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 2024-01-14.

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% 9% 4 8% 7% 4 6% 6% 6% 7% 34%	74580
	Night	8% 12% 5% 9% 7% 4 6% 5% 6% 7% 33%	75183
Transparency	Day	50% 3 13% 20% 11% 2	74275
	Night	53% 4 14% 19% 9% 2	75006
Seeing	Day	14% 8% 24% 29% 18% 7%	13924
	Night	16% 3 15% 41% 22% 3	23177

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	11% 22% 7% 12% 11% 4 5% 6% 5% 5% 11%	1414
	11th to 20th	Day	9% 17% 8% 12% 9% 4 8% 6% 4 6% 16%	1464
	21st to end	Day	7% 15% 7% 12% 11% 5% 7% 6% 6% 8% 17%	1584
February	1st to 10th	Day	7% 11% 6% 14% 10% 5% 8% 6% 7% 7% 20%	1440
	11th to 20th	Day	4 16% 6% 11% 9% 5% 8% 7% 6% 7% 22%	1617
	21st to end	Day	6% 12% 6% 13% 10% 5% 7% 7% 5% 6% 24%	1480
March	1st to 10th	Day	8% 14% 8% 11% 8% 4 7% 6% 6% 8% 21%	1800
	11th to 20th	Day	8% 15% 6% 9% 7% 4 6% 6% 6% 7% 26%	1875
	21st to end	Day	10% 16% 7% 11% 9% 4 6% 6% 6% 6% 18%	2312
April	1st to 10th	Day	8% 13% 6% 10% 9% 5% 6% 7% 6% 7% 23%	2160
	11th to 20th	Day	9% 11% 7% 11% 8% 5% 6% 5% 6% 7% 26%	2246
	21st to end	Day	11% 10% 4 9% 8% 4 7% 6% 6% 7% 28%	2340
May	1st to 10th	Day	8% 9% 5% 7% 7% 4 6% 5% 6% 8% 35%	2473
	11th to 20th	Day	9% 8% 4 7% 7% 4 6% 5% 7% 7% 36%	2477
	21st to end	Day	14% 9% 4 7% 7% 5% 6% 6% 7% 7% 26%	2908
June	1st to 10th	Day	16% 8% 4 7% 7% 4 6% 6% 6% 7% 29%	2700
	11th to 20th	Day	14% 8% 4 8% 9% 6% 7% 7% 6% 8% 24%	2700
	21st to end	Day	7% 4 2 6% 6% 4 5% 6% 7% 8% 44%	2700
July	1st to 10th	Day	3 4 2 5% 5% 3 5% 5% 6% 8% 53%	2700
	11th to 20th	Day	4 3 1 3 4 2 5% 5% 6% 8% 57%	2564
	21st to end	Day	2 1 1 3 4 3 4 4 5% 7% 67%	2741
August	1st to 10th	Day	2 3 2 4 4 3 5% 5% 6% 9% 58%	2507
	11th to 20th	Day	4 2 1 3 3 2 4 4 6% 8% 63%	2372
	21st to end	Day	5% 3 1 4 4 3 5% 6% 7% 8% 54%	2502
September	1st to 10th	Day	6% 3 2 4 5% 3 5% 4 6% 7% 56%	2160
	11th to 20th	Day	7% 6% 3 6% 5% 3 5% 5% 6% 7% 47%	2036
	21st to end	Day	10% 6% 3 7% 5% 3 6% 6% 6% 7% 41%	1980
October	1st to 10th	Day	10% 6% 2 7% 7% 4 6% 5% 6% 7% 39%	1936
	11th to 20th	Day	10% 7% 3 8% 7% 4 7% 6% 6% 7% 37%	1648
	21st to end	Day	12% 11% 4 8% 5% 4 7% 5% 6% 7% 31%	1782
November	1st to 10th	Day	14% 15% 6% 9% 7% 5% 5% 6% 5% 7% 19%	1674
	11th to 20th	Day	13% 14% 5% 8% 8% 4 8% 6% 6% 6% 22%	1504
	21st to end	Day	7% 14% 8% 15% 9% 6% 7% 6% 5% 4 19%	1330
December	1st to 10th	Day	9% 13% 7% 10% 10% 6% 7% 6% 5% 6% 21%	1330
	11th to 20th	Day	11% 20% 9% 13% 9% 5% 6% 6% 6% 4 11%	1682
	21st to end	Day	12% 18% 7% 15% 12% 5% 6% 5% 4 4 13%	2442
January	1st to 10th	Night	11% 24% 10% 12% 9% 5% 5% 5% 5% 4 12%	2768
	11th to 20th	Night	10% 19% 8% 15% 9% 5% 6% 4 5% 5% 13%	2574
	21st to end	Night	7% 19% 8% 14% 9% 5% 7% 6% 5% 6% 13%	2772
