Duncan
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Duncan clear sky chart from 2005-11-06 to 2026-01-17.

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	15% 10% 3 8% 5% 5% 5% 4 5% 6% 35%	131485
	Night	16% 10% 3 7% 5% 5% 4 4 5% 6% 36%	128863
Transparency	Day	47% 8% 16% 19% 8% 1	131191
	Night	48% 8% 15% 20% 8% 1	128734
Seeing	Day	18% 11% 24% 26% 16% 4	16935
	Night	21% 8% 18% 28% 20% 4	25196

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% 18% 4 9% 6% 5% 4 3 4 5% 14%	3249
	11th to 20th	Day	16% 10% 3 6% 5% 4 5% 5% 6% 6% 34%	3201
	21st to end	Day	26% 15% 3 8% 5% 5% 4 3 3 4 25%	4076
February	1st to 10th	Day	20% 16% 5% 10% 7% 6% 5% 4 4 6% 17%	1800
	11th to 20th	Day	20% 17% 4 10% 5% 4 5% 3 5% 6% 22%	1755
	21st to end	Day	17% 17% 4 10% 8% 6% 6% 4 6% 6% 16%	1485
March	1st to 10th	Day	17% 16% 5% 11% 7% 5% 5% 4 6% 6% 18%	4761
	11th to 20th	Day	16% 13% 3 9% 6% 6% 5% 3 5% 6% 26%	5000
	21st to end	Day	21% 14% 4 11% 6% 6% 5% 4 5% 6% 16%	5564
April	1st to 10th	Day	20% 13% 4 10% 7% 5% 5% 5% 6% 6% 19%	2400
	11th to 20th	Day	17% 13% 3 8% 7% 6% 5% 4 6% 6% 24%	2499
	21st to end	Day	20% 11% 3 9% 6% 4 5% 4 5% 6% 26%	2613
May	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 4 5% 6% 40%	6320
	11th to 20th	Day	13% 10% 4 8% 5% 6% 6% 4 7% 9% 29%	6277
	21st to end	Day	16% 9% 3 8% 5% 5% 6% 5% 7% 8% 28%	7082
June	1st to 10th	Day	18% 9% 3 8% 5% 5% 6% 5% 6% 7% 29%	3000
	11th to 20th	Day	15% 9% 3 11% 7% 7% 7% 4 6% 7% 25%	3000
	21st to end	Day	12% 7% 3 7% 7% 5% 4 5% 6% 7% 37%	3000
July	1st to 10th	Day	6% 3 1 4 3 3 3 3 5% 6% 62%	6780
	11th to 20th	Day	4 2 1 3 3 2 2 2 4 5% 72%	6780
	21st to end	Day	5% 3 1 4 3 3 3 3 5% 8% 62%	7042
August	1st to 10th	Day	5% 3 1 4 2 2 3 3 4 6% 67%	4401
	11th to 20th	Day	10% 6% 3 7% 5% 5% 5% 5% 6% 8% 39%	4156
	21st to end	Day	11% 5% 2 6% 5% 6% 6% 3 6% 7% 44%	4586
September	1st to 10th	Day	10% 4 2 6% 4 4 4 3 6% 8% 50%	2400
	11th to 20th	Day	14% 7% 3 6% 5% 4 4 3 5% 6% 42%	2356
	21st to end	Day	16% 7% 3 7% 5% 4 5% 3 5% 7% 38%	2130
October	1st to 10th	Day	15% 8% 3 8% 6% 6% 6% 5% 7% 7% 30%	3280
	11th to 20th	Day	23% 11% 3 9% 6% 4 4 3 4 6% 27%	3261
	21st to end	Day	18% 11% 4 11% 6% 6% 6% 4 4 7% 23%	3301
November	1st to 10th	Day	25% 12% 4 11% 6% 7% 6% 4 4 5% 16%	1648
	11th to 20th	Day	22% 13% 4 10% 5% 6% 4 4 5% 5% 23%	1680
	21st to end	Day	24% 15% 4 9% 5% 5% 4 4 5% 5% 21%	1680
December	1st to 10th	Day	14% 13% 5% 10% 6% 5% 4 3 5% 6% 30%	2632
	11th to 20th	Day	29% 15% 4 8% 5% 5% 5% 4 4 5% 15%	2715
	21st to end	Day	28% 18% 5% 11% 6% 5% 5% 3 4 4 12%	3575
January	1st to 10th	Night	25% 18% 5% 9% 6% 6% 5% 4 5% 4 14%	6916
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 5% 7% 39%	6829
	21st to end	Night	25% 11% 3 7% 5% 5% 4 3 4 4 28%	6869
February	1st to 10th	Night	20% 14% 3 9% 6% 5% 5% 4 6% 5% 22%	2600
	11th to 20th	Night	19% 15% 4 6% 4 4 3 3 5% 6% 30%	2532
