Grayland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grayland clear sky chart from 2005-11-06 to 2025-11-18.

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	23% 13% 3 8% 5% 4 4 3 5% 7% 25%	131181
	Night	27% 14% 3 7% 4 4 4 3 4 5% 25%	127904
Transparency	Day	36% 38% 8% 11% 6% 1	130891
	Night	40% 38% 6% 11% 5% 1	127771
Seeing	Day	31% 11% 23% 23% 10% 2	16775
	Night	35% 8% 15% 24% 15% 3	25011

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	37% 15% 4 10% 5% 4 4 2 4 4 10%	3179
	11th to 20th	Day	25% 11% 2 7% 4 4 3 3 5% 5% 30%	3342
	21st to end	Day	37% 10% 3 6% 3 3 3 2 3 4 26%	4418
February	1st to 10th	Day	36% 13% 3 8% 5% 4 4 3 3 6% 16%	1800
	11th to 20th	Day	35% 12% 4 6% 5% 3 3 3 4 4 21%	1755
	21st to end	Day	32% 17% 4 8% 6% 4 3 3 4 4 16%	1485
March	1st to 10th	Day	26% 15% 3 8% 5% 5% 5% 4 5% 7% 15%	4861
	11th to 20th	Day	29% 12% 3 9% 5% 4 3 3 4 7% 22%	5000
	21st to end	Day	30% 14% 4 10% 6% 5% 5% 4 5% 6% 12%	5501
April	1st to 10th	Day	33% 14% 3 8% 6% 4 4 3 5% 6% 15%	2400
	11th to 20th	Day	27% 10% 3 8% 6% 4 3 3 5% 7% 24%	2479
	21st to end	Day	26% 10% 4 9% 6% 4 4 3 6% 7% 21%	2600
May	1st to 10th	Day	19% 8% 2 7% 4 4 4 3 4 6% 38%	6178
	11th to 20th	Day	18% 15% 4 9% 5% 5% 5% 4 6% 8% 22%	6277
	21st to end	Day	23% 10% 3 7% 5% 4 4 4 6% 9% 26%	6903
June	1st to 10th	Day	25% 13% 3 7% 5% 3 4 4 5% 7% 23%	2944
	11th to 20th	Day	24% 12% 4 10% 6% 5% 5% 4 5% 8% 19%	3000
	21st to end	Day	19% 13% 5% 8% 5% 4 5% 3 5% 7% 26%	3000
July	1st to 10th	Day	10% 15% 3 6% 4 4 4 3 5% 8% 38%	6780
	11th to 20th	Day	9% 15% 3 7% 4 3 3 3 5% 8% 39%	6698
	21st to end	Day	15% 20% 4 8% 4 4 3 3 5% 7% 26%	6874
August	1st to 10th	Day	12% 20% 5% 10% 5% 4 5% 4 5% 9% 23%	4225
	11th to 20th	Day	16% 13% 4 9% 5% 5% 5% 5% 5% 8% 25%	4156
	21st to end	Day	18% 10% 3 8% 4 4 4 3 4 6% 36%	4572
September	1st to 10th	Day	13% 10% 4 7% 4 4 4 4 6% 7% 36%	2400
	11th to 20th	Day	17% 11% 4 8% 4 4 4 4 6% 7% 31%	2325
	21st to end	Day	20% 8% 2 7% 5% 4 3 3 4 7% 35%	2150
October	1st to 10th	Day	18% 7% 3 6% 3 4 4 3 7% 8% 38%	3280
	11th to 20th	Day	32% 9% 2 7% 4 4 3 3 3 6% 26%	3261
	21st to end	Day	23% 9% 3 7% 6% 5% 5% 4 6% 8% 25%	3482
November	1st to 10th	Day	37% 9% 4 7% 5% 4 4 4 4 4 18%	1648
	11th to 20th	Day	37% 10% 3 6% 4 4 3 4 4 6% 19%	1656
	21st to end	Day	37% 11% 3 7% 3 3 4 3 3 5% 21%	1600
December	1st to 10th	Day	23% 9% 3 7% 5% 5% 3 3 4 5% 34%	2620
	11th to 20th	Day	41% 12% 3 9% 4 4 4 4 5% 5% 9%	2834
	21st to end	Day	38% 14% 4 8% 5% 4 4 3 4 4 11%	3498
January	1st to 10th	Night	38% 15% 4 9% 4 4 4 3 4 4 12%	6766
	11th to 20th	Night	24% 11% 3 7% 4 4 4 3 4 6% 30%	6539
	21st to end	Night	38% 11% 3 5% 3 2 2 2 2 4 28%	6541
February	1st to 10th	Night	33% 15% 4 8% 5% 4 4 3 4 5% 16%	2600
	11th to 20th	Night	33% 14% 4 7% 4 3 3 3 3 4 21%	2532
