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 2024-09-29.

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 7% 5% 5% 5% 4 6% 8% 41%	101558
	Night	7% 8% 3 8% 5% 5% 5% 4 6% 8% 41%	96798
Transparency	Day	20% 49% 9% 15% 7% 1	101295
	Night	19% 50% 9% 15% 6% 1	96650
Seeing	Day	12% 11% 31% 29% 14% 3	15941
	Night	11% 10% 33% 34% 11% 1	25352

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% 10% 4 9% 7% 8% 9% 5% 7% 8% 27%	2346
	11th to 20th	Day	3 7% 3 12% 9% 9% 9% 10% 10% 7% 20%	2540
	21st to end	Day	4 9% 3 10% 6% 4 5% 4 6% 8% 39%	2794
February	1st to 10th	Day	7% 13% 5% 13% 8% 7% 6% 4 7% 7% 23%	1642
	11th to 20th	Day	6% 13% 4 12% 8% 8% 6% 5% 7% 6% 26%	1665
	21st to end	Day	4 11% 4 12% 9% 5% 7% 6% 7% 8% 26%	1525
March	1st to 10th	Day	6% 12% 6% 14% 7% 5% 4 3 5% 6% 32%	3130
	11th to 20th	Day	6% 10% 5% 8% 5% 5% 5% 4 6% 9% 38%	3394
	21st to end	Day	9% 16% 7% 12% 6% 5% 5% 4 5% 6% 24%	4065
April	1st to 10th	Day	8% 8% 3 8% 5% 5% 5% 5% 5% 9% 39%	2280
	11th to 20th	Day	12% 14% 5% 9% 5% 5% 5% 5% 6% 7% 27%	2251
	21st to end	Day	14% 9% 3 8% 6% 5% 4 3 5% 6% 36%	2592
May	1st to 10th	Day	16% 11% 3 7% 5% 4 5% 4 6% 7% 32%	4430
	11th to 20th	Day	11% 7% 3 7% 6% 6% 5% 5% 7% 8% 35%	4388
	21st to end	Day	14% 7% 3 7% 5% 5% 6% 4 6% 8% 35%	4825
June	1st to 10th	Day	14% 6% 2 6% 4 5% 5% 5% 6% 8% 39%	2688
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	2850
	21st to end	Day	7% 5% 2 6% 6% 5% 5% 5% 8% 8% 45%	2850
July	1st to 10th	Day	4 3 2 6% 5% 6% 6% 5% 7% 9% 46%	4740
	11th to 20th	Day	3 3 1 3 3 4 5% 5% 6% 9% 57%	4497
	21st to end	Day	4 3 1 4 3 4 4 4 6% 8% 61%	4802
August	1st to 10th	Day	9% 4 1 4 3 3 4 4 6% 8% 54%	4411
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 7% 10% 53%	4197
	21st to end	Day	7% 4 2 4 3 3 4 3 5% 6% 58%	4256
September	1st to 10th	Day	11% 5% 2 5% 3 3 4 4 5% 6% 51%	2280
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 48%	2280
	21st to end	Day	12% 9% 2 4 3 4 4 3 5% 7% 48%	2021
October	1st to 10th	Day	15% 9% 3 7% 4 3 4 3 4 5% 44%	1800
	11th to 20th	Day	10% 6% 3 8% 5% 4 4 4 5% 8% 42%	1775
	21st to end	Day	8% 11% 4 11% 3 4 5% 3 6% 7% 37%	1782
November	1st to 10th	Day	10% 13% 5% 9% 5% 5% 4 3 5% 6% 36%	1566
	11th to 20th	Day	5% 9% 5% 11% 6% 6% 5% 4 7% 6% 36%	1520
	21st to end	Day	7% 11% 5% 9% 6% 5% 5% 5% 6% 8% 34%	1520
December	1st to 10th	Day	6% 13% 5% 11% 8% 7% 7% 6% 5% 7% 26%	1456
	11th to 20th	Day	5% 11% 4 13% 6% 5% 6% 4 7% 7% 32%	1830
	21st to end	Day	4 8% 3 8% 4 5% 3 4 5% 8% 47%	2570
January	1st to 10th	Night	3 8% 4 12% 6% 6% 6% 5% 8% 9% 33%	4596
	11th to 20th	Night	3 6% 3 12% 9% 9% 9% 7% 8% 8% 25%	4430
	21st to end	Night	3 11% 4 10% 5% 4 5% 4 6% 8% 39%	4873
February	1st to 10th	Night	7% 13% 4 10% 7% 6% 5% 5% 7% 8% 27%	2556
	11th to 20th	Night	4 12% 4 12% 7% 6% 6% 5% 7% 9% 29%	2402
