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 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	17% 9% 4 7% 7% 4 5% 5% 5% 7% 31%	74733
	Night	17% 9% 3 6% 6% 3 5% 5% 5% 6% 34%	75029
Transparency	Day	51% 10% 14% 17% 7% 1	74421
	Night	50% 8% 14% 18% 8% 1	74859
Seeing	Day	19% 11% 25% 26% 16% 4	14324
	Night	21% 8% 18% 28% 21% 4	22643

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	28% 17% 5% 8% 6% 4 5% 5% 4 3 15%	1329
	11th to 20th	Day	22% 13% 4 7% 7% 3 7% 6% 5% 6% 21%	1281
	21st to end	Day	20% 15% 6% 10% 8% 5% 6% 4 5% 6% 15%	1614
February	1st to 10th	Day	22% 15% 5% 9% 8% 4 5% 4 4 6% 18%	1620
	11th to 20th	Day	19% 15% 4 8% 6% 3 5% 4 5% 7% 22%	1575
	21st to end	Day	16% 16% 5% 9% 8% 5% 5% 6% 6% 7% 18%	1332
March	1st to 10th	Day	17% 14% 4 9% 8% 4 6% 6% 6% 5% 19%	1887
	11th to 20th	Day	22% 13% 4 8% 8% 4 6% 4 5% 6% 20%	1980
	21st to end	Day	22% 13% 5% 8% 8% 5% 5% 5% 5% 5% 19%	2205
April	1st to 10th	Day	21% 12% 4 9% 8% 4 5% 6% 6% 6% 19%	2160
	11th to 20th	Day	19% 12% 4 8% 7% 4 7% 6% 7% 6% 21%	2245
	21st to end	Day	20% 11% 4 8% 7% 4 5% 5% 5% 6% 24%	2351
May	1st to 10th	Day	16% 7% 3 8% 7% 4 6% 6% 6% 7% 29%	2520
	11th to 20th	Day	14% 8% 4 8% 8% 4 6% 6% 5% 8% 29%	2477
	21st to end	Day	17% 9% 3 7% 7% 3 5% 5% 6% 6% 29%	2790
June	1st to 10th	Day	19% 8% 4 7% 6% 4 6% 6% 6% 7% 27%	2700
	11th to 20th	Day	16% 8% 4 9% 8% 5% 6% 6% 5% 7% 25%	2700
	21st to end	Day	12% 6% 3 6% 7% 3 5% 5% 6% 7% 38%	2700
July	1st to 10th	Day	10% 5% 2 6% 6% 4 4 5% 4 6% 46%	2700
	11th to 20th	Day	8% 3 3 4 5% 3 4 4 5% 8% 52%	2700
	21st to end	Day	7% 3 1 3 4 2 4 4 5% 7% 61%	2811
August	1st to 10th	Day	9% 4 2 5% 4 3 3 4 5% 7% 53%	2436
	11th to 20th	Day	7% 5% 2 4 5% 3 5% 5% 5% 8% 53%	2298
	21st to end	Day	9% 5% 2 6% 6% 4 6% 4 6% 8% 43%	2537
September	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 5% 8% 49%	2160
	11th to 20th	Day	14% 7% 3 5% 6% 3 4 4 5% 6% 42%	2121
	21st to end	Day	17% 6% 3 6% 6% 4 4 5% 5% 7% 39%	1918
October	1st to 10th	Day	19% 7% 3 4 5% 3 4 5% 5% 7% 37%	1800
	11th to 20th	Day	21% 9% 3 7% 7% 4 4 5% 5% 7% 27%	1800
	21st to end	Day	22% 11% 4 7% 6% 3 5% 5% 4 8% 27%	1849
November	1st to 10th	Day	25% 11% 5% 9% 8% 5% 6% 5% 4 5% 17%	1488
	11th to 20th	Day	22% 12% 4 9% 7% 4 5% 5% 4 5% 24%	1520
	21st to end	Day	24% 13% 4 7% 7% 3 5% 5% 5% 5% 22%	1520
December	1st to 10th	Day	18% 13% 7% 9% 6% 4 4 5% 4 6% 24%	1482
	11th to 20th	Day	26% 15% 4 8% 7% 4 6% 5% 5% 5% 16%	1685
	21st to end	Day	26% 17% 5% 9% 8% 4 4 4 5% 5% 14%	2442
January	1st to 10th	Night	26% 13% 4 7% 7% 4 6% 5% 5% 5% 18%	2836
	11th to 20th	Night	23% 13% 4 6% 7% 4 5% 5% 5% 6% 22%	2751
	21st to end	Night	21% 13% 4 8% 6% 4 5% 5% 5% 7% 23%	2737
February	1st to 10th	Night	22% 13% 3 8% 6% 4 6% 5% 6% 6% 22%	2340
	11th to 20th	Night	18% 14% 4 5% 5% 3 4 4 5% 6% 31%	2272
	21st to end	Night	17% 12% 5% 8% 6% 4 6% 3 5% 7% 28%	1924
