Cabin in the Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cabin in the Sky Observatory 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	10% 9% 3 7% 4 4 4 4 5% 7% 42%	105611
	Night	12% 12% 3 7% 4 4 4 3 5% 6% 40%	102492
Transparency	Day	22% 47% 6% 12% 9% 3	105379
	Night	25% 47% 7% 12% 8% 2	102370
Seeing	Day	19% 12% 28% 26% 12% 3	14943
	Night	19% 7% 19% 33% 18% 3	23773

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	17% 24% 7% 11% 6% 4 4 3 4 5% 16%	2539
	11th to 20th	Day	17% 14% 4 7% 5% 5% 4 3 5% 6% 30%	2516
	21st to end	Day	25% 26% 5% 7% 3 3 3 2 4 5% 17%	2794
February	1st to 10th	Day	14% 16% 4 8% 4 5% 5% 3 5% 7% 29%	1567
	11th to 20th	Day	11% 15% 4 8% 5% 5% 5% 4 5% 5% 32%	1674
	21st to end	Day	10% 15% 3 9% 7% 6% 7% 4 6% 6% 27%	1570
March	1st to 10th	Day	11% 17% 5% 9% 4 5% 4 4 5% 7% 29%	3138
	11th to 20th	Day	8% 10% 2 5% 4 3 4 3 5% 6% 49%	3297
	21st to end	Day	12% 14% 4 8% 5% 5% 5% 6% 7% 7% 26%	4150
April	1st to 10th	Day	11% 12% 4 11% 7% 6% 5% 4 6% 7% 26%	2280
	11th to 20th	Day	10% 11% 3 9% 7% 6% 5% 5% 6% 8% 32%	2268
	21st to end	Day	9% 12% 4 9% 5% 5% 5% 4 6% 8% 32%	2560
May	1st to 10th	Day	8% 7% 3 7% 5% 5% 5% 5% 6% 9% 40%	4420
	11th to 20th	Day	8% 7% 3 7% 5% 5% 6% 5% 7% 10% 39%	4377
	21st to end	Day	13% 11% 3 9% 6% 6% 6% 5% 6% 7% 28%	4813
June	1st to 10th	Day	15% 9% 3 7% 5% 5% 6% 5% 6% 7% 33%	2798
	11th to 20th	Day	12% 8% 3 8% 6% 6% 7% 5% 7% 7% 31%	2854
	21st to end	Day	8% 5% 2 7% 4 4 5% 5% 6% 7% 46%	3009
July	1st to 10th	Day	3 2 1 3 2 2 3 3 4 5% 73%	4457
	11th to 20th	Day	2 3 1 3 2 2 3 2 3 4 75%	4420
	21st to end	Day	3 2 1 3 2 3 3 3 5% 7% 69%	4790
August	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 7% 65%	4401
	11th to 20th	Day	3 3 1 3 4 4 4 4 6% 7% 60%	4304
	21st to end	Day	6% 4 2 5% 4 4 5% 4 4 7% 55%	4180
September	1st to 10th	Day	5% 4 1 5% 4 3 3 3 3 8% 62%	2280
	11th to 20th	Day	9% 6% 2 5% 5% 4 4 4 6% 7% 50%	2246
	21st to end	Day	10% 5% 2 6% 4 4 4 3 5% 7% 50%	2090
October	1st to 10th	Day	8% 6% 2 6% 4 4 4 3 5% 7% 50%	3185
	11th to 20th	Day	14% 8% 3 8% 5% 5% 4 4 5% 5% 40%	3066
	21st to end	Day	15% 13% 5% 7% 4 4 3 3 4 5% 37%	3101
November	1st to 10th	Day	21% 11% 3 6% 5% 5% 5% 4 6% 7% 27%	1717
	11th to 20th	Day	18% 16% 3 8% 4 4 5% 2 3 5% 31%	1600
	21st to end	Day	16% 17% 6% 10% 4 4 4 4 5% 5% 25%	1504
December	1st to 10th	Day	16% 11% 5% 9% 5% 4 4 3 5% 7% 30%	1330
	11th to 20th	Day	17% 19% 8% 9% 5% 4 5% 3 5% 7% 19%	1682
	21st to end	Day	15% 18% 6% 12% 6% 5% 4 4 5% 6% 19%	2634
January	1st to 10th	Night	20% 23% 8% 12% 5% 4 3 2 3 4 16%	4406
	11th to 20th	Night	17% 15% 3 7% 5% 3 3 3 4 6% 33%	4382
	21st to end	Night	22% 25% 9% 9% 3 3 3 2 4 4 16%	4862
February	1st to 10th	Night	15% 15% 5% 9% 6% 4 5% 3 5% 6% 28%	2610
	11th to 20th	Night	11% 13% 4 8% 5% 4 4 5% 5% 6% 35%	2393
	21st to end	Night	10% 15% 5% 8% 5% 5% 4 4 6% 7% 29%	1884