	21st to end	Night	34% 15% 4 8% 4 4 4 2 4 4 17%	2144
March	1st to 10th	Night	26% 15% 4 8% 5% 5% 5% 4 6% 6% 17%	5031
	11th to 20th	Night	26% 14% 4 8% 4 4 4 3 4 6% 24%	4999
	21st to end	Night	35% 17% 4 10% 5% 4 4 3 4 4 11%	5473
April	1st to 10th	Night	29% 15% 5% 10% 6% 5% 4 3 4 4 16%	2000
	11th to 20th	Night	24% 14% 4 7% 6% 5% 4 2 4 5% 24%	1869
	21st to end	Night	25% 15% 4 7% 6% 5% 5% 4 5% 5% 20%	1800
May	1st to 10th	Night	19% 10% 3 6% 3 3 3 3 4 6% 41%	3788
	11th to 20th	Night	21% 17% 4 8% 6% 6% 6% 4 5% 5% 19%	3616
	21st to end	Night	20% 15% 4 8% 5% 4 4 3 5% 7% 26%	3972
June	1st to 10th	Night	26% 15% 2 6% 4 4 3 3 3 5% 29%	1452
	11th to 20th	Night	24% 15% 3 7% 4 5% 4 4 5% 6% 23%	1400
	21st to end	Night	19% 16% 3 6% 4 4 4 3 4 6% 31%	1400
July	1st to 10th	Night	10% 14% 2 5% 3 2 3 1 3 5% 52%	3180
	11th to 20th	Night	8% 19% 5% 6% 4 2 3 2 3 5% 43%	3260
	21st to end	Night	14% 32% 2 4 2 1 1 2 2 3 35%	3969
August	1st to 10th	Night	13% 31% 3 7% 3 2 2 2 3 5% 30%	2932
	11th to 20th	Night	20% 17% 3 5% 3 3 4 2 4 6% 34%	2902
	21st to end	Night	18% 10% 3 6% 4 3 4 3 3 5% 41%	3339
September	1st to 10th	Night	14% 11% 3 5% 3 3 3 3 4 7% 44%	2000
	11th to 20th	Night	20% 11% 3 8% 3 3 4 3 4 6% 34%	2072
	21st to end	Night	21% 10% 3 6% 4 3 3 2 3 6% 38%	2249
October	1st to 10th	Night	15% 8% 3 6% 4 3 4 3 5% 10% 38%	3920
	11th to 20th	Night	29% 11% 3 7% 4 4 4 3 4 6% 26%	3897
	21st to end	Night	27% 8% 3 8% 4 5% 5% 4 6% 6% 25%	4323
November	1st to 10th	Night	36% 9% 4 8% 4 5% 5% 3 5% 5% 17%	2870
	11th to 20th	Night	37% 11% 2 8% 4 4 3 3 4 4 18%	2898
	21st to end	Night	30% 11% 4 8% 5% 4 3 3 5% 5% 22%	2800
December	1st to 10th	Night	25% 9% 3 7% 4 4 4 4 5% 6% 31%	4560
	11th to 20th	Night	43% 13% 4 8% 4 4 3 2 3 4 11%	5353
	21st to end	Night	40% 15% 4 8% 5% 3 4 2 3 4 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	59% 28% 4 6% 3 1	3179
	11th to 20th	Day	41% 26% 7% 20% 6%	3364
	21st to end	Day	47% 28% 6% 11% 8%	4418
February	1st to 10th	Day	28% 62% 3 4 2	1800
	11th to 20th	Day	31% 61% 1 4 2	1764
	21st to end	Day	28% 62% 2 5% 2	1485
March	1st to 10th	Day	49% 32% 7% 10% 2	4861
	11th to 20th	Day	46% 30% 6% 14% 4	5000
	21st to end	Day	54% 30% 10% 6%	5501
April	1st to 10th	Day	30% 61% 4 4 1	2400
	11th to 20th	Day	24% 62% 4 7% 2	2479
	21st to end	Day	25% 62% 4 8% 1	2600
May	1st to 10th	Day	31% 30% 12% 17% 10%	6178
	11th to 20th	Day	46% 31% 12% 10% 2	6280
	21st to end	Day	38% 31% 18% 11% 3	6904
June	1st to 10th	Day	19% 62% 7% 8% 3	2944
	11th to 20th	Day	22% 63% 4 9% 2	3000
	21st to end	Day	24% 60% 6% 6% 3	3000
July	1st to 10th	Day	24% 25% 14% 11% 17% 9%	6706
	11th to 20th	Day	24% 25% 8% 18% 23% 1	6617
	21st to end	Day	40% 25% 10% 21% 4	6681
August	1st to 10th	Day	28% 36% 5% 20% 10% 1	4208
	11th to 20th	Day	40% 36% 10% 9% 4 1	4195
	21st to end	Day	32% 35% 7% 15% 11%	4571
