Creston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Creston clear sky chart from 2005-11-06 to 2024-06-30.

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	8% 11% 4 10% 6% 6% 6% 4 6% 7% 32%	89410
	Night	8% 14% 4 10% 6% 5% 5% 4 6% 7% 31%	89776
Transparency	Day	53% 3 13% 20% 10% 2	89129
	Night	55% 3 13% 19% 8% 2	89637
Seeing	Day	14% 8% 24% 29% 18% 7%	14573
	Night	16% 3 15% 41% 22% 3	23710

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	7% 26% 10% 18% 9% 6% 5% 5% 4 3 8%	2356
	11th to 20th	Day	7% 18% 6% 12% 6% 6% 6% 4 4 6% 25%	2516
	21st to end	Day	14% 28% 8% 14% 7% 5% 4 3 4 5% 10%	2794
February	1st to 10th	Day	6% 13% 6% 15% 9% 8% 6% 5% 6% 7% 19%	1520
	11th to 20th	Day	4 17% 5% 12% 7% 6% 8% 6% 6% 6% 22%	1709
	21st to end	Day	5% 13% 5% 14% 9% 6% 8% 5% 5% 6% 23%	1570
March	1st to 10th	Day	5% 16% 6% 11% 6% 6% 6% 5% 8% 8% 23%	3138
	11th to 20th	Day	5% 12% 4 8% 6% 5% 7% 4 5% 6% 38%	3320
	21st to end	Day	10% 17% 6% 13% 8% 6% 6% 5% 5% 6% 17%	4062
April	1st to 10th	Day	8% 15% 5% 11% 7% 7% 6% 6% 6% 7% 23%	2280
	11th to 20th	Day	8% 13% 5% 12% 7% 7% 6% 4 6% 7% 27%	2373
	21st to end	Day	11% 11% 4 10% 7% 6% 6% 5% 6% 7% 27%	2470
May	1st to 10th	Day	10% 12% 5% 11% 7% 6% 6% 4 5% 6% 29%	4347
	11th to 20th	Day	7% 8% 3 8% 6% 6% 6% 5% 8% 9% 33%	4377
	21st to end	Day	12% 11% 5% 10% 7% 7% 8% 5% 7% 7% 21%	5152
June	1st to 10th	Day	15% 9% 3 8% 6% 6% 5% 5% 6% 7% 29%	2850
	11th to 20th	Day	13% 9% 3 9% 8% 7% 7% 5% 6% 8% 25%	2850
	21st to end	Day	7% 4 2 7% 5% 5% 5% 5% 7% 8% 44%	2836
July	1st to 10th	Day	3 4 2 6% 5% 4 4 5% 7% 8% 53%	2700
	11th to 20th	Day	4 3 1 4 4 4 5% 4 7% 7% 57%	2564
	21st to end	Day	2 2 1 3 3 4 4 3 5% 7% 67%	2741
August	1st to 10th	Day	2 3 2 4 4 4 5% 4 7% 9% 57%	2507
	11th to 20th	Day	4 2 1 3 2 3 3 4 6% 8% 62%	2372
	21st to end	Day	5% 3 1 5% 4 5% 5% 4 7% 7% 54%	2502
September	1st to 10th	Day	6% 3 2 5% 4 5% 4 3 6% 7% 56%	2160
	11th to 20th	Day	7% 7% 3 6% 4 4 5% 4 6% 7% 47%	2036
	21st to end	Day	10% 6% 2 8% 4 5% 5% 5% 6% 7% 41%	1980
October	1st to 10th	Day	10% 6% 2 8% 6% 6% 5% 4 6% 7% 39%	1936
	11th to 20th	Day	10% 7% 2 9% 6% 6% 6% 4 7% 6% 37%	1648
	21st to end	Day	12% 12% 4 9% 4 6% 6% 4 6% 6% 30%	1782
November	1st to 10th	Day	14% 16% 5% 11% 6% 6% 5% 5% 6% 7% 18%	1674
	11th to 20th	Day	12% 15% 5% 9% 6% 7% 7% 4 7% 6% 22%	1504
	21st to end	Day	7% 16% 7% 15% 8% 8% 6% 5% 5% 4 19%	1330
December	1st to 10th	Day	9% 14% 6% 12% 9% 8% 5% 5% 6% 6% 21%	1330
	11th to 20th	Day	11% 21% 8% 15% 8% 6% 6% 4 6% 3 11%	1682
	21st to end	Day	11% 19% 7% 18% 9% 7% 6% 3 4 4 13%	2442
January	1st to 10th	Night	8% 32% 10% 16% 7% 5% 4 3 4 3 8%	4619
	11th to 20th	Night	8% 17% 6% 13% 8% 7% 6% 4 6% 5% 21%	4382
	21st to end	Night	17% 30% 8% 13% 5% 5% 4 3 4 4 8%	4862
February	1st to 10th	Night	7% 17% 5% 12% 8% 6% 6% 4 5% 8% 21%	2660
