Visual Reconstructions Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Visual Reconstructions Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	101054
	Night	14% 10% 3 8% 5% 4 4 4 5% 6% 35%	97956
Transparency	Day	46% 7% 13% 21% 12% 1	104682
	Night	46% 6% 12% 21% 12% 1	102469
Seeing	Day	16% 11% 29% 24% 14% 5%	15361
	Night	18% 9% 29% 30% 12% 1	24993

By Month

Percentage Cloud Cover SE (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	16% 17% 6% 12% 5% 5% 5% 3 4 5% 22%	2527
	11th to 20th	Day	15% 17% 6% 13% 8% 9% 7% 4 5% 5% 12%	2533
	21st to end	Day	28% 18% 7% 10% 6% 5% 4 3 3 4 13%	3124
February	1st to 10th	Day	14% 16% 4 12% 7% 6% 5% 4 5% 5% 21%	1710
	11th to 20th	Day	13% 13% 5% 12% 7% 5% 5% 4 5% 5% 25%	1770
	21st to end	Day	14% 15% 4 11% 6% 4 4 4 5% 6% 26%	1570
March	1st to 10th	Day	18% 16% 4 9% 5% 4 4 3 4 6% 27%	3230
	11th to 20th	Day	13% 13% 4 9% 6% 6% 5% 3 4 7% 31%	3480
	21st to end	Day	20% 12% 3 9% 5% 5% 4 3 4 5% 28%	3815
April	1st to 10th	Day	14% 14% 3 7% 5% 4 4 4 5% 7% 31%	2185
	11th to 20th	Day	16% 11% 3 7% 5% 5% 4 4 5% 6% 34%	2428
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 5% 7% 34%	2470
May	1st to 10th	Day	17% 8% 2 7% 5% 4 4 4 6% 7% 35%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 9% 33%	4081
	21st to end	Day	10% 6% 2 7% 5% 4 5% 4 5% 7% 45%	4476
June	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 4 6% 9% 40%	2770
	11th to 20th	Day	10% 5% 2 7% 5% 4 5% 5% 6% 9% 42%	2850
	21st to end	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 9% 36%	2780
July	1st to 10th	Day	10% 5% 2 7% 6% 5% 6% 5% 7% 8% 37%	4440
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 5% 7% 11% 44%	4440
	21st to end	Day	6% 5% 3 7% 6% 6% 6% 5% 7% 9% 40%	4498
August	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 9% 40%	4104
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 5% 9% 10% 37%	4044
	21st to end	Day	9% 6% 2 7% 6% 6% 6% 5% 7% 9% 38%	4532
September	1st to 10th	Day	12% 5% 3 7% 5% 5% 5% 4 5% 8% 42%	2138
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 7% 8% 47%	2090
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	2090
October	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 3 6% 7% 43%	2019
	11th to 20th	Day	17% 10% 3 7% 5% 5% 5% 4 4 7% 35%	1900
	21st to end	Day	19% 11% 3 8% 4 5% 4 4 5% 6% 30%	1957
November	1st to 10th	Day	10% 10% 3 7% 4 5% 5% 4 6% 6% 39%	1708
	11th to 20th	Day	14% 12% 3 7% 6% 6% 4 4 5% 7% 32%	1710
	21st to end	Day	18% 11% 5% 10% 5% 4 4 3 5% 6% 29%	1555
December	1st to 10th	Day	14% 14% 7% 12% 6% 5% 6% 4 4 6% 22%	1520
	11th to 20th	Day	17% 16% 5% 9% 5% 5% 4 4 6% 6% 24%	1792
	21st to end	Day	26% 20% 7% 11% 6% 4 4 3 4 5% 11%	2636
January	1st to 10th	Night	16% 16% 6% 10% 5% 5% 5% 2 4 5% 25%	4428
	11th to 20th	Night	13% 18% 7% 13% 7% 5% 6% 5% 5% 5% 16%	4434
	21st to end	Night	23% 19% 8% 10% 4 4 3 3 3 4 19%	4532