	21st to end	Night	4 10% 4 10% 6% 6% 6% 4 6% 8% 35%	1928
March	1st to 10th	Night	5% 9% 4 11% 7% 5% 5% 4 6% 9% 34%	3734
	11th to 20th	Night	4 8% 3 9% 5% 5% 5% 4 6% 10% 41%	3568
	21st to end	Night	10% 13% 6% 13% 7% 5% 5% 4 6% 7% 24%	3579
April	1st to 10th	Night	8% 8% 3 7% 6% 4 5% 4 8% 7% 41%	1900
	11th to 20th	Night	12% 12% 3 8% 3 5% 5% 3 8% 7% 35%	1877
	21st to end	Night	11% 8% 3 6% 5% 4 4 4 6% 8% 41%	1582
May	1st to 10th	Night	15% 10% 3 7% 4 4 4 4 7% 6% 36%	2540
	11th to 20th	Night	10% 7% 2 6% 5% 5% 6% 4 7% 8% 41%	2517
	21st to end	Night	13% 7% 2 5% 5% 4 4 4 6% 9% 40%	2762
June	1st to 10th	Night	12% 5% 1 5% 5% 4 4 3 6% 8% 47%	1488
	11th to 20th	Night	12% 6% 2 6% 3 4 4 3 6% 7% 47%	1330
	21st to end	Night	6% 5% 1 4 4 5% 3 4 6% 9% 53%	1330
July	1st to 10th	Night	5% 3 2 4 3 4 4 5% 8% 11% 52%	2220
	11th to 20th	Night	5% 3 2 6% 4 4 4 4 6% 8% 56%	2466
	21st to end	Night	2 2 1 4 3 3 3 4 5% 9% 63%	2761
August	1st to 10th	Night	8% 5% 2 4 3 3 3 3 4 6% 60%	2528
	11th to 20th	Night	4 3 1 4 3 3 4 4 6% 9% 58%	2639
	21st to end	Night	6% 4 2 4 4 3 4 3 6% 8% 57%	3409
September	1st to 10th	Night	7% 4 1 4 4 4 3 3 6% 6% 58%	1900
	11th to 20th	Night	10% 5% 2 5% 4 3 4 3 5% 7% 52%	1900
	21st to end	Night	12% 5% 2 5% 3 4 4 3 4 6% 52%	2124
October	1st to 10th	Night	12% 7% 2 7% 3 4 4 3 5% 7% 46%	2160
	11th to 20th	Night	8% 8% 3 7% 5% 3 4 3 5% 6% 47%	2165
	21st to end	Night	9% 10% 3 8% 5% 4 5% 4 6% 8% 38%	2574
November	1st to 10th	Night	12% 12% 4 7% 5% 4 5% 4 6% 8% 35%	2508
	11th to 20th	Night	5% 10% 4 11% 7% 6% 5% 4 7% 7% 34%	2660
	21st to end	Night	5% 10% 5% 10% 5% 5% 6% 4 6% 8% 36%	2660
December	1st to 10th	Night	6% 10% 4 11% 8% 7% 6% 4 6% 7% 30%	2711
	11th to 20th	Night	4 11% 5% 12% 6% 7% 6% 5% 7% 8% 30%	3353
	21st to end	Night	4 8% 3 7% 5% 4 3 3 5% 7% 51%	5068

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	36% 41% 11% 9% 3	2346
	11th to 20th	Day	35% 41% 9% 12% 3	2540
	21st to end	Day	20% 41% 11% 25% 3	2794
February	1st to 10th	Day	22% 66% 4 5% 4	1642
	11th to 20th	Day	20% 64% 4 4 6%	1674
	21st to end	Day	16% 66% 5% 8% 5%	1525
March	1st to 10th	Day	29% 41% 9% 18% 3	3130
	11th to 20th	Day	22% 43% 16% 14% 4	3394
	21st to end	Day	39% 42% 8% 7% 4	4065
April	1st to 10th	Day	14% 66% 8% 10% 2	2280
	11th to 20th	Day	16% 66% 4 10% 4	2254
	21st to end	Day	18% 65% 5% 8% 4	2592
May	1st to 10th	Day	30% 44% 11% 12% 4	4430
	11th to 20th	Day	24% 44% 9% 13% 9%	4392
	21st to end	Day	20% 45% 12% 18% 5%	4825
June	1st to 10th	Day	11% 66% 7% 11% 4 1	2688
	11th to 20th	Day	14% 66% 7% 8% 5% 1	2850
	21st to end	Day	16% 65% 5% 11% 3	2850
July	1st to 10th	Day	20% 40% 14% 14% 11% 1	4666
	11th to 20th	Day	12% 37% 9% 26% 14% 1	4419
	21st to end	Day	12% 36% 5% 24% 14% 9%	4651
August	1st to 10th	Day	19% 38% 8% 22% 13% 1	4396
	11th to 20th	Day	15% 39% 13% 24% 9%	4239
