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 2024-01-13.

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	26% 11% 4 7% 6% 3 4 4 5% 6% 23%	74774
	Night	29% 12% 4 6% 5% 3 4 4 4 5% 24%	74964
Transparency	Day	41% 37% 8% 9% 4 1	74465
	Night	43% 37% 7% 8% 4 1	74791
Seeing	Day	31% 10% 22% 24% 10% 3	14284
	Night	35% 8% 15% 24% 16% 3	22727

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	45% 13% 4 7% 5% 3 3 2 3 3 13%	1329
	11th to 20th	Day	37% 11% 4 7% 6% 3 3 3 3 4 18%	1346
	21st to end	Day	33% 12% 4 7% 5% 3 4 3 4 4 20%	1746
February	1st to 10th	Day	39% 12% 3 7% 5% 3 4 4 3 5% 15%	1620
	11th to 20th	Day	35% 11% 3 6% 6% 2 3 3 4 5% 21%	1575
	21st to end	Day	32% 16% 4 7% 6% 3 3 3 4 4 17%	1332
March	1st to 10th	Day	33% 12% 4 6% 5% 3 4 4 5% 5% 18%	1937
	11th to 20th	Day	36% 12% 3 7% 6% 2 3 4 3 6% 18%	1980
	21st to end	Day	32% 13% 4 9% 7% 4 4 4 5% 6% 12%	2181
April	1st to 10th	Day	34% 12% 4 7% 6% 3 4 4 4 6% 15%	2160
	11th to 20th	Day	29% 10% 4 7% 7% 3 4 4 5% 6% 20%	2227
	21st to end	Day	28% 9% 5% 8% 6% 3 5% 4 6% 7% 20%	2340
May	1st to 10th	Day	21% 10% 4 7% 5% 4 5% 5% 4 6% 29%	2456
	11th to 20th	Day	23% 13% 5% 8% 6% 3 4 5% 5% 8% 20%	2477
	21st to end	Day	26% 14% 5% 9% 6% 3 4 4 4 7% 18%	2723
June	1st to 10th	Day	27% 12% 4 6% 6% 2 4 5% 5% 7% 21%	2649
	11th to 20th	Day	26% 11% 4 9% 7% 3 5% 4 5% 8% 18%	2700
	21st to end	Day	19% 13% 5% 7% 6% 3 4 5% 4 7% 27%	2700
July	1st to 10th	Day	17% 14% 4 6% 6% 2 5% 4 5% 8% 30%	2700
	11th to 20th	Day	17% 14% 3 6% 4 3 4 4 5% 8% 33%	2674
	21st to end	Day	13% 13% 4 6% 5% 2 4 4 7% 9% 32%	2741
August	1st to 10th	Day	18% 14% 5% 7% 6% 3 5% 6% 5% 9% 22%	2331
	11th to 20th	Day	13% 10% 4 7% 5% 3 5% 5% 5% 8% 35%	2298
	21st to end	Day	17% 11% 4 8% 5% 3 5% 5% 5% 8% 29%	2524
September	1st to 10th	Day	14% 9% 3 7% 5% 2 4 5% 6% 8% 38%	2160
	11th to 20th	Day	17% 11% 4 7% 5% 3 4 5% 6% 7% 31%	2093
	21st to end	Day	21% 7% 3 6% 6% 3 4 4 4 7% 35%	1936
October	1st to 10th	Day	24% 7% 3 6% 5% 3 4 4 5% 6% 33%	1800
	11th to 20th	Day	26% 7% 3 7% 5% 3 3 5% 3 7% 31%	1800
	21st to end	Day	30% 8% 4 7% 4 3 4 4 4 6% 25%	1945
November	1st to 10th	Day	38% 8% 4 6% 6% 3 4 5% 4 5% 18%	1488
	11th to 20th	Day	36% 9% 3 6% 5% 3 4 4 4 6% 21%	1520
	21st to end	Day	39% 9% 3 6% 4 2 4 4 3 4 22%	1520
December	1st to 10th	Day	29% 11% 4 6% 6% 3 4 4 3 4 25%	1520
	11th to 20th	Day	37% 11% 4 7% 4 3 4 5% 5% 6% 14%	1804
	21st to end	Day	37% 15% 4 7% 6% 3 4 4 4 4 12%	2442
January	1st to 10th	Night	45% 11% 3 6% 4 3 3 3 3 4 14%	2836
	11th to 20th	Night	39% 11% 3 5% 5% 3 4 4 4 5% 17%	2655
	21st to end	Night	36% 11% 4 6% 5% 2 4 3 4 6% 21%	2608
February	1st to 10th	Night	35% 12% 5% 7% 5% 3 4 4 5% 4 16%	2340
