Suburban Lights Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Suburban Lights Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	13% 8% 3 7% 5% 4 4 4 6% 7% 38%	127014
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 39%	123295
Transparency	Day	43% 5% 12% 25% 13% 1	130495
	Night	45% 5% 11% 23% 15% 2	127887
Seeing	Day	16% 10% 29% 26% 14% 5%	17293
	Night	17% 10% 31% 30% 11% 1	27104

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	8% 10% 6% 16% 8% 7% 5% 3 5% 6% 27%	3617
	11th to 20th	Day	7% 10% 6% 11% 5% 4 5% 7% 7% 8% 28%	3620
	21st to end	Day	12% 13% 6% 9% 4 4 3 3 5% 5% 35%	4148
February	1st to 10th	Day	11% 13% 6% 9% 6% 4 4 4 4 6% 32%	1800
	11th to 20th	Day	13% 11% 3 7% 5% 5% 5% 4 5% 6% 35%	1884
	21st to end	Day	12% 11% 4 10% 7% 4 5% 4 6% 5% 33%	1650
March	1st to 10th	Day	15% 7% 2 6% 3 3 3 3 5% 6% 46%	4480
	11th to 20th	Day	12% 5% 2 7% 5% 4 5% 4 5% 7% 44%	4790
	21st to end	Day	22% 12% 4 9% 5% 4 4 3 4 6% 29%	5913
April	1st to 10th	Day	17% 13% 4 7% 5% 4 4 4 6% 6% 30%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 4 5% 4 6% 7% 33%	2374
	21st to end	Day	18% 10% 3 8% 5% 4 3 4 5% 6% 33%	2475
May	1st to 10th	Day	14% 7% 3 6% 4 4 4 4 6% 7% 41%	6060
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 4 6% 7% 42%	6319
	21st to end	Day	16% 7% 2 5% 4 4 5% 4 7% 9% 39%	6935
June	1st to 10th	Day	11% 4 2 6% 5% 4 5% 4 6% 9% 44%	2800
	11th to 20th	Day	11% 7% 2 6% 5% 4 4 4 7% 8% 42%	2800
	21st to end	Day	12% 6% 2 5% 5% 4 5% 4 7% 10% 40%	2800
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 5% 7% 10% 40%	6360
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 5% 7% 9% 47%	6360
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	6924
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 10% 42%	4513
	11th to 20th	Day	12% 5% 2 5% 3 4 4 4 6% 8% 48%	3925
	21st to end	Day	10% 6% 3 6% 4 4 5% 4 7% 9% 45%	4312
September	1st to 10th	Day	12% 5% 3 6% 4 4 4 5% 6% 8% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 8% 50%	2330
	21st to end	Day	11% 7% 2 7% 4 4 4 3 5% 7% 45%	2200
October	1st to 10th	Day	13% 6% 3 6% 3 3 4 3 5% 5% 48%	2050
	11th to 20th	Day	19% 8% 3 8% 5% 4 4 4 5% 7% 34%	2000
	21st to end	Day	17% 10% 4 6% 4 4 4 2 4 6% 39%	2005
November	1st to 10th	Day	12% 8% 3 6% 4 4 4 3 5% 7% 44%	1780
	11th to 20th	Day	13% 13% 3 9% 7% 5% 3 3 5% 7% 31%	1790
	21st to end	Day	15% 13% 4 8% 6% 4 3 3 4 5% 36%	1600
December	1st to 10th	Day	8% 10% 4 9% 5% 5% 5% 4 6% 8% 36%	2610
	11th to 20th	Day	13% 15% 5% 11% 6% 4 5% 4 6% 7% 25%	3008
	21st to end	Day	20% 21% 8% 12% 5% 4 5% 3 4 5% 13%	3982
January	1st to 10th	Night	8% 10% 6% 13% 7% 4 4 3 5% 5% 33%	6348
	11th to 20th	Night	10% 12% 6% 14% 6% 5% 5% 4 6% 7% 24%	6360
	21st to end	Night	12% 15% 7% 10% 5% 4 4 4 5% 6% 30%	6816
February	1st to 10th	Night	11% 14% 4 9% 6% 5% 4 4 5% 5% 33%	2600
	11th to 20th	Night	12% 10% 3 8% 5% 5% 4 3 5% 6% 39%	2402
