Corona
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Corona clear sky chart from 2005-11-06 to 2025-10-19.

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	3 3 1 3 2 2 3 3 4 6% 70%	135813
	Night	3 3 1 3 2 3 3 3 4 6% 67%	131464
Transparency	Day	16% 3 6% 16% 28% 31%	130903
	Night	17% 4 8% 16% 25% 29%	127347
Seeing	Day	4 6% 19% 35% 25% 11%	17097
	Night	5% 2 10% 36% 38% 9%	26944

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	4 2 1 3 2 2 3 3 3 6% 72%	4098
	11th to 20th	Day	4 2 1 2 3 2 3 3 5% 6% 69%	4358
	21st to end	Day	8% 4 2 4 3 4 4 3 5% 9% 53%	5016
February	1st to 10th	Day	7% 4 2 4 3 3 4 3 5% 5% 61%	2000
	11th to 20th	Day	7% 4 1 4 3 3 3 4 6% 6% 61%	1950
	21st to end	Day	5% 5% 3 6% 6% 3 4 3 4 6% 56%	1650
March	1st to 10th	Day	5% 4 1 5% 5% 5% 5% 5% 7% 8% 50%	4518
	11th to 20th	Day	5% 3 2 5% 4 4 4 4 6% 7% 56%	4705
	21st to end	Day	6% 4 1 3 3 3 4 4 6% 9% 57%	5898
April	1st to 10th	Day	4 4 1 4 4 3 4 3 5% 7% 62%	2400
	11th to 20th	Day	3 3 1 4 4 3 4 3 5% 6% 65%	2369
	21st to end	Day	2 3 1 4 3 3 3 3 4 6% 67%	2400
May	1st to 10th	Day	2 3 2 5% 3 3 3 2 4 5% 69%	5677
	11th to 20th	Day	1 4 1 4 3 2 3 3 5% 7% 66%	5841
	21st to end	Day	1 4 2 4 3 3 3 2 5% 7% 67%	6423
June	1st to 10th	Day	2 5% 2 4 3 3 3 3 5% 7% 64%	2600
	11th to 20th	Day	1 3 1 3 2 2 2 2 3 6% 75%	2657
	21st to end	Day	1 2 1 3 2 2 2 2 3 4 79%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 3 89%	6319
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 5% 81%	5932
	21st to end	Day	1 2 1 2 1 1 2 2 3 4 83%	6100
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 87%	5346
	11th to 20th	Day	1 1 1 1 2 1 2 3 89%	5364
	21st to end	Day	1 1 2 1 1 2 2 3 4 84%	5984
September	1st to 10th	Day	2 2 1 2 2 1 2 2 4 5% 79%	2400
	11th to 20th	Day	2 2 1 3 2 2 2 1 3 4 79%	2218
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 79%	2200
October	1st to 10th	Day	2 1 1 2 2 2 2 2 3 4 80%	3600
	11th to 20th	Day	2 3 1 2 2 2 2 2 3 6% 75%	3448
	21st to end	Day	2 2 1 2 2 3 4 3 5% 7% 70%	3309
November	1st to 10th	Day	3 2 1 2 2 2 2 2 3 7% 73%	1764
	11th to 20th	Day	3 1 1 3 2 2 2 3 5% 6% 71%	1800
	21st to end	Day	3 3 1 5% 4 3 3 4 4 7% 61%	1800
December	1st to 10th	Day	3 2 1 2 3 3 4 3 6% 8% 66%	2950
	11th to 20th	Day	6% 4 1 4 3 4 5% 4 7% 10% 52%	3409
	21st to end	Day	6% 4 2 5% 5% 5% 6% 5% 6% 9% 47%	4510
January	1st to 10th	Night	3 3 1 3 2 3 3 3 4 7% 70%	5867
	11th to 20th	Night	3 3 1 3 3 4 4 3 4 7% 65%	5616
	21st to end	Night	9% 7% 2 6% 3 4 5% 4 6% 6% 49%	5984
February	1st to 10th	Night	7% 5% 2 4 4 2 3 2 5% 6% 60%	2400
	11th to 20th	Night	6% 4 1 4 3 3 3 4 6% 6% 61%	2338
	21st to end	Night	8% 5% 2 4 4 4 4 3 4 5% 56%	1980
March	1st to 10th	Night	6% 5% 2 5% 4 4 5% 4 7% 10% 47%	5376
