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 2024-11-08.

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	24% 13% 3 8% 5% 4 4 3 5% 6% 25%	105595
	Night	27% 14% 3 7% 4 4 4 3 4 5% 25%	102133
Transparency	Day	31% 46% 6% 10% 5% 1	105347
	Night	34% 47% 5% 9% 4 1	102027
Seeing	Day	30% 11% 23% 24% 10% 3	15455
	Night	35% 8% 15% 24% 15% 3	23708

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	39% 15% 4 10% 5% 4 4 2 4 3 9%	2219
	11th to 20th	Day	36% 14% 3 8% 4 4 2 2 3 3 19%	2326
	21st to end	Day	47% 12% 4 7% 4 3 3 2 3 3 12%	3082
February	1st to 10th	Day	38% 12% 3 8% 4 4 4 3 3 5% 16%	1710
	11th to 20th	Day	35% 12% 3 7% 5% 3 3 3 4 4 21%	1665
	21st to end	Day	31% 17% 4 8% 5% 4 3 3 3 4 16%	1413
March	1st to 10th	Day	24% 15% 3 8% 6% 5% 5% 4 6% 8% 16%	3377
	11th to 20th	Day	25% 10% 3 9% 5% 3 3 3 4 7% 29%	3490
	21st to end	Day	27% 15% 4 10% 6% 5% 4 4 5% 6% 15%	3840
April	1st to 10th	Day	33% 14% 4 8% 6% 4 4 3 4 6% 15%	2280
	11th to 20th	Day	28% 11% 4 8% 7% 4 3 3 5% 6% 22%	2353
	21st to end	Day	28% 10% 4 9% 6% 4 4 3 6% 6% 20%	2470
May	1st to 10th	Day	21% 9% 3 7% 3 4 4 3 4 6% 36%	4317
	11th to 20th	Day	15% 15% 4 8% 4 4 4 4 6% 8% 27%	4377
	21st to end	Day	29% 11% 4 8% 5% 4 4 3 5% 7% 20%	4813
June	1st to 10th	Day	27% 13% 3 7% 5% 3 4 4 5% 7% 21%	2797
	11th to 20th	Day	25% 12% 4 10% 6% 4 5% 4 5% 8% 19%	2850
	21st to end	Day	19% 13% 5% 8% 5% 4 5% 3 5% 7% 26%	2850
July	1st to 10th	Day	10% 13% 3 5% 4 3 3 2 5% 7% 45%	4740
	11th to 20th	Day	10% 16% 3 7% 4 3 3 3 5% 8% 37%	4686
	21st to end	Day	18% 17% 3 6% 4 3 3 3 5% 8% 30%	4790
August	1st to 10th	Day	12% 20% 5% 10% 4 4 5% 4 5% 9% 23%	4095
	11th to 20th	Day	16% 13% 4 9% 5% 5% 5% 5% 5% 8% 25%	4026
	21st to end	Day	18% 10% 3 7% 4 4 4 3 4 6% 36%	4432
September	1st to 10th	Day	14% 10% 4 7% 4 4 4 4 6% 7% 37%	2280
	11th to 20th	Day	17% 11% 4 8% 4 5% 4 4 6% 7% 31%	2209
	21st to end	Day	21% 8% 2 7% 5% 4 3 4 4 7% 35%	2043
October	1st to 10th	Day	18% 7% 3 6% 3 3 4 3 6% 9% 38%	3180
	11th to 20th	Day	32% 9% 2 7% 4 4 3 3 3 6% 26%	3161
	21st to end	Day	22% 8% 3 7% 6% 5% 5% 4 6% 8% 25%	3374
November	1st to 10th	Day	37% 9% 4 7% 5% 4 4 4 4 4 18%	1544
	11th to 20th	Day	36% 10% 3 7% 4 4 3 3 4 6% 20%	1520
	21st to end	Day	38% 10% 3 7% 3 3 4 3 3 4 22%	1520
December	1st to 10th	Day	29% 12% 3 7% 5% 4 3 4 3 4 25%	1520
	11th to 20th	Day	37% 12% 3 8% 4 3 4 4 5% 6% 14%	1804
	21st to end	Day	37% 16% 4 8% 5% 4 4 3 4 4 12%	2442
January	1st to 10th	Night	38% 17% 4 10% 5% 5% 4 3 3 4 10%	4726
	11th to 20th	Night	34% 13% 3 7% 4 4 4 3 4 5% 19%	4569
	21st to end	Night	50% 11% 3 7% 4 3 3 2 3 3 12%	4581
February	1st to 10th	Night	34% 14% 4 7% 5% 4 4 3 5% 4 16%	2470
	11th to 20th	Night	33% 15% 4 7% 4 3 3 3 3 4 21%	2402
	21st to end	Night	34% 15% 4 8% 4 4 4 2 4 4 17%	2040
