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 2024-01-14.

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	11% 9% 4 6% 6% 3 5% 5% 5% 7% 39%	74793
	Night	13% 11% 4 6% 5% 3 4 5% 5% 7% 37%	74953
Transparency	Day	46% 6% 13% 20% 12% 2	74495
	Night	47% 6% 14% 20% 11% 2	74769
Seeing	Day	19% 12% 27% 26% 13% 3	13754
	Night	19% 7% 19% 33% 18% 3	22739

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	21% 18% 7% 9% 7% 3 4 4 4 5% 17%	1519
	11th to 20th	Day	16% 14% 5% 8% 5% 5% 5% 4 4 6% 27%	1464
	21st to end	Day	15% 16% 5% 8% 5% 3 4 3 5% 7% 30%	1584
February	1st to 10th	Day	14% 14% 5% 7% 4 4 6% 4 5% 7% 30%	1485
	11th to 20th	Day	11% 14% 5% 8% 6% 3 6% 5% 5% 6% 32%	1584
	21st to end	Day	10% 13% 5% 8% 8% 4 7% 6% 6% 6% 27%	1480
March	1st to 10th	Day	12% 13% 4 7% 6% 4 5% 6% 6% 8% 28%	1800
	11th to 20th	Day	14% 12% 4 7% 6% 4 5% 5% 6% 8% 30%	1865
	21st to end	Day	12% 13% 5% 8% 8% 4 6% 6% 7% 7% 23%	2361
April	1st to 10th	Day	12% 11% 5% 10% 8% 4 6% 5% 6% 7% 26%	2160
	11th to 20th	Day	10% 10% 4 8% 8% 5% 5% 6% 6% 7% 31%	2147
	21st to end	Day	10% 11% 4 8% 7% 4 5% 5% 6% 8% 34%	2425
May	1st to 10th	Day	9% 7% 3 6% 7% 4 5% 5% 6% 8% 38%	2520
	11th to 20th	Day	12% 8% 3 6% 6% 4 6% 6% 6% 8% 34%	2477
	21st to end	Day	14% 8% 3 7% 8% 5% 6% 6% 6% 8% 31%	2723
June	1st to 10th	Day	15% 8% 4 7% 6% 4 5% 6% 6% 7% 33%	2650
	11th to 20th	Day	13% 8% 3 7% 7% 4 7% 7% 6% 8% 31%	2703
	21st to end	Day	8% 5% 3 6% 5% 3 4 6% 6% 8% 46%	2851
July	1st to 10th	Day	5% 4 2 4 4 2 4 6% 6% 8% 56%	2543
	11th to 20th	Day	4 4 2 4 4 2 4 4 4 7% 63%	2520
	21st to end	Day	3 2 1 2 3 2 3 4 5% 6% 69%	2741
August	1st to 10th	Day	3 3 1 3 4 2 4 5% 6% 6% 62%	2507
	11th to 20th	Day	5% 3 1 2 3 2 3 3 4 6% 66%	2462
	21st to end	Day	4 4 2 5% 5% 3 5% 5% 6% 8% 53%	2376
September	1st to 10th	Day	5% 4 1 4 4 3 3 3 3 8% 62%	2160
	11th to 20th	Day	9% 5% 2 4 5% 3 4 5% 6% 7% 49%	2128
	21st to end	Day	10% 5% 2 5% 4 3 4 4 4 7% 49%	1980
October	1st to 10th	Day	11% 7% 3 5% 5% 3 5% 4 5% 7% 46%	1805
	11th to 20th	Day	14% 7% 3 5% 5% 4 4 5% 6% 5% 41%	1746
	21st to end	Day	19% 11% 4 5% 5% 3 4 4 4 6% 35%	1782
November	1st to 10th	Day	22% 9% 4 5% 6% 3 5% 5% 6% 7% 27%	1630
	11th to 20th	Day	18% 14% 5% 7% 5% 4 4 4 3 5% 32%	1520
	21st to end	Day	17% 15% 6% 9% 5% 3 4 5% 5% 5% 26%	1449
December	1st to 10th	Day	17% 9% 6% 9% 5% 3 4 4 5% 7% 31%	1330
	11th to 20th	Day	17% 18% 8% 8% 6% 3 5% 4 5% 7% 19%	1682
	21st to end	Day	16% 18% 7% 11% 7% 3 5% 5% 5% 6% 19%	2634
January	1st to 10th	Night	25% 16% 6% 9% 6% 4 4 4 4 5% 18%	2646
	11th to 20th	Night	18% 15% 5% 9% 6% 3 4 3 4 7% 26%	2574
	21st to end	Night	13% 18% 6% 10% 5% 2 4 4 5% 6% 27%	2772
February	1st to 10th	Night	15% 13% 5% 8% 6% 3 5% 4 5% 7% 29%	2472
	11th to 20th	Night	12% 12% 5% 7% 6% 3 4 5% 5% 6% 35%	2263