March	1st to 10th	Night	17% 10% 3 6% 5% 4 5% 6% 7% 7% 30%	2068
	11th to 20th	Night	17% 10% 4 6% 7% 4 6% 6% 7% 8% 25%	1979
	21st to end	Night	15% 11% 6% 8% 7% 5% 6% 5% 5% 7% 25%	2150
April	1st to 10th	Night	17% 9% 3 7% 7% 5% 6% 5% 6% 7% 29%	1800
	11th to 20th	Night	15% 9% 4 5% 6% 3 5% 4 6% 8% 34%	1662
	21st to end	Night	15% 7% 3 7% 5% 3 5% 6% 5% 8% 34%	1608
May	1st to 10th	Night	11% 4 2 6% 6% 4 5% 5% 6% 6% 45%	1440
	11th to 20th	Night	11% 4 2 6% 5% 3 6% 7% 7% 9% 40%	1416
	21st to end	Night	11% 6% 3 6% 4 3 5% 4 7% 9% 42%	1481
June	1st to 10th	Night	12% 8% 3 7% 6% 3 5% 5% 4 7% 40%	1260
	11th to 20th	Night	10% 5% 3 5% 5% 3 5% 6% 6% 10% 42%	1260
	21st to end	Night	6% 3 2 4 4 3 5% 6% 7% 9% 51%	1260
July	1st to 10th	Night	6% 3 2 4 4 3 4 5% 4 6% 58%	1260
	11th to 20th	Night	4 2 2 2 4 2 4 3 4 7% 66%	1260
	21st to end	Night	3 1 1 2 2 2 2 3 5% 6% 72%	1505
August	1st to 10th	Night	5% 4 1 3 3 2 3 3 4 6% 65%	1501
	11th to 20th	Night	2 3 1 3 3 2 4 3 4 8% 67%	1602
	21st to end	Night	7% 4 2 4 4 3 4 4 5% 9% 54%	1818
September	1st to 10th	Night	5% 3 1 3 4 3 4 5% 4 7% 62%	1800
	11th to 20th	Night	13% 5% 2 5% 5% 3 5% 5% 6% 6% 47%	1836
	21st to end	Night	13% 5% 2 6% 5% 4 4 5% 5% 7% 44%	2041
October	1st to 10th	Night	14% 6% 3 5% 5% 3 4 4 5% 5% 45%	2160
	11th to 20th	Night	19% 7% 4 6% 5% 3 5% 5% 4 8% 34%	2160
	21st to end	Night	23% 8% 3 6% 6% 3 4 4 5% 5% 33%	2505
November	1st to 10th	Night	25% 11% 3 7% 7% 4 6% 5% 5% 5% 23%	2590
	11th to 20th	Night	23% 10% 4 6% 6% 4 5% 4 5% 6% 27%	2660
	21st to end	Night	22% 10% 4 8% 6% 4 5% 5% 4 6% 25%	2660
December	1st to 10th	Night	19% 14% 5% 8% 6% 3 6% 5% 4 5% 25%	2693
	11th to 20th	Night	25% 14% 5% 8% 7% 4 5% 5% 4 6% 17%	3520
	21st to end	Night	25% 15% 5% 9% 7% 4 5% 4 4 5% 17%	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	74% 7% 6% 8% 5%	1329
	11th to 20th	Day	66% 7% 13% 9% 3 1	1281
	21st to end	Day	71% 7% 9% 11% 2	1614
February	1st to 10th	Day	68% 8% 9% 10% 4	1620
	11th to 20th	Day	63% 8% 11% 14% 5% 1	1584
	21st to end	Day	62% 10% 14% 9% 4 2	1332
March	1st to 10th	Day	63% 10% 12% 10% 4 1	1887
	11th to 20th	Day	66% 7% 11% 11% 5% 1	1980
	21st to end	Day	65% 10% 11% 11% 2	2205
April	1st to 10th	Day	62% 11% 12% 12% 3	2160
	11th to 20th	Day	60% 10% 12% 11% 8% 1	2245
	21st to end	Day	57% 10% 12% 14% 6% 1	2351
May	1st to 10th	Day	51% 10% 15% 17% 5% 1	2520
	11th to 20th	Day	50% 10% 17% 18% 5%	2480
	21st to end	Day	50% 12% 17% 18% 3	2791
June	1st to 10th	Day	52% 11% 17% 14% 5% 2	2700
	11th to 20th	Day	53% 12% 19% 13% 3	2700
	21st to end	Day	40% 11% 20% 19% 9% 2	2700
July	1st to 10th	Day	37% 10% 14% 25% 12% 3	2626
	11th to 20th	Day	27% 11% 19% 30% 11% 1	2619
	21st to end	Day	19% 9% 18% 36% 15% 3	2655
August	1st to 10th	Day	28% 9% 18% 29% 13% 2	2419
	11th to 20th	Day	27% 10% 21% 26% 14% 2	2298