March	1st to 10th	Night	28% 16% 3 7% 5% 5% 5% 4 6% 6% 15%	3495
	11th to 20th	Night	22% 13% 3 6% 4 4 3 3 4 7% 31%	3489
	21st to end	Night	30% 19% 4 10% 5% 4 3 3 3 4 15%	3812
April	1st to 10th	Night	29% 16% 5% 10% 6% 4 4 3 4 4 16%	1900
	11th to 20th	Night	25% 15% 4 8% 6% 5% 4 2 4 5% 23%	1775
	21st to end	Night	26% 15% 4 7% 6% 5% 4 4 5% 5% 19%	1710
May	1st to 10th	Night	25% 11% 3 5% 2 2 3 3 4 5% 38%	2646
	11th to 20th	Night	18% 17% 3 7% 4 4 5% 3 5% 6% 26%	2516
	21st to end	Night	22% 17% 4 9% 6% 4 4 2 4 5% 23%	2762
June	1st to 10th	Night	27% 15% 2 6% 4 4 3 3 3 6% 27%	1380
	11th to 20th	Night	25% 15% 3 7% 4 5% 4 4 5% 5% 22%	1330
	21st to end	Night	19% 16% 3 6% 4 4 4 3 4 6% 32%	1330
July	1st to 10th	Night	11% 15% 2 5% 3 2 3 1 2 4 52%	2220
	11th to 20th	Night	9% 21% 4 6% 4 3 2 2 3 5% 41%	2272
	21st to end	Night	17% 23% 2 4 2 1 1 2 3 4 40%	2762
August	1st to 10th	Night	13% 31% 3 7% 3 2 2 2 3 5% 30%	2842
	11th to 20th	Night	19% 17% 3 5% 3 3 4 2 4 6% 34%	2812
	21st to end	Night	18% 10% 3 6% 4 3 4 3 3 5% 42%	3237
September	1st to 10th	Night	13% 10% 3 5% 3 3 2 3 5% 7% 46%	1900
	11th to 20th	Night	20% 10% 4 8% 3 3 4 3 4 6% 33%	1968
	21st to end	Night	21% 9% 3 7% 4 3 3 2 4 7% 37%	2136
October	1st to 10th	Night	15% 8% 3 6% 4 3 5% 3 5% 10% 38%	3800
	11th to 20th	Night	29% 11% 3 7% 4 4 4 3 4 6% 26%	3777
	21st to end	Night	27% 8% 3 8% 4 5% 5% 4 6% 6% 25%	4189
November	1st to 10th	Night	36% 9% 4 8% 4 5% 5% 3 5% 5% 17%	2694
	11th to 20th	Night	36% 11% 3 8% 5% 4 4 3 4 4 20%	2660
	21st to end	Night	30% 11% 4 8% 5% 4 3 3 4 5% 22%	2660
December	1st to 10th	Night	30% 12% 3 7% 5% 4 4 4 4 5% 23%	2660
	11th to 20th	Night	36% 13% 4 8% 5% 4 3 3 4 4 15%	3397
	21st to end	Night	38% 16% 4 9% 5% 3 4 2 3 4 12%	5214

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	51% 39% 5% 3 2 1	2219
	11th to 20th	Day	48% 36% 4 10% 2	2348
	21st to end	Day	54% 40% 2 2 1	3082
February	1st to 10th	Day	27% 65% 3 4 2	1710
	11th to 20th	Day	29% 64% 1 4 1	1674
	21st to end	Day	25% 65% 2 4 2	1413
March	1st to 10th	Day	37% 46% 6% 9% 2	3377
	11th to 20th	Day	30% 40% 7% 18% 5%	3490
	21st to end	Day	41% 42% 9% 7% 1	3840
April	1st to 10th	Day	28% 64% 4 4 1	2280
	11th to 20th	Day	24% 66% 3 5% 2	2353
	21st to end	Day	24% 65% 4 6% 1	2470
May	1st to 10th	Day	27% 42% 10% 14% 8%	4317
	11th to 20th	Day	31% 42% 13% 12% 2	4380
	21st to end	Day	39% 43% 12% 4 2	4814
June	1st to 10th	Day	19% 65% 7% 6% 3	2797
	11th to 20th	Day	20% 66% 4 8% 2	2850
	21st to end	Day	22% 64% 6% 6% 2	2850
July	1st to 10th	Day	20% 36% 3 8% 21% 13%	4666
	11th to 20th	Day	21% 36% 5% 14% 22% 1	4605
	21st to end	Day	34% 35% 5% 21% 5%	4639
August	1st to 10th	Day	27% 37% 4 20% 10% 1	4078
	11th to 20th	Day	40% 37% 10% 9% 3 1	4065
	21st to end	Day	31% 36% 7% 15% 11%	4431