	21st to end	Night	18% 14% 4 9% 5% 5% 5% 2 5% 6% 27%	2145
March	1st to 10th	Night	14% 12% 4 9% 6% 5% 5% 4 6% 7% 26%	5128
	11th to 20th	Night	12% 10% 3 7% 5% 5% 5% 5% 7% 8% 33%	4999
	21st to end	Night	21% 14% 5% 9% 5% 5% 5% 3 5% 6% 23%	5410
April	1st to 10th	Night	16% 10% 3 8% 7% 6% 5% 4 6% 7% 28%	2000
	11th to 20th	Night	13% 9% 4 6% 5% 5% 5% 3 6% 8% 36%	1848
	21st to end	Night	15% 8% 3 8% 5% 5% 5% 5% 6% 8% 34%	1786
May	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 4 5% 53%	3640
	11th to 20th	Night	10% 6% 3 7% 4 4 5% 4 6% 7% 44%	3616
	21st to end	Night	12% 7% 3 8% 6% 5% 5% 4 7% 8% 34%	3789
June	1st to 10th	Night	11% 8% 3 7% 5% 4 5% 4 5% 7% 42%	1400
	11th to 20th	Night	9% 5% 3 6% 4 5% 4 5% 7% 10% 43%	1400
	21st to end	Night	6% 3 2 6% 4 3 5% 5% 7% 9% 50%	1400
July	1st to 10th	Night	4 2 1 3 2 3 3 2 4 5% 70%	3180
	11th to 20th	Night	2 1 1 2 2 1 2 1 3 5% 81%	3180
	21st to end	Night	4 2 1 3 2 2 2 2 5% 5% 71%	3786
August	1st to 10th	Night	3 2 1 2 2 2 2 2 3 6% 75%	2738
	11th to 20th	Night	9% 5% 2 7% 4 4 4 3 4 7% 49%	2902
	21st to end	Night	10% 3 2 4 3 3 4 3 5% 8% 55%	3311
September	1st to 10th	Night	5% 3 1 4 3 3 4 3 5% 8% 61%	2000
	11th to 20th	Night	13% 5% 2 5% 5% 4 4 3 5% 6% 46%	2040
	21st to end	Night	13% 6% 2 7% 4 5% 4 4 5% 7% 44%	2269
October	1st to 10th	Night	11% 7% 2 7% 5% 5% 5% 3 5% 6% 44%	3920
	11th to 20th	Night	19% 7% 4 7% 5% 6% 6% 4 5% 7% 31%	3897
	21st to end	Night	21% 10% 4 9% 5% 4 4 4 5% 4 30%	4487
November	1st to 10th	Night	24% 12% 2 8% 6% 5% 6% 4 5% 5% 22%	2870
	11th to 20th	Night	23% 12% 3 7% 5% 5% 5% 3 5% 5% 25%	2940
	21st to end	Night	22% 11% 4 9% 6% 5% 5% 4 4 6% 24%	2940
December	1st to 10th	Night	15% 13% 4 9% 6% 5% 5% 4 6% 6% 28%	4763
	11th to 20th	Night	30% 17% 4 9% 5% 5% 4 3 4 5% 14%	5710
	21st to end	Night	25% 14% 4 11% 6% 6% 5% 4 5% 5% 16%	7623

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	71% 8% 6% 12% 4	3249
	11th to 20th	Day	49% 9% 19% 19% 4 1	3222
	21st to end	Day	65% 7% 8% 15% 5%	4076
February	1st to 10th	Day	68% 8% 9% 10% 4 1	1800
	11th to 20th	Day	64% 7% 10% 14% 5% 1	1764
	21st to end	Day	64% 9% 13% 8% 4 1	1485
March	1st to 10th	Day	62% 9% 11% 12% 5%	4761
	11th to 20th	Day	56% 6% 11% 21% 5% 1	5000
	21st to end	Day	65% 9% 13% 10% 2	5564
April	1st to 10th	Day	61% 12% 13% 12% 2	2400
	11th to 20th	Day	56% 10% 12% 12% 8% 2	2499
	21st to end	Day	55% 10% 13% 15% 6% 1	2613
May	1st to 10th	Day	42% 6% 18% 22% 11% 1	6320
	11th to 20th	Day	47% 10% 22% 16% 4	6280
	21st to end	Day	48% 11% 21% 16% 3 1	7083
June	1st to 10th	Day	49% 10% 17% 16% 6% 2	3000
	11th to 20th	Day	52% 12% 19% 14% 4	3000
	21st to end	Day	41% 11% 20% 19% 8% 1	3000
July	1st to 10th	Day	21% 6% 21% 28% 16% 8%	6706
	11th to 20th	Day	14% 5% 20% 36% 23% 2	6699
	21st to end	Day	20% 7% 33% 27% 11% 2	6845
August	1st to 10th	Day	18% 6% 14% 43% 18% 1	4384
	11th to 20th	Day	37% 10% 25% 18% 8% 1	4195