	21st to end	Day	18% 38% 9% 27% 8% 1	4255
September	1st to 10th	Day	12% 61% 7% 14% 5% 2	2280
	11th to 20th	Day	17% 61% 6% 10% 4 2	2280
	21st to end	Day	19% 60% 5% 8% 6% 1	2021
October	1st to 10th	Day	20% 63% 6% 8% 3	1800
	11th to 20th	Day	13% 63% 4 12% 7% 1	1775
	21st to end	Day	18% 62% 6% 10% 4 1	1782
November	1st to 10th	Day	23% 60% 6% 9% 3 1	1566
	11th to 20th	Day	17% 60% 9% 9% 4 1	1520
	21st to end	Day	18% 60% 8% 11% 3	1520
December	1st to 10th	Day	20% 62% 8% 8% 3	1456
	11th to 20th	Day	21% 50% 17% 6% 5%	1828
	21st to end	Day	15% 37% 5% 22% 14% 7%	2570
January	1st to 10th	Night	28% 40% 15% 12% 5% 1	4596
	11th to 20th	Night	34% 38% 9% 14% 5%	4430
	21st to end	Night	20% 42% 15% 20% 3	4873
February	1st to 10th	Night	20% 65% 5% 7% 3	2556
	11th to 20th	Night	17% 64% 6% 8% 4	2403
	21st to end	Night	13% 66% 5% 9% 7%	1928
March	1st to 10th	Night	25% 42% 11% 15% 7%	3734
	11th to 20th	Night	20% 43% 18% 15% 4	3568
	21st to end	Night	38% 43% 9% 8% 2	3579
April	1st to 10th	Night	13% 66% 8% 10% 3	1900
	11th to 20th	Night	14% 66% 5% 9% 5%	1877
	21st to end	Night	14% 65% 4 12% 5%	1582
May	1st to 10th	Night	29% 42% 7% 17% 5%	2540
	11th to 20th	Night	21% 44% 12% 14% 9%	2518
	21st to end	Night	18% 43% 9% 23% 6%	2762
June	1st to 10th	Night	9% 64% 7% 12% 6% 2	1488
	11th to 20th	Night	13% 65% 5% 10% 4 1	1330
	21st to end	Night	12% 64% 7% 11% 5% 1	1330
July	1st to 10th	Night	15% 40% 20% 16% 8% 1	2185
	11th to 20th	Night	17% 37% 9% 20% 16% 1	2421
	21st to end	Night	11% 36% 6% 26% 13% 8%	2674
August	1st to 10th	Night	18% 38% 7% 22% 15% 1	2520
	11th to 20th	Night	14% 38% 10% 30% 8%	2663
	21st to end	Night	18% 38% 11% 26% 7% 1	3409
September	1st to 10th	Night	9% 61% 5% 16% 7% 2	1900
	11th to 20th	Night	15% 60% 5% 13% 6% 1	1900
	21st to end	Night	16% 60% 5% 10% 7% 1	2124
October	1st to 10th	Night	18% 64% 5% 9% 4	2160
	11th to 20th	Night	12% 63% 5% 13% 5% 1	2165
	21st to end	Night	18% 63% 6% 9% 4 1	2574
November	1st to 10th	Night	23% 60% 7% 8% 2 1	2508
	11th to 20th	Night	17% 61% 7% 10% 3 1	2660
	21st to end	Night	20% 60% 5% 9% 5%	2660
December	1st to 10th	Night	18% 61% 7% 10% 4	2711
	11th to 20th	Night	25% 52% 8% 11% 4 1	3354
	21st to end	Night	13% 37% 8% 20% 17% 5%	5068

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% 30% 31% 11% 3	216
	11th to 20th	Day	10% 14% 34% 29% 10% 3	270
	21st to end	Day	9% 15% 35% 31% 9%	297
February	1st to 10th	Day	16% 12% 40% 26% 4 1	270
	11th to 20th	Day	14% 6% 42% 26% 10% 1	270
	21st to end	Day	8% 9% 32% 39% 12%	225
March	1st to 10th	Day	15% 13% 36% 27% 7% 1	270
	11th to 20th	Day	11% 17% 35% 24% 11% 2	403
	21st to end	Day	12% 15% 35% 23% 12% 3	506
April	1st to 10th	Day	12% 18% 40% 26% 5%	460
	11th to 20th	Day	19% 17% 34% 21% 7% 1	452
	21st to end	Day	18% 12% 29% 27% 14% 1	615