	11th to 20th	Night	6% 17% 5% 10% 7% 6% 5% 4 6% 8% 27%	2357
	21st to end	Night	5% 14% 4 12% 8% 6% 6% 5% 7% 7% 26%	1884
March	1st to 10th	Night	4 13% 6% 11% 7% 6% 6% 5% 7% 7% 28%	3736
	11th to 20th	Night	7% 14% 3 7% 4 4 7% 6% 5% 6% 36%	3659
	21st to end	Night	11% 15% 4 12% 6% 6% 6% 4 6% 8% 22%	3587
April	1st to 10th	Night	6% 11% 3 7% 5% 5% 6% 5% 8% 9% 36%	1900
	11th to 20th	Night	9% 8% 3 9% 5% 6% 4 6% 6% 7% 37%	1758
	21st to end	Night	5% 10% 4 8% 6% 6% 6% 4 7% 7% 38%	1710
May	1st to 10th	Night	8% 9% 4 9% 7% 6% 6% 4 7% 8% 33%	2613
	11th to 20th	Night	5% 6% 2 7% 4 5% 6% 5% 7% 9% 44%	2516
	21st to end	Night	12% 11% 4 10% 5% 5% 6% 5% 7% 8% 28%	2423
June	1st to 10th	Night	13% 6% 2 8% 5% 5% 5% 5% 6% 8% 37%	1330
	11th to 20th	Night	11% 5% 2 7% 5% 6% 6% 6% 9% 9% 34%	1330
	21st to end	Night	3 3 2 5% 4 4 3 5% 7% 9% 56%	1323
July	1st to 10th	Night	2 2 1 3 3 3 3 4 8% 8% 63%	1260
	11th to 20th	Night	2 2 1 1 2 2 3 4 7% 9% 67%	1393
	21st to end	Night	1 1 1 2 3 4 3 5% 6% 74%	1576
August	1st to 10th	Night	3 2 1 3 2 2 3 2 5% 8% 68%	1432
	11th to 20th	Night	3 2 1 3 3 2 3 3 5% 8% 69%	1527
	21st to end	Night	4 3 1 3 2 3 3 3 6% 7% 65%	1850
September	1st to 10th	Night	4 2 1 4 3 3 3 4 5% 8% 63%	1800
	11th to 20th	Night	6% 4 2 5% 5% 4 5% 4 6% 9% 50%	1923
	21st to end	Night	8% 5% 2 4 3 5% 5% 3 6% 7% 51%	1980
October	1st to 10th	Night	9% 6% 3 7% 4 4 6% 4 5% 6% 45%	2021
	11th to 20th	Night	10% 8% 2 7% 4 5% 5% 5% 6% 7% 41%	2312
	21st to end	Night	12% 11% 4 8% 5% 4 5% 4 6% 8% 33%	2574
November	1st to 10th	Night	14% 15% 4 11% 7% 5% 5% 4 6% 9% 19%	2405
	11th to 20th	Night	11% 17% 4 11% 7% 6% 6% 4 6% 7% 20%	2650
	21st to end	Night	9% 21% 6% 12% 8% 6% 6% 4 5% 5% 19%	2850
December	1st to 10th	Night	10% 18% 5% 15% 8% 6% 6% 5% 5% 4 18%	2850
	11th to 20th	Night	11% 23% 7% 19% 8% 5% 5% 3 5% 4 9%	3510
	21st to end	Night	10% 22% 8% 15% 9% 7% 6% 4 5% 4 10%	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	82% 3 6% 7% 2	2356
	11th to 20th	Day	63% 3 17% 10% 3 4	2540
	21st to end	Day	81% 3 6% 7% 2	2794
February	1st to 10th	Day	65% 5% 13% 8% 6% 2	1520
	11th to 20th	Day	64% 4 13% 13% 5%	1718
	21st to end	Day	62% 3 12% 14% 8%	1570
March	1st to 10th	Day	60% 2 12% 16% 9%	3138
	11th to 20th	Day	48% 3 23% 22% 4	3321
	21st to end	Day	69% 2 10% 14% 4	4062
April	1st to 10th	Day	60% 3 15% 17% 5%	2280
	11th to 20th	Day	58% 3 13% 18% 7% 1	2373
	21st to end	Day	58% 3 13% 20% 5% 1	2470
May	1st to 10th	Day	57% 2 10% 22% 8% 1	4347
	11th to 20th	Day	47% 1 15% 28% 8% 1	4380
	21st to end	Day	62% 2 14% 17% 5%	5153
June	1st to 10th	Day	55% 2 14% 18% 10% 1	2850
	11th to 20th	Day	56% 3 12% 19% 8% 1	2850
	21st to end	Day	36% 3 15% 28% 15% 2	2836
July	1st to 10th	Day	29% 2 12% 31% 21% 6%	2626
	11th to 20th	Day	25% 4 11% 25% 27% 8%	2486
	21st to end	Day	19% 2 7% 29% 34% 9%	2589
August	1st to 10th	Day	24% 3 13% 31% 22% 8%	2490
