Clearwater
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clearwater clear sky chart from 2005-11-06 to 2025-12-03.

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	10% 11% 4 9% 6% 6% 6% 4 6% 7% 31%	130875
	Night	10% 14% 5% 11% 6% 6% 5% 4 6% 7% 26%	128509
Transparency	Day	52% 7% 14% 18% 7% 1	130579
	Night	56% 7% 13% 16% 6% 1	128382
Seeing	Day	17% 15% 27% 25% 13% 3	17330
	Night	19% 12% 24% 28% 15% 2	25548

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	10% 22% 7% 17% 9% 7% 6% 3 4 6% 9%	3014
	11th to 20th	Day	9% 12% 5% 10% 5% 4 7% 7% 9% 10% 23%	3223
	21st to end	Day	8% 22% 6% 11% 6% 5% 5% 3 4 6% 24%	4004
February	1st to 10th	Day	12% 22% 5% 11% 8% 6% 6% 3 4 5% 18%	1608
	11th to 20th	Day	9% 17% 7% 11% 7% 6% 4 5% 5% 7% 23%	1755
	21st to end	Day	6% 17% 6% 11% 8% 6% 6% 5% 7% 7% 20%	1580
March	1st to 10th	Day	4 13% 6% 14% 9% 7% 6% 5% 7% 7% 23%	4518
	11th to 20th	Day	4 13% 4 10% 7% 7% 7% 5% 6% 8% 28%	4797
	21st to end	Day	6% 16% 5% 11% 8% 7% 7% 5% 6% 7% 23%	5878
April	1st to 10th	Day	6% 18% 6% 12% 9% 7% 7% 5% 7% 6% 18%	2400
	11th to 20th	Day	8% 15% 5% 10% 7% 7% 6% 5% 6% 7% 24%	2421
	21st to end	Day	9% 14% 4 10% 7% 6% 6% 5% 7% 7% 24%	2800
May	1st to 10th	Day	9% 11% 4 9% 6% 5% 5% 4 6% 6% 36%	6320
	11th to 20th	Day	11% 13% 4 10% 7% 7% 6% 6% 7% 8% 22%	6277
	21st to end	Day	14% 13% 4 8% 6% 6% 6% 4 6% 7% 26%	7363
June	1st to 10th	Day	17% 10% 3 9% 7% 6% 5% 5% 6% 7% 26%	3200
	11th to 20th	Day	19% 12% 4 10% 8% 7% 6% 4 6% 6% 17%	3200
	21st to end	Day	12% 9% 4 10% 7% 6% 6% 4 6% 8% 28%	3200
July	1st to 10th	Day	6% 6% 2 6% 4 4 4 4 6% 8% 49%	7240
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 5% 7% 54%	6782
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 8% 10% 47%	6927
August	1st to 10th	Day	5% 4 1 5% 4 4 5% 4 7% 9% 51%	4541
	11th to 20th	Day	5% 4 2 6% 4 5% 6% 6% 8% 11% 42%	4470
	21st to end	Day	12% 6% 2 6% 5% 4 4 3 6% 8% 43%	4466
September	1st to 10th	Day	10% 6% 3 7% 5% 5% 4 4 5% 7% 45%	2400
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 4 5% 8% 38%	2400
	21st to end	Day	15% 10% 4 9% 6% 5% 5% 4 7% 6% 29%	2149
October	1st to 10th	Day	14% 11% 3 8% 6% 5% 5% 5% 6% 8% 29%	3280
	11th to 20th	Day	20% 13% 4 8% 5% 4 4 3 4 6% 29%	3140
	21st to end	Day	11% 12% 4 10% 7% 7% 7% 5% 5% 7% 26%	3200
November	1st to 10th	Day	16% 18% 5% 11% 7% 9% 6% 3 4 5% 16%	1673
	11th to 20th	Day	14% 16% 5% 11% 7% 6% 6% 5% 6% 7% 17%	1653
	21st to end	Day	13% 19% 4 12% 8% 8% 6% 4 6% 5% 13%	1470
December	1st to 10th	Day	11% 13% 5% 13% 8% 7% 7% 5% 6% 8% 17%	2272
	11th to 20th	Day	11% 20% 6% 14% 8% 7% 6% 5% 6% 8% 11%	2262
	21st to end	Day	8% 18% 6% 13% 8% 7% 7% 5% 6% 7% 16%	2992
January	1st to 10th	Night	12% 25% 7% 16% 6% 6% 5% 4 5% 6% 8%	6973
	11th to 20th	Night	7% 12% 6% 12% 7% 6% 7% 6% 8% 7% 21%	6675
	21st to end	Night	11% 22% 6% 11% 7% 6% 4 3 5% 5% 20%	6952
February	1st to 10th	Night	11% 22% 6% 14% 7% 5% 5% 4 5% 5% 17%	2790