September	1st to 10th	Day	24% 57% 6% 7% 4 1	2400
	11th to 20th	Day	23% 57% 5% 7% 7% 1	2325
	21st to end	Day	19% 57% 7% 11% 6%	2150
October	1st to 10th	Day	26% 35% 12% 24% 4	3280
	11th to 20th	Day	42% 36% 6% 11% 4	3261
	21st to end	Day	40% 37% 9% 7% 6% 1	3482
November	1st to 10th	Day	35% 53% 6% 5% 1	1648
	11th to 20th	Day	31% 55% 4 5% 3 2	1656
	21st to end	Day	28% 56% 2 5% 8% 1	1600
December	1st to 10th	Day	34% 37% 3 12% 13% 1	2620
	11th to 20th	Day	56% 32% 8% 4 1	2836
	21st to end	Day	60% 27% 7% 4 2	3498
January	1st to 10th	Night	59% 27% 5% 5% 4 1	6766
	11th to 20th	Night	40% 26% 7% 22% 5%	6579
	21st to end	Night	47% 28% 3 13% 8% 1	6541
February	1st to 10th	Night	29% 62% 3 4 2	2600
	11th to 20th	Night	31% 60% 2 3 3 1	2534
	21st to end	Night	24% 62% 5% 6% 2	2144
March	1st to 10th	Night	45% 33% 7% 11% 3	5031
	11th to 20th	Night	44% 30% 6% 15% 5%	5000
	21st to end	Night	58% 29% 8% 4 1	5473
April	1st to 10th	Night	28% 62% 4 5%	2000
	11th to 20th	Night	23% 62% 4 8% 3	1869
	21st to end	Night	23% 62% 5% 7% 3	1800
May	1st to 10th	Night	29% 29% 11% 23% 8%	3785
	11th to 20th	Night	50% 31% 7% 7% 4	3616
	21st to end	Night	41% 32% 14% 13% 1	3972
June	1st to 10th	Night	20% 63% 7% 8% 2	1452
	11th to 20th	Night	20% 63% 5% 11% 2	1400
	21st to end	Night	22% 61% 8% 8% 2	1400
July	1st to 10th	Night	23% 24% 14% 14% 14% 11%	3145
	11th to 20th	Night	34% 25% 7% 17% 17% 1	3218
	21st to end	Night	43% 25% 8% 20% 5% 1	3857
August	1st to 10th	Night	39% 36% 6% 13% 6% 1	2924
	11th to 20th	Night	38% 36% 12% 9% 4 1	2929
	21st to end	Night	32% 35% 7% 13% 12% 1	3339
September	1st to 10th	Night	21% 57% 6% 10% 5% 1	2000
	11th to 20th	Night	23% 58% 5% 7% 6% 2	2072
	21st to end	Night	21% 56% 4 12% 7%	2249
October	1st to 10th	Night	26% 35% 10% 25% 4	3920
	11th to 20th	Night	43% 35% 6% 12% 4	3897
	21st to end	Night	41% 35% 6% 10% 6% 1	4323
November	1st to 10th	Night	34% 53% 5% 6% 2	2870
	11th to 20th	Night	32% 55% 4 5% 2 2	2898
	21st to end	Night	28% 57% 3 4 8% 1	2800
December	1st to 10th	Night	34% 36% 4 13% 13% 1	4560
	11th to 20th	Night	58% 32% 3 5% 2	5350
	21st to end	Night	59% 28% 7% 5% 1	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	47% 14% 24% 12% 4 1	200
	11th to 20th	Day	44% 14% 22% 13% 8% 2	200
	21st to end	Day	43% 10% 17% 19% 11% 2	400
February	1st to 10th	Day	44% 13% 18% 19% 6% 1	400
	11th to 20th	Day	45% 12% 25% 13% 4 1	395
	21st to end	Day	43% 14% 20% 17% 5% 1	331
March	1st to 10th	Day	41% 12% 22% 17% 8%	500
	11th to 20th	Day	39% 11% 22% 17% 7% 3	500
	21st to end	Day	41% 14% 19% 18% 7% 1	550
April	1st to 10th	Day	40% 13% 21% 20% 5% 1	500
	11th to 20th	Day	31% 15% 28% 17% 7% 3	492
	21st to end	Day	32% 14% 27% 17% 7% 2	500
May	1st to 10th	Day	23% 15% 25% 24% 11% 2	503
	11th to 20th	Day	37% 11% 22% 20% 7% 3	528