	11th to 20th	Day	21% 2 15% 33% 27% 3	2372
	21st to end	Day	29% 3 13% 33% 20% 2	2502
September	1st to 10th	Day	30% 2 12% 33% 19% 5%	2160
	11th to 20th	Day	37% 2 14% 28% 16% 2	2036
	21st to end	Day	43% 3 21% 21% 10% 1	1980
October	1st to 10th	Day	45% 3 16% 29% 7%	1936
	11th to 20th	Day	48% 4 18% 22% 7%	1648
	21st to end	Day	55% 5% 16% 18% 6%	1782
November	1st to 10th	Day	67% 6% 12% 12% 3	1674
	11th to 20th	Day	63% 6% 14% 10% 6%	1504
	21st to end	Day	69% 4 14% 10% 3	1330
December	1st to 10th	Day	67% 5% 11% 15% 2	1330
	11th to 20th	Day	79% 3 9% 8% 2	1684
	21st to end	Day	77% 3 8% 8% 4	2442
January	1st to 10th	Night	84% 2 6% 6% 2	4619
	11th to 20th	Night	67% 3 8% 16% 4 3	4420
	21st to end	Night	84% 3 6% 6% 2	4862
February	1st to 10th	Night	65% 4 13% 10% 7% 1	2660
	11th to 20th	Night	59% 5% 15% 15% 6%	2359
	21st to end	Night	58% 3 16% 13% 8% 1	1884
March	1st to 10th	Night	57% 2 15% 18% 8%	3736
	11th to 20th	Night	52% 3 23% 18% 4 1	3659
	21st to end	Night	63% 3 15% 15% 3	3587
April	1st to 10th	Night	45% 5% 22% 21% 8% 1	1900
	11th to 20th	Night	47% 2 17% 23% 10%	1758
	21st to end	Night	47% 2 16% 25% 7% 1	1710
May	1st to 10th	Night	51% 2 14% 25% 8%	2613
	11th to 20th	Night	38% 2 16% 33% 10% 1	2516
	21st to end	Night	55% 2 17% 20% 6%	2423
June	1st to 10th	Night	46% 2 16% 24% 10% 2	1330
	11th to 20th	Night	44% 3 15% 27% 8% 2	1330
	21st to end	Night	25% 2 18% 31% 21% 2	1323
July	1st to 10th	Night	18% 2 15% 34% 24% 7%	1225
	11th to 20th	Night	14% 3 14% 30% 27% 11%	1348
	21st to end	Night	14% 1 7% 32% 33% 13%	1488
August	1st to 10th	Night	17% 2 12% 33% 26% 10%	1424
	11th to 20th	Night	18% 1 12% 36% 29% 4	1527
	21st to end	Night	19% 2 14% 36% 24% 4	1850
September	1st to 10th	Night	21% 3 12% 36% 22% 6%	1800
	11th to 20th	Night	33% 3 15% 31% 16% 3	1923
	21st to end	Night	33% 3 24% 25% 12% 2	1980
October	1st to 10th	Night	41% 3 18% 30% 8%	2021
	11th to 20th	Night	43% 4 19% 23% 10% 1	2312
	21st to end	Night	50% 4 19% 20% 6% 1	2574
November	1st to 10th	Night	65% 6% 12% 13% 4	2405
	11th to 20th	Night	65% 6% 12% 10% 6%	2650
	21st to end	Night	69% 6% 12% 11% 2	2850
December	1st to 10th	Night	70% 3 10% 14% 2	2850
	11th to 20th	Night	82% 3 7% 6% 1	3507
	21st to end	Night	80% 3 7% 7% 3	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	18% 6% 18% 33% 16% 8%	180
	11th to 20th	Day	18% 3 16% 40% 16% 8%	180
	21st to end	Day	19% 3 23% 34% 14% 8%	198
February	1st to 10th	Day	13% 4 31% 41% 10% 2	180
	11th to 20th	Day	15% 7% 23% 34% 19% 2	349
	21st to end	Day	13% 8% 30% 30% 16% 3	382
March	1st to 10th	Day	18% 10% 31% 24% 13% 3	460
	11th to 20th	Day	16% 12% 23% 23% 18% 8%	460
	21st to end	Day	21% 11% 28% 25% 12% 4	506
April	1st to 10th	Day	21% 13% 26% 24% 12% 4	460
	11th to 20th	Day	20% 17% 25% 23% 11% 5%	452
	21st to end	Day	18% 13% 28% 21% 16% 4	460