	11th to 20th	Night	10% 17% 7% 12% 6% 5% 6% 3 5% 6% 23%	2532
	21st to end	Night	6% 17% 6% 11% 8% 6% 6% 5% 6% 8% 20%	2049
March	1st to 10th	Night	5% 16% 6% 11% 6% 6% 6% 4 7% 8% 25%	5376
	11th to 20th	Night	3 11% 5% 12% 7% 6% 6% 6% 8% 9% 27%	5187
	21st to end	Night	5% 13% 5% 12% 8% 7% 6% 5% 6% 8% 24%	5096
April	1st to 10th	Night	7% 17% 5% 10% 6% 6% 6% 4 6% 9% 26%	2000
	11th to 20th	Night	7% 13% 4 9% 7% 7% 6% 5% 7% 8% 29%	1925
	21st to end	Night	8% 11% 4 9% 7% 7% 6% 4 7% 7% 31%	1600
May	1st to 10th	Night	9% 12% 4 8% 5% 5% 6% 4 5% 5% 37%	3640
	11th to 20th	Night	11% 9% 4 10% 6% 6% 5% 5% 7% 7% 29%	3616
	21st to end	Night	14% 12% 3 9% 5% 5% 5% 4 6% 9% 30%	3506
June	1st to 10th	Night	16% 9% 3 8% 6% 5% 4 4 7% 8% 31%	1200
	11th to 20th	Night	17% 10% 4 11% 6% 5% 6% 4 7% 7% 23%	1200
	21st to end	Night	8% 9% 4 8% 8% 7% 5% 5% 7% 7% 33%	1200
July	1st to 10th	Night	9% 5% 2 6% 3 3 3 4 6% 6% 53%	2720
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 5% 7% 54%	3175
	21st to end	Night	8% 6% 2 6% 5% 4 5% 5% 6% 8% 45%	3913
August	1st to 10th	Night	5% 3 1 6% 4 4 5% 5% 7% 9% 51%	2609
	11th to 20th	Night	7% 5% 3 7% 5% 6% 6% 6% 8% 10% 39%	2577
	21st to end	Night	11% 8% 3 7% 5% 6% 5% 4 6% 6% 40%	3433
September	1st to 10th	Night	9% 6% 3 7% 4 4 4 4 5% 7% 47%	2000
	11th to 20th	Night	14% 8% 2 5% 4 5% 4 3 6% 8% 41%	2000
	21st to end	Night	13% 10% 3 8% 5% 5% 4 4 6% 7% 36%	2250
October	1st to 10th	Night	14% 13% 3 9% 5% 5% 6% 4 6% 7% 30%	3920
	11th to 20th	Night	13% 12% 4 9% 6% 5% 5% 5% 6% 6% 28%	4014
	21st to end	Night	11% 13% 4 11% 6% 5% 5% 4 7% 6% 28%	4600
November	1st to 10th	Night	18% 17% 5% 10% 6% 6% 6% 4 5% 6% 16%	2843
	11th to 20th	Night	15% 18% 6% 11% 6% 6% 5% 4 5% 6% 17%	2945
	21st to end	Night	13% 19% 6% 12% 8% 7% 5% 4 6% 7% 13%	3150
December	1st to 10th	Night	10% 13% 4 10% 7% 7% 7% 5% 8% 10% 19%	4949
	11th to 20th	Night	13% 22% 8% 16% 8% 6% 5% 4 4 5% 10%	5930
	21st to end	Night	7% 19% 5% 15% 8% 7% 6% 5% 6% 6% 16%	7964

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% 8% 7%	3014
	11th to 20th	Day	54% 8% 14% 11% 9% 5%	3244
	21st to end	Day	65% 6% 15% 12% 2	4004
February	1st to 10th	Day	68% 6% 9% 10% 6% 1	1608
	11th to 20th	Day	61% 8% 13% 10% 6% 2	1764
	21st to end	Day	62% 10% 12% 9% 6% 1	1580
March	1st to 10th	Day	59% 8% 11% 14% 8%	4518
	11th to 20th	Day	53% 6% 12% 22% 6%	4797
	21st to end	Day	59% 9% 11% 14% 6%	5878
April	1st to 10th	Day	66% 10% 10% 12% 2	2400
	11th to 20th	Day	58% 9% 13% 14% 5% 1	2421
	21st to end	Day	55% 11% 12% 17% 5%	2800
May	1st to 10th	Day	45% 8% 10% 27% 10% 1	6320
	11th to 20th	Day	56% 8% 14% 18% 4	6280
	21st to end	Day	54% 8% 16% 19% 3	7364
June	1st to 10th	Day	58% 10% 10% 11% 9% 3	3200
	11th to 20th	Day	66% 10% 9% 12% 3	3200
	21st to end	Day	54% 10% 14% 14% 8% 1	3200
July	1st to 10th	Day	34% 4 13% 25% 18% 5%	7161
	11th to 20th	Day	33% 5% 14% 28% 18% 4	6700
	21st to end	Day	33% 4 22% 27% 11% 3	6732
