Starr Butte
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starr Butte clear sky chart from 2005-11-06 to 2024-01-13.

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	13% 9% 3 6% 5% 3 4 4 5% 6% 41%	74568
	Night	14% 10% 3 6% 5% 3 4 4 5% 6% 38%	75159
Transparency	Day	44% 6% 13% 19% 13% 4	74271
	Night	46% 6% 13% 18% 12% 4	74974
Seeing	Day	17% 11% 23% 25% 18% 6%	13665
	Night	20% 7% 17% 32% 20% 3	22996

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	26% 16% 6% 7% 5% 3 5% 4 5% 6% 17%	1519
	11th to 20th	Day	25% 14% 7% 8% 6% 3 4 5% 4 6% 19%	1456
	21st to end	Day	19% 17% 5% 9% 6% 3 4 4 4 5% 23%	1584
February	1st to 10th	Day	20% 15% 5% 7% 5% 3 6% 5% 5% 6% 23%	1440
	11th to 20th	Day	18% 17% 6% 8% 5% 2 4 4 6% 6% 24%	1695
	21st to end	Day	18% 19% 7% 9% 5% 4 4 3 5% 5% 22%	1480
March	1st to 10th	Day	17% 16% 5% 8% 7% 4 4 5% 5% 6% 24%	1800
	11th to 20th	Day	15% 15% 5% 6% 6% 4 6% 5% 5% 6% 26%	1860
	21st to end	Day	19% 17% 5% 8% 8% 4 5% 4 4 5% 21%	2335
April	1st to 10th	Day	15% 13% 4 7% 7% 4 6% 6% 6% 8% 25%	2160
	11th to 20th	Day	14% 10% 4 7% 6% 4 4 4 5% 7% 33%	2199
	21st to end	Day	14% 8% 4 6% 6% 4 5% 5% 6% 7% 36%	2340
May	1st to 10th	Day	9% 7% 3 5% 5% 3 5% 5% 7% 8% 42%	2440
	11th to 20th	Day	13% 6% 3 7% 6% 3 5% 5% 7% 9% 36%	2477
	21st to end	Day	12% 7% 3 7% 7% 4 5% 6% 6% 7% 36%	2727
June	1st to 10th	Day	16% 8% 3 6% 5% 4 4 4 5% 7% 38%	2700
	11th to 20th	Day	15% 7% 3 5% 5% 3 5% 5% 6% 8% 39%	2700
	21st to end	Day	7% 3 2 3 4 3 4 5% 5% 7% 57%	2700
July	1st to 10th	Day	4 3 1 2 3 2 3 5% 4 6% 67%	2532
	11th to 20th	Day	3 2 1 2 3 1 3 3 4 5% 71%	2520
	21st to end	Day	1 1 1 2 2 1 2 2 3 4 81%	2741
August	1st to 10th	Day	3 2 1 2 3 2 3 3 4 6% 70%	2507
	11th to 20th	Day	5% 2 1 2 2 2 3 3 4 7% 68%	2350
	21st to end	Day	5% 4 1 4 4 2 3 4 6% 6% 61%	2435
September	1st to 10th	Day	6% 3 1 3 2 2 3 4 3 7% 66%	2160
	11th to 20th	Day	8% 5% 2 5% 5% 3 5% 4 5% 8% 51%	2048
	21st to end	Day	9% 6% 2 5% 5% 4 4 5% 5% 7% 48%	1980
October	1st to 10th	Day	13% 6% 3 6% 4 2 4 4 5% 6% 46%	1927
	11th to 20th	Day	16% 7% 2 4 5% 3 4 4 5% 6% 44%	1686
	21st to end	Day	19% 10% 4 6% 5% 3 4 5% 4 6% 35%	1782
November	1st to 10th	Day	24% 12% 4 6% 5% 3 4 4 4 8% 26%	1674
	11th to 20th	Day	22% 15% 4 7% 5% 3 5% 4 5% 5% 24%	1644
	21st to end	Day	18% 15% 5% 9% 6% 3 4 4 5% 6% 23%	1388
December	1st to 10th	Day	19% 15% 6% 11% 6% 3 4 4 5% 6% 20%	1330
	11th to 20th	Day	22% 21% 6% 10% 8% 3 5% 4 5% 3 13%	1682
	21st to end	Day	19% 18% 5% 9% 6% 4 4 5% 5% 7% 19%	2570
January	1st to 10th	Night	26% 17% 6% 7% 6% 3 4 4 4 5% 18%	2646
	11th to 20th	Night	24% 14% 4 8% 6% 4 4 4 4 6% 22%	2548
	21st to end	Night	18% 14% 5% 8% 6% 3 5% 5% 5% 5% 25%	2772
February	1st to 10th	Night	19% 18% 4 7% 5% 3 4 4 4 6% 26%	2520
	11th to 20th	Night	17% 14% 4 8% 6% 3 4 4 5% 5% 30%	2149
	21st to end	Night	14% 15% 3 7% 6% 4 6% 5% 6% 6% 28%	1776