March	1st to 10th	Night	8% 13% 4 10% 5% 5% 5% 4 6% 8% 32%	3736
	11th to 20th	Night	9% 10% 3 5% 4 3 3 3 5% 6% 51%	3680
	21st to end	Night	12% 16% 5% 9% 6% 4 5% 5% 5% 6% 27%	3502
April	1st to 10th	Night	12% 10% 3 8% 4 4 5% 4 7% 8% 35%	1900
	11th to 20th	Night	8% 8% 3 8% 6% 5% 7% 4 5% 6% 40%	1862
	21st to end	Night	9% 9% 3 7% 5% 4 6% 4 6% 8% 40%	1618
May	1st to 10th	Night	7% 8% 2 7% 5% 4 5% 4 5% 7% 46%	2540
	11th to 20th	Night	6% 5% 2 6% 5% 5% 4 4 7% 9% 49%	2516
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 5% 7% 8% 38%	2762
June	1st to 10th	Night	12% 6% 2 5% 4 5% 6% 5% 7% 7% 39%	1378
	11th to 20th	Night	10% 7% 2 6% 6% 7% 5% 4 6% 7% 39%	1316
	21st to end	Night	7% 4 2 5% 4 4 5% 4 6% 8% 52%	1166
July	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 76%	2494
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 77%	2540
	21st to end	Night	2 2 1 3 3 3 3 3 4 6% 71%	2762
August	1st to 10th	Night	3 2 1 5% 3 3 4 3 5% 5% 66%	2529
	11th to 20th	Night	2 2 1 4 3 3 4 3 7% 10% 61%	2520
	21st to end	Night	8% 4 2 4 3 3 4 4 5% 7% 56%	3498
September	1st to 10th	Night	3 4 2 3 2 3 3 4 5% 7% 63%	1900
	11th to 20th	Night	8% 5% 1 5% 3 3 3 3 5% 8% 55%	1933
	21st to end	Night	8% 5% 3 6% 4 4 3 3 5% 7% 51%	2090
October	1st to 10th	Night	8% 5% 2 5% 4 4 5% 4 5% 6% 51%	3791
	11th to 20th	Night	16% 9% 3 7% 4 3 4 3 5% 6% 41%	3869
	21st to end	Night	18% 9% 3 7% 4 4 3 3 5% 4 42%	4457
November	1st to 10th	Night	21% 14% 3 6% 4 4 3 3 6% 7% 29%	2580
	11th to 20th	Night	21% 15% 3 7% 4 4 4 4 5% 6% 28%	2800
	21st to end	Night	19% 17% 4 10% 5% 3 4 4 4 5% 25%	2806
December	1st to 10th	Night	15% 15% 4 10% 5% 4 4 3 4 5% 30%	2850
	11th to 20th	Night	18% 23% 6% 10% 4 4 4 3 4 5% 19%	3510
	21st to end	Night	16% 25% 5% 9% 4 4 3 3 5% 5% 21%	5050

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	45% 39% 5% 7% 4	2539
	11th to 20th	Day	36% 37% 9% 10% 2 5%	2540
	21st to end	Day	52% 39% 4 3 1	2794
February	1st to 10th	Day	21% 66% 3 5% 3 1	1567
	11th to 20th	Day	21% 65% 4 7% 3	1683
	21st to end	Day	17% 66% 4 8% 5%	1570
March	1st to 10th	Day	34% 42% 5% 9% 10%	3138
	11th to 20th	Day	15% 41% 13% 27% 3	3297
	21st to end	Day	35% 43% 9% 8% 6%	4150
April	1st to 10th	Day	18% 67% 6% 7% 2	2280
	11th to 20th	Day	16% 67% 4 9% 3	2268
	21st to end	Day	17% 66% 4 7% 5%	2560
May	1st to 10th	Day	22% 43% 6% 14% 13% 1	4420
	11th to 20th	Day	22% 42% 9% 19% 9%	4380
	21st to end	Day	34% 41% 9% 13% 3	4814
June	1st to 10th	Day	16% 66% 4 8% 4 1	2798
	11th to 20th	Day	16% 67% 4 9% 4 1	2854
	21st to end	Day	14% 64% 4 10% 7% 1	3009
July	1st to 10th	Day	7% 37% 3 9% 23% 21%	4388
	11th to 20th	Day	7% 36% 2 6% 23% 26%	4345
	21st to end	Day	10% 37% 5% 16% 21% 11%	4639
August	1st to 10th	Day	9% 38% 3 17% 26% 8%	4384
	11th to 20th	Day	14% 41% 9% 27% 6% 1	4346
	21st to end	Day	19% 39% 9% 17% 13% 3	4179
