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 2024-11-28.

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	7% 10% 4 9% 6% 5% 5% 4 6% 7% 37%	105593
	Night	8% 12% 4 9% 5% 5% 5% 4 6% 7% 35%	102533
Transparency	Day	47% 4 13% 21% 12% 3	105354
	Night	51% 4 14% 20% 9% 2	102418
Seeing	Day	14% 8% 24% 29% 18% 7%	15129
	Night	16% 3 15% 41% 22% 3	24221

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% 6% 5% 5% 5% 7% 8% 44%	2850
July	1st to 10th	Day	2 2 1 4 4 4 4 4 5% 6% 64%	4740
	11th to 20th	Day	2 2 1 2 2 2 3 3 5% 5% 73%	4492
	21st to end	Day	2 1 1 3 3 3 3 3 5% 7% 68%	4790
August	1st to 10th	Day	2 2 1 3 3 4 5% 4 6% 8% 61%	4401
	11th to 20th	Day	5% 2 1 3 3 3 4 4 6% 7% 63%	4149
	21st to end	Day	4 2 1 5% 4 4 5% 5% 7% 8% 55%	4397
September	1st to 10th	Day	5% 3 2 5% 4 5% 4 3 6% 7% 57%	2280
	11th to 20th	Day	7% 7% 3 6% 4 4 5% 4 6% 7% 47%	2149
	21st to end	Day	9% 6% 2 8% 4 5% 5% 5% 6% 8% 43%	2090
October	1st to 10th	Day	6% 4 1 6% 4 4 5% 4 7% 8% 52%	3407
	11th to 20th	Day	7% 5% 1 7% 5% 5% 6% 5% 7% 9% 43%	2884
	21st to end	Day	9% 10% 4 9% 6% 6% 7% 5% 7% 6% 31%	3101
November	1st to 10th	Day	14% 16% 5% 11% 6% 6% 6% 5% 6% 7% 19%	1764
	11th to 20th	Day	12% 15% 5% 9% 6% 7% 7% 4 7% 6% 21%	1582
	21st to end	Day	7% 17% 7% 15% 8% 8% 6% 5% 5% 4 18%	1379
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 4 5% 7% 9% 55%	1330
July	1st to 10th	Night	1 1 1 2 2 2 3 4 6% 8% 70%	2220
	11th to 20th	Night	1 1 1 2 2 2 2 5% 5% 77%	2465
	21st to end	Night	2 1 2 2 2 3 2 4 6% 77%	2762
August	1st to 10th	Night	2 2 1 3 2 3 3 2 5% 9% 68%	2529
	11th to 20th	Night	2 2 1 3 3 3 3 5% 6% 8% 64%	2684
	21st to end	Night	3 2 1 3 2 4 5% 4 7% 9% 61%	3270
September	1st to 10th	Night	4 2 1 4 3 3 3 4 5% 8% 64%	1900
	11th to 20th	Night	6% 4 2 6% 5% 4 5% 4 6% 9% 50%	2030
	21st to end	Night	8% 5% 2 4 3 5% 5% 4 6% 7% 52%	2090
October	1st to 10th	Night	5% 4 2 4 3 3 4 3 5% 8% 60%	3570
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 5% 6% 8% 48%	4045
	21st to end	Night	8% 8% 3 8% 6% 5% 7% 6% 7% 8% 35%	4457
November	1st to 10th	Night	14% 15% 4 12% 7% 5% 5% 4 6% 9% 20%	2535
	11th to 20th	Night	11% 17% 4 11% 7% 6% 7% 4 6% 7% 19%	2790
	21st to end	Night	10% 21% 6% 12% 7% 6% 6% 4 5% 5% 18%	2956
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	2850
July	1st to 10th	Day	21% 4 9% 29% 30% 9%	4666
	11th to 20th	Day	15% 3 6% 17% 38% 21%	4414
	21st to end	Day	17% 3 7% 26% 30% 16%	4639
August	1st to 10th	Day	20% 4 12% 34% 24% 6%	4384
	11th to 20th	Day	21% 3 18% 39% 17% 2	4191
	21st to end	Day	25% 6% 18% 34% 15% 2	4396
September	1st to 10th	Day	29% 2 12% 33% 19% 5%	2280
	11th to 20th	Day	37% 2 15% 28% 16% 2	2149
