Diener Foundation Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Diener Foundation Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	20% 8% 3 6% 4 4 4 3 5% 7% 36%	154126
	Night	25% 9% 3 7% 4 4 4 3 4 6% 32%	146686
Transparency	Day	46% 10% 16% 18% 8% 1	153829
	Night	52% 9% 14% 17% 7% 1	146565
Seeing	Day	26% 9% 21% 25% 16% 3	18110
	Night	32% 7% 16% 25% 17% 3	26085

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	40% 12% 3 7% 4 4 5% 3 4 4 13%	4125
	11th to 20th	Day	21% 11% 3 7% 4 4 4 3 5% 6% 32%	4119
	21st to end	Day	41% 10% 3 6% 4 3 3 2 3 5% 22%	5099
February	1st to 10th	Day	32% 12% 4 7% 6% 4 5% 4 5% 5% 17%	1890
	11th to 20th	Day	27% 11% 3 8% 4 5% 4 3 5% 7% 23%	1845
	21st to end	Day	25% 11% 3 9% 5% 6% 5% 4 4 6% 22%	1557
March	1st to 10th	Day	26% 15% 4 9% 5% 5% 5% 3 5% 5% 19%	4814
	11th to 20th	Day	23% 13% 3 8% 5% 4 4 3 5% 5% 25%	5110
	21st to end	Day	27% 13% 4 9% 6% 5% 4 3 4 5% 19%	5691
April	1st to 10th	Day	25% 11% 4 8% 4 4 5% 4 7% 6% 23%	2520
	11th to 20th	Day	20% 10% 3 7% 5% 4 5% 4 5% 6% 30%	2641
	21st to end	Day	21% 10% 3 6% 5% 4 4 3 5% 6% 33%	2764
May	1st to 10th	Day	10% 7% 3 6% 4 4 4 4 5% 6% 47%	8195
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 7% 11% 32%	8160
	21st to end	Day	18% 7% 2 6% 4 4 5% 4 7% 8% 35%	9369
June	1st to 10th	Day	20% 7% 3 6% 5% 4 4 4 6% 8% 33%	3150
	11th to 20th	Day	16% 8% 2 7% 5% 6% 5% 5% 6% 7% 33%	3150
	21st to end	Day	14% 7% 2 7% 5% 5% 4 4 6% 7% 40%	3150
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 4 6% 54%	8820
	11th to 20th	Day	6% 4 1 3 2 2 3 2 4 6% 67%	8820
	21st to end	Day	10% 4 2 5% 3 3 3 3 5% 7% 54%	9254
August	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 5% 70%	6313
	11th to 20th	Day	9% 7% 2 6% 3 3 4 4 6% 9% 47%	5923
	21st to end	Day	15% 6% 2 5% 3 4 4 4 5% 8% 43%	6488
September	1st to 10th	Day	11% 6% 2 6% 3 4 4 3 6% 7% 48%	2520
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 4 7% 41%	2478
	21st to end	Day	24% 5% 2 6% 3 4 5% 3 6% 8% 34%	2237
October	1st to 10th	Day	24% 9% 2 7% 4 4 5% 4 6% 7% 28%	3380
	11th to 20th	Day	33% 7% 2 6% 4 4 4 3 4 6% 25%	3361
	21st to end	Day	30% 10% 3 7% 5% 4 4 3 5% 7% 20%	3345
November	1st to 10th	Day	38% 10% 3 7% 4 3 5% 4 4 4 17%	1648
	11th to 20th	Day	35% 12% 2 6% 3 4 4 3 5% 5% 20%	1680
	21st to end	Day	37% 11% 3 9% 4 3 3 3 3 5% 19%	1680
December	1st to 10th	Day	25% 13% 3 6% 5% 4 5% 4 5% 6% 24%	2540
	11th to 20th	Day	43% 14% 4 8% 4 4 4 2 3 3 12%	2715
	21st to end	Day	42% 14% 3 9% 5% 4 4 2 3 4 10%	3575
January	1st to 10th	Night	38% 12% 4 9% 5% 4 4 3 4 4 14%	8770
	11th to 20th	Night	24% 8% 3 6% 4 4 3 3 5% 6% 35%	8779
	21st to end	Night	36% 8% 2 6% 3 4 4 3 3 5% 26%	9085
February	1st to 10th	Night	31% 12% 4 7% 4 4 4 4 4 5% 20%	2730
	11th to 20th	Night	24% 12% 3 8% 5% 4 3 3 4 4 28%	2662
	21st to end	Night	29% 13% 3 7% 4 3 4 3 5% 5% 24%	2249
