Sun, Moon and Stars Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sun, Moon and Stars Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	21% 8% 3 7% 4 4 4 3 5% 6% 33%	123909
	Night	26% 10% 3 7% 4 4 4 3 4 5% 30%	123924
Transparency	Day	49% 11% 15% 17% 7% 1	123655
	Night	54% 9% 13% 16% 7% 1	123821
Seeing	Day	25% 8% 21% 25% 17% 3	16395
	Night	32% 7% 16% 25% 17% 3	24470

By Month

Percentage Cloud Cover NW (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	38% 12% 3 7% 5% 4 5% 4 4 5% 14%	3179
	11th to 20th	Day	20% 11% 3 8% 5% 4 5% 3 5% 7% 28%	3159
	21st to end	Day	43% 10% 2 5% 3 3 3 2 4 4 22%	3945
February	1st to 10th	Day	31% 11% 4 8% 5% 5% 6% 5% 5% 4 17%	1800
	11th to 20th	Day	28% 10% 3 7% 4 6% 5% 4 5% 6% 23%	1755
	21st to end	Day	24% 11% 3 10% 5% 5% 6% 3 6% 4 23%	1485
March	1st to 10th	Day	25% 13% 4 10% 6% 5% 4 4 6% 5% 18%	4721
	11th to 20th	Day	24% 12% 3 9% 5% 4 4 4 5% 5% 26%	5000
	21st to end	Day	28% 13% 4 10% 6% 5% 5% 3 4 5% 19%	5572
April	1st to 10th	Day	25% 11% 3 8% 5% 4 5% 4 7% 7% 22%	2400
	11th to 20th	Day	22% 8% 3 7% 5% 5% 5% 3 6% 6% 30%	2509
	21st to end	Day	22% 9% 4 7% 3 4 5% 4 5% 6% 31%	2635
May	1st to 10th	Day	12% 8% 3 7% 4 3 4 3 5% 6% 45%	6320
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 4 7% 10% 32%	6277
	21st to end	Day	20% 7% 3 7% 5% 5% 6% 4 6% 8% 30%	7177
June	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 4 6% 8% 32%	3000
	11th to 20th	Day	16% 8% 3 7% 7% 6% 6% 5% 7% 8% 29%	3000
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	3000
July	1st to 10th	Day	6% 4 1 4 3 3 3 3 4 5% 64%	4890
	11th to 20th	Day	5% 3 1 2 2 2 2 3 4 6% 70%	4890
	21st to end	Day	10% 5% 2 5% 3 3 4 3 5% 7% 53%	4962
August	1st to 10th	Day	5% 2 1 3 2 2 3 2 4 7% 68%	4319
	11th to 20th	Day	11% 8% 3 8% 3 4 4 4 6% 8% 42%	4026
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 7% 44%	4464
September	1st to 10th	Day	11% 6% 2 6% 3 4 4 3 5% 8% 48%	2280
	11th to 20th	Day	18% 7% 2 6% 4 4 3 3 5% 6% 41%	2245
	21st to end	Day	24% 6% 2 6% 3 4 4 3 6% 8% 35%	2022
October	1st to 10th	Day	24% 8% 2 7% 5% 5% 5% 4 6% 6% 27%	3180
	11th to 20th	Day	34% 7% 3 6% 4 4 5% 3 4 7% 24%	3161
	21st to end	Day	30% 11% 3 8% 5% 5% 4 3 5% 6% 18%	3155
November	1st to 10th	Day	39% 10% 3 7% 4 3 4 3 4 6% 17%	1568
	11th to 20th	Day	35% 11% 3 6% 4 3 4 3 4 6% 22%	1600
	21st to end	Day	37% 11% 3 7% 4 4 3 3 5% 4 20%	1600
December	1st to 10th	Day	23% 12% 3 6% 5% 6% 4 4 5% 6% 25%	2470
	11th to 20th	Day	43% 14% 3 8% 5% 4 3 2 3 3 12%	2645
	21st to end	Day	41% 15% 4 10% 5% 4 3 2 3 3 10%	3498
January	1st to 10th	Night	36% 12% 4 10% 5% 4 3 2 3 4 16%	6766
	11th to 20th	Night	22% 8% 3 8% 4 4 4 4 6% 7% 30%	6739
	21st to end	Night	39% 9% 2 6% 3 3 3 2 4 4 24%	7010
February	1st to 10th	Night	30% 11% 4 8% 5% 4 5% 3 5% 5% 20%	2600
	11th to 20th	Night	26% 12% 3 8% 4 4 4 4 4 5% 28%	2532