May	1st to 10th	Day	14% 9% 27% 29% 17% 4	653
	11th to 20th	Day	15% 11% 23% 27% 18% 5%	675
	21st to end	Day	16% 11% 26% 26% 15% 6%	734
June	1st to 10th	Day	15% 14% 25% 27% 16% 3	680
	11th to 20th	Day	16% 14% 32% 24% 11% 2	680
	21st to end	Day	8% 10% 26% 33% 20% 3	680
July	1st to 10th	Day	6% 5% 28% 38% 20% 4	671
	11th to 20th	Day	6% 7% 31% 35% 17% 4	668
	21st to end	Day	4 8% 26% 36% 21% 5%	739
August	1st to 10th	Day	9% 6% 23% 36% 19% 6%	676
	11th to 20th	Day	8% 7% 24% 32% 23% 5%	584
	21st to end	Day	7% 8% 28% 38% 15% 4	539
September	1st to 10th	Day	11% 9% 31% 33% 13% 3	490
	11th to 20th	Day	16% 10% 29% 29% 16% 1	490
	21st to end	Day	18% 11% 29% 28% 11% 3	307
October	1st to 10th	Day	20% 6% 31% 23% 17% 3	270
	11th to 20th	Day	10% 10% 44% 20% 12% 4	270
	21st to end	Day	13% 19% 40% 20% 7% 2	297
November	1st to 10th	Day	16% 14% 36% 23% 10% 1	270
	11th to 20th	Day	10% 16% 34% 30% 9% 1	270
	21st to end	Day	10% 14% 39% 26% 10% 1	270
December	1st to 10th	Day	9% 13% 44% 24% 8% 2	270
	11th to 20th	Day	13% 14% 43% 21% 8% 1	270
	21st to end	Day	11% 8% 41% 26% 10% 3	234
January	1st to 10th	Night	8% 14% 39% 28% 10% 1	950
	11th to 20th	Night	10% 16% 36% 30% 8% 1	950
	21st to end	Night	10% 13% 40% 28% 10%	1045
February	1st to 10th	Night	11% 11% 33% 31% 12% 1	846
	11th to 20th	Night	10% 11% 41% 28% 9% 1	744
	21st to end	Night	7% 7% 35% 32% 16% 3	628
March	1st to 10th	Night	9% 12% 34% 34% 10% 1	760
	11th to 20th	Night	11% 10% 37% 31% 8% 1	760
	21st to end	Night	13% 8% 29% 37% 12% 2	690
April	1st to 10th	Night	11% 10% 33% 38% 8%	570
	11th to 20th	Night	16% 9% 39% 29% 7%	563
	21st to end	Night	15% 8% 34% 35% 8%	570
May	1st to 10th	Night	12% 8% 29% 38% 13% 1	570
	11th to 20th	Night	14% 6% 29% 35% 16% 1	561
	21st to end	Night	17% 8% 24% 33% 17% 1	617
June	1st to 10th	Night	13% 8% 29% 36% 12% 1	538
	11th to 20th	Night	14% 6% 29% 40% 10% 1	380
	21st to end	Night	8% 9% 27% 42% 14% 1	380
July	1st to 10th	Night	10% 3 21% 48% 17% 1	370
	11th to 20th	Night	6% 5% 31% 45% 13%	507
	21st to end	Night	3 4 30% 46% 17% 1	624
August	1st to 10th	Night	6% 4 25% 49% 14% 2	567
	11th to 20th	Night	5% 4 26% 43% 21% 1	564
	21st to end	Night	10% 4 25% 42% 18% 1	627
September	1st to 10th	Night	9% 4 25% 44% 16% 2	570
	11th to 20th	Night	10% 6% 26% 43% 15% 1	570
	21st to end	Night	12% 8% 29% 39% 11% 1	626
October	1st to 10th	Night	16% 9% 27% 33% 15% 1	720
	11th to 20th	Night	13% 12% 29% 33% 11% 2	725
	21st to end	Night	13% 12% 39% 28% 8%	990
November	1st to 10th	Night	18% 11% 35% 29% 6% 1	925
	11th to 20th	Night	9% 14% 40% 26% 10% 1	950
	21st to end	Night	10% 14% 41% 28% 7% 1	950
December	1st to 10th	Night	12% 10% 37% 29% 11% 1	950
	11th to 20th	Night	10% 14% 38% 29% 8% 1	950
	21st to end	Night	11% 10% 39% 27% 11% 1	1045

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