March	1st to 10th	Night	27% 12% 3 7% 4 4 4 3 4 7% 24%	5286
	11th to 20th	Night	24% 12% 3 7% 4 4 4 3 5% 5% 28%	5109
	21st to end	Night	34% 13% 4 8% 5% 4 4 3 4 5% 18%	5525
April	1st to 10th	Night	24% 11% 3 8% 5% 5% 6% 4 5% 7% 23%	2100
	11th to 20th	Night	18% 11% 4 8% 5% 5% 4 3 5% 7% 30%	1927
	21st to end	Night	21% 10% 2 7% 4 4 4 4 5% 7% 32%	1855
May	1st to 10th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	4724
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 9% 34%	4700
	21st to end	Night	17% 9% 3 8% 5% 5% 4 3 5% 8% 32%	4786
June	1st to 10th	Night	20% 8% 4 9% 5% 4 4 3 4 5% 34%	1470
	11th to 20th	Night	15% 11% 2 7% 5% 6% 4 3 7% 7% 34%	1470
	21st to end	Night	13% 6% 2 8% 3 4 5% 4 5% 8% 41%	1470
July	1st to 10th	Night	10% 4 2 6% 4 4 5% 3 4 7% 51%	4140
	11th to 20th	Night	4 2 1 3 2 2 3 2 4 6% 71%	4140
	21st to end	Night	9% 4 2 5% 4 3 4 4 5% 7% 53%	4811
August	1st to 10th	Night	4 2 1 3 2 1 2 2 4 6% 74%	3839
	11th to 20th	Night	10% 4 2 5% 4 3 4 4 6% 8% 50%	4142
	21st to end	Night	15% 5% 2 6% 4 4 5% 3 5% 6% 44%	4663
September	1st to 10th	Night	8% 5% 2 4 3 4 4 3 5% 6% 56%	2100
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 5% 6% 43%	2140
	21st to end	Night	23% 8% 2 6% 4 3 4 3 4 6% 38%	2383
October	1st to 10th	Night	24% 9% 3 6% 4 4 4 3 5% 7% 30%	4040
	11th to 20th	Night	30% 9% 3 6% 4 4 4 3 4 6% 29%	4017
	21st to end	Night	31% 9% 2 7% 4 4 4 3 4 6% 27%	4632
November	1st to 10th	Night	38% 10% 3 7% 5% 4 3 3 4 5% 18%	2870
	11th to 20th	Night	35% 10% 3 6% 5% 3 4 2 4 4 22%	2940
	21st to end	Night	35% 11% 3 7% 4 3 4 2 4 5% 21%	2940
December	1st to 10th	Night	27% 11% 3 6% 4 5% 5% 4 5% 6% 24%	4859
	11th to 20th	Night	42% 14% 3 7% 4 3 4 3 4 4 12%	5710
	21st to end	Night	40% 14% 4 8% 4 4 3 2 4 4 13%	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	72% 10% 7% 8% 2	4125
	11th to 20th	Day	47% 10% 13% 20% 9%	4140
	21st to end	Day	68% 5% 7% 15% 4	5100
February	1st to 10th	Day	67% 10% 9% 7% 3 2	1890
	11th to 20th	Day	59% 10% 12% 13% 4 1	1854
	21st to end	Day	61% 12% 11% 9% 5% 1	1557
March	1st to 10th	Day	63% 12% 11% 11% 4	4814
	11th to 20th	Day	59% 9% 11% 18% 3	5110
	21st to end	Day	66% 10% 14% 8% 2	5691
April	1st to 10th	Day	57% 13% 15% 11% 3	2520
	11th to 20th	Day	52% 10% 13% 14% 9% 1	2641
	21st to end	Day	52% 10% 14% 16% 7% 1	2764
May	1st to 10th	Day	36% 9% 18% 23% 13% 1	8195
	11th to 20th	Day	42% 14% 22% 18% 4	8163
	21st to end	Day	43% 13% 21% 18% 5%	9370
June	1st to 10th	Day	48% 11% 19% 14% 6% 2	3150
	11th to 20th	Day	46% 14% 19% 15% 5%	3150
	21st to end	Day	41% 12% 19% 17% 8% 1	3150
July	1st to 10th	Day	30% 11% 18% 22% 15% 5%	8746
	11th to 20th	Day	19% 8% 18% 35% 19% 2	8739
	21st to end	Day	28% 10% 26% 23% 11% 1	9053
August	1st to 10th	Day	16% 8% 14% 42% 17% 3	6296
	11th to 20th	Day	31% 13% 29% 20% 6% 1	5962