	21st to end	Night	29% 12% 4 8% 4 4 3 3 5% 5% 24%	2145
March	1st to 10th	Night	27% 12% 4 7% 5% 5% 4 3 5% 6% 23%	5168
	11th to 20th	Night	24% 11% 3 8% 4 4 4 3 5% 6% 28%	4999
	21st to end	Night	35% 13% 4 8% 5% 4 4 3 4 4 17%	5402
April	1st to 10th	Night	24% 12% 3 8% 5% 5% 6% 4 5% 6% 23%	2000
	11th to 20th	Night	19% 10% 3 8% 6% 5% 3 4 5% 7% 30%	1837
	21st to end	Night	21% 9% 3 7% 4 5% 4 4 5% 7% 31%	1765
May	1st to 10th	Night	15% 8% 2 6% 3 3 3 3 4 5% 48%	3640
	11th to 20th	Night	14% 8% 3 8% 4 5% 5% 3 6% 8% 37%	3616
	21st to end	Night	17% 10% 4 9% 6% 6% 4 4 5% 7% 28%	3682
June	1st to 10th	Night	20% 7% 4 8% 5% 4 4 4 4 5% 36%	1400
	11th to 20th	Night	15% 9% 4 7% 6% 5% 4 4 6% 8% 32%	1400
	21st to end	Night	11% 7% 3 8% 5% 5% 4 4 5% 7% 42%	1400
July	1st to 10th	Night	6% 3 2 4 3 2 4 2 4 6% 64%	2290
	11th to 20th	Night	4 2 1 3 2 2 2 2 3 5% 74%	2290
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 6% 53%	2589
August	1st to 10th	Night	4 3 1 2 1 2 2 2 3 6% 73%	2612
	11th to 20th	Night	11% 7% 2 7% 3 4 4 4 5% 7% 46%	2812
	21st to end	Night	14% 6% 3 6% 4 4 4 3 5% 6% 45%	3204
September	1st to 10th	Night	8% 5% 1 3 4 4 4 4 5% 6% 57%	1900
	11th to 20th	Night	19% 6% 2 5% 3 4 3 3 5% 5% 43%	1933
	21st to end	Night	21% 7% 2 6% 4 4 4 3 4 6% 39%	2157
October	1st to 10th	Night	23% 9% 3 8% 4 4 4 3 5% 6% 30%	3800
	11th to 20th	Night	30% 10% 2 6% 3 4 4 3 4 6% 27%	3777
	21st to end	Night	32% 9% 2 7% 4 3 4 3 4 5% 26%	4402
November	1st to 10th	Night	37% 11% 3 7% 4 4 3 2 4 4 19%	2730
	11th to 20th	Night	34% 10% 3 7% 4 4 3 3 5% 4 23%	2800
	21st to end	Night	35% 10% 3 7% 4 4 4 3 4 5% 21%	2800
December	1st to 10th	Night	26% 10% 3 7% 4 5% 5% 4 4 6% 25%	4709
	11th to 20th	Night	41% 13% 3 9% 4 3 4 2 4 4 12%	5560
	21st to end	Night	41% 14% 4 9% 4 4 3 2 3 4 12%	7458

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	70% 10% 8% 9% 2 1	3179
	11th to 20th	Day	51% 11% 16% 15% 7%	3180
	21st to end	Day	68% 5% 8% 15% 3	3945
February	1st to 10th	Day	67% 11% 9% 8% 3 2	1800
	11th to 20th	Day	59% 11% 12% 13% 4 1	1764
	21st to end	Day	61% 13% 11% 9% 4 1	1485
March	1st to 10th	Day	63% 12% 11% 10% 4	4721
	11th to 20th	Day	58% 9% 11% 19% 3	5000
	21st to end	Day	66% 11% 14% 8% 2	5572
April	1st to 10th	Day	59% 13% 14% 10% 4	2400
	11th to 20th	Day	52% 10% 13% 15% 9% 1	2509
	21st to end	Day	53% 10% 14% 15% 7% 1	2635
May	1st to 10th	Day	39% 9% 18% 22% 12% 1	6320
	11th to 20th	Day	44% 15% 20% 18% 3	6280
	21st to end	Day	49% 14% 18% 16% 3	7178
June	1st to 10th	Day	48% 11% 20% 14% 5% 2	3000
	11th to 20th	Day	48% 15% 20% 13% 5%	3000
	21st to end	Day	42% 12% 20% 17% 8% 2	3000
July	1st to 10th	Day	22% 9% 13% 24% 23% 9%	4816
	11th to 20th	Day	16% 8% 16% 37% 22% 1	4809
	21st to end	Day	29% 11% 21% 25% 12% 2	4805
August	1st to 10th	Day	17% 9% 14% 46% 13% 1	4302
	11th to 20th	Day	38% 13% 23% 16% 8% 1	4065