February	1st to 10th	Night	7% 16% 6% 10% 9% 4 7% 6% 5% 8% 22%	2520
	11th to 20th	Night	6% 16% 5% 9% 8% 5% 6% 5% 6% 8% 27%	2229
	21st to end	Night	5% 11% 5% 11% 10% 5% 6% 6% 7% 7% 27%	1776
March	1st to 10th	Night	8% 13% 6% 9% 8% 5% 7% 6% 6% 7% 27%	2160
	11th to 20th	Night	9% 13% 5% 7% 6% 3 7% 5% 6% 7% 32%	2084
	21st to end	Night	9% 11% 4 9% 7% 4 6% 6% 6% 10% 28%	2040
April	1st to 10th	Night	6% 9% 4 6% 6% 3 7% 7% 8% 9% 36%	1800
	11th to 20th	Night	10% 7% 4 7% 6% 4 5% 7% 6% 7% 37%	1665
	21st to end	Night	5% 9% 4 8% 6% 5% 6% 5% 7% 8% 38%	1620
May	1st to 10th	Night	6% 7% 3 6% 5% 3 6% 6% 8% 9% 40%	1485
	11th to 20th	Night	6% 6% 2 5% 5% 3 6% 6% 7% 8% 45%	1416
	21st to end	Night	14% 8% 4 9% 5% 3 5% 5% 6% 8% 34%	1367
June	1st to 10th	Night	13% 5% 2 7% 6% 4 5% 7% 6% 8% 37%	1260
	11th to 20th	Night	11% 5% 3 6% 6% 4 6% 7% 8% 10% 35%	1260
	21st to end	Night	3 3 2 4 4 3 4 5% 7% 9% 55%	1260
July	1st to 10th	Night	2 2 1 2 3 2 3 5% 8% 8% 63%	1260
	11th to 20th	Night	2 2 1 1 3 1 3 5% 6% 9% 68%	1393
	21st to end	Night	1 1 1 1 2 4 3 5% 6% 74%	1576
August	1st to 10th	Night	3 2 1 3 2 2 2 3 5% 9% 68%	1432
	11th to 20th	Night	3 2 1 2 3 2 3 3 5% 8% 69%	1527
	21st to end	Night	4 2 1 3 2 2 3 4 5% 8% 65%	1850
September	1st to 10th	Night	4 2 1 3 4 2 3 4 5% 8% 63%	1800
	11th to 20th	Night	6% 3 3 5% 5% 3 5% 5% 6% 9% 50%	1923
	21st to end	Night	8% 5% 2 4 4 3 5% 5% 6% 7% 51%	1980
October	1st to 10th	Night	9% 6% 3 6% 5% 3 5% 5% 5% 6% 45%	2021
	11th to 20th	Night	10% 8% 2 6% 4 4 5% 6% 6% 7% 41%	2312
	21st to end	Night	12% 11% 4 7% 6% 3 5% 5% 5% 8% 33%	2574
November	1st to 10th	Night	15% 14% 5% 10% 8% 4 5% 5% 6% 9% 20%	2405
	11th to 20th	Night	12% 16% 5% 10% 8% 5% 7% 6% 6% 7% 20%	2650
	21st to end	Night	10% 17% 8% 11% 8% 4 6% 6% 4 5% 19%	2850
December	1st to 10th	Night	10% 15% 7% 12% 10% 5% 6% 6% 5% 4 18%	2850
	11th to 20th	Night	11% 22% 8% 17% 9% 4 6% 4 5% 4 9%	3510
	21st to end	Night	11% 21% 8% 13% 11% 5% 7% 5% 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	76% 5% 7% 9% 3	1414
	11th to 20th	Day	70% 5% 14% 8% 2 1	1464
	21st to end	Day	68% 5% 10% 13% 4	1584
February	1st to 10th	Day	64% 5% 14% 9% 7% 2	1440
	11th to 20th	Day	64% 5% 14% 13% 4	1626
	21st to end	Day	62% 3 12% 14% 8%	1480
March	1st to 10th	Day	63% 3 16% 13% 5%	1800
	11th to 20th	Day	57% 4 14% 20% 4	1876
	21st to end	Day	67% 2 14% 13% 4	2312
April	1st to 10th	Day	60% 4 15% 17% 5%	2160
	11th to 20th	Day	59% 3 13% 18% 6% 1	2246
	21st to end	Day	57% 3 13% 21% 5% 1	2340
May	1st to 10th	Day	49% 3 15% 23% 10% 1	2473
	11th to 20th	Day	48% 2 18% 24% 8% 1	2480
	21st to end	Day	57% 3 17% 18% 6%	2909
June	1st to 10th	Day	55% 2 14% 19% 9% 1	2700
