 7% 28% 30% 9%	2090
October	1st to 10th	Day	25% 2 6% 22% 33% 13%	3306
	11th to 20th	Day	27% 3 9% 37% 18% 6%	3150
	21st to end	Day	38% 5% 13% 27% 17% 1	3292
November	1st to 10th	Day	45% 4 14% 23% 12% 2	1764
	11th to 20th	Day	46% 4 17% 24% 8% 2	1670
	21st to end	Day	47% 3 17% 22% 9% 2	1600
December	1st to 10th	Day	38% 4 22% 30% 4 2	2620
	11th to 20th	Day	66% 4 13% 14% 3	2952
	21st to end	Day	60% 3 12% 17% 7%	3498
January	1st to 10th	Night	58% 4 10% 21% 6%	6447
	11th to 20th	Night	60% 4 13% 17% 4 2	6164
	21st to end	Night	47% 2 14% 21% 13% 3	6468
February	1st to 10th	Night	56% 5% 14% 18% 7% 1	2600
	11th to 20th	Night	53% 4 13% 20% 9% 1	2518
	21st to end	Night	55% 5% 12% 19% 7% 2	1980
March	1st to 10th	Night	47% 5% 9% 24% 12% 4	5122
	11th to 20th	Night	40% 3 11% 28% 17% 1	5000
	21st to end	Night	45% 4 15% 28% 7%	5489
April	1st to 10th	Night	44% 5% 12% 21% 15% 3	2127
	11th to 20th	Night	42% 4 11% 24% 16% 2	1781
	21st to end	Night	39% 4 10% 28% 18% 1	1800
May	1st to 10th	Night	36% 5% 7% 24% 24% 3	4097
	11th to 20th	Night	32% 6% 8% 30% 19% 5%	4075
	21st to end	Night	27% 5% 7% 29% 29% 4	4026
June	1st to 10th	Night	31% 5% 13% 25% 24% 2	1481
	11th to 20th	Night	28% 4 8% 21% 28% 11%	1400
	21st to end	Night	17% 2 4 19% 33% 24%	1400
July	1st to 10th	Night	15% 3 3 11% 24% 45%	2287
	11th to 20th	Night	13% 3 2 11% 22% 49%	2623
	21st to end	Night	16% 3 4 13% 31% 33%	3011
August	1st to 10th	Night	21% 3 4 13% 28% 32%	2838
	11th to 20th	Night	19% 3 3 18% 34% 23%	2838
	21st to end	Night	16% 3 4 16% 29% 32%	3146
September	1st to 10th	Night	20% 2 4 13% 32% 29%	1971
	11th to 20th	Night	25% 3 6% 23% 26% 17%	2090
	21st to end	Night	24% 3 4 26% 32% 11%	2090
October	1st to 10th	Night	20% 1 6% 20% 36% 16%	3667
	11th to 20th	Night	33% 2 7% 32% 17% 8%	3778
	21st to end	Night	37% 4 11% 28% 18% 2	4262
November	1st to 10th	Night	41% 4 13% 26% 14% 2	2535
	11th to 20th	Night	46% 4 14% 25% 10% 2	2707
	21st to end	Night	46% 3 15% 23% 11% 2	2800
December	1st to 10th	Night	40% 4 21% 30% 4 1	4570
	11th to 20th	Night	56% 5% 19% 16% 3	5243
	21st to end	Night	60% 3 12% 17% 8% 1	7458