February	1st to 10th	Night	14% 16% 4 10% 7% 5% 5% 4 5% 5% 23%	2470
	11th to 20th	Night	13% 13% 4 10% 7% 5% 5% 5% 5% 6% 27%	2291
	21st to end	Night	18% 12% 3 9% 5% 4 4 4 5% 6% 29%	1884
March	1st to 10th	Night	16% 14% 3 8% 4 4 4 3 4 6% 35%	3622
	11th to 20th	Night	10% 9% 2 6% 5% 5% 5% 4 6% 7% 41%	3480
	21st to end	Night	16% 9% 3 9% 5% 5% 4 3 4 5% 37%	3817
April	1st to 10th	Night	17% 10% 3 6% 5% 4 4 4 6% 7% 35%	2010
	11th to 20th	Night	18% 9% 3 5% 4 4 3 4 6% 6% 38%	1701
	21st to end	Night	17% 7% 2 7% 5% 4 4 4 5% 7% 39%	1710
May	1st to 10th	Night	17% 8% 3 7% 4 3 4 4 5% 8% 37%	2813
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 3 5% 8% 41%	2812
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 8% 49%	3079
June	1st to 10th	Night	11% 6% 2 7% 3 4 5% 3 6% 7% 47%	1400
	11th to 20th	Night	9% 5% 2 6% 5% 6% 4 4 7% 8% 45%	1330
	21st to end	Night	9% 6% 2 5% 4 4 4 4 6% 8% 49%	1393
July	1st to 10th	Night	8% 4 2 4 4 4 5% 4 7% 10% 49%	2530
	11th to 20th	Night	9% 4 1 5% 3 4 4 4 6% 8% 53%	2530
	21st to end	Night	5% 4 2 5% 4 4 6% 6% 8% 9% 46%	3053
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 8% 46%	2825
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 6% 7% 47%	2785
	21st to end	Night	8% 5% 2 6% 5% 4 5% 3 6% 7% 48%	3124
September	1st to 10th	Night	10% 6% 2 6% 5% 4 5% 3 5% 7% 48%	2019
	11th to 20th	Night	9% 5% 1 5% 4 3 4 3 5% 7% 55%	2090
	21st to end	Night	15% 6% 2 6% 4 3 4 3 5% 7% 44%	2090
October	1st to 10th	Night	13% 6% 3 6% 4 3 3 3 5% 7% 47%	2159
	11th to 20th	Night	18% 8% 3 7% 4 5% 4 4 4 6% 37%	2280
	21st to end	Night	21% 9% 2 7% 5% 4 5% 4 5% 6% 32%	2638
November	1st to 10th	Night	12% 8% 3 7% 4 4 4 4 5% 7% 43%	2470
	11th to 20th	Night	15% 11% 3 8% 6% 5% 5% 4 6% 6% 31%	2470
	21st to end	Night	19% 12% 4 8% 5% 4 4 4 5% 6% 29%	2622
December	1st to 10th	Night	18% 14% 6% 9% 6% 5% 5% 4 5% 5% 23%	2660
	11th to 20th	Night	14% 15% 5% 9% 6% 5% 4 3 6% 7% 25%	3393
	21st to end	Night	28% 21% 6% 11% 5% 4 3 3 4 4 13%	5012

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	62% 8% 12% 12% 5% 1	2528
	11th to 20th	Day	71% 7% 12% 7% 3	2533
	21st to end	Day	73% 8% 8% 6% 3 1	3124
February	1st to 10th	Day	62% 8% 9% 14% 6%	1710
	11th to 20th	Day	60% 5% 10% 17% 8% 1	1779
	21st to end	Day	57% 7% 12% 17% 7% 1	1570
March	1st to 10th	Day	58% 6% 12% 18% 5% 1	3230
	11th to 20th	Day	51% 6% 14% 19% 7% 1	3480
	21st to end	Day	56% 7% 13% 14% 9% 2	3815
April	1st to 10th	Day	51% 6% 12% 19% 12% 1	2185
	11th to 20th	Day	48% 6% 12% 20% 13% 2	2428
	21st to end	Day	49% 6% 13% 17% 13% 2	2470
May	1st to 10th	Day	45% 7% 12% 22% 11% 2	4120
	11th to 20th	Day	44% 9% 15% 22% 9% 1	4085
	21st to end	Day	35% 8% 11% 18% 23% 5%	4489
June	1st to 10th	Day	39% 7% 13% 26% 13% 3	2770
	11th to 20th	Day	37% 9% 13% 23% 16% 3	2850
	21st to end	Day	39% 8% 14% 22% 15% 1	2780
July	1st to 10th	Day	40% 7% 14% 21% 15% 2	4371
	11th to 20th	Day	30% 7% 15% 32% 15% 1	4365