	21st to end	Night	16% 12% 4 8% 4 4 3 3 5% 6% 36%	1980
March	1st to 10th	Night	16% 7% 3 7% 4 4 3 3 4 5% 43%	5373
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 5% 8% 47%	5214
	21st to end	Night	22% 12% 4 10% 4 4 4 2 4 5% 29%	5015
April	1st to 10th	Night	17% 9% 3 6% 4 4 5% 3 5% 6% 37%	2000
	11th to 20th	Night	19% 8% 3 6% 5% 4 3 4 5% 5% 38%	1975
	21st to end	Night	17% 8% 3 6% 4 4 4 4 5% 7% 38%	1923
May	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 5% 7% 40%	3840
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 5% 7% 46%	3596
	21st to end	Night	15% 8% 3 7% 5% 4 5% 4 5% 7% 38%	3942
June	1st to 10th	Night	8% 4 2 5% 5% 5% 4 4 6% 10% 46%	1600
	11th to 20th	Night	13% 6% 2 5% 4 3 4 4 6% 8% 45%	1600
	21st to end	Night	10% 7% 2 6% 4 4 4 4 4 8% 49%	1600
July	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 8% 44%	3618
	11th to 20th	Night	6% 5% 2 5% 5% 4 4 3 5% 8% 54%	3617
	21st to end	Night	6% 5% 2 5% 4 4 4 4 6% 7% 51%	3947
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 49%	2643
	11th to 20th	Night	10% 7% 2 5% 4 4 4 3 5% 8% 49%	3124
	21st to end	Night	10% 4 2 5% 4 4 4 3 6% 9% 50%	3586
September	1st to 10th	Night	9% 4 2 4 4 3 4 3 5% 7% 55%	2000
	11th to 20th	Night	11% 3 2 5% 4 4 5% 3 4 7% 52%	2050
	21st to end	Night	11% 5% 2 5% 4 4 5% 3 5% 7% 50%	2200
October	1st to 10th	Night	12% 6% 2 6% 3 3 3 2 6% 7% 51%	2348
	11th to 20th	Night	16% 8% 2 6% 4 4 4 2 4 7% 43%	2400
	21st to end	Night	17% 9% 3 6% 4 4 4 4 6% 7% 36%	2835
November	1st to 10th	Night	12% 8% 3 6% 4 4 3 3 5% 6% 45%	2563
	11th to 20th	Night	14% 11% 4 8% 4 4 5% 4 5% 5% 34%	2605
	21st to end	Night	17% 10% 3 7% 5% 4 4 4 5% 5% 36%	2800
December	1st to 10th	Night	9% 8% 3 7% 5% 4 4 4 5% 7% 43%	4580
	11th to 20th	Night	16% 17% 5% 10% 5% 5% 4 4 4 5% 26%	5199
	21st to end	Night	20% 20% 6% 11% 6% 4 4 3 4 4 18%	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	59% 4 12% 18% 6%	3618
	11th to 20th	Day	52% 4 12% 13% 18% 1	3620
	21st to end	Day	52% 3 12% 20% 13%	4148
February	1st to 10th	Day	54% 3 13% 19% 11% 1	1800
	11th to 20th	Day	49% 5% 11% 23% 10% 2	1893
	21st to end	Day	53% 5% 10% 19% 11% 2	1650
March	1st to 10th	Day	39% 4 12% 29% 12% 4	4480
	11th to 20th	Day	39% 4 12% 29% 14% 1	4790
	21st to end	Day	58% 4 12% 15% 10% 2	5915
April	1st to 10th	Day	53% 5% 11% 20% 11%	2400
	11th to 20th	Day	49% 6% 10% 21% 11% 2	2374
	21st to end	Day	51% 6% 11% 20% 12% 1	2475
May	1st to 10th	Day	41% 5% 10% 25% 17% 2	6095
	11th to 20th	Day	40% 7% 10% 26% 16% 1	6323
	21st to end	Day	41% 6% 10% 28% 13% 2	6949
June	1st to 10th	Day	36% 7% 14% 26% 16% 1	2800
	11th to 20th	Day	38% 7% 13% 24% 17% 1	2800
	21st to end	Day	38% 7% 15% 24% 14% 2	2800
July	1st to 10th	Day	38% 7% 14% 31% 10% 1	6291
	11th to 20th	Day	32% 7% 14% 32% 14% 1	6285
	21st to end	Day	38% 7% 15% 22% 16% 1	6728
August	1st to 10th	Day	37% 7% 15% 29% 11% 1	4498