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	27% 19% 15% 25% 11% 4	200
	11th to 20th	Day	19% 9% 29% 23% 18% 2	235
	21st to end	Day	18% 9% 27% 32% 13% 2	330
February	1st to 10th	Day	23% 20% 30% 21% 6% 1	300
	11th to 20th	Day	20% 18% 33% 19% 9% 1	310
	21st to end	Day	17% 21% 29% 23% 9% 1	414
March	1st to 10th	Day	18% 22% 30% 22% 8% 1	500
	11th to 20th	Day	14% 20% 29% 27% 9% 1	500
	21st to end	Day	20% 21% 33% 18% 7% 1	550
April	1st to 10th	Day	18% 19% 26% 24% 10% 2	500
	11th to 20th	Day	17% 18% 33% 22% 9% 1	492
	21st to end	Day	16% 15% 29% 26% 11% 3	500
May	1st to 10th	Day	12% 15% 27% 30% 12% 3	503
	11th to 20th	Day	13% 16% 29% 24% 16% 3	528
	21st to end	Day	14% 9% 22% 30% 18% 7%	759
June	1st to 10th	Day	8% 12% 26% 28% 19% 7%	730
	11th to 20th	Day	13% 13% 28% 30% 14% 2	730
	21st to end	Day	5% 7% 28% 34% 20% 6%	730
July	1st to 10th	Day	4 7% 23% 37% 25% 4	696
	11th to 20th	Day	3 7% 27% 40% 19% 4	524
	21st to end	Day	4 3 21% 34% 30% 8%	532
August	1st to 10th	Day	6% 5% 28% 32% 23% 6%	487
	11th to 20th	Day	5% 5% 25% 34% 24% 7%	477
	21st to end	Day	5% 11% 25% 29% 22% 8%	539
September	1st to 10th	Day	4 7% 25% 29% 27% 7%	490
	11th to 20th	Day	9% 12% 26% 29% 19% 5%	490
	21st to end	Day	10% 9% 26% 26% 23% 6%	490
October	1st to 10th	Day	12% 11% 18% 29% 25% 6%	384
	11th to 20th	Day	11% 17% 19% 22% 20% 10%	300
	21st to end	Day	16% 15% 26% 29% 11% 3	330
November	1st to 10th	Day	22% 11% 27% 25% 9% 6%	300
	11th to 20th	Day	16% 15% 30% 24% 12% 3	300
	21st to end	Day	17% 12% 29% 30% 12% 2	300
December	1st to 10th	Day	21% 11% 23% 26% 14% 5%	300
	11th to 20th	Day	26% 10% 33% 20% 11% 1	283
	21st to end	Day	26% 14% 25% 20% 10% 3	220
January	1st to 10th	Night	22% 9% 20% 31% 17% 1	863
	11th to 20th	Night	22% 7% 23% 29% 16% 3	800
	21st to end	Night	21% 6% 21% 37% 14% 2	880
February	1st to 10th	Night	20% 12% 28% 27% 12% 1	800
	11th to 20th	Night	21% 10% 25% 30% 11% 3	784
	21st to end	Night	14% 13% 27% 32% 11% 2	660
March	1st to 10th	Night	16% 12% 27% 32% 12% 2	800
	11th to 20th	Night	12% 9% 29% 34% 15% 2	800
	21st to end	Night	13% 10% 29% 36% 11% 1	880
April	1st to 10th	Night	15% 10% 26% 35% 13% 1	750
	11th to 20th	Night	15% 11% 31% 34% 8% 2	593
	21st to end	Night	13% 9% 25% 36% 16% 2	600
May	1st to 10th	Night	12% 8% 24% 35% 19% 1	600
	11th to 20th	Night	11% 7% 22% 34% 22% 3	591
	21st to end	Night	12% 7% 21% 39% 18% 3	650
June	1st to 10th	Night	9% 8% 22% 35% 23% 3	481
	11th to 20th	Night	10% 7% 29% 36% 18% 1	400
	21st to end	Night	5% 2 22% 42% 24% 6%	400
July	1st to 10th	Night	3 1 21% 44% 25% 5%	390
	11th to 20th	Night	2 1 22% 50% 24% 1	413
	21st to end	Night	3 3 16% 46% 29% 4	624
August	1st to 10th	Night	6% 2 16% 47% 26% 2	567
	11th to 20th	Night	3 2 18% 46% 28% 3	564
	21st to end	Night	5% 4 21% 41% 26% 3	627
September	1st to 10th	Night	5% 3 20% 42% 25% 5%	662
	11th to 20th	Night	7% 4 21% 37% 26% 6%	760
	21st to end	Night	10% 3 21% 38% 21% 7%	760
October	1st to 10th	Night	12% 5% 19% 39% 23% 2	760
	11th to 20th	Night	12% 8% 20% 36% 20% 4	760
	21st to end	Night	12% 9% 22% 40% 13% 4	836
November	1st to 10th	Night	16% 10% 25% 31% 16% 2	780
	11th to 20th	Night	17% 10% 29% 31% 13% 1	907
	21st to end	Night	15% 9% 23% 35% 16% 2	1000
December	1st to 10th	Night	19% 9% 24% 35% 12% 1	1000
	11th to 20th	Night	24% 9% 23% 30% 12% 1	1000
	21st to end	Night	27% 12% 23% 28% 9% 2	1100

Page generated at 2025-07-17 23:10:00. Version ac07

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