August	1st to 10th	Day	30% 6% 10% 37% 16%	4524
	11th to 20th	Day	35% 6% 24% 27% 7% 1	4512
	21st to end	Day	41% 6% 23% 23% 5% 1	4465
September	1st to 10th	Day	38% 8% 18% 26% 9% 1	2400
	11th to 20th	Day	45% 9% 12% 23% 9% 1	2400
	21st to end	Day	53% 10% 15% 14% 8% 1	2149
October	1st to 10th	Day	53% 7% 18% 18% 4	3280
	11th to 20th	Day	59% 6% 18% 14% 2	3140
	21st to end	Day	57% 6% 21% 13% 3	3200
November	1st to 10th	Day	70% 8% 9% 10% 3	1673
	11th to 20th	Day	66% 12% 10% 8% 3 1	1653
	21st to end	Day	72% 8% 9% 6% 4 1	1470
December	1st to 10th	Day	66% 7% 18% 7% 1	2272
	11th to 20th	Day	73% 5% 12% 8% 2	2264
	21st to end	Day	65% 10% 12% 10% 2	2992
January	1st to 10th	Night	77% 6% 10% 6% 1	6973
	11th to 20th	Night	59% 7% 12% 13% 4 4	6716
	21st to end	Night	69% 4 12% 12% 2	6952
February	1st to 10th	Night	68% 6% 9% 9% 8% 1	2790
	11th to 20th	Night	61% 7% 11% 13% 5% 2	2534
	21st to end	Night	59% 10% 13% 12% 5% 1	2049
March	1st to 10th	Night	56% 7% 10% 17% 8% 1	5376
	11th to 20th	Night	50% 8% 14% 21% 7%	5188
	21st to end	Night	55% 10% 12% 16% 6% 1	5096
April	1st to 10th	Night	57% 10% 13% 14% 6%	2000
	11th to 20th	Night	53% 9% 12% 16% 10% 1	1925
	21st to end	Night	51% 11% 10% 21% 7%	1600
May	1st to 10th	Night	48% 7% 9% 24% 11%	3637
	11th to 20th	Night	50% 7% 13% 24% 6%	3616
	21st to end	Night	51% 7% 16% 23% 3	3506
June	1st to 10th	Night	50% 10% 11% 18% 9% 2	1200
	11th to 20th	Night	60% 11% 11% 13% 6%	1200
	21st to end	Night	50% 10% 16% 15% 8% 1	1200
July	1st to 10th	Night	33% 4 10% 28% 17% 8%	2690
	11th to 20th	Night	32% 5% 12% 28% 21% 2	3134
	21st to end	Night	38% 4 16% 28% 11% 3	3803
August	1st to 10th	Night	29% 5% 11% 39% 15% 1	2601
	11th to 20th	Night	39% 6% 21% 25% 7% 1	2601
	21st to end	Night	44% 5% 28% 16% 5% 1	3433
September	1st to 10th	Night	37% 8% 16% 26% 11% 2	2000
	11th to 20th	Night	41% 7% 14% 24% 11% 2	2000
	21st to end	Night	47% 9% 17% 15% 11% 1	2250
October	1st to 10th	Night	54% 4 18% 21% 3	3920
	11th to 20th	Night	56% 5% 15% 21% 3	4014
	21st to end	Night	56% 5% 19% 17% 3	4600
November	1st to 10th	Night	67% 9% 10% 10% 3 1	2843
	11th to 20th	Night	68% 7% 11% 8% 5% 1	2945
	21st to end	Night	68% 9% 11% 8% 4 1	3150
December	1st to 10th	Night	59% 9% 21% 9% 2	4949
	11th to 20th	Night	78% 4 10% 6% 2	5927
	21st to end	Night	67% 10% 12% 9% 2	7964

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	20% 15% 33% 21% 8% 4	200
	11th to 20th	Day	16% 17% 35% 23% 9% 2	200
	21st to end	Day	21% 16% 35% 20% 7%	220
February	1st to 10th	Day	18% 25% 30% 16% 8% 3	208
	11th to 20th	Day	20% 18% 28% 28% 6% 1	395
	21st to end	Day	14% 21% 33% 23% 7% 2	379
March	1st to 10th	Day	18% 20% 30% 22% 9% 1	500
	11th to 20th	Day	18% 23% 26% 22% 10% 1	500
	21st to end	Day	20% 21% 28% 17% 11% 3	550
April	1st to 10th	Day	21% 20% 29% 21% 7% 2	500
	11th to 20th	Day	17% 21% 25% 23% 10% 3	492
	21st to end	Day	16% 18% 28% 22% 12% 4	500