	21st to end	Night	10% 14% 6% 7% 6% 4 4 5% 5% 8% 30%	1776
March	1st to 10th	Night	13% 12% 4 8% 5% 3 5% 5% 6% 8% 31%	2160
	11th to 20th	Night	13% 11% 4 7% 6% 3 5% 5% 6% 6% 33%	2092
	21st to end	Night	11% 12% 5% 8% 6% 3 6% 6% 5% 8% 31%	1994
April	1st to 10th	Night	12% 8% 4 7% 5% 4 5% 6% 7% 8% 35%	1800
	11th to 20th	Night	8% 8% 4 7% 6% 4 7% 6% 5% 6% 39%	1763
	21st to end	Night	9% 8% 4 6% 6% 3 5% 6% 6% 8% 40%	1533
May	1st to 10th	Night	8% 6% 2 6% 6% 4 5% 6% 5% 7% 45%	1440
	11th to 20th	Night	9% 7% 2 6% 6% 4 4 6% 7% 9% 40%	1416
	21st to end	Night	11% 9% 3 6% 5% 4 6% 5% 7% 7% 37%	1552
June	1st to 10th	Night	13% 6% 3 5% 5% 3 6% 6% 8% 8% 39%	1306
	11th to 20th	Night	10% 6% 3 5% 6% 4 7% 5% 6% 8% 40%	1247
	21st to end	Night	6% 4 2 4 5% 3 5% 5% 7% 8% 52%	1104
July	1st to 10th	Night	4 4 1 3 3 3 4 4 6% 7% 61%	1408
	11th to 20th	Night	2 3 1 2 3 2 4 4 5% 8% 67%	1440
	21st to end	Night	1 1 1 2 2 2 3 3 4 5% 75%	1576
August	1st to 10th	Night	3 2 1 4 3 3 3 4 5% 7% 65%	1432
	11th to 20th	Night	2 2 3 3 2 3 3 5% 9% 68%	1433
	21st to end	Night	5% 3 2 2 3 2 4 4 6% 8% 61%	1980
September	1st to 10th	Night	4 4 2 3 2 2 4 4 5% 8% 62%	1800
	11th to 20th	Night	8% 5% 1 4 4 3 3 4 5% 8% 55%	1831
	21st to end	Night	8% 5% 3 5% 5% 3 3 4 5% 8% 50%	1980
October	1st to 10th	Night	12% 7% 2 4 4 3 4 5% 6% 6% 46%	2151
	11th to 20th	Night	15% 8% 2 6% 5% 3 4 4 6% 5% 41%	2214
	21st to end	Night	18% 10% 3 5% 4 3 4 4 5% 6% 38%	2574
November	1st to 10th	Night	22% 13% 4 5% 5% 3 4 3 6% 7% 29%	2447
	11th to 20th	Night	20% 13% 4 6% 4 3 5% 4 4 7% 29%	2660
	21st to end	Night	19% 14% 5% 9% 6% 2 5% 5% 4 5% 25%	2707
December	1st to 10th	Night	16% 14% 5% 9% 6% 3 4 4 4 6% 30%	2850
	11th to 20th	Night	18% 22% 7% 9% 5% 3 4 4 4 5% 19%	3510
	21st to end	Night	16% 24% 6% 8% 5% 3 3 4 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	72% 5% 10% 8% 4	1519
	11th to 20th	Day	58% 7% 14% 15% 4 2	1464
	21st to end	Day	55% 5% 15% 16% 8%	1584
February	1st to 10th	Day	55% 7% 15% 14% 7% 2	1485
	11th to 20th	Day	53% 8% 15% 17% 7%	1593
	21st to end	Day	57% 8% 11% 16% 8%	1480
March	1st to 10th	Day	55% 7% 14% 14% 8% 1	1800
	11th to 20th	Day	55% 7% 13% 17% 7%	1865
	21st to end	Day	61% 7% 16% 13% 4	2361
April	1st to 10th	Day	58% 8% 12% 16% 6%	2160
	11th to 20th	Day	53% 7% 12% 17% 9% 1	2147
	21st to end	Day	50% 6% 13% 20% 10% 1	2425
May	1st to 10th	Day	45% 5% 15% 24% 9% 1	2520
	11th to 20th	Day	50% 7% 15% 21% 7% 1	2480
	21st to end	Day	53% 6% 13% 20% 7%	2724
June	1st to 10th	Day	54% 5% 10% 17% 10% 4	2650
	11th to 20th	Day	53% 7% 12% 20% 7% 1	2703
	21st to end	Day	38% 6% 13% 25% 16% 2	2851
July	1st to 10th	Day	30% 5% 11% 25% 22% 7%	2474
	11th to 20th	Day	25% 4 9% 24% 29% 10%	2445
	21st to end	Day	17% 4 7% 26% 34% 11%	2589