	11th to 20th	Night	33% 14% 4 7% 5% 2 4 3 3 4 21%	2272
	21st to end	Night	34% 14% 4 7% 5% 3 4 3 4 4 18%	1924
March	1st to 10th	Night	34% 13% 4 5% 5% 3 4 4 4 5% 19%	2021
	11th to 20th	Night	35% 12% 4 7% 5% 3 4 3 4 5% 19%	1979
	21st to end	Night	33% 15% 4 8% 6% 3 4 4 4 5% 14%	2174
April	1st to 10th	Night	30% 15% 5% 9% 6% 3 4 4 4 4 15%	1800
	11th to 20th	Night	27% 13% 5% 7% 6% 4 5% 3 4 5% 21%	1681
	21st to end	Night	26% 14% 4 7% 6% 4 4 5% 5% 5% 20%	1620
May	1st to 10th	Night	22% 14% 4 6% 4 2 4 4 4 6% 30%	1504
	11th to 20th	Night	30% 13% 4 6% 5% 3 5% 5% 4 5% 21%	1416
	21st to end	Night	26% 16% 4 7% 5% 3 4 4 4 6% 20%	1552
June	1st to 10th	Night	27% 13% 4 5% 6% 3 3 3 3 5% 28%	1308
	11th to 20th	Night	27% 14% 4 7% 5% 3 5% 4 4 5% 20%	1260
	21st to end	Night	20% 16% 3 5% 4 3 3 3 4 6% 33%	1260
July	1st to 10th	Night	17% 12% 3 6% 4 2 6% 3 3 6% 37%	1260
	11th to 20th	Night	14% 14% 5% 5% 4 2 3 3 4 7% 39%	1284
	21st to end	Night	12% 17% 3 5% 3 1 2 3 4 6% 43%	1576
August	1st to 10th	Night	19% 18% 4 6% 4 2 2 3 4 7% 32%	1605
	11th to 20th	Night	12% 12% 3 4 4 1 3 3 4 7% 46%	1602
	21st to end	Night	16% 11% 4 5% 5% 3 4 4 4 7% 39%	1831
September	1st to 10th	Night	13% 9% 3 4 3 2 3 4 4 8% 47%	1800
	11th to 20th	Night	20% 10% 4 7% 5% 3 4 4 4 6% 34%	1864
	21st to end	Night	21% 9% 3 6% 5% 2 3 3 3 6% 38%	2023
October	1st to 10th	Night	19% 10% 4 6% 5% 2 4 4 5% 6% 34%	2160
	11th to 20th	Night	25% 8% 3 5% 4 3 3 4 4 7% 32%	2160
	21st to end	Night	31% 9% 3 5% 3 3 4 4 4 5% 29%	2408
November	1st to 10th	Night	36% 8% 4 7% 5% 3 5% 4 5% 5% 17%	2590
	11th to 20th	Night	36% 10% 3 7% 5% 3 4 3 4 4 20%	2660
	21st to end	Night	31% 10% 4 7% 6% 3 4 3 4 5% 22%	2660
December	1st to 10th	Night	30% 10% 3 7% 5% 3 4 4 4 5% 23%	2660
	11th to 20th	Night	36% 13% 4 7% 6% 3 4 3 4 4 16%	3397
	21st to end	Night	38% 15% 4 8% 6% 2 4 3 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	54% 38% 3 4 1	1329
	11th to 20th	Day	43% 42% 5% 2 5% 3	1346
	21st to end	Day	46% 37% 4 8% 5% 1	1746
February	1st to 10th	Day	48% 36% 7% 6% 2	1620
	11th to 20th	Day	43% 38% 6% 8% 4 1	1584
	21st to end	Day	51% 36% 5% 5% 3	1332
March	1st to 10th	Day	47% 39% 6% 7% 1	1937
	11th to 20th	Day	49% 36% 6% 8% 2	1980
	21st to end	Day	51% 38% 6% 4 1	2181
April	1st to 10th	Day	48% 38% 7% 7% 1	2160
	11th to 20th	Day	43% 38% 7% 6% 5% 1	2227
	21st to end	Day	44% 37% 9% 7% 3	2340
May	1st to 10th	Day	40% 37% 9% 11% 3	2456
	11th to 20th	Day	39% 38% 10% 8% 4 1	2480
	21st to end	Day	47% 38% 8% 7%	2724
June	1st to 10th	Day	45% 38% 8% 6% 3	2649
	11th to 20th	Day	47% 38% 8% 6% 1	2700
	21st to end	Day	36% 37% 12% 10% 3 2	2700
July	1st to 10th	Day	33% 37% 11% 11% 6% 1	2626