	21st to end	Day	41% 4 21% 23% 10% 1	2090
October	1st to 10th	Day	30% 7% 21% 36% 7%	3407
	11th to 20th	Day	39% 8% 18% 26% 8%	2884
	21st to end	Day	50% 11% 16% 17% 6%	3101
November	1st to 10th	Day	66% 6% 12% 12% 3	1764
	11th to 20th	Day	63% 6% 14% 10% 6%	1582
	21st to end	Day	70% 4 14% 9% 3	1379
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	1330
July	1st to 10th	Night	12% 3 13% 32% 30% 9%	2185
	11th to 20th	Night	11% 2 8% 20% 32% 27%	2420
	21st to end	Night	12% 2 5% 29% 29% 22%	2674
August	1st to 10th	Night	16% 3 13% 33% 27% 8%	2521
	11th to 20th	Night	19% 3 22% 34% 20% 2	2708
	21st to end	Night	20% 5% 19% 35% 18% 3	3270
September	1st to 10th	Night	21% 3 12% 36% 22% 6%	1900
	11th to 20th	Night	33% 3 16% 31% 15% 3	2030
	21st to end	Night	32% 4 24% 26% 12% 2	2090
October	1st to 10th	Night	24% 6% 23% 39% 8%	3570
	11th to 20th	Night	36% 8% 16% 32% 9%	4045
	21st to end	Night	43% 10% 22% 20% 4	4457
November	1st to 10th	Night	64% 7% 13% 13% 3	2535
	11th to 20th	Night	65% 7% 12% 10% 6%	2790
	21st to end	Night	69% 6% 12% 11% 2	2956
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% 21% 31% 22% 10%	680
July	1st to 10th	Day	5% 6% 23% 32% 27% 7%	671
	11th to 20th	Day	5% 9% 28% 31% 20% 7%	527
	21st to end	Day	4 8% 30% 26% 22% 10%	531
August	1st to 10th	Day	6% 7% 23% 33% 22% 10%	487
	11th to 20th	Day	6% 5% 22% 33% 20% 14%	477
	21st to end	Day	7% 10% 24% 27% 22% 10%	539
September	1st to 10th	Day	11% 7% 24% 26% 22% 10%	490
	11th to 20th	Day	12% 8% 22% 32% 18% 9%	490
	21st to end	Day	16% 8% 27% 27% 15% 6%	490
October	1st to 10th	Day	14% 7% 25% 33% 14% 7%	490
	11th to 20th	Day	14% 7% 21% 32% 18% 9%	329
	21st to end	Day	23% 6% 27% 27% 14% 3	330
November	1st to 10th	Day	19% 6% 21% 34% 18% 1	300
	11th to 20th	Day	23% 5% 21% 33% 15% 3	243
	21st to end	Day	14% 3 22% 35% 16% 10%	194
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	380
July	1st to 10th	Night	4 1 6% 45% 43% 2	370
	11th to 20th	Night	4 1 9% 51% 33% 2	368
	21st to end	Night	3 8% 45% 42% 2	416
August	1st to 10th	Night	3 1 7% 40% 45% 4	378
	11th to 20th	Night	4 1 8% 39% 44% 4	485
	21st to end	Night	5% 2 11% 42% 36% 3	699
September	1st to 10th	Night	6% 1 12% 45% 29% 7%	760
	11th to 20th	Night	8% 1 11% 38% 36% 6%	760
	21st to end	Night	13% 1 14% 42% 25% 5%	760
October	1st to 10th	Night	14% 3 14% 40% 25% 3	760
	11th to 20th	Night	17% 1 15% 40% 23% 4	760
	21st to end	Night	19% 5% 18% 40% 15% 3	836
November	1st to 10th	Night	25% 3 16% 43% 12% 1	780
	11th to 20th	Night	26% 5% 18% 36% 14% 1	880
	21st to end	Night	22% 3 16% 39% 18% 2	986
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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