	21st to end	Day	34% 11% 20% 28% 6% 1	2537
September	1st to 10th	Day	31% 11% 17% 27% 11% 3	2160
	11th to 20th	Day	42% 8% 14% 20% 15% 2	2121
	21st to end	Day	44% 10% 15% 20% 10% 1	1918
October	1st to 10th	Day	47% 9% 15% 20% 9%	1800
	11th to 20th	Day	57% 9% 14% 16% 4	1800
	21st to end	Day	57% 7% 12% 15% 8%	1849
November	1st to 10th	Day	69% 8% 10% 9% 4 1	1488
	11th to 20th	Day	64% 7% 9% 12% 7% 1	1520
	21st to end	Day	64% 8% 9% 14% 5%	1520
December	1st to 10th	Day	63% 9% 9% 12% 5% 1	1482
	11th to 20th	Day	68% 10% 12% 7% 2	1688
	21st to end	Day	72% 7% 10% 7% 4	2442
January	1st to 10th	Night	69% 7% 8% 10% 5%	2836
	11th to 20th	Night	64% 9% 12% 10% 4 2	2751
	21st to end	Night	62% 8% 8% 16% 5% 1	2737
February	1st to 10th	Night	63% 8% 12% 11% 5% 1	2340
	11th to 20th	Night	55% 9% 13% 16% 8%	2274
	21st to end	Night	57% 7% 11% 16% 7% 2	1924
March	1st to 10th	Night	53% 9% 12% 18% 8%	2068
	11th to 20th	Night	55% 11% 12% 15% 6% 1	1980
	21st to end	Night	58% 10% 13% 16% 3	2150
April	1st to 10th	Night	54% 7% 17% 17% 6%	1800
	11th to 20th	Night	48% 9% 13% 19% 9% 1	1662
	21st to end	Night	47% 9% 16% 17% 9% 1	1608
May	1st to 10th	Night	38% 9% 19% 22% 11% 1	1440
	11th to 20th	Night	38% 11% 18% 25% 8%	1416
	21st to end	Night	38% 10% 22% 24% 5%	1481
June	1st to 10th	Night	43% 7% 19% 22% 6% 3	1260
	11th to 20th	Night	35% 10% 26% 22% 6%	1260
	21st to end	Night	27% 10% 26% 22% 11% 4	1260
July	1st to 10th	Night	27% 8% 18% 29% 12% 5%	1225
	11th to 20th	Night	20% 6% 23% 32% 17% 2	1218
	21st to end	Night	12% 6% 17% 36% 26% 4	1421
August	1st to 10th	Night	22% 6% 18% 33% 18% 3	1493
	11th to 20th	Night	17% 8% 22% 30% 16% 7%	1602
	21st to end	Night	25% 8% 21% 34% 10% 1	1818
September	1st to 10th	Night	22% 9% 19% 30% 17% 5%	1800
	11th to 20th	Night	35% 8% 15% 19% 17% 5%	1836
	21st to end	Night	38% 8% 17% 22% 12% 3	2041
October	1st to 10th	Night	41% 7% 16% 22% 13% 1	2160
	11th to 20th	Night	49% 8% 16% 18% 9%	2160
	21st to end	Night	52% 8% 12% 17% 11%	2505
November	1st to 10th	Night	63% 8% 10% 14% 5% 1	2590
	11th to 20th	Night	59% 9% 11% 12% 8% 1	2660
	21st to end	Night	60% 8% 12% 13% 8%	2660
December	1st to 10th	Night	62% 8% 12% 13% 5% 1	2693
	11th to 20th	Night	68% 10% 12% 7% 3	3516
	21st to end	Night	68% 7% 10% 10% 5%	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	36% 21% 16% 14% 8% 6%	180
	11th to 20th	Day	40% 15% 14% 13% 14% 3	166
	21st to end	Day	30% 16% 23% 18% 11% 2	244
February	1st to 10th	Day	29% 22% 25% 17% 6% 1	340
	11th to 20th	Day	28% 19% 31% 15% 6% 1	335
	21st to end	Day	28% 21% 26% 17% 7% 1	280
March	1st to 10th	Day	26% 17% 24% 23% 7% 2	418
	11th to 20th	Day	25% 19% 23% 22% 9% 2	420
	21st to end	Day	27% 17% 25% 23% 7% 2	462
April	1st to 10th	Day	27% 15% 28% 19% 9% 1	420
	11th to 20th	Day	22% 15% 30% 20% 11% 2	412
	21st to end	Day	20% 16% 31% 20% 11% 2	420