	11th to 20th	Night	4 5% 2 5% 4 4 5% 4 6% 8% 53%	5293
	21st to end	Night	4 4 1 5% 3 4 4 4 6% 8% 56%	5060
April	1st to 10th	Night	5% 5% 1 5% 3 3 3 3 6% 8% 58%	2000
	11th to 20th	Night	4 5% 1 4 3 2 4 2 4 6% 65%	1979
	21st to end	Night	3 5% 2 3 3 3 3 3 6% 6% 64%	2000
May	1st to 10th	Night	4 6% 1 5% 3 3 3 2 4 5% 64%	4297
	11th to 20th	Night	1 4 1 4 2 3 3 3 5% 7% 67%	4074
	21st to end	Night	1 4 1 3 2 2 2 2 4 5% 72%	4474
June	1st to 10th	Night	2 4 1 3 2 2 3 3 5% 7% 68%	1800
	11th to 20th	Night	2 2 2 2 2 2 2 4 5% 77%	1732
	21st to end	Night	1 1 1 2 1 2 2 2 4 4 80%	1600
July	1st to 10th	Night	1 1 1 2 1 2 3 90%	3640
	11th to 20th	Night	1 1 1 2 2 1 3 4 83%	4026
	21st to end	Night	1 2 1 2 1 1 2 1 2 3 84%	4782
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	4486
	11th to 20th	Night	1 1 1 1 1 1 2 4 87%	4506
	21st to end	Night	1 1 1 1 2 2 2 3 4 83%	5015
September	1st to 10th	Night	2 1 1 4 2 2 1 2 3 5% 76%	2000
	11th to 20th	Night	1 3 1 2 1 1 1 2 2 4 82%	2180
	21st to end	Night	1 3 1 2 1 1 2 2 2 4 80%	2200
October	1st to 10th	Night	1 1 1 2 2 2 2 1 3 4 82%	3600
	11th to 20th	Night	3 6% 2 4 2 3 2 2 3 6% 68%	3663
	21st to end	Night	1 2 1 2 2 2 3 3 4 5% 75%	4242
November	1st to 10th	Night	3 3 1 3 2 3 3 3 4 6% 70%	2535
	11th to 20th	Night	2 2 2 4 2 2 3 2 4 6% 70%	2600
	21st to end	Night	5% 3 2 4 3 3 4 4 5% 9% 57%	2600
December	1st to 10th	Night	4 2 1 3 3 3 3 3 5% 6% 67%	4240
	11th to 20th	Night	6% 5% 1 5% 4 4 5% 4 7% 8% 51%	4811
	21st to end	Night	5% 5% 2 6% 4 4 5% 4 8% 12% 44%	6468

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	15% 3 5% 19% 32% 26%	4099
	11th to 20th	Day	16% 5% 8% 19% 19% 33%	4358
	21st to end	Day	26% 4 6% 13% 27% 24%	5016
February	1st to 10th	Day	26% 5% 5% 13% 24% 27%	2000
	11th to 20th	Day	24% 4 5% 18% 25% 24%	1960
	21st to end	Day	31% 4 7% 15% 21% 22%	1650
March	1st to 10th	Day	26% 6% 9% 18% 23% 18%	4518
	11th to 20th	Day	24% 5% 10% 28% 21% 12%	4705
	21st to end	Day	23% 4 12% 31% 19% 11%	5898
April	1st to 10th	Day	22% 5% 11% 23% 25% 15%	2400
	11th to 20th	Day	19% 4 7% 22% 27% 20%	2372
	21st to end	Day	18% 4 8% 22% 34% 14%	2400
May	1st to 10th	Day	18% 4 7% 21% 31% 21%	5677
	11th to 20th	Day	15% 3 9% 22% 37% 13%	5844
	21st to end	Day	15% 4 7% 23% 39% 13%	6424
June	1st to 10th	Day	18% 3 10% 22% 31% 16%	2600
	11th to 20th	Day	12% 2 6% 14% 30% 37%	2657
	21st to end	Day	10% 2 4 9% 27% 48%	2800
July	1st to 10th	Day	5% 1 2 5% 18% 70%	6251
	11th to 20th	Day	7% 2 3 11% 37% 40%	5861
	21st to end	Day	7% 2 4 15% 34% 38%	5925
August	1st to 10th	Day	8% 1 2 11% 43% 35%	3890
	11th to 20th	Day	4 1 3 13% 28% 50%	3880
	21st to end	Day	6% 1 2 7% 28% 55%	4311
September	1st to 10th	Day	9% 3 5% 8% 32% 43%	2400
	11th to 20th	Day	10% 2 4 9% 36% 40%	2218
