SOLO Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SOLO Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	12% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	114035
	Night	12% 12% 5% 11% 6% 6% 5% 4 5% 6% 30%	115122
Transparency	Day	48% 7% 14% 21% 9% 1	117547
	Night	53% 7% 12% 19% 9% 1	119772
Seeing	Day	16% 6% 23% 34% 17% 4	16743
	Night	18% 7% 26% 32% 15% 2	26782

By Month

Percentage Cloud Cover NE (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% 7% 21% 9% 10% 6% 3 4 4 16%	3617
	11th to 20th	Day	5% 14% 7% 15% 10% 15% 11% 10% 4 2 7%	3589
	21st to end	Day	5% 18% 8% 15% 7% 5% 6% 4 6% 5% 20%	4097
February	1st to 10th	Day	8% 13% 4 11% 7% 8% 6% 6% 6% 7% 23%	1800
	11th to 20th	Day	7% 12% 6% 10% 6% 6% 7% 5% 5% 7% 29%	1754
	21st to end	Day	9% 12% 4 11% 7% 5% 5% 5% 6% 7% 30%	1605
March	1st to 10th	Day	10% 10% 3 7% 5% 4 5% 4 5% 8% 38%	4524
	11th to 20th	Day	8% 11% 5% 12% 7% 5% 5% 4 5% 7% 32%	4980
	21st to end	Day	20% 11% 4 9% 4 3 4 3 5% 5% 34%	5732
April	1st to 10th	Day	15% 12% 4 9% 5% 5% 5% 4 5% 6% 32%	2400
	11th to 20th	Day	19% 12% 3 7% 4 4 5% 4 5% 6% 31%	2374
	21st to end	Day	21% 11% 3 7% 4 3 5% 3 4 6% 34%	2612
May	1st to 10th	Day	17% 8% 3 6% 4 4 4 3 5% 6% 41%	4568
	11th to 20th	Day	13% 7% 2 6% 5% 5% 5% 3 6% 7% 41%	4400
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 5% 7% 35%	4837
June	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 6% 9% 45%	2660
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	2660
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 39%	2660
July	1st to 10th	Day	9% 6% 2 5% 3 4 4 4 6% 10% 48%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 7% 9% 51%	4440
	21st to end	Day	7% 5% 2 4 4 3 4 4 6% 9% 54%	4814
August	1st to 10th	Day	12% 7% 3 6% 4 5% 5% 4 6% 8% 41%	4394
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 7% 52%	4041
	21st to end	Day	11% 8% 2 7% 4 4 5% 4 6% 8% 42%	4249
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 3 6% 9% 46%	2280
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 3 6% 8% 42%	2280
	21st to end	Day	17% 8% 3 8% 4 4 4 2 4 6% 40%	2013
October	1st to 10th	Day	18% 10% 3 7% 5% 4 4 4 5% 7% 35%	1900
	11th to 20th	Day	18% 14% 4 8% 5% 4 5% 4 4 6% 28%	1900
	21st to end	Day	21% 15% 4 8% 5% 4 4 3 4 5% 27%	1932
November	1st to 10th	Day	13% 17% 5% 10% 4 5% 4 3 5% 6% 28%	1687
	11th to 20th	Day	14% 18% 8% 15% 7% 5% 4 3 4 4 19%	1600
	21st to end	Day	15% 14% 6% 13% 8% 5% 5% 4 5% 5% 20%	1600
December	1st to 10th	Day	9% 19% 8% 26% 10% 5% 4 2 3 3 11%	2610
	11th to 20th	Day	9% 18% 7% 16% 11% 6% 9% 3 3 3 14%	3004
	21st to end	Day	17% 18% 6% 19% 7% 6% 4 3 3 3 14%	3982
January	1st to 10th	Night	7% 14% 7% 21% 8% 8% 6% 3 3 4 19%	6348
	11th to 20th	Night	7% 14% 7% 16% 11% 16% 9% 8% 3 3 8%	6316
	21st to end	Night	7% 19% 7% 16% 7% 6% 5% 4 6% 4 19%	6876
February	1st to 10th	Night	7% 14% 5% 13% 8% 8% 7% 4 6% 5% 24%	2600