	21st to end	Day	39% 11% 19% 21% 10% 1	4463
September	1st to 10th	Day	34% 11% 17% 23% 12% 3	2280
	11th to 20th	Day	43% 11% 15% 17% 13% 2	2245
	21st to end	Day	47% 11% 17% 16% 9% 1	2022
October	1st to 10th	Day	53% 11% 17% 14% 5%	3180
	11th to 20th	Day	60% 9% 12% 14% 5%	3161
	21st to end	Day	62% 11% 12% 11% 4	3155
November	1st to 10th	Day	69% 8% 11% 8% 5% 1	1568
	11th to 20th	Day	65% 10% 10% 9% 4 2	1600
	21st to end	Day	68% 7% 9% 10% 6%	1600
December	1st to 10th	Day	56% 9% 13% 17% 4 1	2470
	11th to 20th	Day	76% 7% 9% 5% 2	2648
	21st to end	Day	78% 10% 6% 5% 1 1	3498
January	1st to 10th	Night	71% 7% 7% 11% 2	6766
	11th to 20th	Night	48% 11% 19% 17% 5%	6780
	21st to end	Night	63% 7% 9% 18% 3	7010
February	1st to 10th	Night	65% 8% 10% 10% 5% 2	2600
	11th to 20th	Night	59% 10% 11% 12% 7% 1	2534
	21st to end	Night	61% 8% 12% 13% 6% 1	2145
March	1st to 10th	Night	58% 10% 14% 12% 6%	5168
	11th to 20th	Night	53% 10% 12% 21% 3	5000
	21st to end	Night	68% 9% 12% 10% 2	5402
April	1st to 10th	Night	58% 10% 14% 15% 3	2000
	11th to 20th	Night	52% 8% 12% 16% 9% 2	1837
	21st to end	Night	52% 9% 15% 16% 8%	1765
May	1st to 10th	Night	38% 6% 15% 27% 13% 1	3637
	11th to 20th	Night	41% 12% 20% 21% 5%	3616
	21st to end	Night	54% 10% 17% 17% 2	3682
June	1st to 10th	Night	50% 8% 17% 17% 6% 3	1400
	11th to 20th	Night	47% 12% 18% 18% 5%	1400
	21st to end	Night	40% 11% 19% 19% 9% 3	1400
July	1st to 10th	Night	22% 6% 12% 27% 25% 7%	2255
	11th to 20th	Night	15% 5% 15% 38% 25% 2	2248
	21st to end	Night	28% 7% 18% 28% 16% 3	2507
August	1st to 10th	Night	14% 5% 15% 40% 24% 2	2604
	11th to 20th	Night	35% 11% 27% 16% 9% 3	2839
	21st to end	Night	39% 9% 18% 24% 10%	3204
September	1st to 10th	Night	27% 9% 17% 29% 14% 4	1900
	11th to 20th	Night	42% 9% 13% 17% 16% 4	1933
	21st to end	Night	45% 9% 16% 20% 8% 2	2157
October	1st to 10th	Night	52% 10% 17% 14% 7%	3800
	11th to 20th	Night	57% 7% 12% 18% 5%	3777
	21st to end	Night	57% 9% 13% 15% 5%	4402
November	1st to 10th	Night	69% 7% 10% 9% 4 1	2730
	11th to 20th	Night	64% 9% 11% 9% 5% 2	2800
	21st to end	Night	66% 8% 10% 10% 6% 1	2800
December	1st to 10th	Night	58% 8% 14% 15% 4	4709
	11th to 20th	Night	75% 8% 8% 6% 2	5556
	21st to end	Night	74% 9% 8% 7% 2	7458

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	42% 9% 21% 17% 8% 4	200
	11th to 20th	Day	37% 17% 17% 18% 11% 1	200
	21st to end	Day	45% 9% 20% 15% 9% 2	266
February	1st to 10th	Day	37% 11% 25% 17% 9% 1	400
	11th to 20th	Day	36% 9% 25% 20% 9% 1	395
	21st to end	Day	32% 15% 27% 15% 10% 1	331
March	1st to 10th	Day	34% 11% 25% 20% 8% 1	496
	11th to 20th	Day	33% 12% 20% 21% 13% 1	500
	21st to end	Day	35% 12% 25% 19% 7% 1	550
April	1st to 10th	Day	34% 12% 24% 20% 9% 1	500
	11th to 20th	Day	26% 12% 28% 20% 14% 1	492
	21st to end	Day	26% 11% 26% 22% 14% 2	500
May	1st to 10th	Day	19% 10% 22% 31% 18% 1	503