	21st to end	Day	30% 11% 25% 22% 8% 4	573
June	1st to 10th	Day	30% 10% 26% 24% 8% 3	674
	11th to 20th	Day	28% 14% 30% 21% 6% 1	730
	21st to end	Day	21% 12% 25% 30% 10% 2	730
July	1st to 10th	Day	22% 7% 27% 31% 11% 3	721
	11th to 20th	Day	18% 7% 28% 32% 11% 4	688
	21st to end	Day	17% 7% 21% 35% 15% 6%	575
August	1st to 10th	Day	22% 6% 19% 32% 17% 5%	527
	11th to 20th	Day	17% 4 21% 34% 16% 7%	517
	21st to end	Day	20% 9% 23% 30% 13% 5%	583
September	1st to 10th	Day	22% 6% 24% 30% 15% 3	530
	11th to 20th	Day	22% 9% 23% 28% 15% 3	530
	21st to end	Day	23% 11% 25% 26% 14% 2	530
October	1st to 10th	Day	27% 10% 22% 25% 14% 3	530
	11th to 20th	Day	35% 10% 19% 21% 13% 2	530
	21st to end	Day	34% 14% 21% 20% 9% 2	554
November	1st to 10th	Day	45% 11% 20% 15% 6% 1	220
	11th to 20th	Day	44% 8% 20% 19% 8% 1	214
	21st to end	Day	40% 10% 16% 23% 12% 1	200
December	1st to 10th	Day	40% 11% 23% 22% 5% 1	200
	11th to 20th	Day	47% 13% 20% 14% 6% 1	200
	21st to end	Day	45% 14% 24% 14% 2	220
January	1st to 10th	Night	49% 13% 17% 15% 5%	900
	11th to 20th	Night	43% 8% 14% 25% 9%	857
	21st to end	Night	42% 7% 15% 22% 13% 1	880
February	1st to 10th	Night	39% 10% 19% 22% 9% 1	800
	11th to 20th	Night	42% 9% 17% 23% 7% 1	784
	21st to end	Night	44% 10% 17% 20% 8% 2	660
March	1st to 10th	Night	41% 9% 14% 24% 11% 2	800
	11th to 20th	Night	41% 8% 18% 22% 10% 2	800
	21st to end	Night	42% 10% 15% 21% 10% 2	880
April	1st to 10th	Night	37% 7% 15% 26% 12% 2	800
	11th to 20th	Night	31% 9% 19% 27% 11% 3	681
	21st to end	Night	34% 10% 16% 25% 13% 2	600
May	1st to 10th	Night	28% 7% 15% 28% 19% 3	470
	11th to 20th	Night	31% 6% 13% 24% 24% 2	394
	21st to end	Night	33% 7% 12% 24% 20% 3	433
June	1st to 10th	Night	32% 6% 14% 25% 20% 3	400
	11th to 20th	Night	26% 10% 14% 32% 17% 2	400
	21st to end	Night	26% 4 14% 29% 24% 3	400
July	1st to 10th	Night	18% 6% 12% 28% 32% 3	390
	11th to 20th	Night	21% 4 14% 33% 23% 4	388
	21st to end	Night	20% 5% 15% 26% 30% 5%	438
August	1st to 10th	Night	27% 5% 8% 23% 28% 9%	591
	11th to 20th	Night	22% 4 7% 20% 35% 12%	594
	21st to end	Night	23% 5% 14% 26% 26% 6%	715
September	1st to 10th	Night	19% 5% 10% 30% 30% 7%	800
	11th to 20th	Night	24% 3 14% 25% 27% 8%	800
	21st to end	Night	27% 5% 15% 31% 19% 2	800
October	1st to 10th	Night	23% 5% 13% 29% 25% 4	800
	11th to 20th	Night	33% 4 13% 28% 19% 3	800
	21st to end	Night	36% 8% 18% 25% 13% 1	880
November	1st to 10th	Night	42% 10% 17% 24% 8%	820
	11th to 20th	Night	45% 9% 17% 23% 6%	828
	21st to end	Night	36% 9% 19% 24% 10% 2	800
December	1st to 10th	Night	36% 8% 18% 28% 11% 1	800
	11th to 20th	Night	47% 11% 18% 16% 7%	838
	21st to end	Night	52% 10% 16% 16% 5% 1	990

Page generated at 2025-11-14 21:56:00. Version ac07

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