	11th to 20th	Day	34% 6% 14% 33% 13% 1	3962
	21st to end	Day	35% 6% 14% 29% 16% 1	4311
September	1st to 10th	Day	37% 7% 12% 27% 17% 1	2400
	11th to 20th	Day	32% 5% 13% 32% 16% 2	2330
	21st to end	Day	39% 5% 9% 31% 15% 1	2200
October	1st to 10th	Day	24% 3 6% 39% 26% 1	3336
	11th to 20th	Day	36% 3 10% 28% 22% 1	3240
	21st to end	Day	42% 4 9% 30% 13% 2	3217
November	1st to 10th	Day	41% 4 15% 24% 13% 2	1780
	11th to 20th	Day	53% 5% 12% 21% 9% 1	1790
	21st to end	Day	53% 4 15% 22% 7% 1	1600
December	1st to 10th	Day	45% 5% 20% 23% 6% 1	2610
	11th to 20th	Day	58% 4 11% 18% 7% 2	3005
	21st to end	Day	74% 4 7% 10% 6%	3982
January	1st to 10th	Night	52% 4 9% 23% 11%	6348
	11th to 20th	Night	59% 4 8% 11% 17% 1	6360
	21st to end	Night	56% 3 12% 17% 11%	6816
February	1st to 10th	Night	52% 4 11% 19% 12% 2	2600
	11th to 20th	Night	47% 5% 10% 23% 14% 1	2403
	21st to end	Night	49% 5% 11% 20% 12% 3	1980
March	1st to 10th	Night	44% 3 12% 23% 10% 7%	5373
	11th to 20th	Night	34% 5% 12% 32% 13% 4	5214
	21st to end	Night	59% 4 8% 16% 12% 1	5015
April	1st to 10th	Night	47% 6% 9% 21% 15% 1	2000
	11th to 20th	Night	47% 5% 7% 21% 17% 3	1975
	21st to end	Night	44% 6% 12% 24% 12% 2	1923
May	1st to 10th	Night	43% 5% 11% 18% 21% 2	3867
	11th to 20th	Night	36% 8% 15% 22% 17% 2	3597
	21st to end	Night	44% 5% 10% 23% 14% 3	3950
June	1st to 10th	Night	33% 6% 13% 28% 19% 1	1600
	11th to 20th	Night	35% 6% 10% 25% 22% 2	1600
	21st to end	Night	35% 6% 16% 22% 19% 3	1600
July	1st to 10th	Night	37% 6% 14% 29% 12% 2	3580
	11th to 20th	Night	29% 6% 14% 31% 19% 2	3572
	21st to end	Night	30% 7% 15% 24% 21% 2	3832
August	1st to 10th	Night	32% 6% 14% 27% 19% 2	2634
	11th to 20th	Night	34% 6% 13% 32% 15% 1	3153
	21st to end	Night	32% 6% 13% 28% 18% 3	3586
September	1st to 10th	Night	29% 6% 11% 29% 23% 2	2000
	11th to 20th	Night	32% 6% 11% 32% 18% 1	2050
	21st to end	Night	34% 5% 10% 29% 20% 1	2200
October	1st to 10th	Night	25% 4 6% 37% 27% 1	3842
	11th to 20th	Night	34% 3 10% 28% 22% 2	3899
	21st to end	Night	43% 4 9% 27% 15% 1	4573
November	1st to 10th	Night	40% 4 10% 27% 17% 2	2563
	11th to 20th	Night	51% 5% 11% 20% 11% 1	2605
	21st to end	Night	50% 4 14% 22% 10% 1	2800
December	1st to 10th	Night	40% 5% 17% 27% 8% 2	4580
	11th to 20th	Night	62% 4 9% 15% 9% 1	5201
	21st to end	Night	70% 4 7% 13% 6%	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	16% 13% 36% 24% 9% 2	300
	11th to 20th	Day	14% 15% 39% 24% 7% 1	300
	21st to end	Day	22% 12% 35% 25% 5% 1	330
February	1st to 10th	Day	15% 15% 41% 23% 4 1	300
	11th to 20th	Day	19% 18% 35% 22% 6%	300
	21st to end	Day	18% 16% 45% 18% 2	249
March	1st to 10th	Day	16% 16% 43% 21% 4	300
	11th to 20th	Day	20% 18% 33% 20% 6% 3	300
	21st to end	Day	29% 11% 30% 21% 7% 2	522
April	1st to 10th	Day	24% 15% 35% 22% 4	500
	11th to 20th	Day	25% 17% 27% 19% 10% 2	492
