Coronach
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coronach clear sky chart from 2005-11-06 to 2026-07-01.

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	8% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	147234
	Night	6% 8% 3 9% 6% 5% 5% 4 6% 8% 40%	142459
Transparency	Day	26% 37% 10% 18% 9% 1	141820
	Night	27% 36% 10% 17% 9% 1	138883
Seeing	Day	12% 12% 32% 29% 13% 3	18322
	Night	11% 10% 34% 34% 11% 1	27781

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	5% 12% 4 12% 8% 8% 8% 5% 7% 7% 25%	4377
	11th to 20th	Day	2 6% 3 14% 9% 8% 8% 9% 8% 8% 24%	4740
	21st to end	Day	4 9% 4 13% 7% 5% 5% 4 7% 7% 35%	5188
February	1st to 10th	Day	7% 13% 5% 13% 8% 6% 6% 4 7% 8% 23%	1816
	11th to 20th	Day	6% 12% 4 11% 7% 8% 6% 5% 7% 6% 29%	1845
	21st to end	Day	4 10% 4 12% 9% 5% 7% 6% 7% 8% 28%	1681
March	1st to 10th	Day	5% 10% 5% 16% 7% 5% 4 3 5% 7% 34%	4600
	11th to 20th	Day	5% 9% 5% 9% 5% 5% 6% 5% 7% 9% 35%	4969
	21st to end	Day	7% 14% 7% 14% 7% 5% 5% 4 6% 7% 24%	5959
April	1st to 10th	Day	7% 9% 3 8% 5% 5% 5% 5% 5% 8% 39%	2520
	11th to 20th	Day	12% 13% 5% 9% 5% 5% 5% 4 6% 7% 28%	2491
	21st to end	Day	13% 9% 4 9% 6% 5% 5% 3 5% 6% 36%	2865
May	1st to 10th	Day	10% 7% 2 6% 5% 5% 5% 4 6% 7% 42%	8224
	11th to 20th	Day	16% 9% 3 7% 6% 5% 5% 4 6% 7% 31%	8191
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 44%	9026
June	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 5% 6% 8% 39%	2971
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 5% 7% 9% 35%	3150
	21st to end	Day	8% 5% 2 6% 6% 5% 5% 5% 8% 8% 44%	3150
July	1st to 10th	Day	4 3 2 5% 5% 5% 6% 5% 7% 9% 49%	6780
	11th to 20th	Day	5% 4 2 5% 4 5% 6% 5% 8% 10% 46%	6433
	21st to end	Day	5% 4 2 6% 4 4 5% 4 6% 8% 53%	6903
August	1st to 10th	Day	11% 6% 2 6% 4 4 4 3 6% 7% 47%	6232
	11th to 20th	Day	4 3 1 5% 3 4 4 4 7% 10% 55%	6051
	21st to end	Day	5% 4 2 4 3 4 4 3 5% 5% 61%	6099
September	1st to 10th	Day	10% 5% 2 5% 3 4 5% 4 5% 6% 51%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 7% 47%	2400
	21st to end	Day	11% 8% 2 4 3 4 3 3 5% 7% 50%	2148
October	1st to 10th	Day	9% 6% 2 5% 3 3 3 3 4 6% 56%	3270
	11th to 20th	Day	7% 6% 3 6% 4 3 3 4 6% 11% 48%	3197
	21st to end	Day	4 9% 3 8% 4 4 5% 4 7% 8% 44%	3190
November	1st to 10th	Day	9% 13% 5% 9% 5% 5% 5% 3 5% 6% 35%	1734
	11th to 20th	Day	5% 10% 5% 11% 6% 5% 5% 4 7% 6% 35%	1680
	21st to end	Day	7% 11% 5% 9% 7% 6% 6% 5% 6% 8% 31%	1680
December	1st to 10th	Day	4 11% 4 14% 6% 7% 7% 5% 5% 7% 30%	2576
	11th to 20th	Day	8% 14% 5% 12% 6% 6% 7% 6% 6% 7% 25%	2935
	21st to end	Day	5% 7% 3 9% 7% 5% 4 4 5% 9% 42%	3763
January	1st to 10th	Night	2 9% 4 14% 9% 7% 7% 5% 7% 7% 29%	8508
	11th to 20th	Night	2 5% 4 14% 8% 8% 8% 6% 9% 9% 27%	8250
	21st to end	Night	3 10% 4 12% 7% 5% 5% 4 7% 9% 34%	9026
February	1st to 10th	Night	6% 12% 4 10% 7% 6% 5% 5% 8% 9% 27%	2824