March	1st to 10th	Night	18% 10% 5% 7% 6% 3 5% 5% 6% 6% 29%	2160
	11th to 20th	Night	15% 11% 4 6% 6% 4 5% 5% 6% 6% 32%	2098
	21st to end	Night	17% 9% 3 8% 7% 4 7% 5% 6% 9% 26%	2016
April	1st to 10th	Night	11% 10% 4 6% 6% 3 6% 5% 8% 9% 32%	1800
	11th to 20th	Night	13% 9% 4 7% 6% 3 5% 4 4 5% 39%	1711
	21st to end	Night	10% 9% 2 7% 6% 3 5% 5% 6% 7% 39%	1620
May	1st to 10th	Night	9% 6% 3 5% 6% 4 5% 4 4 8% 45%	1517
	11th to 20th	Night	9% 6% 3 5% 5% 3 5% 5% 6% 9% 44%	1416
	21st to end	Night	12% 6% 2 5% 5% 4 5% 6% 6% 9% 41%	1547
June	1st to 10th	Night	13% 6% 2 6% 5% 3 5% 5% 4 9% 42%	1260
	11th to 20th	Night	11% 6% 2 6% 5% 3 4 6% 7% 7% 42%	1260
	21st to end	Night	7% 3 2 4 4 3 5% 5% 5% 7% 54%	1260
July	1st to 10th	Night	3 1 1 3 2 1 3 3 4 7% 70%	1423
	11th to 20th	Night	3 2 1 2 3 1 3 2 4 6% 73%	1440
	21st to end	Night	1 1 1 2 2 1 3 3 4 5% 77%	1576
August	1st to 10th	Night	3 2 1 3 3 1 2 3 6% 7% 69%	1432
	11th to 20th	Night	3 2 1 2 3 2 3 4 4 6% 70%	1549
	21st to end	Night	4 3 1 3 3 2 3 4 5% 5% 66%	1917
September	1st to 10th	Night	5% 3 2 3 3 2 3 3 4 6% 67%	1800
	11th to 20th	Night	6% 5% 3 4 4 3 5% 4 5% 6% 56%	1912
	21st to end	Night	10% 4 2 5% 5% 2 3 4 5% 8% 53%	1980
October	1st to 10th	Night	12% 5% 2 5% 5% 2 3 5% 5% 6% 48%	2029
	11th to 20th	Night	14% 7% 3 5% 4 3 4 4 4 7% 44%	2272
	21st to end	Night	17% 7% 3 6% 4 3 4 5% 4 6% 41%	2574
November	1st to 10th	Night	22% 11% 3 7% 5% 3 4 4 5% 7% 29%	2405
	11th to 20th	Night	21% 12% 5% 7% 7% 3 4 4 4 5% 28%	2531
	21st to end	Night	21% 11% 5% 7% 5% 3 5% 5% 5% 6% 28%	2769
December	1st to 10th	Night	19% 13% 5% 8% 5% 3 4 5% 5% 6% 28%	2850
	11th to 20th	Night	22% 18% 6% 9% 6% 4 5% 4 5% 5% 15%	3510
	21st to end	Night	19% 14% 5% 9% 6% 4 6% 6% 6% 7% 19%	5114

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	70% 8% 11% 8% 2 1	1519
	11th to 20th	Day	67% 7% 13% 9% 2 1	1456
	21st to end	Day	64% 7% 14% 11% 2 1	1584
February	1st to 10th	Day	61% 9% 14% 10% 5%	1440
	11th to 20th	Day	62% 8% 13% 11% 6%	1705
	21st to end	Day	66% 6% 8% 15% 5%	1480
March	1st to 10th	Day	61% 7% 13% 14% 5% 1	1800
	11th to 20th	Day	60% 7% 13% 16% 5%	1860
	21st to end	Day	68% 6% 15% 9% 2	2335
April	1st to 10th	Day	57% 7% 16% 15% 4	2160
	11th to 20th	Day	51% 7% 15% 16% 10% 1	2199
	21st to end	Day	48% 7% 15% 20% 8% 1	2340
May	1st to 10th	Day	37% 7% 18% 27% 10%	2440
	11th to 20th	Day	44% 7% 17% 23% 9%	2480
	21st to end	Day	45% 8% 17% 23% 6% 1	2728
June	1st to 10th	Day	46% 6% 14% 23% 11%	2700
	11th to 20th	Day	41% 7% 17% 23% 10% 2	2700
	21st to end	Day	26% 6% 15% 30% 20% 3	2700
July	1st to 10th	Day	19% 5% 11% 27% 26% 13%	2463
	11th to 20th	Day	16% 4 10% 21% 35% 14%	2445
	21st to end	Day	10% 3 5% 21% 44% 17%	2589
August	1st to 10th	Day	16% 4 8% 24% 34% 14%	2490
	11th to 20th	Day	17% 4 9% 27% 34% 8%	2350
	21st to end	Day	23% 4 10% 29% 27% 7%	2435