	11th to 20th	Day	34% 37% 8% 15% 5%	2593
	21st to end	Day	30% 38% 9% 15% 7% 1	2589
August	1st to 10th	Day	37% 36% 11% 11% 4	2314
	11th to 20th	Day	29% 35% 12% 16% 7% 2	2298
	21st to end	Day	31% 37% 12% 16% 4	2524
September	1st to 10th	Day	25% 36% 9% 17% 8% 4	2160
	11th to 20th	Day	32% 38% 10% 13% 7% 1	2093
	21st to end	Day	36% 37% 9% 11% 4 2	1936
October	1st to 10th	Day	35% 37% 8% 12% 7% 1	1800
	11th to 20th	Day	36% 37% 12% 11% 4 1	1800
	21st to end	Day	39% 38% 7% 9% 8%	1945
November	1st to 10th	Day	44% 41% 5% 6% 3	1488
	11th to 20th	Day	43% 40% 7% 7% 3 1	1520
	21st to end	Day	43% 39% 5% 8% 4	1520
December	1st to 10th	Day	40% 41% 6% 9% 5%	1520
	11th to 20th	Day	49% 33% 8% 8% 2	1806
	21st to end	Day	60% 26% 7% 6% 1	2442
January	1st to 10th	Night	52% 39% 4 4 1	2836
	11th to 20th	Night	44% 40% 6% 4 3 3	2655
	21st to end	Night	44% 37% 4 8% 6% 1	2608
February	1st to 10th	Night	48% 36% 6% 7% 2	2340
	11th to 20th	Night	42% 38% 7% 7% 5% 1	2274
	21st to end	Night	50% 36% 5% 6% 3	1924
March	1st to 10th	Night	47% 37% 7% 8% 1	2021
	11th to 20th	Night	48% 36% 6% 7% 3	1980
	21st to end	Night	50% 37% 6% 5% 1	2174
April	1st to 10th	Night	47% 36% 8% 7% 2	1800
	11th to 20th	Night	43% 37% 7% 7% 3 2	1681
	21st to end	Night	46% 37% 7% 6% 3 1	1620
May	1st to 10th	Night	41% 37% 8% 11% 3	1504
	11th to 20th	Night	39% 38% 10% 7% 5%	1416
	21st to end	Night	45% 38% 9% 7% 1	1552
June	1st to 10th	Night	42% 36% 7% 11% 4	1308
	11th to 20th	Night	43% 39% 12% 6%	1260
	21st to end	Night	35% 36% 13% 9% 5% 2	1260
July	1st to 10th	Night	33% 37% 8% 13% 7% 2	1225
	11th to 20th	Night	31% 37% 8% 16% 7% 1	1242
	21st to end	Night	31% 37% 9% 12% 8% 3	1488
August	1st to 10th	Night	38% 35% 9% 11% 6% 1	1597
	11th to 20th	Night	27% 35% 10% 16% 8% 3	1602
	21st to end	Night	29% 36% 11% 15% 7% 1	1831
September	1st to 10th	Night	22% 37% 9% 17% 11% 4	1800
	11th to 20th	Night	35% 37% 9% 9% 7% 2	1864
	21st to end	Night	36% 36% 6% 11% 8% 2	2023
October	1st to 10th	Night	35% 36% 8% 12% 8% 1	2160
	11th to 20th	Night	35% 36% 9% 13% 6% 1	2160
	21st to end	Night	39% 37% 7% 9% 7%	2408
November	1st to 10th	Night	43% 41% 6% 6% 3 1	2590
	11th to 20th	Night	45% 39% 7% 6% 2 1	2660
	21st to end	Night	41% 40% 7% 8% 4	2660
December	1st to 10th	Night	41% 41% 6% 7% 4	2660
	11th to 20th	Night	50% 33% 5% 8% 4	3394
	21st to end	Night	60% 27% 6% 5% 1	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	49% 13% 18% 10% 7% 2	164
	21st to end	Day	43% 9% 17% 19% 10% 2	338
February	1st to 10th	Day	49% 11% 16% 20% 5%	340
	11th to 20th	Day	44% 11% 25% 14% 5% 1	335
	21st to end	Day	41% 14% 20% 18% 6% 1	280
March	1st to 10th	Day	41% 12% 23% 16% 8%	420