	21st to end	Day	39% 11% 20% 21% 8% 1	6487
September	1st to 10th	Day	34% 11% 18% 23% 11% 3	2520
	11th to 20th	Day	42% 12% 15% 18% 12% 2	2478
	21st to end	Day	47% 10% 17% 16% 9% 1	2237
October	1st to 10th	Day	52% 12% 17% 14% 5%	3380
	11th to 20th	Day	58% 8% 12% 15% 6%	3361
	21st to end	Day	62% 11% 12% 11% 3	3345
November	1st to 10th	Day	68% 8% 11% 8% 4 1	1648
	11th to 20th	Day	66% 10% 10% 9% 4 2	1680
	21st to end	Day	69% 7% 9% 10% 5%	1680
December	1st to 10th	Day	57% 9% 12% 16% 4 1	2540
	11th to 20th	Day	76% 7% 9% 5% 2	2718
	21st to end	Day	78% 10% 6% 5% 1 1	3575
January	1st to 10th	Night	72% 7% 8% 10% 3	8770
	11th to 20th	Night	45% 10% 16% 18% 11%	8820
	21st to end	Night	62% 7% 9% 20% 3	9085
February	1st to 10th	Night	65% 8% 10% 10% 6% 2	2730
	11th to 20th	Night	59% 10% 11% 12% 7% 1	2664
	21st to end	Night	61% 8% 12% 12% 6% 1	2249
March	1st to 10th	Night	58% 10% 14% 12% 6%	5286
	11th to 20th	Night	54% 10% 12% 20% 4	5110
	21st to end	Night	68% 9% 12% 10% 2	5525
April	1st to 10th	Night	57% 10% 14% 16% 2	2100
	11th to 20th	Night	52% 8% 13% 16% 9% 2	1927
	21st to end	Night	50% 9% 15% 16% 9%	1855
May	1st to 10th	Night	35% 7% 16% 28% 14%	4721
	11th to 20th	Night	41% 12% 20% 23% 5%	4700
	21st to end	Night	47% 9% 21% 18% 4	4790
June	1st to 10th	Night	50% 8% 17% 17% 6% 3	1470
	11th to 20th	Night	45% 11% 17% 20% 6%	1470
	21st to end	Night	39% 11% 19% 19% 9% 3	1470
July	1st to 10th	Night	29% 9% 18% 25% 15% 4	4105
	11th to 20th	Night	15% 6% 18% 36% 22% 3	4098
	21st to end	Night	27% 8% 27% 23% 13% 2	4707
August	1st to 10th	Night	13% 5% 19% 37% 23% 4	3831
	11th to 20th	Night	29% 11% 32% 20% 6% 2	4169
	21st to end	Night	38% 11% 19% 24% 8% 1	4663
September	1st to 10th	Night	28% 9% 18% 28% 13% 4	2100
	11th to 20th	Night	40% 9% 14% 18% 15% 4	2140
	21st to end	Night	47% 9% 16% 19% 8% 2	2383
October	1st to 10th	Night	51% 10% 18% 14% 7%	4040
	11th to 20th	Night	56% 7% 12% 19% 6%	4017
	21st to end	Night	57% 9% 13% 15% 5%	4632
November	1st to 10th	Night	69% 8% 10% 9% 4 1	2870
	11th to 20th	Night	65% 9% 11% 10% 5% 2	2940
	21st to end	Night	66% 8% 9% 10% 6% 1	2940
December	1st to 10th	Night	59% 8% 14% 15% 4	4859
	11th to 20th	Night	75% 8% 8% 6% 3	5706
	21st to end	Night	73% 9% 8% 7% 2	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	44% 9% 21% 15% 7% 4	220
	11th to 20th	Day	36% 16% 16% 18% 12% 2	220
	21st to end	Day	46% 8% 20% 15% 8% 2	295
February	1st to 10th	Day	38% 11% 24% 17% 9% 1	430
	11th to 20th	Day	36% 9% 25% 20% 9% 1	425
	21st to end	Day	33% 17% 26% 15% 9% 1	355
March	1st to 10th	Day	35% 12% 23% 20% 8% 1	532
	11th to 20th	Day	36% 13% 19% 20% 12% 1	540
	21st to end	Day	36% 12% 25% 19% 7% 1	594
April	1st to 10th	Day	33% 11% 24% 21% 9% 1	540
	11th to 20th	Day	25% 12% 28% 20% 14% 1	532
	21st to end	Day	25% 12% 26% 21% 14% 2	540
May	1st to 10th	Day	18% 11% 22% 31% 18% 1	543