	11th to 20th	Night	4 11% 4 12% 7% 6% 6% 5% 7% 9% 31%	2662
	21st to end	Night	4 9% 4 9% 6% 6% 6% 4 7% 8% 36%	2124
March	1st to 10th	Night	5% 8% 3 10% 6% 5% 5% 4 7% 10% 38%	5494
	11th to 20th	Night	5% 8% 3 10% 6% 6% 6% 5% 7% 9% 37%	5230
	21st to end	Night	8% 13% 7% 14% 7% 5% 6% 4 6% 7% 23%	5241
April	1st to 10th	Night	8% 8% 3 7% 6% 4 5% 4 8% 7% 40%	2100
	11th to 20th	Night	12% 11% 3 8% 3 5% 5% 4 8% 7% 35%	2077
	21st to end	Night	10% 9% 3 6% 5% 4 5% 3 6% 7% 41%	1749
May	1st to 10th	Night	8% 6% 2 5% 4 4 5% 4 8% 9% 46%	4723
	11th to 20th	Night	14% 8% 3 7% 5% 5% 5% 4 6% 7% 35%	4701
	21st to end	Night	10% 6% 2 5% 4 4 4 4 6% 10% 45%	5181
June	1st to 10th	Night	12% 5% 2 5% 5% 4 4 3 6% 8% 46%	1644
	11th to 20th	Night	12% 6% 2 7% 3 4 4 4 6% 7% 44%	1470
	21st to end	Night	7% 5% 2 5% 3 4 3 4 6% 9% 51%	1470
July	1st to 10th	Night	5% 4 2 5% 4 4 4 5% 7% 10% 51%	3180
	11th to 20th	Night	8% 4 2 6% 4 3 4 4 6% 8% 52%	3538
	21st to end	Night	5% 3 2 5% 3 4 3 3 5% 9% 56%	3970
August	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 5% 8% 50%	3580
	11th to 20th	Night	3 3 1 4 4 3 4 4 7% 10% 58%	3795
	21st to end	Night	5% 4 2 5% 4 3 4 3 5% 7% 58%	4876
September	1st to 10th	Night	7% 4 1 4 4 4 3 3 5% 6% 59%	2000
	11th to 20th	Night	10% 5% 2 5% 4 3 4 3 6% 7% 52%	2000
	21st to end	Night	11% 5% 2 5% 3 4 4 3 4 6% 54%	2249
October	1st to 10th	Night	8% 5% 2 6% 3 4 4 3 5% 7% 54%	3920
	11th to 20th	Night	5% 6% 3 5% 4 3 4 4 6% 9% 52%	3917
	21st to end	Night	6% 7% 3 7% 5% 4 5% 4 7% 8% 44%	4610
November	1st to 10th	Night	12% 12% 4 7% 4 4 5% 4 6% 7% 35%	2780
	11th to 20th	Night	5% 10% 4 11% 7% 6% 6% 4 6% 7% 34%	2940
	21st to end	Night	5% 10% 5% 11% 6% 5% 6% 4 6% 8% 34%	2940
December	1st to 10th	Night	5% 12% 6% 15% 8% 6% 6% 3 5% 6% 27%	4806
	11th to 20th	Night	4 12% 6% 13% 8% 8% 6% 5% 6% 7% 25%	5463
	21st to end	Night	3 8% 4 10% 7% 5% 4 3 5% 7% 44%	7421

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	48% 24% 10% 12% 7%	4378
	11th to 20th	Day	43% 23% 13% 13% 7%	4740
	21st to end	Day	35% 24% 16% 20% 5%	5188
February	1st to 10th	Day	24% 60% 4 7% 5%	1816
	11th to 20th	Day	22% 58% 5% 7% 8%	1854
	21st to end	Day	18% 60% 6% 11% 6%	1681
March	1st to 10th	Day	34% 29% 13% 18% 7%	4600
	11th to 20th	Day	30% 31% 15% 19% 5%	4969
	21st to end	Day	44% 31% 10% 9% 5%	5959
April	1st to 10th	Day	17% 60% 8% 12% 2 1	2520
	11th to 20th	Day	19% 60% 6% 11% 4	2494
	21st to end	Day	22% 59% 5% 10% 4	2865
May	1st to 10th	Day	28% 25% 9% 24% 13% 1	8224
	11th to 20th	Day	39% 25% 10% 14% 11%	8195
	21st to end	Day	23% 26% 10% 25% 16% 1	9027
June	1st to 10th	Day	14% 60% 7% 12% 5% 1	2971
	11th to 20th	Day	17% 61% 7% 9% 5% 1	3150
	21st to end	Day	19% 60% 6% 12% 4	3150
July	1st to 10th	Day	22% 30% 16% 17% 14% 2	6707
	11th to 20th	Day	24% 29% 13% 23% 10% 1	6355
	21st to end	Day	23% 27% 9% 22% 12% 6%	6705
August	1st to 10th	Day	20% 37% 8% 22% 13% 1	4536