September	1st to 10th	Day	22% 60% 6% 7% 4 1	2280
	11th to 20th	Day	22% 60% 5% 6% 7% 1	2209
	21st to end	Day	18% 60% 7% 10% 5%	2043
October	1st to 10th	Day	25% 36% 12% 23% 4	3180
	11th to 20th	Day	42% 37% 6% 11% 4	3161
	21st to end	Day	39% 38% 9% 7% 7% 1	3374
November	1st to 10th	Day	32% 56% 5% 5% 1	1544
	11th to 20th	Day	26% 60% 4 5% 3 2	1520
	21st to end	Day	25% 59% 1 5% 9% 1	1520
December	1st to 10th	Day	28% 60% 3 5% 4 1	1520
	11th to 20th	Day	38% 49% 6% 5% 1	1806
	21st to end	Day	49% 38% 6% 4 2	2442
January	1st to 10th	Night	51% 38% 4 4 3 1	4726
	11th to 20th	Night	44% 37% 5% 13% 2	4609
	21st to end	Night	55% 39% 2 3 1	4581
February	1st to 10th	Night	27% 65% 3 3 2	2470
	11th to 20th	Night	29% 63% 2 3 2	2404
	21st to end	Night	22% 65% 4 6% 2	2040
March	1st to 10th	Night	36% 47% 6% 9% 2	3495
	11th to 20th	Night	26% 42% 8% 18% 6%	3490
	21st to end	Night	45% 41% 6% 5% 2	3812
April	1st to 10th	Night	26% 65% 4 4	1900
	11th to 20th	Night	22% 66% 3 6% 3	1775
	21st to end	Night	22% 65% 3 6% 3	1710
May	1st to 10th	Night	25% 41% 9% 18% 6%	2646
	11th to 20th	Night	33% 44% 8% 10% 6%	2516
	21st to end	Night	38% 43% 10% 8% 1	2762
June	1st to 10th	Night	19% 66% 6% 7% 1	1380
	11th to 20th	Night	18% 66% 5% 9% 2	1330
	21st to end	Night	20% 64% 7% 7% 2	1330
July	1st to 10th	Night	21% 35% 3 10% 15% 15%	2185
	11th to 20th	Night	31% 35% 4 14% 15% 1	2230
	21st to end	Night	32% 36% 5% 21% 6% 1	2674
August	1st to 10th	Night	38% 37% 5% 13% 6% 1	2834
	11th to 20th	Night	37% 37% 13% 9% 3 1	2839
	21st to end	Night	31% 37% 7% 13% 12% 1	3237
September	1st to 10th	Night	17% 60% 6% 10% 5% 1	1900
	11th to 20th	Night	22% 60% 5% 6% 5% 2	1968
	21st to end	Night	19% 59% 4 11% 7% 1	2136
October	1st to 10th	Night	26% 36% 10% 25% 4	3800
	11th to 20th	Night	43% 36% 6% 12% 3	3777
	21st to end	Night	41% 37% 6% 9% 7% 1	4189
November	1st to 10th	Night	31% 57% 5% 6% 2	2694
	11th to 20th	Night	27% 60% 4 5% 2 2	2660
	21st to end	Night	25% 60% 2 4 8% 1	2660
December	1st to 10th	Night	28% 60% 3 5% 4 1	2660
	11th to 20th	Night	38% 49% 3 6% 3	3394
	21st to end	Night	48% 39% 6% 5% 2	5214

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	46% 14% 22% 13% 4 1	180
	11th to 20th	Day	48% 13% 20% 11% 7% 2	180
	21st to end	Day	46% 10% 16% 18% 9% 2	369
February	1st to 10th	Day	46% 12% 16% 20% 5% 1	370
	11th to 20th	Day	44% 12% 25% 14% 5% 1	365
	21st to end	Day	41% 15% 20% 18% 6% 1	307
March	1st to 10th	Day	40% 13% 22% 17% 8%	460
	11th to 20th	Day	37% 11% 22% 17% 8% 3	460
	21st to end	Day	40% 15% 18% 19% 7% 1	506
April	1st to 10th	Day	40% 13% 21% 18% 6% 2	460
	11th to 20th	Day	33% 16% 27% 15% 6% 3	452
	21st to end	Day	34% 15% 27% 17% 6% 1	460
May	1st to 10th	Day	24% 14% 25% 24% 11% 2	463
	11th to 20th	Day	36% 11% 22% 21% 8% 3	488
	21st to end	Day	31% 11% 24% 23% 8% 3	529