May	1st to 10th	Day	15% 10% 27% 27% 14% 6%	463
	11th to 20th	Day	18% 8% 22% 27% 16% 8%	488
	21st to end	Day	19% 6% 18% 26% 22% 8%	725
June	1st to 10th	Day	19% 8% 21% 29% 18% 4	680
	11th to 20th	Day	15% 10% 28% 26% 16% 4	680
	21st to end	Day	9% 8% 20% 31% 22% 10%	676
July	1st to 10th	Day	6% 6% 23% 31% 27% 7%	621
	11th to 20th	Day	6% 10% 28% 31% 18% 7%	485
	21st to end	Day	3 9% 30% 25% 22% 11%	487
August	1st to 10th	Day	6% 6% 23% 34% 21% 10%	447
	11th to 20th	Day	6% 4 22% 34% 19% 15%	437
	21st to end	Day	7% 10% 24% 27% 21% 10%	495
September	1st to 10th	Day	12% 7% 25% 27% 20% 9%	450
	11th to 20th	Day	12% 8% 23% 30% 18% 9%	450
	21st to end	Day	16% 8% 27% 27% 16% 7%	450
October	1st to 10th	Day	14% 6% 25% 33% 14% 7%	450
	11th to 20th	Day	15% 6% 22% 31% 17% 9%	298
	21st to end	Day	25% 3 28% 27% 15% 3	297
November	1st to 10th	Day	19% 6% 21% 33% 19% 1	270
	11th to 20th	Day	25% 2 21% 33% 16% 3	219
	21st to end	Day	11% 3 22% 37% 17% 11%	180
December	1st to 10th	Day	18% 3 14% 33% 22% 9%	180
	11th to 20th	Day	13% 8% 18% 42% 16% 4	180
	21st to end	Day	12% 4 16% 40% 20% 8%	198
January	1st to 10th	Night	26% 4 16% 38% 13% 3	882
	11th to 20th	Night	21% 5% 15% 39% 17% 3	760
	21st to end	Night	21% 3 16% 42% 15% 3	836
February	1st to 10th	Night	17% 4 19% 44% 14% 3	760
	11th to 20th	Night	17% 3 16% 46% 15% 2	744
	21st to end	Night	15% 3 17% 41% 20% 4	628
March	1st to 10th	Night	17% 3 18% 41% 18% 2	760
	11th to 20th	Night	18% 3 19% 36% 22% 2	760
	21st to end	Night	19% 3 17% 39% 19% 3	836
April	1st to 10th	Night	13% 2 16% 44% 22% 2	760
	11th to 20th	Night	15% 2 20% 43% 19% 1	630
	21st to end	Night	13% 5% 19% 41% 21% 1	570
May	1st to 10th	Night	9% 3 15% 47% 23% 2	427
	11th to 20th	Night	12% 1 12% 37% 35% 3	374
	21st to end	Night	19% 3 10% 37% 28% 4	411
June	1st to 10th	Night	17% 2 15% 39% 26% 2	380
	11th to 20th	Night	15% 2 15% 44% 22% 2	380
	21st to end	Night	5% 2 14% 42% 33% 4	378
July	1st to 10th	Night	4 1 6% 44% 43% 2	350
	11th to 20th	Night	4 1 9% 51% 33% 2	348
	21st to end	Night	3 7% 43% 44% 3	394
August	1st to 10th	Night	3 1 6% 38% 47% 4	358
	11th to 20th	Night	4 1 7% 39% 44% 5%	459
	21st to end	Night	5% 2 11% 41% 38% 3	662
September	1st to 10th	Night	6% 1 12% 46% 28% 7%	720
	11th to 20th	Night	8% 1 10% 37% 37% 7%	720
	21st to end	Night	14% 1 13% 42% 26% 5%	720
October	1st to 10th	Night	15% 3 12% 40% 26% 3	720
	11th to 20th	Night	17% 1 15% 40% 23% 4	720
	21st to end	Night	20% 4 17% 41% 15% 3	792
November	1st to 10th	Night	26% 3 16% 44% 12% 1	740
	11th to 20th	Night	26% 4 18% 37% 14% 2	836
	21st to end	Night	22% 3 16% 39% 18% 2	950
December	1st to 10th	Night	20% 2 15% 43% 18% 3	950
	11th to 20th	Night	22% 5% 17% 43% 13% 1	950
	21st to end	Night	22% 4 16% 41% 15% 1	1045

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