	11th to 20th	Day	16% 38% 13% 24% 9%	4375
	21st to end	Day	18% 37% 9% 27% 9% 1	4390
September	1st to 10th	Day	13% 58% 7% 15% 6% 2	2400
	11th to 20th	Day	18% 58% 7% 11% 4 1	2400
	21st to end	Day	18% 57% 5% 10% 8% 1	2148
October	1st to 10th	Day	19% 35% 5% 24% 13% 3	3270
	11th to 20th	Day	16% 36% 7% 29% 10% 1	3197
	21st to end	Day	22% 36% 9% 26% 7%	3190
November	1st to 10th	Day	25% 54% 6% 11% 3 1	1734
	11th to 20th	Day	21% 55% 9% 11% 4 1	1680
	21st to end	Day	23% 55% 8% 11% 4	1680
December	1st to 10th	Day	32% 37% 17% 11% 3	2576
	11th to 20th	Day	40% 34% 17% 5% 3	2933
	21st to end	Day	26% 26% 6% 26% 12% 5%	3763
January	1st to 10th	Night	43% 23% 12% 14% 7%	8509
	11th to 20th	Night	40% 23% 12% 13% 11% 1	8250
	21st to end	Night	34% 24% 16% 17% 7%	9026
February	1st to 10th	Night	22% 60% 5% 8% 5%	2824
	11th to 20th	Night	18% 58% 6% 10% 7% 1	2663
	21st to end	Night	14% 60% 7% 12% 7%	2124
March	1st to 10th	Night	26% 30% 15% 19% 10%	5494
	11th to 20th	Night	30% 31% 15% 18% 6%	5230
	21st to end	Night	45% 31% 11% 10% 3	5241
April	1st to 10th	Night	16% 60% 9% 11% 4	2100
	11th to 20th	Night	16% 60% 6% 11% 6%	2077
	21st to end	Night	17% 59% 5% 14% 5%	1749
May	1st to 10th	Night	24% 24% 9% 25% 16% 1	4724
	11th to 20th	Night	37% 26% 12% 14% 11%	4702
	21st to end	Night	22% 25% 9% 28% 14% 1	5182
June	1st to 10th	Night	12% 59% 7% 14% 7% 2	1644
	11th to 20th	Night	18% 60% 6% 12% 4 1	1470
	21st to end	Night	15% 59% 8% 13% 6% 1	1470
July	1st to 10th	Night	21% 29% 19% 19% 12% 1	3145
	11th to 20th	Night	25% 27% 14% 22% 11% 1	3493
	21st to end	Night	20% 27% 11% 25% 12% 5%	3856
August	1st to 10th	Night	18% 37% 8% 22% 15% 1	2600
	11th to 20th	Night	14% 37% 11% 30% 8%	2747
	21st to end	Night	18% 37% 11% 26% 8% 1	3516
September	1st to 10th	Night	9% 58% 6% 19% 7% 2	2000
	11th to 20th	Night	16% 57% 6% 13% 6% 1	2000
	21st to end	Night	16% 57% 5% 12% 9% 1	2249
October	1st to 10th	Night	20% 36% 6% 22% 14% 2	3920
	11th to 20th	Night	15% 36% 8% 30% 10% 1	3917
	21st to end	Night	22% 37% 6% 26% 9% 1	4610
November	1st to 10th	Night	25% 54% 7% 10% 3 1	2780
	11th to 20th	Night	21% 56% 8% 11% 4 1	2940
	21st to end	Night	25% 55% 6% 10% 5%	2940
December	1st to 10th	Night	39% 36% 13% 9% 3	4806
	11th to 20th	Night	41% 34% 13% 9% 3	5464
	21st to end	Night	27% 26% 9% 19% 17% 4	7421

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	11% 14% 32% 31% 9% 2	265
	11th to 20th	Day	12% 15% 37% 26% 8% 2	330
	21st to end	Day	11% 17% 35% 29% 8%	363
February	1st to 10th	Day	16% 14% 41% 24% 5% 1	330
	11th to 20th	Day	13% 6% 42% 27% 11% 2	330
	21st to end	Day	7% 12% 35% 35% 11%	273
March	1st to 10th	Day	15% 13% 38% 25% 8% 2	330
	11th to 20th	Day	12% 17% 36% 24% 10% 2	477
	21st to end	Day	12% 17% 36% 22% 10% 2	594
April	1st to 10th	Day	12% 16% 40% 27% 5%	540
	11th to 20th	Day	19% 17% 36% 21% 6% 1	532
	21st to end	Day	16% 13% 29% 27% 14% 1	711