September	1st to 10th	Day	12% 60% 6% 13% 8% 1	2280
	11th to 20th	Day	16% 62% 4 10% 6% 1	2246
	21st to end	Day	16% 61% 7% 10% 5% 1	2090
October	1st to 10th	Day	18% 39% 13% 24% 6%	3185
	11th to 20th	Day	29% 41% 5% 18% 8%	3066
	21st to end	Day	31% 40% 7% 16% 7%	3101
November	1st to 10th	Day	24% 58% 8% 8% 2	1717
	11th to 20th	Day	26% 58% 6% 6% 3 1	1600
	21st to end	Day	25% 58% 6% 6% 4 1	1504
December	1st to 10th	Day	26% 60% 6% 6% 2	1330
	11th to 20th	Day	37% 50% 7% 4 2	1684
	21st to end	Day	46% 37% 8% 6% 3	2634
January	1st to 10th	Night	49% 37% 5% 6% 2	4406
	11th to 20th	Night	34% 37% 6% 12% 7% 4	4420
	21st to end	Night	53% 40% 4 2 1	4862
February	1st to 10th	Night	22% 66% 3 4 3 1	2610
	11th to 20th	Night	21% 65% 4 7% 2 1	2395
	21st to end	Night	16% 66% 4 8% 6%	1884
March	1st to 10th	Night	30% 43% 5% 12% 10% 1	3736
	11th to 20th	Night	15% 41% 11% 26% 6%	3681
	21st to end	Night	37% 42% 8% 10% 4	3502
April	1st to 10th	Night	16% 67% 6% 8% 2 1	1900
	11th to 20th	Night	16% 67% 3 8% 5%	1862
	21st to end	Night	15% 66% 3 9% 7%	1618
May	1st to 10th	Night	20% 43% 6% 15% 16%	2540
	11th to 20th	Night	16% 40% 10% 24% 6% 2	2516
	21st to end	Night	23% 41% 14% 17% 4	2762
June	1st to 10th	Night	12% 66% 6% 11% 5% 1	1378
	11th to 20th	Night	15% 66% 5% 8% 6% 1	1316
	21st to end	Night	12% 63% 4 13% 6% 1	1166
July	1st to 10th	Night	6% 35% 2 9% 31% 17%	2454
	11th to 20th	Night	7% 35% 3 5% 23% 26%	2492
	21st to end	Night	10% 36% 3 19% 22% 11%	2674
August	1st to 10th	Night	12% 37% 3 15% 28% 5%	2521
	11th to 20th	Night	11% 40% 11% 27% 10% 1	2544
	21st to end	Night	18% 38% 9% 22% 11% 1	3498
September	1st to 10th	Night	10% 61% 6% 13% 9% 1	1900
	11th to 20th	Night	14% 61% 5% 12% 7%	1933
	21st to end	Night	15% 61% 7% 12% 5%	2090
October	1st to 10th	Night	16% 38% 14% 23% 8%	3791
	11th to 20th	Night	27% 38% 7% 18% 10%	3869
	21st to end	Night	26% 39% 10% 21% 5%	4457
November	1st to 10th	Night	25% 58% 7% 8% 2	2580
	11th to 20th	Night	24% 58% 6% 6% 4 2	2800
	21st to end	Night	25% 59% 5% 6% 4 1	2806
December	1st to 10th	Night	25% 60% 5% 9% 2	2850
	11th to 20th	Night	39% 51% 5% 3 3	3507
	21st to end	Night	42% 38% 9% 7% 4	5050

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	31% 9% 21% 30% 6% 4	180
	11th to 20th	Day	34% 3 23% 26% 11% 3	180
	21st to end	Day	29% 8% 26% 24% 10% 3	198
February	1st to 10th	Day	22% 13% 34% 25% 4 1	227
	11th to 20th	Day	22% 14% 33% 21% 9% 1	369
	21st to end	Day	17% 11% 33% 30% 8% 1	382
March	1st to 10th	Day	25% 18% 29% 17% 10% 1	460
	11th to 20th	Day	24% 14% 24% 22% 14% 2	460
	21st to end	Day	26% 17% 25% 19% 11% 3	506
April	1st to 10th	Day	27% 14% 30% 19% 9% 2	460
	11th to 20th	Day	24% 18% 27% 18% 9% 4	452
	21st to end	Day	20% 18% 26% 22% 10% 4	460
May	1st to 10th	Day	19% 16% 28% 24% 11% 3	463