September	1st to 10th	Day	20% 4 11% 29% 24% 10%	2160
	11th to 20th	Day	31% 6% 12% 23% 23% 5%	2048
	21st to end	Day	35% 7% 18% 24% 12% 4	1980
October	1st to 10th	Day	40% 5% 14% 28% 13% 1	1927
	11th to 20th	Day	42% 6% 15% 27% 10% 1	1686
	21st to end	Day	52% 7% 14% 19% 8%	1782
November	1st to 10th	Day	60% 6% 14% 15% 3 1	1674
	11th to 20th	Day	63% 7% 16% 9% 3 1	1644
	21st to end	Day	61% 8% 13% 14% 3 1	1388
December	1st to 10th	Day	65% 9% 12% 11% 3 1	1330
	11th to 20th	Day	74% 7% 11% 7% 1	1684
	21st to end	Day	63% 7% 16% 10% 5%	2570
January	1st to 10th	Night	69% 6% 11% 10% 2 1	2646
	11th to 20th	Night	63% 7% 14% 13% 2 1	2548
	21st to end	Night	60% 8% 15% 13% 4 1	2772
February	1st to 10th	Night	61% 7% 13% 13% 5% 1	2520
	11th to 20th	Night	57% 7% 16% 13% 7%	2150
	21st to end	Night	55% 7% 13% 17% 7% 1	1776
March	1st to 10th	Night	55% 7% 15% 15% 8% 1	2160
	11th to 20th	Night	52% 7% 11% 20% 9%	2099
	21st to end	Night	55% 9% 15% 16% 5%	2016
April	1st to 10th	Night	47% 7% 15% 22% 8%	1800
	11th to 20th	Night	46% 5% 11% 23% 13% 1	1711
	21st to end	Night	42% 7% 14% 23% 13% 2	1620
May	1st to 10th	Night	36% 6% 13% 30% 14% 1	1517
	11th to 20th	Night	36% 6% 15% 27% 14% 1	1416
	21st to end	Night	38% 8% 17% 27% 9% 1	1547
June	1st to 10th	Night	39% 7% 10% 22% 20% 2	1260
	11th to 20th	Night	38% 7% 13% 26% 13% 4	1260
	21st to end	Night	28% 6% 10% 27% 22% 7%	1260
July	1st to 10th	Night	14% 3 8% 22% 34% 19%	1383
	11th to 20th	Night	15% 3 8% 18% 32% 24%	1392
	21st to end	Night	10% 3 5% 14% 39% 29%	1488
August	1st to 10th	Night	16% 3 7% 16% 38% 20%	1424
	11th to 20th	Night	16% 4 8% 22% 36% 15%	1549
	21st to end	Night	19% 4 10% 23% 31% 13%	1917
September	1st to 10th	Night	20% 3 10% 27% 26% 14%	1800
	11th to 20th	Night	29% 4 11% 23% 25% 8%	1912
	21st to end	Night	30% 5% 15% 28% 15% 7%	1980
October	1st to 10th	Night	37% 4 11% 30% 16% 2	2029
	11th to 20th	Night	40% 6% 13% 24% 15% 2	2272
	21st to end	Night	45% 7% 13% 21% 13% 2	2574
November	1st to 10th	Night	55% 7% 16% 16% 5% 1	2405
	11th to 20th	Night	59% 6% 15% 13% 6% 1	2531
	21st to end	Night	56% 7% 15% 16% 6%	2769
December	1st to 10th	Night	58% 7% 16% 15% 4	2850
	11th to 20th	Night	70% 8% 12% 9% 1	3507
	21st to end	Night	61% 8% 17% 11% 4	5114

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% 18% 15% 16% 16% 4	180
	11th to 20th	Day	32% 13% 19% 17% 10% 8%	164
	21st to end	Day	28% 13% 30% 16% 11% 1	176
February	1st to 10th	Day	28% 18% 30% 16% 8% 1	160
	11th to 20th	Day	33% 16% 23% 18% 8% 1	387
	21st to end	Day	26% 15% 26% 22% 10% 1	346
March	1st to 10th	Day	27% 19% 24% 19% 8% 3	420
	11th to 20th	Day	23% 14% 21% 25% 15% 2	420
	21st to end	Day	28% 19% 23% 21% 8% 2	462
April	1st to 10th	Day	22% 14% 30% 22% 8% 4	420
	11th to 20th	Day	20% 19% 27% 18% 11% 5%	412
	21st to end	Day	18% 14% 27% 24% 15% 3	420