May	1st to 10th	Day	17% 15% 29% 25% 12% 2	503
	11th to 20th	Day	15% 15% 24% 26% 17% 3	528
	21st to end	Day	21% 13% 24% 24% 15% 4	760
June	1st to 10th	Day	21% 18% 29% 25% 7% 1	930
	11th to 20th	Day	21% 15% 26% 24% 12% 2	930
	21st to end	Day	14% 11% 26% 29% 17% 3	930
July	1st to 10th	Day	15% 11% 23% 30% 18% 3	916
	11th to 20th	Day	10% 13% 27% 30% 18% 3	719
	21st to end	Day	8% 7% 26% 34% 19% 5%	602
August	1st to 10th	Day	10% 7% 23% 31% 22% 7%	527
	11th to 20th	Day	9% 9% 28% 28% 19% 5%	517
	21st to end	Day	13% 11% 25% 29% 18% 5%	583
September	1st to 10th	Day	16% 12% 27% 23% 15% 7%	530
	11th to 20th	Day	15% 13% 25% 27% 17% 3	530
	21st to end	Day	23% 16% 28% 23% 8% 2	530
October	1st to 10th	Day	22% 14% 27% 22% 12% 2	530
	11th to 20th	Day	25% 18% 23% 22% 11% 2	460
	21st to end	Day	25% 17% 26% 20% 11% 1	363
November	1st to 10th	Day	27% 20% 31% 15% 5% 2	234
	11th to 20th	Day	25% 15% 28% 22% 8% 1	220
	21st to end	Day	22% 16% 28% 22% 8% 4	220
December	1st to 10th	Day	20% 11% 21% 28% 17% 3	204
	11th to 20th	Day	20% 18% 29% 19% 14% 2	200
	21st to end	Day	17% 15% 35% 24% 8% 1	220
January	1st to 10th	Night	28% 15% 28% 21% 7% 1	990
	11th to 20th	Night	20% 15% 28% 28% 9%	887
	21st to end	Night	21% 14% 28% 26% 10% 1	880
February	1st to 10th	Night	23% 16% 24% 25% 12% 1	800
	11th to 20th	Night	19% 13% 23% 35% 10%	784
	21st to end	Night	16% 12% 25% 28% 17% 2	660
March	1st to 10th	Night	18% 14% 24% 29% 13% 1	800
	11th to 20th	Night	14% 15% 27% 28% 15% 1	800
	21st to end	Night	14% 14% 24% 27% 18% 3	880
April	1st to 10th	Night	17% 11% 23% 29% 18% 3	800
	11th to 20th	Night	13% 12% 27% 32% 15% 1	737
	21st to end	Night	14% 12% 25% 31% 17% 2	400
May	1st to 10th	Night	16% 9% 20% 34% 19% 2	400
	11th to 20th	Night	11% 8% 22% 36% 20% 2	394
	21st to end	Night	20% 9% 18% 31% 19% 3	433
June	1st to 10th	Night	21% 10% 23% 32% 14% 1	400
	11th to 20th	Night	20% 8% 20% 32% 18% 2	400
	21st to end	Night	14% 9% 20% 29% 26% 3	400
July	1st to 10th	Night	14% 4 19% 36% 26% 2	390
	11th to 20th	Night	11% 5% 18% 38% 24% 3	388
	21st to end	Night	9% 5% 15% 36% 30% 5%	438
August	1st to 10th	Night	9% 3 11% 32% 40% 5%	398
	11th to 20th	Night	9% 4 14% 38% 32% 4	396
	21st to end	Night	13% 6% 16% 34% 28% 2	770
September	1st to 10th	Night	13% 6% 20% 31% 28% 2	800
	11th to 20th	Night	17% 6% 15% 30% 28% 5%	800
	21st to end	Night	21% 10% 21% 29% 16% 2	800
October	1st to 10th	Night	22% 9% 18% 31% 19% 2	800
	11th to 20th	Night	23% 12% 24% 25% 15% 1	800
	21st to end	Night	26% 12% 28% 23% 11% 1	880
November	1st to 10th	Night	30% 14% 27% 22% 7% 1	820
	11th to 20th	Night	29% 16% 23% 24% 8% 1	845
	21st to end	Night	23% 15% 30% 24% 7% 1	1050
December	1st to 10th	Night	20% 11% 29% 28% 10% 1	1018
	11th to 20th	Night	24% 14% 28% 26% 8%	1100
	21st to end	Night	21% 15% 31% 25% 6%	1210

Page generated at 2025-11-29 22:00:57. Version ac07

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