	11th to 20th	Night	7% 11% 4 10% 7% 8% 7% 5% 7% 6% 28%	2533
	21st to end	Night	10% 15% 5% 9% 6% 6% 5% 3 5% 6% 31%	2025
March	1st to 10th	Night	9% 8% 3 7% 4 4 4 4 5% 6% 45%	5348
	11th to 20th	Night	11% 13% 5% 13% 6% 4 5% 3 5% 5% 28%	4979
	21st to end	Night	21% 13% 5% 10% 4 3 3 2 4 4 30%	5198
April	1st to 10th	Night	17% 14% 4 8% 5% 4 5% 3 5% 5% 31%	2000
	11th to 20th	Night	19% 11% 3 7% 4 4 4 4 5% 6% 32%	1975
	21st to end	Night	20% 12% 3 6% 4 4 4 4 5% 5% 34%	1783
May	1st to 10th	Night	16% 7% 4 8% 4 4 4 4 5% 8% 37%	2603
	11th to 20th	Night	13% 7% 3 5% 5% 4 5% 4 5% 6% 44%	2511
	21st to end	Night	14% 8% 4 7% 7% 5% 5% 5% 6% 7% 32%	2750
June	1st to 10th	Night	14% 6% 1 7% 4 5% 5% 4 6% 7% 40%	1520
	11th to 20th	Night	12% 6% 2 7% 4 5% 4 4 8% 8% 39%	1520
	21st to end	Night	12% 6% 2 7% 6% 5% 4 5% 6% 8% 40%	1520
July	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 10% 48%	2530
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 5% 9% 54%	2530
	21st to end	Night	7% 6% 1 4 3 3 4 3 6% 8% 56%	2750
August	1st to 10th	Night	13% 6% 2 5% 3 4 4 4 5% 7% 44%	2543
	11th to 20th	Night	12% 5% 2 5% 3 4 4 3 6% 8% 48%	2798
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 43%	3408
September	1st to 10th	Night	11% 5% 2 7% 4 3 4 4 4 8% 49%	1900
	11th to 20th	Night	11% 6% 2 6% 4 3 5% 4 5% 7% 47%	1900
	21st to end	Night	17% 7% 2 6% 4 3 4 3 5% 7% 41%	2144
October	1st to 10th	Night	16% 10% 3 6% 3 3 3 3 5% 6% 41%	2280
	11th to 20th	Night	17% 14% 3 8% 5% 3 4 3 5% 5% 31%	2280
	21st to end	Night	21% 15% 4 9% 5% 4 4 3 5% 6% 26%	2665
November	1st to 10th	Night	14% 15% 6% 9% 5% 4 4 3 5% 6% 28%	2610
	11th to 20th	Night	14% 18% 7% 14% 6% 5% 4 3 4 5% 21%	2800
	21st to end	Night	14% 13% 5% 16% 7% 5% 4 4 5% 5% 22%	2800
December	1st to 10th	Night	11% 17% 9% 24% 9% 6% 3 3 4 3 11%	4580
	11th to 20th	Night	11% 16% 8% 16% 8% 8% 10% 3 3 3 12%	5203
	21st to end	Night	20% 18% 5% 13% 7% 6% 5% 3 3 4 15%	6996

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	69% 6% 8% 13% 5%	3618
	11th to 20th	Day	71% 8% 12% 8% 1	3620
	21st to end	Day	62% 8% 10% 16% 3	4097
February	1st to 10th	Day	56% 7% 13% 17% 6%	1800
	11th to 20th	Day	52% 8% 13% 21% 7%	1763
	21st to end	Day	52% 7% 14% 15% 11% 1	1605
March	1st to 10th	Day	42% 8% 14% 22% 11% 2	4526
	11th to 20th	Day	49% 8% 14% 17% 11%	4980
	21st to end	Day	53% 5% 12% 18% 11%	5732
April	1st to 10th	Day	53% 7% 11% 17% 11% 1	2400
	11th to 20th	Day	53% 7% 11% 17% 10% 2	2374
	21st to end	Day	52% 5% 11% 20% 11% 1	2612
May	1st to 10th	Day	43% 7% 12% 23% 14% 1	4580
	11th to 20th	Day	40% 10% 15% 25% 9% 1	4404
	21st to end	Day	47% 9% 12% 18% 13% 1	4837
June	1st to 10th	Day	35% 9% 19% 24% 12% 1	2660
	11th to 20th	Day	41% 9% 14% 21% 14% 1	2660
	21st to end	Day	38% 9% 19% 24% 8% 1	2660
July	1st to 10th	Day	32% 8% 22% 24% 14% 1	4371
	11th to 20th	Day	26% 8% 19% 33% 12% 1	4365