	21st to end	Day	10% 2 3 15% 31% 39%	2200
October	1st to 10th	Day	10% 2 3 11% 17% 57%	3600
	11th to 20th	Day	13% 2 3 13% 35% 34%	3448
	21st to end	Day	13% 3 3 9% 31% 41%	3309
November	1st to 10th	Day	14% 3 6% 15% 27% 36%	1764
	11th to 20th	Day	16% 3 6% 14% 22% 39%	1800
	21st to end	Day	23% 4 8% 12% 21% 32%	1800
December	1st to 10th	Day	17% 3 5% 13% 25% 37%	2950
	11th to 20th	Day	25% 4 8% 16% 25% 21%	3408
	21st to end	Day	32% 4 8% 23% 23% 9%	4510
January	1st to 10th	Night	16% 3 5% 15% 34% 26%	5867
	11th to 20th	Night	18% 5% 9% 20% 17% 31%	5616
	21st to end	Night	30% 5% 7% 12% 22% 23%	5984
February	1st to 10th	Night	26% 3 7% 15% 23% 27%	2400
	11th to 20th	Night	24% 5% 7% 17% 21% 25%	2339
	21st to end	Night	30% 4 7% 16% 19% 24%	1980
March	1st to 10th	Night	27% 7% 13% 17% 21% 15%	5376
	11th to 20th	Night	25% 7% 11% 25% 20% 11%	5293
	21st to end	Night	22% 7% 17% 23% 22% 9%	5060
April	1st to 10th	Night	23% 6% 12% 22% 20% 17%	2000
	11th to 20th	Night	19% 4 11% 21% 23% 20%	1979
	21st to end	Night	19% 4 12% 23% 27% 14%	2000
May	1st to 10th	Night	20% 5% 11% 21% 29% 14%	4294
	11th to 20th	Night	15% 3 17% 27% 30% 9%	4075
	21st to end	Night	13% 5% 13% 27% 30% 13%	4474
June	1st to 10th	Night	15% 6% 16% 21% 26% 16%	1800
	11th to 20th	Night	11% 3 8% 17% 28% 34%	1732
	21st to end	Night	7% 2 6% 13% 26% 46%	1600
July	1st to 10th	Night	3 1 3 6% 18% 70%	3600
	11th to 20th	Night	6% 2 4 13% 36% 40%	3974
	21st to end	Night	7% 2 4 15% 34% 38%	4645
August	1st to 10th	Night	6% 1 3 14% 43% 33%	3256
	11th to 20th	Night	4 1 4 14% 28% 49%	3256
	21st to end	Night	5% 1 3 8% 25% 58%	3608
September	1st to 10th	Night	12% 2 5% 10% 28% 42%	2000
	11th to 20th	Night	9% 2 6% 11% 32% 40%	2180
	21st to end	Night	11% 2 4 13% 32% 37%	2200
October	1st to 10th	Night	9% 2 4 11% 18% 57%	3600
	11th to 20th	Night	18% 2 5% 11% 29% 34%	3663
	21st to end	Night	11% 3 3 9% 28% 46%	4242
November	1st to 10th	Night	15% 3 7% 14% 24% 37%	2535
	11th to 20th	Night	17% 3 5% 13% 21% 40%	2600
	21st to end	Night	24% 5% 8% 14% 18% 32%	2600
December	1st to 10th	Night	17% 3 4 13% 23% 39%	4240
	11th to 20th	Night	27% 4 9% 14% 25% 20%	4811
	21st to end	Night	28% 6% 12% 23% 22% 9%	6468

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% 5% 17% 40% 22% 5%	300
	11th to 20th	Day	10% 9% 20% 34% 21% 5%	415
	21st to end	Day	9% 10% 26% 32% 20% 3	550
February	1st to 10th	Day	10% 10% 26% 31% 16% 7%	500
	11th to 20th	Day	8% 11% 29% 30% 18% 3	495
	21st to end	Day	8% 14% 28% 33% 15% 2	414
March	1st to 10th	Day	7% 17% 27% 32% 14% 3	500
	11th to 20th	Day	8% 12% 29% 33% 14% 4	500
	21st to end	Day	7% 13% 31% 32% 13% 4	550
April	1st to 10th	Day	7% 9% 26% 32% 19% 7%	500
	11th to 20th	Day	3 10% 35% 32% 15% 6%	492
	21st to end	Day	2 11% 28% 30% 18% 10%	500
