Stargazer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stargazer Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	18% 9% 3 7% 5% 4 4 4 5% 6% 34%	130705
	Night	21% 11% 3 8% 5% 4 4 3 5% 6% 30%	127363
Transparency	Day	50% 9% 15% 17% 8% 1	130417
	Night	55% 8% 13% 16% 7% 1	127228
Seeing	Day	24% 9% 20% 26% 16% 5%	16381
	Night	30% 9% 14% 25% 19% 4	24616

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	32% 17% 4 10% 6% 5% 5% 4 4 4 9%	3179
	11th to 20th	Day	18% 12% 3 8% 6% 6% 5% 4 6% 8% 24%	3432
	21st to end	Day	27% 12% 3 7% 5% 5% 5% 4 5% 5% 22%	4078
February	1st to 10th	Day	29% 14% 4 9% 5% 4 4 4 5% 6% 17%	1800
	11th to 20th	Day	26% 14% 3 10% 5% 5% 4 4 4 5% 21%	1755
	21st to end	Day	25% 16% 5% 8% 5% 4 5% 3 5% 4 20%	1536
March	1st to 10th	Day	19% 14% 4 11% 6% 6% 6% 4 5% 7% 18%	4638
	11th to 20th	Day	18% 14% 4 9% 5% 5% 5% 4 5% 6% 25%	5000
	21st to end	Day	23% 12% 4 11% 6% 7% 6% 4 6% 6% 16%	5658
April	1st to 10th	Day	23% 14% 3 7% 6% 5% 5% 4 6% 6% 20%	2400
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 4 5% 6% 27%	2419
	21st to end	Day	22% 10% 3 8% 5% 5% 4 4 6% 6% 27%	2655
May	1st to 10th	Day	15% 8% 3 7% 4 4 4 3 5% 6% 41%	6320
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 4 6% 8% 30%	6277
	21st to end	Day	23% 7% 3 6% 4 4 4 4 6% 8% 30%	6903
June	1st to 10th	Day	20% 8% 3 6% 5% 4 5% 4 5% 7% 35%	2926
	11th to 20th	Day	19% 9% 3 7% 6% 5% 5% 5% 6% 7% 29%	3000
	21st to end	Day	12% 8% 3 7% 5% 4 4 4 7% 8% 40%	3000
July	1st to 10th	Day	7% 3 1 4 2 2 3 3 4 6% 66%	6780
	11th to 20th	Day	4 3 2 3 2 2 2 2 4 5% 70%	6645
	21st to end	Day	9% 4 2 5% 3 3 3 3 4 7% 58%	6874
August	1st to 10th	Day	6% 3 1 4 3 3 4 3 4 7% 61%	4488
	11th to 20th	Day	10% 7% 2 6% 4 4 5% 4 6% 7% 45%	4156
	21st to end	Day	14% 8% 3 7% 4 4 4 3 5% 7% 42%	4490
September	1st to 10th	Day	11% 6% 3 6% 4 3 3 3 5% 7% 50%	2400
	11th to 20th	Day	13% 8% 3 7% 4 5% 4 4 5% 7% 41%	2400
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 4 8% 37%	2113
October	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 5% 6% 10% 29%	3211
	11th to 20th	Day	28% 12% 3 8% 4 4 3 3 4 6% 24%	3161
	21st to end	Day	24% 11% 5% 10% 5% 6% 5% 3 5% 6% 19%	3237
November	1st to 10th	Day	31% 9% 4 8% 5% 4 5% 4 5% 5% 17%	1585
	11th to 20th	Day	31% 10% 3 8% 6% 4 5% 4 5% 5% 20%	1600
	21st to end	Day	30% 12% 4 7% 6% 4 5% 4 5% 6% 17%	1600
December	1st to 10th	Day	20% 10% 3 9% 7% 6% 7% 4 5% 7% 22%	2620
	11th to 20th	Day	29% 15% 5% 11% 7% 5% 4 4 4 4 11%	2871
	21st to end	Day	28% 18% 5% 12% 6% 5% 4 4 4 4 10%	3498
January	1st to 10th	Night	29% 15% 5% 11% 6% 6% 5% 3 4 4 11%	6766
	11th to 20th	Night	20% 11% 3 7% 5% 5% 5% 4 6% 9% 27%	6450
	21st to end	Night	24% 12% 3 8% 5% 5% 4 3 5% 7% 24%	6867
February	1st to 10th	Night	28% 15% 4 8% 5% 4 4 3 4 5% 20%	2600
	11th to 20th	Night	26% 14% 4 9% 5% 4 4 3 4 5% 23%	2532