	11th to 20th	Day	20% 8% 22% 29% 18% 3	568
	21st to end	Day	19% 9% 25% 28% 16% 3	746
June	1st to 10th	Day	23% 9% 24% 25% 17% 1	780
	11th to 20th	Day	18% 10% 26% 29% 16% 2	780
	21st to end	Day	18% 6% 21% 32% 20% 3	780
July	1st to 10th	Day	15% 6% 19% 32% 24% 3	771
	11th to 20th	Day	8% 5% 21% 39% 23% 4	768
	21st to end	Day	9% 4 15% 34% 31% 6%	740
August	1st to 10th	Day	10% 5% 17% 30% 30% 9%	567
	11th to 20th	Day	12% 4 16% 33% 29% 6%	557
	21st to end	Day	16% 7% 18% 36% 20% 4	627
September	1st to 10th	Day	16% 7% 24% 30% 19% 3	570
	11th to 20th	Day	23% 8% 18% 27% 18% 5%	570
	21st to end	Day	30% 9% 24% 21% 14% 3	570
October	1st to 10th	Day	30% 8% 20% 22% 16% 4	570
	11th to 20th	Day	35% 10% 18% 19% 14% 4	570
	21st to end	Day	39% 14% 18% 19% 7% 3	468
November	1st to 10th	Day	42% 13% 23% 16% 6%	220
	11th to 20th	Day	42% 10% 14% 19% 10% 5%	220
	21st to end	Day	48% 10% 17% 11% 8% 5%	220
December	1st to 10th	Day	43% 7% 21% 11% 12% 5%	220
	11th to 20th	Day	49% 10% 17% 18% 5% 1	220
	21st to end	Day	46% 10% 27% 14% 3	242
January	1st to 10th	Night	49% 10% 18% 15% 8% 1	950
	11th to 20th	Night	41% 7% 18% 22% 12%	950
	21st to end	Night	41% 7% 19% 22% 10%	968
February	1st to 10th	Night	37% 8% 23% 22% 10%	840
	11th to 20th	Night	33% 8% 21% 28% 9% 1	824
	21st to end	Night	37% 12% 19% 20% 10% 2	692
March	1st to 10th	Night	35% 10% 20% 22% 11% 2	840
	11th to 20th	Night	41% 8% 16% 23% 11% 1	840
	21st to end	Night	37% 9% 19% 20% 13% 2	924
April	1st to 10th	Night	31% 9% 16% 27% 13% 4	840
	11th to 20th	Night	25% 10% 22% 27% 15% 1	679
	21st to end	Night	27% 6% 21% 30% 15% 2	595
May	1st to 10th	Night	18% 5% 17% 33% 23% 4	420
	11th to 20th	Night	21% 6% 10% 30% 28% 5%	414
	21st to end	Night	20% 4 15% 31% 25% 4	455
June	1st to 10th	Night	24% 5% 13% 32% 23% 3	420
	11th to 20th	Night	22% 5% 12% 32% 23% 6%	420
	21st to end	Night	16% 4 10% 36% 29% 5%	420
July	1st to 10th	Night	13% 2 10% 26% 41% 9%	410
	11th to 20th	Night	9% 3 9% 35% 37% 8%	408
	21st to end	Night	8% 2 5% 27% 46% 11%	460
August	1st to 10th	Night	10% 2 6% 24% 45% 14%	470
	11th to 20th	Night	9% 2 6% 25% 48% 10%	624
	21st to end	Night	15% 4 10% 29% 37% 6%	725
September	1st to 10th	Night	12% 3 11% 35% 32% 8%	840
	11th to 20th	Night	21% 5% 13% 23% 30% 9%	840
	21st to end	Night	30% 4 15% 27% 21% 3	840
October	1st to 10th	Night	26% 5% 16% 30% 21% 2	840
	11th to 20th	Night	36% 6% 14% 26% 16% 2	840
	21st to end	Night	40% 8% 19% 24% 8% 2	912
November	1st to 10th	Night	44% 8% 20% 23% 5%	820
	11th to 20th	Night	42% 8% 21% 19% 8% 1	840
	21st to end	Night	44% 7% 20% 21% 7% 1	840
December	1st to 10th	Night	38% 8% 18% 26% 11% 1	890
	11th to 20th	Night	50% 10% 19% 17% 4	950
	21st to end	Night	45% 13% 23% 15% 4	1045

Page generated at 2026-10-26 01:37:49. Version ac07

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