August	1st to 10th	Day	24% 5% 10% 26% 25% 9%	2490
	11th to 20th	Day	21% 6% 10% 30% 28% 5%	2462
	21st to end	Day	30% 6% 11% 32% 19% 2	2376
September	1st to 10th	Day	27% 4 13% 33% 20% 4	2160
	11th to 20th	Day	37% 5% 11% 25% 19% 3	2128
	21st to end	Day	36% 7% 17% 25% 14% 2	1980
October	1st to 10th	Day	39% 6% 14% 30% 11%	1805
	11th to 20th	Day	46% 6% 15% 23% 10%	1746
	21st to end	Day	52% 6% 13% 22% 7%	1782
November	1st to 10th	Day	57% 7% 14% 15% 7%	1630
	11th to 20th	Day	59% 5% 14% 16% 5% 1	1520
	21st to end	Day	60% 6% 14% 13% 5% 1	1449
December	1st to 10th	Day	55% 7% 15% 18% 6%	1330
	11th to 20th	Day	67% 5% 14% 10% 5%	1684
	21st to end	Day	69% 4 12% 11% 4	2634
January	1st to 10th	Night	70% 5% 10% 10% 4 1	2646
	11th to 20th	Night	60% 5% 14% 13% 5% 1	2574
	21st to end	Night	59% 5% 15% 14% 6% 1	2772
February	1st to 10th	Night	56% 6% 15% 14% 7% 2	2472
	11th to 20th	Night	49% 6% 17% 20% 7%	2265
	21st to end	Night	52% 8% 14% 15% 10%	1776
March	1st to 10th	Night	50% 8% 16% 16% 9% 1	2160
	11th to 20th	Night	51% 7% 12% 23% 8%	2093
	21st to end	Night	52% 7% 16% 18% 6%	1994
April	1st to 10th	Night	45% 9% 16% 22% 7% 1	1800
	11th to 20th	Night	46% 6% 14% 21% 11% 1	1763
	21st to end	Night	44% 6% 11% 26% 12% 1	1533
May	1st to 10th	Night	39% 5% 15% 27% 13% 1	1440
	11th to 20th	Night	40% 6% 19% 24% 11% 1	1416
	21st to end	Night	43% 8% 18% 23% 8% 1	1552
June	1st to 10th	Night	42% 6% 14% 24% 12% 3	1306
	11th to 20th	Night	42% 6% 16% 24% 11%	1247
	21st to end	Night	29% 7% 15% 28% 17% 3	1104
July	1st to 10th	Night	23% 6% 12% 29% 23% 8%	1368
	11th to 20th	Night	20% 4 13% 26% 27% 10%	1392
	21st to end	Night	13% 4 6% 30% 36% 11%	1488
August	1st to 10th	Night	20% 5% 11% 26% 29% 9%	1424
	11th to 20th	Night	16% 4 11% 34% 31% 4	1433
	21st to end	Night	21% 5% 13% 35% 21% 4	1980
September	1st to 10th	Night	22% 5% 13% 31% 24% 4	1800
	11th to 20th	Night	30% 5% 12% 26% 23% 5%	1831
	21st to end	Night	33% 5% 16% 29% 13% 3	1980
October	1st to 10th	Night	37% 6% 15% 27% 14% 1	2151
	11th to 20th	Night	43% 6% 13% 25% 11% 1	2214
	21st to end	Night	47% 6% 13% 24% 9% 1	2574
November	1st to 10th	Night	57% 7% 13% 14% 7% 1	2447
	11th to 20th	Night	57% 6% 14% 15% 6% 2	2660
	21st to end	Night	63% 5% 13% 12% 6% 1	2707
December	1st to 10th	Night	56% 5% 15% 19% 5%	2850
	11th to 20th	Night	70% 6% 10% 8% 5%	3507
	21st to end	Night	66% 4 12% 12% 6% 1	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% 4 21% 27% 11% 4	166
	21st to end	Day	27% 8% 26% 26% 10% 3	176
February	1st to 10th	Day	22% 13% 35% 25% 3 1	205
	11th to 20th	Day	22% 14% 34% 22% 8% 1	339
	21st to end	Day	16% 12% 33% 31% 9% 1	346
March	1st to 10th	Day	26% 19% 29% 15% 10% 1	420
	11th to 20th	Day	25% 15% 25% 21% 13% 1	420
	21st to end	Day	26% 17% 25% 18% 11% 3	462
