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 2025-02-25.

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% 41%	112566
	Night	6% 8% 3 9% 6% 5% 5% 4 6% 8% 40%	114806
Transparency	Day	22% 45% 9% 16% 7% 1	112303
	Night	24% 42% 10% 16% 7% 1	114658
Seeing	Day	12% 11% 31% 29% 14% 3	16376
	Night	11% 10% 34% 34% 11% 1	26054

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% 13% 5% 12% 7% 9% 8% 6% 7% 6% 22%	3376
	11th to 20th	Day	2 7% 4 13% 8% 7% 9% 10% 8% 7% 25%	3640
	21st to end	Day	4 8% 4 13% 5% 4 4 4 6% 7% 41%	4004
February	1st to 10th	Day	7% 13% 5% 13% 8% 6% 6% 4 7% 8% 23%	1729
	11th to 20th	Day	5% 12% 4 11% 7% 8% 6% 5% 7% 6% 28%	1755
	21st to end	Day	4 11% 4 11% 9% 5% 7% 6% 7% 8% 26%	1563
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%	2041
October	1st to 10th	Day	9% 6% 2 5% 3 3 3 3 4 6% 55%	3170
	11th to 20th	Day	7% 5% 2 6% 4 3 3 4 6% 11% 49%	3099
	21st to end	Day	4 9% 3 8% 4 4 5% 4 7% 8% 43%	3091
November	1st to 10th	Day	9% 13% 5% 9% 5% 5% 5% 3 5% 6% 36%	1650
	11th to 20th	Day	5% 10% 5% 11% 6% 6% 5% 4 7% 6% 36%	1600
	21st to end	Day	7% 11% 5% 9% 7% 5% 5% 5% 6% 8% 33%	1600
December	1st to 10th	Day	4 11% 4 13% 6% 7% 7% 5% 5% 7% 31%	2500
	11th to 20th	Day	8% 14% 5% 12% 6% 6% 7% 6% 6% 7% 24%	2860
	21st to end	Day	5% 7% 3 9% 7% 5% 4 3 5% 9% 42%	3682
January	1st to 10th	Night	3 9% 4 15% 8% 6% 6% 5% 7% 7% 29%	6571
	11th to 20th	Night	3 6% 3 14% 9% 8% 9% 6% 9% 8% 26%	6340
	21st to end	Night	2 9% 4 12% 7% 4 5% 4 6% 8% 38%	6974
February	1st to 10th	Night	6% 13% 4 10% 7% 6% 5% 5% 8% 8% 27%	2690
	11th to 20th	Night	4 11% 4 12% 7% 6% 6% 4 7% 9% 30%	2532
	21st to end	Night	4 10% 4 10% 6% 6% 6% 4 7% 8% 35%	1984
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%	2136
October	1st to 10th	Night	8% 5% 2 6% 3 4 4 3 5% 7% 53%	3800
	11th to 20th	Night	5% 5% 2 5% 4 3 4 4 6% 9% 52%	3796
	21st to end	Night	6% 7% 3 7% 5% 4 5% 4 7% 8% 44%	4467
November	1st to 10th	Night	12% 12% 4 7% 4 4 5% 4 6% 8% 35%	2644
	11th to 20th	Night	6% 10% 4 11% 7% 6% 5% 4 6% 7% 34%	2800
	21st to end	Night	5% 11% 5% 11% 6% 6% 6% 4 6% 8% 34%	2800
December	1st to 10th	Night	5% 12% 6% 15% 8% 6% 6% 3 5% 6% 27%	4663
	11th to 20th	Night	4 12% 6% 13% 8% 8% 6% 5% 6% 7% 25%	5319
	21st to end	Night	3 8% 4 10% 7% 4 3 3 5% 7% 45%	7260