	21st to end	Day	27% 7% 16% 33% 16% 1	4662
August	1st to 10th	Day	40% 7% 18% 24% 10% 1	4379
	11th to 20th	Day	31% 7% 21% 32% 8%	4080
	21st to end	Day	37% 10% 18% 24% 10%	4248
September	1st to 10th	Day	35% 7% 18% 30% 10%	2280
	11th to 20th	Day	40% 9% 18% 27% 6%	2280
	21st to end	Day	46% 7% 14% 26% 8%	2013
October	1st to 10th	Day	38% 5% 17% 29% 9% 1	3160
	11th to 20th	Day	49% 6% 11% 25% 10%	3140
	21st to end	Day	54% 7% 14% 20% 5%	3157
November	1st to 10th	Day	57% 6% 11% 17% 8% 1	1687
	11th to 20th	Day	69% 6% 11% 11% 4	1600
	21st to end	Day	64% 9% 11% 12% 3 1	1600
December	1st to 10th	Day	79% 5% 8% 7% 1	2610
	11th to 20th	Day	71% 8% 8% 8% 4	3005
	21st to end	Day	74% 6% 7% 12% 2	3982
January	1st to 10th	Night	66% 6% 10% 11% 7%	6348
	11th to 20th	Night	72% 11% 11% 5% 1	6360
	21st to end	Night	64% 7% 12% 14% 4	6876
February	1st to 10th	Night	58% 7% 12% 17% 6% 1	2600
	11th to 20th	Night	52% 7% 15% 19% 7%	2534
	21st to end	Night	52% 6% 12% 20% 8% 2	2025
March	1st to 10th	Night	38% 6% 16% 19% 18% 4	5359
	11th to 20th	Night	55% 7% 11% 20% 7%	4980
	21st to end	Night	58% 5% 8% 15% 12% 1	5198
April	1st to 10th	Night	55% 6% 9% 16% 13% 1	2000
	11th to 20th	Night	51% 7% 10% 19% 11% 2	1975
	21st to end	Night	50% 5% 8% 24% 12% 1	1783
May	1st to 10th	Night	43% 9% 14% 23% 11% 1	2610
	11th to 20th	Night	38% 8% 10% 27% 14% 3	2512
	21st to end	Night	46% 11% 13% 18% 10% 2	2751
June	1st to 10th	Night	40% 8% 15% 23% 12% 2	1520
	11th to 20th	Night	38% 9% 15% 19% 17% 2	1520
	21st to end	Night	40% 9% 12% 28% 11% 1	1520
July	1st to 10th	Night	31% 9% 21% 25% 12% 3	2490
	11th to 20th	Night	24% 8% 17% 37% 13% 1	2482
	21st to end	Night	24% 7% 14% 34% 18% 2	2662
August	1st to 10th	Night	37% 7% 13% 25% 15% 2	2534
	11th to 20th	Night	33% 8% 17% 33% 9% 1	2825
	21st to end	Night	38% 9% 18% 24% 10% 1	3408
September	1st to 10th	Night	33% 7% 15% 32% 12% 1	1900
	11th to 20th	Night	33% 7% 19% 32% 8%	1900
	21st to end	Night	42% 7% 15% 27% 9%	2144
October	1st to 10th	Night	31% 4 10% 34% 19% 1	3800
	11th to 20th	Night	48% 6% 12% 20% 13%	3779
	21st to end	Night	56% 5% 11% 21% 6%	4390
November	1st to 10th	Night	55% 8% 12% 18% 7%	2610
	11th to 20th	Night	65% 6% 11% 11% 6%	2800
	21st to end	Night	62% 8% 15% 11% 4	2800
December	1st to 10th	Night	76% 6% 9% 7% 1	4580
	11th to 20th	Night	73% 8% 8% 9% 2	5201
	21st to end	Night	70% 7% 8% 11% 3	6996

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	12% 10% 34% 28% 14% 1	300
	11th to 20th	Day	17% 9% 29% 31% 13% 1	300
	21st to end	Day	16% 10% 30% 32% 12% 1	330
February	1st to 10th	Day	11% 11% 36% 33% 8% 1	300
	11th to 20th	Day	11% 12% 42% 22% 12% 1	300
	21st to end	Day	13% 9% 40% 33% 4	249
March	1st to 10th	Day	14% 8% 30% 36% 12% 1	300
	11th to 20th	Day	14% 6% 33% 36% 10% 2	300
	21st to end	Day	24% 7% 27% 28% 11% 2	443
April	1st to 10th	Day	17% 9% 31% 33% 8% 1	500
	11th to 20th	Day	22% 7% 23% 35% 11% 2	492