April	1st to 10th	Day	27% 13% 30% 19% 10% 1	420
	11th to 20th	Day	25% 18% 26% 17% 9% 5%	412
	21st to end	Day	20% 18% 27% 21% 10% 4	420
May	1st to 10th	Day	19% 16% 27% 25% 10% 2	423
	11th to 20th	Day	24% 8% 26% 27% 10% 5%	448
	21st to end	Day	22% 6% 25% 27% 16% 4	485
June	1st to 10th	Day	19% 12% 28% 27% 11% 3	580
	11th to 20th	Day	20% 10% 27% 27% 13% 3	633
	21st to end	Day	13% 9% 26% 30% 18% 4	781
July	1st to 10th	Day	8% 12% 28% 36% 12% 3	469
	11th to 20th	Day	8% 16% 29% 31% 13% 4	444
	21st to end	Day	5% 10% 30% 33% 18% 5%	487
August	1st to 10th	Day	8% 6% 28% 35% 16% 7%	447
	11th to 20th	Day	7% 7% 29% 30% 18% 9%	437
	21st to end	Day	13% 11% 26% 29% 15% 6%	495
September	1st to 10th	Day	12% 13% 28% 28% 15% 4	450
	11th to 20th	Day	20% 8% 24% 29% 16% 3	450
	21st to end	Day	20% 12% 28% 24% 13% 3	450
October	1st to 10th	Day	18% 13% 28% 26% 14% 2	450
	11th to 20th	Day	19% 13% 29% 24% 13% 3	396
	21st to end	Day	29% 6% 31% 22% 10% 2	297
November	1st to 10th	Day	27% 12% 24% 27% 10%	248
	11th to 20th	Day	31% 12% 23% 21% 13%	180
	21st to end	Day	26% 6% 31% 21% 13% 4	180
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% 18% 33% 13% 2	736
	21st to end	Night	21% 5% 22% 35% 16% 1	792
February	1st to 10th	Night	22% 8% 24% 32% 11% 3	720
	11th to 20th	Night	18% 7% 23% 35% 14% 2	704
	21st to end	Night	18% 6% 20% 35% 17% 3	592
March	1st to 10th	Night	21% 8% 20% 31% 16% 3	720
	11th to 20th	Night	19% 9% 21% 34% 16% 2	720
	21st to end	Night	22% 9% 20% 31% 15% 3	792
April	1st to 10th	Night	18% 6% 18% 35% 19% 4	720
	11th to 20th	Night	13% 9% 23% 34% 17% 4	695
	21st to end	Night	14% 8% 25% 35% 16% 2	453
May	1st to 10th	Night	13% 8% 20% 37% 19% 3	360
	11th to 20th	Night	14% 4 18% 31% 27% 5%	354
	21st to end	Night	16% 4 17% 31% 27% 5%	389
June	1st to 10th	Night	16% 6% 19% 35% 20% 3	360
	11th to 20th	Night	16% 7% 21% 33% 21% 2	360
	21st to end	Night	8% 5% 20% 35% 28% 4	360
July	1st to 10th	Night	7% 5% 18% 37% 30% 4	350
	11th to 20th	Night	6% 7% 18% 42% 24% 4	348
	21st to end	Night	4 5% 15% 40% 31% 5%	394
August	1st to 10th	Night	4 4 15% 32% 39% 7%	358
	11th to 20th	Night	4 3 17% 35% 35% 7%	365
	21st to end	Night	8% 5% 16% 39% 27% 5%	792
September	1st to 10th	Night	6% 5% 17% 39% 26% 7%	720
	11th to 20th	Night	12% 4 13% 32% 30% 9%	720
	21st to end	Night	14% 5% 15% 38% 24% 5%	720
October	1st to 10th	Night	20% 6% 12% 33% 26% 2	720
	11th to 20th	Night	22% 6% 18% 33% 18% 3	720
	21st to end	Night	26% 6% 18% 33% 14% 3	792
November	1st to 10th	Night	31% 8% 22% 31% 8% 1	740
	11th to 20th	Night	29% 10% 19% 26% 15% 2	760
	21st to end	Night	28% 7% 18% 30% 13% 3	807
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 2024-01-11 10:31:58. Version ac07

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