	21st to end	Day	37% 9% 14% 17% 22% 1	4356
August	1st to 10th	Day	38% 8% 13% 25% 15% 1	4091
	11th to 20th	Day	37% 9% 13% 30% 10% 1	4083
	21st to end	Day	39% 7% 13% 26% 14% 1	4531
September	1st to 10th	Day	38% 8% 15% 24% 14% 2	2138
	11th to 20th	Day	32% 7% 13% 32% 15% 1	2090
	21st to end	Day	43% 8% 14% 23% 12%	2090
October	1st to 10th	Day	30% 6% 12% 31% 19% 1	3357
	11th to 20th	Day	39% 6% 11% 26% 17%	3140
	21st to end	Day	45% 5% 13% 23% 12% 1	3207
November	1st to 10th	Day	42% 8% 13% 24% 11% 1	1708
	11th to 20th	Day	50% 6% 15% 18% 10% 1	1710
	21st to end	Day	56% 6% 13% 18% 7% 1	1555
December	1st to 10th	Day	63% 6% 11% 15% 4	1520
	11th to 20th	Day	59% 7% 14% 14% 5%	1788
	21st to end	Day	75% 6% 10% 6% 3	2636
January	1st to 10th	Night	59% 6% 13% 15% 5% 1	4428
	11th to 20th	Night	67% 8% 12% 10% 3	4434
	21st to end	Night	69% 5% 10% 13% 3	4532
February	1st to 10th	Night	60% 8% 11% 14% 7% 1	2470
	11th to 20th	Night	55% 7% 11% 17% 9% 1	2293
	21st to end	Night	54% 6% 13% 18% 8% 1	1884
March	1st to 10th	Night	51% 5% 14% 23% 7%	3622
	11th to 20th	Night	38% 7% 17% 18% 18% 1	3480
	21st to end	Night	47% 6% 12% 22% 12% 1	3817
April	1st to 10th	Night	48% 6% 11% 20% 13% 2	2010
	11th to 20th	Night	45% 7% 11% 18% 17% 2	1701
	21st to end	Night	44% 5% 11% 20% 16% 4	1710
May	1st to 10th	Night	45% 6% 11% 21% 14% 4	2840
	11th to 20th	Night	41% 8% 12% 26% 12% 2	2813
	21st to end	Night	32% 8% 13% 21% 21% 6%	3087
June	1st to 10th	Night	35% 6% 12% 28% 17% 3	1400
	11th to 20th	Night	35% 8% 13% 25% 18% 2	1330
	21st to end	Night	33% 7% 13% 26% 21% 1	1393
July	1st to 10th	Night	29% 5% 13% 32% 15% 6%	2490
	11th to 20th	Night	28% 6% 14% 33% 17% 2	2482
	21st to end	Night	29% 10% 18% 21% 20% 2	2956
August	1st to 10th	Night	36% 6% 14% 27% 16% 1	2814
	11th to 20th	Night	34% 8% 11% 34% 12% 2	2812
	21st to end	Night	33% 8% 12% 32% 14% 1	3124
September	1st to 10th	Night	35% 7% 13% 26% 16% 2	2019
	11th to 20th	Night	28% 6% 13% 31% 20% 2	2090
	21st to end	Night	40% 6% 12% 29% 12% 1	2090
October	1st to 10th	Night	26% 4 10% 32% 27% 1	3601
	11th to 20th	Night	43% 5% 12% 23% 17%	3779
	21st to end	Night	42% 4 12% 28% 13% 1	4338
November	1st to 10th	Night	41% 6% 13% 26% 14% 1	2470
	11th to 20th	Night	49% 6% 15% 20% 9% 1	2470
	21st to end	Night	55% 7% 13% 19% 6%	2622
December	1st to 10th	Night	61% 8% 11% 14% 5%	2660
	11th to 20th	Night	56% 7% 15% 14% 6% 1	3396
	21st to end	Night	75% 5% 10% 7% 2	5012

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	22% 19% 34% 17% 7%	270
	11th to 20th	Day	20% 18% 41% 16% 4 1	270
	21st to end	Day	27% 11% 37% 18% 6%	297
February	1st to 10th	Day	22% 18% 40% 17% 3 1	270
	11th to 20th	Day	20% 20% 38% 20% 2	270
	21st to end	Day	18% 24% 43% 12% 4	225
March	1st to 10th	Day	23% 19% 35% 17% 5%	328
	11th to 20th	Day	20% 15% 33% 25% 6% 1	460
	21st to end	Day	26% 14% 32% 21% 5% 1	506
April	1st to 10th	Day	15% 17% 38% 20% 9% 1	460