	21st to end	Day	37% 9% 20% 25% 9% 1	4585
September	1st to 10th	Day	29% 11% 19% 26% 12% 3	2400
	11th to 20th	Day	41% 8% 15% 21% 14% 1	2356
	21st to end	Day	45% 10% 15% 19% 10% 1	2130
October	1st to 10th	Day	49% 9% 16% 20% 6%	3280
	11th to 20th	Day	59% 7% 12% 16% 6%	3261
	21st to end	Day	59% 8% 12% 14% 6%	3301
November	1st to 10th	Day	69% 8% 10% 8% 4 1	1648
	11th to 20th	Day	64% 7% 9% 12% 6% 1	1680
	21st to end	Day	65% 8% 10% 13% 4	1680
December	1st to 10th	Day	56% 8% 11% 19% 6%	2632
	11th to 20th	Day	70% 9% 10% 9% 2	2718
	21st to end	Day	75% 8% 9% 6% 3	3575
January	1st to 10th	Night	71% 8% 7% 11% 3	6916
	11th to 20th	Night	45% 7% 19% 22% 6% 1	6870
	21st to end	Night	59% 7% 10% 17% 7%	6869
February	1st to 10th	Night	63% 8% 12% 11% 6% 1	2600
	11th to 20th	Night	55% 8% 12% 15% 8% 1	2534
	21st to end	Night	58% 7% 11% 15% 7% 1	2145
March	1st to 10th	Night	52% 8% 14% 17% 7%	5128
	11th to 20th	Night	46% 10% 13% 25% 6%	5000
	21st to end	Night	61% 9% 12% 15% 3	5410
April	1st to 10th	Night	54% 8% 17% 16% 5%	2000
	11th to 20th	Night	46% 9% 14% 20% 10% 2	1848
	21st to end	Night	46% 9% 17% 17% 9% 1	1786
May	1st to 10th	Night	35% 5% 13% 29% 17% 1	3637
	11th to 20th	Night	36% 8% 24% 26% 6%	3616
	21st to end	Night	44% 8% 21% 24% 3	3789
June	1st to 10th	Night	41% 7% 20% 22% 7% 3	1400
	11th to 20th	Night	34% 10% 25% 24% 7%	1400
	21st to end	Night	28% 10% 26% 22% 11% 3	1400
July	1st to 10th	Night	18% 5% 18% 34% 14% 10%	3145
	11th to 20th	Night	10% 3 21% 34% 29% 3	3138
	21st to end	Night	16% 4 29% 32% 17% 2	3678
August	1st to 10th	Night	14% 4 14% 40% 26% 2	2730
	11th to 20th	Night	35% 8% 26% 18% 9% 4	2929
	21st to end	Night	27% 7% 21% 32% 12% 1	3311
September	1st to 10th	Night	20% 9% 21% 29% 16% 5%	2000
	11th to 20th	Night	36% 8% 15% 19% 17% 5%	2040
	21st to end	Night	38% 8% 18% 22% 11% 3	2269
October	1st to 10th	Night	41% 7% 20% 23% 9% 1	3920
	11th to 20th	Night	52% 6% 14% 21% 7%	3897
	21st to end	Night	54% 7% 14% 16% 9%	4487
November	1st to 10th	Night	64% 8% 10% 13% 4 1	2870
	11th to 20th	Night	60% 8% 11% 12% 7% 1	2940
	21st to end	Night	61% 8% 12% 12% 7%	2940
December	1st to 10th	Night	55% 10% 12% 19% 5%	4763
	11th to 20th	Night	73% 8% 11% 6% 2	5706
	21st to end	Night	67% 9% 11% 10% 3	7623

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	36% 18% 17% 17% 7% 5%	220
	11th to 20th	Day	35% 14% 14% 20% 15% 2	212
	21st to end	Day	30% 14% 23% 19% 10% 4	295
February	1st to 10th	Day	27% 21% 24% 20% 7% 1	400
	11th to 20th	Day	29% 18% 29% 17% 6% 1	395
	21st to end	Day	30% 21% 25% 17% 6% 1	331
March	1st to 10th	Day	25% 18% 24% 22% 8% 2	498
	11th to 20th	Day	25% 19% 22% 23% 10% 2	500
	21st to end	Day	26% 16% 25% 24% 7% 1	550
April	1st to 10th	Day	25% 15% 29% 20% 10% 1	500
	11th to 20th	Day	20% 15% 32% 20% 10% 2	492
	21st to end	Day	21% 15% 29% 21% 12% 2	500
May	1st to 10th	Day	13% 11% 31% 29% 14% 2	503