	11th to 20th	Day	23% 8% 27% 27% 10% 5%	488
	21st to end	Day	22% 6% 27% 26% 16% 4	529
June	1st to 10th	Day	20% 11% 29% 27% 11% 3	628
	11th to 20th	Day	20% 11% 26% 26% 13% 3	684
	21st to end	Day	13% 10% 26% 29% 17% 4	839
July	1st to 10th	Day	7% 14% 28% 36% 11% 4	510
	11th to 20th	Day	7% 15% 30% 32% 12% 4	484
	21st to end	Day	6% 9% 30% 33% 17% 5%	531
August	1st to 10th	Day	7% 7% 29% 34% 17% 7%	487
	11th to 20th	Day	8% 7% 28% 30% 18% 9%	477
	21st to end	Day	14% 12% 26% 28% 14% 6%	539
September	1st to 10th	Day	11% 12% 28% 29% 15% 5%	490
	11th to 20th	Day	21% 8% 24% 28% 16% 3	490
	21st to end	Day	20% 13% 29% 23% 12% 3	490
October	1st to 10th	Day	17% 14% 28% 25% 13% 2	490
	11th to 20th	Day	21% 12% 27% 24% 12% 3	433
	21st to end	Day	28% 7% 33% 21% 9% 2	330
November	1st to 10th	Day	30% 11% 25% 26% 9%	275
	11th to 20th	Day	32% 14% 22% 21% 12%	200
	21st to end	Day	25% 7% 30% 20% 13% 5%	194
December	1st to 10th	Day	24% 6% 21% 26% 17% 5%	180
	11th to 20th	Day	24% 11% 28% 27% 7% 4	180
	21st to end	Day	26% 7% 21% 32% 11% 3	198
January	1st to 10th	Night	33% 9% 20% 26% 11% 2	760
	11th to 20th	Night	28% 6% 19% 32% 13% 2	760
	21st to end	Night	23% 5% 21% 34% 15% 2	836
February	1st to 10th	Night	23% 7% 24% 32% 11% 3	760
	11th to 20th	Night	18% 7% 23% 35% 14% 2	744
	21st to end	Night	19% 7% 20% 34% 18% 3	628
March	1st to 10th	Night	20% 8% 20% 31% 17% 3	760
	11th to 20th	Night	18% 9% 21% 34% 16% 2	760
	21st to end	Night	22% 8% 20% 31% 15% 3	836
April	1st to 10th	Night	18% 6% 18% 35% 19% 3	760
	11th to 20th	Night	13% 9% 23% 34% 17% 3	734
	21st to end	Night	14% 8% 24% 35% 17% 2	478
May	1st to 10th	Night	13% 8% 20% 36% 20% 3	380
	11th to 20th	Night	14% 5% 18% 32% 26% 5%	374
	21st to end	Night	16% 5% 18% 31% 27% 5%	411
June	1st to 10th	Night	16% 7% 20% 35% 19% 2	380
	11th to 20th	Night	16% 7% 22% 33% 20% 2	380
	21st to end	Night	9% 6% 21% 34% 27% 4	380
July	1st to 10th	Night	7% 5% 18% 37% 29% 4	370
	11th to 20th	Night	5% 7% 18% 41% 25% 4	368
	21st to end	Night	4 6% 15% 40% 31% 5%	416
August	1st to 10th	Night	4 4 16% 32% 38% 6%	378
	11th to 20th	Night	4 3 16% 35% 35% 6%	386
	21st to end	Night	9% 5% 16% 39% 27% 5%	836
September	1st to 10th	Night	5% 5% 16% 38% 28% 7%	760
	11th to 20th	Night	13% 4 14% 32% 29% 9%	760
	21st to end	Night	14% 6% 16% 37% 23% 4	760
October	1st to 10th	Night	19% 6% 14% 33% 25% 2	760
	11th to 20th	Night	23% 6% 18% 33% 18% 3	760
	21st to end	Night	26% 6% 20% 32% 13% 3	836
November	1st to 10th	Night	30% 8% 22% 32% 8% 1	780
	11th to 20th	Night	30% 9% 19% 26% 14% 2	800
	21st to end	Night	28% 7% 18% 30% 13% 3	836
December	1st to 10th	Night	21% 9% 18% 37% 13% 2	950
	11th to 20th	Night	29% 8% 20% 34% 9% 1	950
	21st to end	Night	29% 8% 24% 29% 10% 1	946

Page generated at 2024-11-24 21:22:18. Version ac07

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