	11th to 20th	Day	24% 17% 29% 15% 9% 6%	452
	21st to end	Day	23% 13% 29% 23% 10% 2	460
May	1st to 10th	Day	21% 12% 29% 24% 11% 3	463
	11th to 20th	Day	17% 10% 27% 26% 13% 6%	487
	21st to end	Day	13% 6% 22% 29% 21% 8%	529
June	1st to 10th	Day	12% 8% 30% 30% 16% 3	638
	11th to 20th	Day	10% 6% 32% 31% 17% 3	680
	21st to end	Day	11% 7% 30% 30% 17% 4	680
July	1st to 10th	Day	8% 5% 22% 30% 23% 11%	671
	11th to 20th	Day	10% 5% 25% 32% 19% 8%	668
	21st to end	Day	8% 4 22% 31% 24% 11%	551
August	1st to 10th	Day	10% 3 22% 26% 26% 14%	486
	11th to 20th	Day	11% 3 19% 31% 23% 14%	479
	21st to end	Day	12% 4 20% 30% 23% 12%	539
September	1st to 10th	Day	12% 5% 24% 25% 23% 11%	490
	11th to 20th	Day	9% 5% 19% 25% 28% 14%	490
	21st to end	Day	18% 8% 25% 22% 20% 8%	490
October	1st to 10th	Day	14% 10% 27% 22% 17% 9%	490
	11th to 20th	Day	21% 14% 30% 24% 8% 2	490
	21st to end	Day	24% 16% 35% 19% 5%	406
November	1st to 10th	Day	14% 19% 34% 22% 10% 2	282
	11th to 20th	Day	20% 21% 34% 18% 7% 1	270
	21st to end	Day	22% 18% 38% 18% 4	196
December	1st to 10th	Day	22% 17% 41% 13% 7% 1	180
	11th to 20th	Day	27% 17% 39% 14% 3 1	180
	21st to end	Day	26% 21% 33% 17% 3	258
January	1st to 10th	Night	22% 14% 36% 22% 5% 1	950
	11th to 20th	Night	24% 14% 33% 24% 4 1	944
	21st to end	Night	20% 11% 34% 29% 6%	836
February	1st to 10th	Night	22% 12% 39% 24% 4	760
	11th to 20th	Night	20% 14% 35% 26% 5%	743
	21st to end	Night	24% 13% 40% 21% 2	628
March	1st to 10th	Night	20% 11% 36% 28% 5%	760
	11th to 20th	Night	16% 14% 38% 26% 5% 1	760
	21st to end	Night	22% 10% 32% 27% 8% 1	836
April	1st to 10th	Night	20% 14% 32% 29% 4 1	680
	11th to 20th	Night	21% 10% 33% 27% 8% 1	563
	21st to end	Night	19% 8% 32% 32% 9%	570
May	1st to 10th	Night	18% 7% 29% 35% 10% 1	570
	11th to 20th	Night	21% 5% 25% 32% 16% 1	561
	21st to end	Night	10% 2 24% 41% 21% 2	617
June	1st to 10th	Night	13% 4 26% 41% 16%	413
	11th to 20th	Night	10% 4 24% 47% 15%	380
	21st to end	Night	10% 4 21% 45% 19% 1	443
July	1st to 10th	Night	7% 3 18% 40% 28% 4	555
	11th to 20th	Night	9% 3 19% 41% 25% 4	552
	21st to end	Night	5% 3 18% 48% 24% 2	624
August	1st to 10th	Night	8% 4 17% 36% 32% 3	567
	11th to 20th	Night	7% 2 18% 37% 30% 5%	564
	21st to end	Night	9% 3 17% 37% 31% 3	627
September	1st to 10th	Night	11% 5% 19% 34% 24% 6%	698
	11th to 20th	Night	11% 3 17% 34% 30% 5%	760
	21st to end	Night	18% 6% 20% 31% 20% 5%	760
October	1st to 10th	Night	15% 8% 26% 30% 18% 3	760
	11th to 20th	Night	23% 12% 27% 26% 9% 2	760
	21st to end	Night	25% 11% 31% 28% 6%	836
November	1st to 10th	Night	17% 10% 32% 26% 14% 1	760
	11th to 20th	Night	20% 16% 32% 24% 7% 1	760
	21st to end	Night	24% 13% 34% 22% 6%	760
December	1st to 10th	Night	25% 12% 37% 20% 6%	760
	11th to 20th	Night	22% 14% 37% 23% 3	831
	21st to end	Night	28% 12% 34% 22% 3	1045

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