	11th to 20th	Day	39% 11% 23% 16% 7% 3	420
	21st to end	Day	41% 15% 17% 18% 8% 2	462
April	1st to 10th	Day	42% 14% 20% 18% 5% 1	420
	11th to 20th	Day	34% 15% 26% 15% 7% 3	412
	21st to end	Day	34% 14% 27% 18% 6% 1	420
May	1st to 10th	Day	23% 13% 26% 25% 11% 2	423
	11th to 20th	Day	38% 10% 21% 21% 8% 3	448
	21st to end	Day	30% 10% 24% 24% 8% 4	485
June	1st to 10th	Day	31% 10% 26% 23% 8% 2	579
	11th to 20th	Day	29% 13% 29% 20% 7% 1	630
	21st to end	Day	20% 13% 24% 31% 10% 2	630
July	1st to 10th	Day	23% 7% 25% 31% 11% 2	621
	11th to 20th	Day	19% 7% 28% 32% 11% 4	592
	21st to end	Day	14% 6% 21% 36% 17% 6%	487
August	1st to 10th	Day	21% 5% 18% 33% 18% 5%	447
	11th to 20th	Day	14% 4 20% 36% 18% 7%	437
	21st to end	Day	19% 9% 24% 31% 12% 5%	495
September	1st to 10th	Day	21% 6% 25% 30% 15% 3	450
	11th to 20th	Day	20% 8% 22% 29% 17% 4	450
	21st to end	Day	23% 10% 23% 27% 15% 2	450
October	1st to 10th	Day	28% 9% 21% 24% 15% 3	450
	11th to 20th	Day	33% 10% 17% 23% 15% 2	450
	21st to end	Day	34% 14% 19% 21% 10% 2	471
November	1st to 10th	Day	43% 13% 19% 17% 7% 1	180
	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	46% 8% 12% 24% 10%	776
	21st to end	Night	42% 7% 15% 23% 13% 1	792
February	1st to 10th	Night	40% 10% 19% 23% 8% 1	720
	11th to 20th	Night	41% 9% 17% 24% 8% 1	704
	21st to end	Night	43% 9% 17% 20% 9% 2	592
March	1st to 10th	Night	41% 9% 13% 23% 12% 2	720
	11th to 20th	Night	41% 8% 18% 22% 10% 2	720
	21st to end	Night	40% 11% 15% 21% 11% 2	792
April	1st to 10th	Night	39% 8% 16% 23% 12% 3	720
	11th to 20th	Night	34% 8% 17% 25% 11% 3	613
	21st to end	Night	33% 10% 16% 25% 14% 2	540
May	1st to 10th	Night	29% 7% 15% 27% 19% 3	424
	11th to 20th	Night	32% 5% 11% 23% 26% 1	354
	21st to end	Night	34% 7% 13% 23% 21% 3	389
June	1st to 10th	Night	31% 6% 15% 25% 20% 3	360
	11th to 20th	Night	27% 10% 13% 31% 16% 2	360
	21st to end	Night	27% 3 15% 27% 24% 4	360
July	1st to 10th	Night	19% 7% 10% 29% 32% 4	350
	11th to 20th	Night	21% 5% 14% 32% 24% 5%	348
	21st to end	Night	19% 5% 14% 27% 30% 5%	394
August	1st to 10th	Night	27% 5% 7% 23% 28% 10%	531
	11th to 20th	Night	20% 3 7% 20% 36% 13%	534
	21st to end	Night	24% 5% 15% 25% 25% 7%	643
September	1st to 10th	Night	17% 5% 11% 31% 29% 7%	720
	11th to 20th	Night	24% 3 13% 24% 28% 8%	720
	21st to end	Night	27% 5% 15% 31% 20% 3	720
October	1st to 10th	Night	23% 5% 13% 29% 26% 5%	720
	11th to 20th	Night	33% 4 11% 28% 21% 3	720
	21st to end	Night	36% 7% 18% 24% 14% 1	792
November	1st to 10th	Night	41% 9% 16% 24% 8% 1	740
	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 2024-01-09 22:25:00. Version ac07

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