May	1st to 10th	Day	13% 11% 29% 30% 14% 2	423
	11th to 20th	Day	17% 8% 23% 28% 21% 4	448
	21st to end	Day	13% 10% 25% 32% 15% 5%	552
June	1st to 10th	Day	17% 10% 30% 27% 14% 3	630
	11th to 20th	Day	13% 12% 33% 25% 15% 2	630
	21st to end	Day	10% 9% 24% 32% 22% 3	630
July	1st to 10th	Day	11% 6% 19% 36% 25% 4	621
	11th to 20th	Day	6% 5% 24% 37% 23% 6%	618
	21st to end	Day	5% 2 20% 36% 29% 7%	557
August	1st to 10th	Day	9% 4 17% 31% 30% 9%	447
	11th to 20th	Day	5% 5% 19% 36% 28% 9%	437
	21st to end	Day	9% 6% 25% 36% 18% 6%	495
September	1st to 10th	Day	10% 7% 28% 36% 16% 3	450
	11th to 20th	Day	20% 8% 22% 27% 19% 4	450
	21st to end	Day	17% 13% 25% 25% 16% 3	450
October	1st to 10th	Day	22% 7% 27% 22% 19% 3	450
	11th to 20th	Day	27% 12% 20% 23% 15% 4	450
	21st to end	Day	25% 19% 26% 16% 9% 4	391
November	1st to 10th	Day	31% 14% 26% 21% 7% 1	180
	11th to 20th	Day	28% 13% 20% 21% 11% 7%	180
	21st to end	Day	32% 14% 22% 14% 9% 9%	180
December	1st to 10th	Day	33% 12% 19% 13% 15% 8%	180
	11th to 20th	Day	32% 12% 19% 22% 13% 2	180
	21st to end	Day	33% 14% 25% 19% 8% 2	198
January	1st to 10th	Night	33% 15% 21% 18% 11% 3	850
	11th to 20th	Night	31% 12% 16% 23% 17% 2	817
	21st to end	Night	29% 8% 17% 27% 15% 3	808
February	1st to 10th	Night	29% 14% 21% 21% 13% 3	720
	11th to 20th	Night	24% 12% 20% 29% 15% 1	704
	21st to end	Night	24% 13% 19% 28% 13% 4	592
March	1st to 10th	Night	20% 13% 24% 25% 16% 2	720
	11th to 20th	Night	23% 12% 23% 26% 15% 1	720
	21st to end	Night	23% 10% 20% 29% 14% 4	792
April	1st to 10th	Night	20% 10% 22% 27% 17% 4	720
	11th to 20th	Night	18% 9% 24% 29% 18% 2	594
	21st to end	Night	18% 5% 21% 38% 16% 2	528
May	1st to 10th	Night	12% 8% 14% 39% 24% 3	360
	11th to 20th	Night	13% 5% 10% 35% 30% 7%	354
	21st to end	Night	12% 3 19% 37% 27% 3	389
June	1st to 10th	Night	16% 4 19% 32% 27% 2	360
	11th to 20th	Night	10% 6% 22% 36% 23% 3	360
	21st to end	Night	6% 4 18% 36% 30% 6%	360
July	1st to 10th	Night	6% 2 14% 28% 46% 5%	350
	11th to 20th	Night	3 3 13% 33% 41% 7%	348
	21st to end	Night	3 1 9% 28% 48% 11%	394
August	1st to 10th	Night	7% 1 7% 26% 44% 15%	427
	11th to 20th	Night	3 1 7% 26% 52% 11%	534
	21st to end	Night	8% 3 14% 31% 38% 7%	630
September	1st to 10th	Night	7% 2 12% 37% 34% 8%	720
	11th to 20th	Night	14% 4 13% 24% 31% 14%	720
	21st to end	Night	16% 4 18% 29% 28% 6%	720
October	1st to 10th	Night	18% 4 14% 32% 27% 6%	720
	11th to 20th	Night	21% 9% 15% 27% 20% 7%	720
	21st to end	Night	27% 9% 21% 24% 15% 4	792
November	1st to 10th	Night	32% 9% 21% 27% 10% 1	740
	11th to 20th	Night	29% 12% 21% 24% 11% 2	760
	21st to end	Night	25% 10% 24% 23% 14% 4	760
December	1st to 10th	Night	26% 10% 20% 26% 17% 2	775
	11th to 20th	Night	32% 10% 22% 25% 9% 1	850
	21st to end	Night	34% 13% 22% 23% 7% 1	935

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