	21st to end	Night	26% 14% 4 7% 5% 5% 4 4 5% 5% 21%	2093
March	1st to 10th	Night	16% 14% 5% 12% 7% 6% 6% 4 6% 6% 19%	5254
	11th to 20th	Night	20% 15% 4 8% 4 4 4 4 4 6% 27%	4999
	21st to end	Night	29% 14% 4 10% 6% 4 5% 4 5% 5% 15%	5316
April	1st to 10th	Night	24% 12% 4 8% 6% 5% 4 5% 7% 5% 20%	2000
	11th to 20th	Night	20% 10% 3 8% 7% 6% 4 4 4 6% 29%	1929
	21st to end	Night	24% 12% 4 8% 5% 5% 4 3 5% 5% 24%	1744
May	1st to 10th	Night	17% 9% 3 6% 4 3 3 3 3 5% 44%	3640
	11th to 20th	Night	19% 9% 3 8% 5% 5% 4 3 4 6% 34%	3616
	21st to end	Night	19% 9% 3 8% 5% 5% 6% 4 7% 7% 27%	3972
June	1st to 10th	Night	20% 7% 3 7% 6% 4 4 4 5% 7% 32%	1470
	11th to 20th	Night	21% 10% 3 7% 5% 6% 5% 4 5% 6% 29%	1400
	21st to end	Night	16% 6% 2 6% 5% 5% 3 4 6% 7% 41%	1400
July	1st to 10th	Night	10% 4 2 4 2 2 2 2 2 5% 64%	3180
	11th to 20th	Night	5% 3 1 3 2 2 2 2 3 4 75%	3313
	21st to end	Night	8% 4 1 4 3 3 4 3 5% 6% 59%	3969
August	1st to 10th	Night	6% 3 1 2 2 2 3 3 5% 6% 67%	2661
	11th to 20th	Night	14% 7% 2 6% 4 3 3 2 4 5% 51%	2902
	21st to end	Night	15% 6% 2 6% 3 3 3 3 4 6% 48%	3424
September	1st to 10th	Night	9% 5% 2 6% 3 3 4 3 5% 6% 54%	2000
	11th to 20th	Night	16% 8% 2 7% 4 5% 4 4 4 5% 42%	2000
	21st to end	Night	17% 7% 3 7% 4 4 4 3 4 7% 41%	2285
October	1st to 10th	Night	15% 7% 2 7% 5% 5% 4 4 6% 8% 36%	3836
	11th to 20th	Night	27% 9% 3 7% 4 3 3 3 5% 6% 31%	3777
	21st to end	Night	25% 13% 4 9% 5% 4 4 3 5% 6% 22%	4323
November	1st to 10th	Night	33% 10% 4 7% 5% 5% 5% 3 4 5% 20%	2711
	11th to 20th	Night	31% 12% 4 7% 5% 4 4 3 4 5% 21%	2800
	21st to end	Night	31% 10% 3 8% 6% 5% 4 3 5% 5% 19%	2800
December	1st to 10th	Night	21% 11% 3 8% 6% 5% 5% 5% 7% 7% 23%	4560
	11th to 20th	Night	28% 15% 5% 11% 6% 5% 4 3 5% 5% 13%	5316
	21st to end	Night	27% 17% 6% 12% 6% 5% 5% 3 4 5% 11%	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	77% 8% 7% 6% 2	3179
	11th to 20th	Day	54% 8% 12% 18% 7% 1	3456
	21st to end	Day	63% 7% 8% 14% 8%	4078
February	1st to 10th	Day	68% 9% 10% 8% 4 2	1800
	11th to 20th	Day	66% 8% 9% 9% 7% 1	1764
	21st to end	Day	67% 9% 9% 10% 4 1	1536
March	1st to 10th	Day	61% 10% 12% 13% 4	4638
	11th to 20th	Day	60% 9% 12% 17% 2	5000
	21st to end	Day	66% 12% 12% 9% 1	5658
April	1st to 10th	Day	64% 11% 14% 9% 1	2400
	11th to 20th	Day	59% 8% 11% 13% 7% 1	2419
	21st to end	Day	59% 9% 12% 14% 5%	2655
May	1st to 10th	Day	45% 8% 13% 24% 10%	6320
	11th to 20th	Day	51% 13% 18% 16% 3	6280
	21st to end	Day	52% 10% 21% 15% 1	6904
June	1st to 10th	Day	52% 9% 18% 15% 6% 1	2926
	11th to 20th	Day	56% 12% 16% 13% 4	3000
	21st to end	Day	44% 10% 19% 20% 7% 1	3000
July	1st to 10th	Day	22% 7% 22% 21% 20% 8%	6706
	11th to 20th	Day	19% 4 17% 30% 28% 2	6564
	21st to end	Day	27% 7% 25% 29% 10% 1	6681
August	1st to 10th	Day	22% 7% 17% 36% 17% 1	4471
	11th to 20th	Day	38% 10% 20% 24% 7% 2	4195