May	1st to 10th	Day	5% 9% 27% 36% 18% 5%	503
	11th to 20th	Day	3 7% 26% 38% 20% 7%	528
	21st to end	Day	4 6% 23% 38% 21% 7%	573
June	1st to 10th	Day	3 3 16% 37% 28% 13%	530
	11th to 20th	Day	2 2 18% 36% 28% 14%	530
	21st to end	Day	1 1 13% 38% 32% 16%	530
July	1st to 10th	Day	2 1 8% 32% 37% 20%	526
	11th to 20th	Day	1 1 3 33% 37% 25%	524
	21st to end	Day	1 1 5% 33% 37% 23%	575
August	1st to 10th	Day	2 6% 38% 35% 20%	527
	11th to 20th	Day	2 5% 33% 37% 23%	517
	21st to end	Day	2 1 7% 35% 35% 21%	583
September	1st to 10th	Day	2 1 9% 41% 30% 17%	530
	11th to 20th	Day	3 2 14% 43% 28% 11%	530
	21st to end	Day	1 2 18% 33% 29% 16%	530
October	1st to 10th	Day	3 3 18% 36% 28% 12%	530
	11th to 20th	Day	2 5% 19% 33% 29% 12%	523
	21st to end	Day	4 3 18% 34% 26% 15%	462
November	1st to 10th	Day	4 4 19% 32% 30% 11%	300
	11th to 20th	Day	2 3 17% 37% 29% 11%	300
	21st to end	Day	4 7% 17% 36% 26% 10%	300
December	1st to 10th	Day	7% 5% 14% 37% 29% 9%	300
	11th to 20th	Day	4 6% 17% 33% 30% 10%	300
	21st to end	Day	9% 10% 21% 35% 19% 5%	330
January	1st to 10th	Night	9% 3 15% 40% 24% 8%	800
	11th to 20th	Night	6% 3 15% 43% 28% 6%	800
	21st to end	Night	9% 4 17% 41% 25% 5%	880
February	1st to 10th	Night	8% 3 17% 40% 24% 8%	800
	11th to 20th	Night	7% 3 17% 42% 24% 6%	784
	21st to end	Night	10% 6% 17% 42% 21% 5%	660
March	1st to 10th	Night	6% 3 17% 48% 22% 4	800
	11th to 20th	Night	6% 5% 15% 40% 27% 7%	800
	21st to end	Night	6% 4 17% 42% 28% 4	880
April	1st to 10th	Night	7% 3 12% 42% 32% 4	800
	11th to 20th	Night	5% 3 14% 42% 31% 4	791
	21st to end	Night	6% 4 9% 40% 35% 7%	800
May	1st to 10th	Night	6% 3 11% 40% 36% 4	693
	11th to 20th	Night	2 2 16% 36% 40% 5%	591
	21st to end	Night	3 2 12% 42% 37% 4	650
June	1st to 10th	Night	2 3 10% 37% 43% 5%	600
	11th to 20th	Night	1 2 6% 38% 50% 4	532
	21st to end	Night	1 1 3 27% 62% 7%	400
July	1st to 10th	Night	1 17% 71% 11%	390
	11th to 20th	Night	2 1 21% 65% 11%	549
	21st to end	Night	2 1 16% 65% 14%	788
August	1st to 10th	Night	2 1 1 22% 58% 16%	796
	11th to 20th	Night	1 1 16% 64% 17%	792
	21st to end	Night	1 1 18% 60% 19%	880
September	1st to 10th	Night	3 3 22% 55% 17%	800
	11th to 20th	Night	3 5% 29% 51% 12%	800
	21st to end	Night	3 1 6% 31% 47% 12%	800
October	1st to 10th	Night	3 1 6% 41% 43% 7%	800
	11th to 20th	Night	4 1 8% 34% 42% 10%	792
	21st to end	Night	3 2 11% 36% 36% 13%	836
November	1st to 10th	Night	4 2 9% 37% 37% 11%	780
	11th to 20th	Night	4 1 9% 42% 34% 11%	800
	21st to end	Night	6% 3 11% 43% 31% 7%	800
December	1st to 10th	Night	7% 3 13% 40% 29% 8%	800
	11th to 20th	Night	10% 3 15% 41% 26% 6%	800
	21st to end	Night	8% 4 18% 40% 23% 6%	880

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