	11th to 20th	Day	58% 3 13% 19% 7% 1	2700
	21st to end	Day	37% 2 16% 28% 15% 2	2700
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	77% 3 8% 8% 3	2768
	11th to 20th	Night	74% 4 10% 9% 2 1	2574
	21st to end	Night	74% 5% 9% 9% 4	2772
February	1st to 10th	Night	63% 5% 13% 11% 8% 1	2520
	11th to 20th	Night	59% 5% 16% 15% 5%	2231
	21st to end	Night	58% 4 16% 14% 8% 1	1776
March	1st to 10th	Night	58% 3 18% 14% 7%	2160
	11th to 20th	Night	52% 3 16% 22% 5% 1	2084
	21st to end	Night	54% 5% 19% 17% 5% 1	2040
April	1st to 10th	Night	44% 5% 21% 21% 8% 1	1800
	11th to 20th	Night	47% 2 17% 23% 10%	1665
	21st to end	Night	47% 3 17% 26% 7% 1	1620
May	1st to 10th	Night	40% 3 18% 26% 13%	1485
	11th to 20th	Night	36% 3 23% 28% 9% 1	1416
	21st to end	Night	50% 3 16% 25% 6%	1367
June	1st to 10th	Night	46% 2 16% 24% 9% 2	1260
	11th to 20th	Night	44% 3 16% 27% 8% 2	1260
	21st to end	Night	25% 3 19% 31% 20% 2	1260
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	17% 3 16% 39% 17% 8%	166
	21st to end	Day	16% 3 26% 35% 13% 8%	176
February	1st to 10th	Day	11% 4 33% 43% 9% 1	160
	11th to 20th	Day	16% 7% 24% 35% 17% 1	317
	21st to end	Day	12% 7% 31% 32% 17% 3	346
March	1st to 10th	Day	19% 10% 32% 24% 13% 3	420
	11th to 20th	Day	17% 12% 24% 23% 16% 8%	420
	21st to end	Day	21% 11% 27% 26% 11% 4	462
April	1st to 10th	Day	20% 13% 26% 25% 12% 5%	420
	11th to 20th	Day	21% 17% 25% 21% 10% 5%	412
	21st to end	Day	19% 13% 26% 22% 15% 5%	420
May	1st to 10th	Day	15% 10% 27% 27% 15% 6%	423
	11th to 20th	Day	19% 8% 21% 28% 16% 8%	448
	21st to end	Day	19% 6% 17% 26% 23% 8%	670
June	1st to 10th	Day	19% 8% 20% 30% 18% 5%	630
	11th to 20th	Day	15% 10% 28% 27% 17% 4	630
	21st to end	Day	9% 7% 20% 31% 23% 10%	630
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	22% 5% 16% 39% 17% 3	736
	21st to end	Night	20% 2 17% 42% 15% 3	792
February	1st to 10th	Night	18% 4 20% 44% 13% 2	720
	11th to 20th	Night	18% 3 16% 46% 14% 2	704
	21st to end	Night	14% 3 17% 43% 20% 4	592
March	1st to 10th	Night	18% 3 18% 41% 18% 3	720
	11th to 20th	Night	18% 3 19% 36% 21% 2	720
	21st to end	Night	19% 3 17% 39% 19% 3	792
April	1st to 10th	Night	13% 2 16% 45% 23% 3	720
	11th to 20th	Night	16% 2 20% 43% 18% 1	597
	21st to end	Night	12% 5% 19% 42% 21% 1	540
May	1st to 10th	Night	9% 3 15% 47% 24% 1	405
	11th to 20th	Night	12% 1 12% 37% 36% 3	354
	21st to end	Night	19% 3 10% 37% 28% 4	389
June	1st to 10th	Night	17% 2 14% 39% 26% 2	360
	11th to 20th	Night	16% 2 14% 44% 22% 2	360
	21st to end	Night	6% 2 13% 42% 34% 4	360
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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