June	1st to 10th	Day	31% 10% 26% 24% 7% 2	627
	11th to 20th	Day	29% 14% 29% 21% 6% 1	680
	21st to end	Day	20% 13% 25% 30% 10% 2	680
July	1st to 10th	Day	22% 7% 26% 31% 11% 3	671
	11th to 20th	Day	18% 7% 28% 32% 11% 4	640
	21st to end	Day	16% 7% 21% 35% 16% 6%	531
August	1st to 10th	Day	21% 5% 19% 33% 17% 5%	487
	11th to 20th	Day	16% 5% 20% 35% 18% 7%	477
	21st to end	Day	20% 10% 23% 31% 12% 4	539
September	1st to 10th	Day	22% 6% 24% 30% 15% 3	490
	11th to 20th	Day	21% 9% 22% 29% 16% 3	490
	21st to end	Day	22% 10% 25% 27% 14% 2	490
October	1st to 10th	Day	27% 9% 21% 24% 15% 3	490
	11th to 20th	Day	35% 10% 18% 22% 14% 2	490
	21st to end	Day	34% 14% 20% 21% 10% 2	512
November	1st to 10th	Day	44% 12% 19% 17% 7% 1	194
	11th to 20th	Day	41% 7% 21% 22% 8% 1	180
	21st to end	Day	39% 11% 16% 21% 12% 1	180
December	1st to 10th	Day	42% 12% 22% 20% 4 1	180
	11th to 20th	Day	46% 13% 18% 15% 7% 1	180
	21st to end	Day	42% 14% 25% 16% 3 1	198
January	1st to 10th	Night	50% 12% 17% 15% 6%	850
	11th to 20th	Night	45% 8% 14% 24% 9%	812
	21st to end	Night	43% 7% 15% 21% 12% 1	836
February	1st to 10th	Night	40% 10% 19% 22% 9% 1	760
	11th to 20th	Night	42% 10% 17% 23% 8% 1	744
	21st to end	Night	43% 10% 17% 20% 8% 2	628
March	1st to 10th	Night	41% 9% 13% 24% 11% 2	760
	11th to 20th	Night	41% 8% 18% 22% 10% 2	760
	21st to end	Night	40% 10% 15% 21% 11% 2	836
April	1st to 10th	Night	38% 8% 15% 24% 12% 3	760
	11th to 20th	Night	33% 9% 19% 26% 11% 3	647
	21st to end	Night	33% 10% 16% 25% 13% 2	570
May	1st to 10th	Night	28% 7% 15% 28% 19% 3	447
	11th to 20th	Night	31% 6% 12% 24% 25% 2	374
	21st to end	Night	33% 7% 13% 24% 20% 3	411
June	1st to 10th	Night	32% 6% 14% 24% 21% 3	380
	11th to 20th	Night	26% 10% 14% 32% 16% 2	380
	21st to end	Night	27% 3 14% 28% 23% 3	380
July	1st to 10th	Night	19% 6% 11% 28% 32% 4	370
	11th to 20th	Night	21% 4 14% 32% 24% 4	368
	21st to end	Night	20% 5% 15% 26% 29% 5%	416
August	1st to 10th	Night	27% 5% 8% 23% 28% 9%	561
	11th to 20th	Night	21% 3 7% 20% 36% 13%	564
	21st to end	Night	24% 5% 14% 25% 25% 6%	679
September	1st to 10th	Night	18% 5% 10% 31% 30% 6%	760
	11th to 20th	Night	24% 3 14% 24% 27% 8%	760
	21st to end	Night	26% 5% 15% 31% 20% 3	760
October	1st to 10th	Night	22% 5% 13% 29% 26% 4	760
	11th to 20th	Night	34% 4 12% 28% 20% 3	760
	21st to end	Night	36% 7% 18% 25% 13% 1	836
November	1st to 10th	Night	41% 9% 16% 24% 8% 1	770
	11th to 20th	Night	42% 9% 18% 24% 6%	760
	21st to end	Night	35% 9% 20% 24% 11% 2	760
December	1st to 10th	Night	37% 8% 18% 27% 10% 1	760
	11th to 20th	Night	46% 12% 18% 17% 7% 1	794
	21st to end	Night	52% 10% 16% 16% 5% 1	935

Page generated at 2024-11-05 05:15:52. Version ac07

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