May	1st to 10th	Day	13% 10% 23% 35% 16% 2	423
	11th to 20th	Day	15% 9% 24% 23% 19% 10%	448
	21st to end	Day	12% 10% 19% 27% 21% 10%	489
June	1st to 10th	Day	17% 9% 20% 27% 20% 8%	630
	11th to 20th	Day	15% 13% 28% 24% 15% 4	630
	21st to end	Day	7% 6% 23% 27% 29% 9%	630
July	1st to 10th	Day	5% 3 22% 35% 30% 5%	458
	11th to 20th	Day	4 7% 23% 38% 23% 5%	444
	21st to end	Day	2 6% 21% 34% 30% 9%	487
August	1st to 10th	Day	6% 4 20% 32% 28% 10%	447
	11th to 20th	Day	4 2 19% 31% 29% 15%	437
	21st to end	Day	8% 6% 26% 26% 23% 11%	495
September	1st to 10th	Day	9% 7% 24% 27% 23% 10%	450
	11th to 20th	Day	10% 9% 24% 26% 21% 10%	450
	21st to end	Day	12% 11% 23% 28% 20% 6%	450
October	1st to 10th	Day	15% 9% 27% 26% 17% 6%	450
	11th to 20th	Day	20% 14% 20% 22% 17% 7%	336
	21st to end	Day	27% 14% 21% 22% 12% 4	297
November	1st to 10th	Day	30% 18% 17% 25% 10% 1	270
	11th to 20th	Day	35% 14% 13% 20% 15% 3	239
	21st to end	Day	27% 9% 27% 19% 17% 2	180
December	1st to 10th	Day	29% 14% 21% 18% 10% 8%	180
	11th to 20th	Day	41% 13% 22% 16% 8% 2	180
	21st to end	Day	33% 11% 22% 21% 11% 3	198
January	1st to 10th	Night	37% 11% 17% 22% 11% 2	760
	11th to 20th	Night	30% 9% 15% 27% 15% 3	728
	21st to end	Night	25% 7% 18% 30% 18% 2	792
February	1st to 10th	Night	30% 11% 20% 27% 10% 2	720
	11th to 20th	Night	26% 10% 18% 29% 15% 2	704
	21st to end	Night	22% 11% 20% 28% 16% 3	592
March	1st to 10th	Night	22% 10% 19% 30% 18% 1	720
	11th to 20th	Night	20% 12% 18% 30% 19% 1	720
	21st to end	Night	22% 7% 24% 33% 12% 2	792
April	1st to 10th	Night	18% 8% 20% 33% 18% 2	720
	11th to 20th	Night	16% 10% 25% 32% 14% 2	643
	21st to end	Night	15% 7% 24% 33% 19% 2	540
May	1st to 10th	Night	10% 7% 23% 31% 26% 3	437
	11th to 20th	Night	12% 6% 14% 30% 29% 9%	354
	21st to end	Night	15% 5% 15% 31% 28% 6%	389
June	1st to 10th	Night	15% 4 16% 33% 25% 6%	360
	11th to 20th	Night	13% 8% 24% 31% 21% 3	360
	21st to end	Night	9% 2 14% 39% 31% 5%	360
July	1st to 10th	Night	2 1 13% 42% 38% 4	350
	11th to 20th	Night	3 3 14% 48% 30% 1	348
	21st to end	Night	1 1 13% 41% 38% 6%	394
August	1st to 10th	Night	4 3 11% 37% 40% 6%	358
	11th to 20th	Night	3 2 7% 37% 41% 10%	481
	21st to end	Night	5% 4 12% 37% 36% 5%	729
September	1st to 10th	Night	6% 3 15% 40% 29% 7%	720
	11th to 20th	Night	8% 4 15% 31% 33% 10%	720
	21st to end	Night	11% 5% 19% 33% 27% 6%	720
October	1st to 10th	Night	15% 6% 13% 37% 26% 3	720
	11th to 20th	Night	16% 8% 17% 30% 25% 4	720
	21st to end	Night	21% 8% 20% 30% 16% 5%	792
November	1st to 10th	Night	29% 8% 16% 33% 13% 1	740
	11th to 20th	Night	30% 9% 19% 28% 13% 1	760
	21st to end	Night	27% 9% 17% 28% 16% 3	869
December	1st to 10th	Night	26% 8% 19% 31% 16% 1	950
	11th to 20th	Night	34% 9% 16% 29% 11%	950
	21st to end	Night	36% 11% 17% 26% 10% 1	984

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