	21st to end	Day	43% 7% 13% 25% 12% 1	4489
September	1st to 10th	Day	35% 9% 17% 23% 12% 4	2400
	11th to 20th	Day	43% 8% 15% 18% 15% 1	2400
	21st to end	Day	45% 9% 18% 18% 9% 2	2113
October	1st to 10th	Day	46% 8% 19% 22% 4	3211
	11th to 20th	Day	60% 6% 13% 13% 7%	3161
	21st to end	Day	65% 8% 11% 10% 6%	3237
November	1st to 10th	Day	67% 8% 13% 7% 4	1585
	11th to 20th	Day	66% 9% 8% 10% 5% 2	1600
	21st to end	Day	67% 7% 9% 9% 7% 1	1600
December	1st to 10th	Day	55% 6% 14% 15% 9%	2620
	11th to 20th	Day	75% 9% 8% 6% 2	2873
	21st to end	Day	76% 9% 9% 5% 1	3498
January	1st to 10th	Night	73% 9% 8% 7% 3	6766
	11th to 20th	Night	50% 8% 15% 19% 7% 1	6488
	21st to end	Night	58% 7% 10% 16% 8%	6867
February	1st to 10th	Night	67% 6% 11% 9% 5% 2	2600
	11th to 20th	Night	65% 7% 10% 11% 6% 2	2534
	21st to end	Night	65% 7% 9% 14% 4 1	2093
March	1st to 10th	Night	62% 9% 12% 12% 5%	5254
	11th to 20th	Night	57% 10% 14% 17% 2	5000
	21st to end	Night	71% 10% 11% 8%	5316
April	1st to 10th	Night	64% 10% 13% 12% 2	2000
	11th to 20th	Night	58% 8% 13% 12% 8% 2	1929
	21st to end	Night	62% 8% 11% 13% 5%	1744
May	1st to 10th	Night	44% 6% 12% 30% 7%	3637
	11th to 20th	Night	50% 11% 18% 17% 4	3616
	21st to end	Night	55% 10% 21% 13% 1	3972
June	1st to 10th	Night	52% 7% 18% 16% 6% 1	1470
	11th to 20th	Night	55% 8% 18% 15% 3	1400
	21st to end	Night	42% 7% 22% 21% 5% 3	1400
July	1st to 10th	Night	27% 4 17% 23% 18% 11%	3145
	11th to 20th	Night	16% 5% 16% 29% 32% 2	3271
	21st to end	Night	26% 6% 21% 32% 14% 2	3857
August	1st to 10th	Night	17% 6% 17% 41% 18% 2	2653
	11th to 20th	Night	36% 7% 22% 24% 9% 2	2929
	21st to end	Night	38% 7% 18% 27% 10% 1	3424
September	1st to 10th	Night	31% 7% 17% 28% 12% 4	2000
	11th to 20th	Night	45% 8% 10% 19% 15% 3	2000
	21st to end	Night	45% 7% 14% 23% 9% 1	2285
October	1st to 10th	Night	45% 8% 19% 23% 4	3836
	11th to 20th	Night	55% 5% 16% 17% 7%	3777
	21st to end	Night	64% 8% 10% 12% 5%	4323
November	1st to 10th	Night	67% 7% 11% 11% 3 1	2711
	11th to 20th	Night	67% 7% 9% 10% 5% 2	2800
	21st to end	Night	67% 7% 10% 9% 6% 1	2800
December	1st to 10th	Night	54% 7% 14% 20% 6%	4560
	11th to 20th	Night	73% 8% 9% 7% 2	5313
	21st to end	Night	75% 12% 6% 6% 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	40% 12% 16% 16% 12% 6%	200
	11th to 20th	Day	33% 15% 16% 17% 15% 6%	200
	21st to end	Day	41% 11% 15% 16% 13% 4	255
February	1st to 10th	Day	40% 12% 19% 19% 9% 2	400
	11th to 20th	Day	37% 13% 21% 19% 9% 1	395
	21st to end	Day	35% 14% 21% 19% 9% 1	358
March	1st to 10th	Day	32% 14% 23% 20% 10% 1	500
	11th to 20th	Day	33% 15% 14% 20% 15% 4	500
	21st to end	Day	33% 13% 24% 19% 9% 1	550
April	1st to 10th	Day	29% 14% 24% 19% 11% 3	500
	11th to 20th	Day	22% 16% 25% 22% 11% 4	492
	21st to end	Day	26% 13% 23% 23% 14% 3	500
May	1st to 10th	Day	18% 9% 24% 32% 14% 2	503