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% 30% 9% 9% 4	3376
	11th to 20th	Day	39% 29% 10% 13% 9%	3640
	21st to end	Day	27% 29% 19% 22% 2	4004
February	1st to 10th	Day	23% 62% 4 6% 5%	1729
	11th to 20th	Day	21% 61% 5% 5% 8%	1764
	21st to end	Day	17% 64% 6% 8% 5%	1563
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	2041
October	1st to 10th	Day	19% 36% 5% 23% 13% 3	3170
	11th to 20th	Day	15% 37% 7% 29% 10% 1	3099
	21st to end	Day	22% 37% 9% 26% 6%	3091
November	1st to 10th	Day	23% 57% 6% 11% 3	1650
	11th to 20th	Day	19% 58% 8% 10% 4 1	1600
	21st to end	Day	21% 58% 8% 11% 3	1600
December	1st to 10th	Day	31% 38% 16% 12% 3	2500
	11th to 20th	Day	40% 34% 17% 5% 3	2858
	21st to end	Day	25% 27% 6% 26% 12% 5%	3682
January	1st to 10th	Night	41% 29% 12% 12% 5%	6571
	11th to 20th	Night	39% 28% 10% 11% 11% 1	6340
	21st to end	Night	28% 30% 18% 21% 2	6974
February	1st to 10th	Night	21% 62% 5% 7% 4	2690
	11th to 20th	Night	18% 61% 6% 8% 7% 1	2533
	21st to end	Night	13% 64% 6% 10% 7%	1984
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	17% 60% 5% 11% 7% 1	2136
October	1st to 10th	Night	20% 37% 5% 22% 14% 2	3800
	11th to 20th	Night	14% 37% 8% 31% 10% 1	3796
	21st to end	Night	22% 38% 6% 26% 8%	4467
November	1st to 10th	Night	23% 57% 7% 9% 2 1	2644
	11th to 20th	Night	19% 59% 8% 10% 4 1	2800
	21st to end	Night	23% 57% 6% 9% 5%	2800
December	1st to 10th	Night	38% 37% 13% 9% 3	4663
	11th to 20th	Night	40% 35% 12% 9% 2	5320
	21st to end	Night	26% 26% 8% 19% 17% 4	7260

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% 15% 31% 32% 10% 2	241
	11th to 20th	Day	9% 14% 37% 28% 9% 2	300
	21st to end	Day	10% 16% 37% 29% 8%	330
February	1st to 10th	Day	16% 13% 40% 25% 5% 1	300
	11th to 20th	Day	13% 6% 43% 27% 11% 1	300
	21st to end	Day	8% 11% 32% 38% 11%	237
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% 27% 11% 3	313
October	1st to 10th	Day	19% 8% 31% 23% 17% 3	300
	11th to 20th	Day	9% 11% 43% 21% 12% 4	300
	21st to end	Day	12% 18% 41% 21% 8% 2	330
November	1st to 10th	Day	15% 13% 36% 25% 10% 1	300
	11th to 20th	Day	13% 15% 35% 28% 8% 1	300
	21st to end	Day	11% 16% 38% 26% 9% 1	300
December	1st to 10th	Day	9% 15% 45% 22% 7% 2	300
	11th to 20th	Day	15% 13% 42% 22% 7% 1	300
	21st to end	Day	11% 7% 40% 29% 10% 3	260
January	1st to 10th	Night	9% 14% 39% 28% 10% 1	1000
	11th to 20th	Night	9% 16% 37% 30% 8% 1	1000
	21st to end	Night	9% 14% 40% 27% 9%	1100
February	1st to 10th	Night	11% 13% 33% 30% 11% 1	890
	11th to 20th	Night	9% 11% 40% 29% 9% 1	784
	21st to end	Night	7% 7% 35% 32% 16% 3	646
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% 10% 1	630
October	1st to 10th	Night	16% 8% 27% 33% 15% 1	760
	11th to 20th	Night	12% 12% 30% 34% 11% 1	766
	21st to end	Night	12% 12% 40% 28% 8%	1045
November	1st to 10th	Night	18% 11% 34% 30% 6% 1	975
	11th to 20th	Night	10% 13% 40% 26% 9% 1	1000
	21st to end	Night	10% 14% 40% 29% 7% 1	1000
December	1st to 10th	Night	12% 10% 37% 29% 10% 1	1000
	11th to 20th	Night	10% 14% 38% 29% 8% 1	1000
	21st to end	Night	11% 10% 39% 28% 11% 2	1100

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