	21st to end	Day	25% 5% 21% 33% 14% 2	645
May	1st to 10th	Day	19% 5% 24% 35% 13% 3	703
	11th to 20th	Day	18% 4 21% 35% 18% 4	676
	21st to end	Day	17% 7% 16% 34% 22% 4	734
June	1st to 10th	Day	11% 4 22% 39% 20% 4	680
	11th to 20th	Day	14% 4 21% 42% 17% 2	680
	21st to end	Day	15% 4 18% 37% 21% 5%	680
July	1st to 10th	Day	10% 1 11% 40% 30% 9%	671
	11th to 20th	Day	9% 3 16% 37% 24% 10%	668
	21st to end	Day	8% 2 12% 41% 27% 9%	739
August	1st to 10th	Day	13% 3 15% 35% 28% 7%	676
	11th to 20th	Day	12% 5% 11% 32% 34% 6%	665
	21st to end	Day	15% 5% 15% 33% 25% 6%	582
September	1st to 10th	Day	12% 3 17% 40% 25% 4	490
	11th to 20th	Day	13% 7% 17% 35% 24% 4	490
	21st to end	Day	21% 7% 23% 26% 18% 5%	390
October	1st to 10th	Day	20% 5% 26% 31% 14% 4	300
	11th to 20th	Day	20% 11% 30% 30% 8% 1	300
	21st to end	Day	27% 9% 32% 26% 7%	330
November	1st to 10th	Day	20% 8% 33% 28% 11%	300
	11th to 20th	Day	18% 15% 31% 27% 8% 1	300
	21st to end	Day	20% 10% 33% 30% 6% 1	300
December	1st to 10th	Day	17% 12% 36% 26% 8% 2	300
	11th to 20th	Day	18% 16% 32% 26% 7%	300
	21st to end	Day	24% 12% 30% 26% 6% 1	330
January	1st to 10th	Night	14% 11% 35% 29% 11% 1	1000
	11th to 20th	Night	16% 9% 36% 29% 9% 1	1000
	21st to end	Night	17% 9% 35% 30% 8% 1	1100
February	1st to 10th	Night	14% 10% 33% 33% 9% 1	1000
	11th to 20th	Night	13% 8% 36% 31% 12% 1	979
	21st to end	Night	17% 6% 30% 37% 10%	705
March	1st to 10th	Night	16% 7% 27% 39% 11% 1	799
	11th to 20th	Night	18% 5% 32% 34% 10% 1	600
	21st to end	Night	20% 5% 26% 32% 14% 3	660
April	1st to 10th	Night	22% 9% 25% 34% 9% 2	600
	11th to 20th	Night	25% 5% 19% 36% 12% 2	593
	21st to end	Night	27% 3 19% 33% 15% 4	600
May	1st to 10th	Night	19% 5% 22% 37% 16% 2	600
	11th to 20th	Night	17% 3 18% 40% 18% 4	563
	21st to end	Night	17% 6% 14% 36% 27% 1	617
June	1st to 10th	Night	15% 4 17% 37% 24% 2	570
	11th to 20th	Night	17% 3 13% 44% 22% 1	570
	21st to end	Night	14% 4 14% 40% 24% 4	570
July	1st to 10th	Night	7% 3 10% 37% 36% 6%	555
	11th to 20th	Night	10% 2 16% 37% 29% 5%	552
	21st to end	Night	9% 2 11% 41% 30% 6%	624
August	1st to 10th	Night	11% 3 10% 41% 32% 3	567
	11th to 20th	Night	13% 4 13% 32% 34% 5%	564
	21st to end	Night	15% 3 14% 34% 30% 4	627
September	1st to 10th	Night	13% 3 18% 34% 27% 5%	570
	11th to 20th	Night	14% 4 17% 38% 25% 4	570
	21st to end	Night	19% 5% 24% 26% 22% 3	717
October	1st to 10th	Night	22% 5% 24% 29% 17% 3	760
	11th to 20th	Night	23% 9% 29% 28% 11% 1	760
	21st to end	Night	29% 7% 26% 29% 9%	836
November	1st to 10th	Night	21% 10% 28% 32% 8% 1	854
	11th to 20th	Night	23% 12% 32% 22% 11% 1	1000
	21st to end	Night	21% 14% 32% 26% 7%	1000
December	1st to 10th	Night	20% 13% 35% 24% 8%	1000
	11th to 20th	Night	22% 14% 31% 24% 8% 1	1000
	21st to end	Night	27% 10% 33% 22% 7% 1	1100

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