	11th to 20th	Day	25% 8% 19% 25% 17% 5%	528
	21st to end	Day	25% 9% 21% 25% 15% 6%	573
June	1st to 10th	Day	19% 9% 26% 29% 14% 3	656
	11th to 20th	Day	21% 9% 28% 27% 13% 3	730
	21st to end	Day	14% 7% 24% 30% 21% 5%	730
July	1st to 10th	Day	12% 5% 20% 37% 23% 4	721
	11th to 20th	Day	8% 6% 20% 38% 23% 6%	661
	21st to end	Day	8% 2 15% 37% 27% 10%	575
August	1st to 10th	Day	12% 3 13% 35% 27% 10%	527
	11th to 20th	Day	9% 3 16% 34% 28% 10%	517
	21st to end	Day	12% 7% 21% 31% 21% 9%	583
September	1st to 10th	Day	15% 6% 18% 34% 20% 6%	530
	11th to 20th	Day	15% 8% 22% 29% 23% 4	530
	21st to end	Day	22% 9% 23% 24% 17% 5%	530
October	1st to 10th	Day	25% 11% 17% 25% 16% 7%	503
	11th to 20th	Day	33% 8% 19% 18% 17% 5%	490
	21st to end	Day	33% 12% 19% 19% 12% 5%	416
November	1st to 10th	Day	38% 11% 21% 17% 10% 2	208
	11th to 20th	Day	38% 13% 15% 15% 12% 9%	200
	21st to end	Day	35% 12% 20% 12% 11% 11%	200
December	1st to 10th	Day	43% 12% 10% 11% 12% 14%	200
	11th to 20th	Day	43% 15% 17% 17% 8% 1	200
	21st to end	Day	44% 10% 16% 20% 6% 2	220
January	1st to 10th	Night	44% 11% 14% 17% 11% 2	900
	11th to 20th	Night	39% 11% 12% 20% 17% 2	837
	21st to end	Night	37% 9% 15% 20% 15% 3	880
February	1st to 10th	Night	38% 11% 13% 26% 10% 2	800
	11th to 20th	Night	35% 11% 13% 26% 14% 1	784
	21st to end	Night	37% 10% 17% 23% 12% 2	660
March	1st to 10th	Night	33% 14% 16% 23% 13% 2	800
	11th to 20th	Night	37% 13% 14% 20% 14% 3	800
	21st to end	Night	36% 12% 19% 22% 9% 1	880
April	1st to 10th	Night	31% 11% 17% 24% 14% 3	800
	11th to 20th	Night	26% 10% 21% 26% 15% 3	741
	21st to end	Night	30% 10% 15% 29% 13% 3	544
May	1st to 10th	Night	20% 7% 17% 32% 23% 2	400
	11th to 20th	Night	22% 6% 13% 25% 29% 4	394
	21st to end	Night	24% 6% 13% 27% 25% 5%	433
June	1st to 10th	Night	23% 6% 17% 30% 23% 2	400
	11th to 20th	Night	23% 7% 14% 34% 20% 2	400
	21st to end	Night	18% 4 12% 34% 28% 5%	400
July	1st to 10th	Night	12% 4 8% 36% 38% 3	390
	11th to 20th	Night	9% 3 12% 37% 35% 4	388
	21st to end	Night	8% 2 9% 33% 42% 7%	438
August	1st to 10th	Night	10% 3 6% 27% 43% 11%	424
	11th to 20th	Night	11% 3 6% 23% 45% 13%	594
	21st to end	Night	14% 4 9% 30% 36% 7%	761
September	1st to 10th	Night	11% 3 11% 35% 33% 8%	800
	11th to 20th	Night	20% 5% 11% 23% 32% 9%	800
	21st to end	Night	22% 7% 13% 27% 27% 5%	800
October	1st to 10th	Night	22% 7% 14% 25% 26% 6%	772
	11th to 20th	Night	31% 8% 13% 23% 20% 5%	760
	21st to end	Night	32% 10% 17% 24% 14% 4	836
November	1st to 10th	Night	39% 11% 17% 23% 10%	780
	11th to 20th	Night	40% 11% 17% 17% 12% 3	800
	21st to end	Night	38% 9% 16% 19% 12% 5%	800
December	1st to 10th	Night	34% 8% 14% 24% 16% 4	800
	11th to 20th	Night	42% 13% 15% 20% 9% 1	830
	21st to end	Night	43% 13% 18% 17% 8%	990

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