	21st to end	Day	25% 13% 28% 22% 10% 2	500
May	1st to 10th	Day	19% 10% 32% 26% 10% 2	584
	11th to 20th	Day	21% 10% 28% 26% 12% 3	725
	21st to end	Day	17% 10% 28% 25% 15% 6%	789
June	1st to 10th	Day	9% 9% 28% 33% 16% 4	730
	11th to 20th	Day	13% 8% 29% 30% 15% 5%	730
	21st to end	Day	13% 8% 29% 29% 16% 4	730
July	1st to 10th	Day	8% 5% 19% 31% 25% 12%	721
	11th to 20th	Day	9% 6% 26% 30% 21% 8%	718
	21st to end	Day	7% 3 24% 37% 23% 7%	794
August	1st to 10th	Day	8% 3 22% 33% 24% 11%	726
	11th to 20th	Day	14% 5% 17% 27% 25% 13%	571
	21st to end	Day	10% 7% 21% 29% 23% 10%	583
September	1st to 10th	Day	14% 8% 23% 27% 19% 9%	530
	11th to 20th	Day	11% 4 21% 29% 26% 9%	530
	21st to end	Day	13% 11% 26% 25% 17% 7%	530
October	1st to 10th	Day	16% 11% 26% 27% 17% 3	380
	11th to 20th	Day	22% 15% 35% 19% 8% 1	330
	21st to end	Day	24% 16% 34% 20% 6% 1	363
November	1st to 10th	Day	16% 15% 37% 23% 6% 3	306
	11th to 20th	Day	18% 19% 34% 20% 9%	300
	21st to end	Day	24% 11% 40% 19% 5%	300
December	1st to 10th	Day	17% 18% 41% 18% 6% 1	300
	11th to 20th	Day	17% 14% 37% 21% 10% 1	300
	21st to end	Day	25% 16% 37% 17% 4	330
January	1st to 10th	Night	15% 16% 36% 24% 8% 1	1000
	11th to 20th	Night	18% 14% 36% 26% 5% 1	1000
	21st to end	Night	19% 12% 37% 26% 5% 1	1100
February	1st to 10th	Night	17% 12% 40% 24% 6% 1	1000
	11th to 20th	Night	16% 15% 37% 27% 5%	847
	21st to end	Night	22% 13% 38% 22% 3 1	660
March	1st to 10th	Night	21% 11% 36% 25% 7% 1	800
	11th to 20th	Night	17% 12% 36% 28% 7%	704
	21st to end	Night	23% 10% 30% 29% 7% 1	660
April	1st to 10th	Night	21% 14% 36% 25% 4	600
	11th to 20th	Night	23% 12% 30% 28% 7%	593
	21st to end	Night	21% 10% 32% 30% 8% 1	600
May	1st to 10th	Night	21% 9% 33% 29% 8% 1	600
	11th to 20th	Night	19% 7% 31% 32% 11%	591
	21st to end	Night	15% 5% 26% 39% 15% 1	650
June	1st to 10th	Night	9% 4 30% 43% 14% 1	600
	11th to 20th	Night	16% 4 29% 39% 11%	600
	21st to end	Night	11% 6% 28% 40% 16%	600
July	1st to 10th	Night	9% 2 15% 39% 31% 3	585
	11th to 20th	Night	7% 5% 20% 43% 24% 2	582
	21st to end	Night	5% 2 20% 50% 21% 2	657
August	1st to 10th	Night	6% 2 17% 47% 26% 2	597
	11th to 20th	Night	10% 3 16% 42% 25% 5%	594
	21st to end	Night	11% 2 17% 45% 23% 2	660
September	1st to 10th	Night	11% 5% 22% 34% 24% 5%	600
	11th to 20th	Night	11% 3 21% 36% 25% 4	650
	21st to end	Night	12% 8% 25% 33% 20% 2	800
October	1st to 10th	Night	14% 7% 30% 33% 17% 1	800
	11th to 20th	Night	20% 12% 31% 29% 9%	800
	21st to end	Night	22% 11% 31% 28% 6% 1	880
November	1st to 10th	Night	17% 10% 37% 27% 9% 1	789
	11th to 20th	Night	21% 14% 38% 22% 6%	805
	21st to end	Night	21% 15% 34% 23% 7%	1000
December	1st to 10th	Night	20% 15% 39% 21% 6%	1000
	11th to 20th	Night	23% 13% 35% 24% 5% 1	1000
	21st to end	Night	28% 14% 32% 22% 3	1100

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