	11th to 20th	Day	20% 7% 21% 29% 20% 3	528
	21st to end	Day	19% 9% 24% 29% 16% 3	691
June	1st to 10th	Day	23% 8% 24% 26% 18% 1	730
	11th to 20th	Day	18% 9% 25% 29% 16% 2	730
	21st to end	Day	18% 5% 21% 33% 20% 3	730
July	1st to 10th	Day	15% 5% 18% 32% 26% 4	721
	11th to 20th	Day	8% 5% 20% 40% 24% 4	718
	21st to end	Day	8% 5% 15% 33% 33% 6%	638
August	1st to 10th	Day	10% 3 16% 29% 32% 9%	487
	11th to 20th	Day	11% 4 14% 34% 30% 6%	477
	21st to end	Day	17% 5% 18% 36% 21% 3	539
September	1st to 10th	Day	17% 6% 24% 31% 20% 3	490
	11th to 20th	Day	26% 7% 19% 26% 17% 5%	490
	21st to end	Day	28% 8% 22% 23% 15% 3	490
October	1st to 10th	Day	31% 7% 19% 22% 17% 4	490
	11th to 20th	Day	38% 7% 18% 19% 14% 4	490
	21st to end	Day	37% 14% 18% 20% 7% 3	403
November	1st to 10th	Day	42% 12% 23% 17% 7%	200
	11th to 20th	Day	42% 10% 14% 19% 11% 5%	200
	21st to end	Day	48% 10% 17% 12% 8% 6%	200
December	1st to 10th	Day	41% 7% 22% 12% 13% 6%	200
	11th to 20th	Day	46% 10% 18% 20% 6% 1	200
	21st to end	Day	46% 10% 27% 14% 3	220
January	1st to 10th	Night	48% 10% 18% 15% 8% 1	900
	11th to 20th	Night	41% 7% 18% 22% 12%	900
	21st to end	Night	42% 7% 19% 22% 10%	917
February	1st to 10th	Night	37% 7% 23% 22% 10%	800
	11th to 20th	Night	34% 8% 21% 28% 9% 1	784
	21st to end	Night	37% 11% 18% 21% 11% 2	660
March	1st to 10th	Night	35% 10% 21% 21% 12% 2	800
	11th to 20th	Night	40% 8% 16% 24% 11% 1	800
	21st to end	Night	38% 9% 18% 20% 14% 2	880
April	1st to 10th	Night	32% 9% 16% 27% 13% 4	800
	11th to 20th	Night	26% 9% 23% 26% 15% 1	649
	21st to end	Night	28% 5% 20% 30% 15% 2	565
May	1st to 10th	Night	18% 5% 18% 33% 23% 4	400
	11th to 20th	Night	22% 5% 10% 29% 29% 5%	394
	21st to end	Night	21% 5% 15% 30% 25% 4	433
June	1st to 10th	Night	24% 6% 12% 31% 24% 4	400
	11th to 20th	Night	23% 4 11% 32% 24% 6%	400
	21st to end	Night	17% 4 10% 35% 30% 5%	400
July	1st to 10th	Night	13% 2 10% 25% 41% 9%	390
	11th to 20th	Night	9% 2 8% 33% 39% 9%	388
	21st to end	Night	8% 2 5% 27% 46% 12%	416
August	1st to 10th	Night	10% 2 5% 22% 45% 15%	422
	11th to 20th	Night	9% 1 5% 26% 48% 11%	564
	21st to end	Night	16% 4 10% 29% 37% 5%	659
September	1st to 10th	Night	12% 2 11% 35% 32% 8%	760
	11th to 20th	Night	23% 5% 12% 22% 29% 10%	760
	21st to end	Night	29% 4 14% 27% 23% 3	760
October	1st to 10th	Night	26% 5% 16% 30% 21% 3	760
	11th to 20th	Night	36% 5% 13% 26% 17% 2	760
	21st to end	Night	40% 8% 19% 24% 8% 2	836
November	1st to 10th	Night	44% 8% 19% 23% 5%	780
	11th to 20th	Night	42% 8% 21% 19% 9% 1	800
	21st to end	Night	43% 7% 20% 21% 7% 1	800
December	1st to 10th	Night	36% 8% 18% 27% 11% 1	843
	11th to 20th	Night	49% 9% 20% 18% 4	900
	21st to end	Night	46% 12% 22% 15% 4	990

Page generated at 2025-07-17 22:10:00. Version ac07

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