	11th to 20th	Day	17% 10% 24% 26% 19% 4	528
	21st to end	Day	14% 12% 25% 30% 15% 4	648
June	1st to 10th	Day	16% 10% 29% 28% 15% 3	730
	11th to 20th	Day	13% 13% 32% 25% 15% 3	730
	21st to end	Day	10% 9% 24% 33% 22% 3	730
July	1st to 10th	Day	10% 6% 20% 37% 23% 5%	721
	11th to 20th	Day	5% 4 23% 38% 23% 6%	718
	21st to end	Day	5% 3 21% 36% 28% 8%	652
August	1st to 10th	Day	7% 4 17% 32% 30% 9%	527
	11th to 20th	Day	5% 5% 20% 34% 27% 9%	517
	21st to end	Day	9% 6% 23% 35% 20% 7%	583
September	1st to 10th	Day	9% 6% 25% 35% 20% 6%	530
	11th to 20th	Day	19% 7% 21% 28% 20% 5%	530
	21st to end	Day	17% 13% 27% 25% 15% 3	530
October	1st to 10th	Day	21% 8% 28% 23% 18% 3	530
	11th to 20th	Day	27% 12% 21% 23% 14% 4	530
	21st to end	Day	25% 19% 27% 17% 9% 3	463
November	1st to 10th	Day	29% 15% 27% 22% 7%	220
	11th to 20th	Day	28% 14% 20% 20% 12% 6%	220
	21st to end	Day	32% 15% 20% 15% 9% 8%	220
December	1st to 10th	Day	33% 12% 17% 14% 16% 8%	220
	11th to 20th	Day	34% 16% 16% 20% 11% 2	220
	21st to end	Day	32% 16% 24% 19% 7% 2	242
January	1st to 10th	Night	33% 14% 20% 19% 10% 3	950
	11th to 20th	Night	29% 11% 17% 26% 15% 2	930
	21st to end	Night	30% 8% 17% 27% 14% 3	900
February	1st to 10th	Night	28% 13% 21% 22% 13% 3	800
	11th to 20th	Night	26% 11% 19% 29% 14% 1	784
	21st to end	Night	25% 14% 20% 26% 12% 3	660
March	1st to 10th	Night	20% 13% 23% 26% 16% 2	800
	11th to 20th	Night	22% 12% 24% 26% 16% 1	800
	21st to end	Night	24% 10% 20% 28% 14% 3	880
April	1st to 10th	Night	19% 11% 22% 28% 16% 4	800
	11th to 20th	Night	16% 9% 25% 31% 18% 2	660
	21st to end	Night	18% 5% 22% 38% 16% 2	586
May	1st to 10th	Night	11% 7% 16% 40% 23% 3	400
	11th to 20th	Night	13% 6% 12% 36% 27% 6%	394
	21st to end	Night	12% 4 19% 37% 25% 3	433
June	1st to 10th	Night	15% 5% 19% 34% 26% 2	400
	11th to 20th	Night	9% 6% 22% 37% 23% 3	400
	21st to end	Night	7% 5% 17% 36% 30% 6%	400
July	1st to 10th	Night	6% 2 14% 30% 44% 4	390
	11th to 20th	Night	3 3 12% 33% 42% 7%	388
	21st to end	Night	4 2 9% 28% 47% 10%	438
August	1st to 10th	Night	7% 1 7% 26% 45% 14%	475
	11th to 20th	Night	4 1 8% 26% 50% 11%	594
	21st to end	Night	8% 2 13% 32% 39% 6%	700
September	1st to 10th	Night	6% 2 12% 35% 37% 8%	800
	11th to 20th	Night	14% 4 12% 24% 32% 13%	800
	21st to end	Night	16% 4 18% 30% 27% 5%	800
October	1st to 10th	Night	17% 4 14% 34% 26% 5%	800
	11th to 20th	Night	21% 9% 16% 29% 19% 6%	800
	21st to end	Night	26% 9% 21% 26% 14% 4	880
November	1st to 10th	Night	31% 9% 22% 28% 10% 1	820
	11th to 20th	Night	29% 12% 21% 26% 11% 2	840
	21st to end	Night	26% 10% 24% 24% 14% 3	840
December	1st to 10th	Night	26% 10% 19% 26% 17% 2	859
	11th to 20th	Night	34% 11% 22% 24% 9% 1	950
	21st to end	Night	33% 13% 23% 24% 6% 1	1045

Page generated at 2026-01-13 22:14:25. Version ac07

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