May	1st to 10th	Day	13% 9% 27% 31% 16% 3	753
	11th to 20th	Day	18% 12% 24% 25% 17% 5%	775
	21st to end	Day	15% 11% 28% 27% 14% 5%	844
June	1st to 10th	Day	16% 15% 26% 26% 16% 3	780
	11th to 20th	Day	15% 15% 32% 25% 10% 2	780
	21st to end	Day	10% 10% 27% 33% 18% 2	780
July	1st to 10th	Day	6% 5% 29% 37% 20% 4	722
	11th to 20th	Day	6% 8% 31% 34% 17% 4	718
	21st to end	Day	5% 7% 26% 36% 21% 5%	794
August	1st to 10th	Day	9% 7% 23% 36% 20% 6%	726
	11th to 20th	Day	7% 7% 25% 33% 23% 5%	630
	21st to end	Day	7% 8% 28% 37% 16% 4	583
September	1st to 10th	Day	11% 8% 30% 33% 14% 4	530
	11th to 20th	Day	15% 10% 28% 29% 17% 1	530
	21st to end	Day	17% 10% 27% 30% 12% 3	343
October	1st to 10th	Day	17% 8% 32% 24% 16% 3	330
	11th to 20th	Day	12% 12% 42% 20% 11% 4	330
	21st to end	Day	12% 18% 40% 21% 8% 1	363
November	1st to 10th	Day	15% 13% 38% 24% 9% 1	330
	11th to 20th	Day	12% 14% 34% 30% 8% 1	330
	21st to end	Day	11% 15% 39% 26% 9% 1	330
December	1st to 10th	Day	11% 14% 46% 21% 7% 2	330
	11th to 20th	Day	15% 14% 42% 21% 7% 1	330
	21st to end	Day	11% 8% 43% 27% 9% 3	286
January	1st to 10th	Night	8% 13% 40% 28% 10% 1	1050
	11th to 20th	Night	10% 17% 36% 29% 7% 1	1050
	21st to end	Night	10% 14% 40% 26% 9%	1155
February	1st to 10th	Night	11% 13% 34% 31% 11% 1	934
	11th to 20th	Night	10% 11% 40% 29% 9% 1	824
	21st to end	Night	7% 9% 35% 32% 15% 2	692
March	1st to 10th	Night	9% 12% 34% 34% 10% 1	840
	11th to 20th	Night	11% 10% 38% 32% 8% 1	840
	21st to end	Night	13% 9% 30% 35% 11% 2	762
April	1st to 10th	Night	11% 9% 33% 38% 8%	630
	11th to 20th	Night	15% 9% 40% 29% 6%	623
	21st to end	Night	14% 9% 36% 35% 7%	630
May	1st to 10th	Night	11% 8% 29% 38% 13% 1	630
	11th to 20th	Night	16% 8% 28% 33% 15%	621
	21st to end	Night	16% 8% 26% 33% 16% 1	683
June	1st to 10th	Night	13% 9% 30% 35% 12% 1	594
	11th to 20th	Night	15% 6% 29% 40% 10%	420
	21st to end	Night	9% 8% 28% 41% 12% 1	420
July	1st to 10th	Night	10% 4 21% 47% 17% 1	390
	11th to 20th	Night	7% 5% 31% 45% 13%	535
	21st to end	Night	4 4 29% 46% 17% 1	657
August	1st to 10th	Night	6% 4 25% 50% 14% 2	597
	11th to 20th	Night	5% 4 27% 43% 21% 1	594
	21st to end	Night	9% 4 25% 42% 18% 1	660
September	1st to 10th	Night	8% 5% 25% 45% 16% 2	600
	11th to 20th	Night	10% 6% 26% 43% 15% 1	600
	21st to end	Night	12% 8% 29% 40% 11% 1	663
October	1st to 10th	Night	16% 8% 26% 35% 15% 1	800
	11th to 20th	Night	13% 11% 31% 33% 10% 1	807
	21st to end	Night	12% 12% 40% 28% 7%	1100
November	1st to 10th	Night	18% 11% 35% 29% 6% 1	1025
	11th to 20th	Night	10% 13% 41% 26% 9% 1	1050
	21st to end	Night	10% 14% 40% 29% 7% 1	1050
December	1st to 10th	Night	12% 10% 38% 29% 10% 1	1050
	11th to 20th	Night	10% 15% 38% 28% 8% 1	1050
	21st to end	Night	11% 10% 39% 28% 10% 1	1155

Page generated at 2026-06-28 00:10:49. Version ac07

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