Dordt College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dordt College Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	12% 8% 3 8% 5% 4 4 4 5% 7% 41%	131747
	Night	10% 7% 3 7% 5% 4 4 3 5% 7% 44%	129130
Transparency	Day	41% 5% 13% 25% 14% 2	135335
	Night	39% 5% 13% 26% 16% 2	133608
Seeing	Day	15% 10% 31% 29% 13% 3	17618
	Night	14% 9% 35% 33% 9% 1	27652

By Month

Percentage Cloud Cover SE (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	8% 14% 5% 16% 8% 7% 5% 3 4 6% 22%	4782
	11th to 20th	Day	4 10% 4 11% 8% 7% 8% 4 7% 8% 30%	5310
	21st to end	Day	4 10% 5% 13% 7% 6% 5% 4 5% 6% 34%	5812
February	1st to 10th	Day	9% 12% 5% 13% 8% 7% 6% 3 5% 7% 25%	1890
	11th to 20th	Day	8% 11% 6% 10% 5% 5% 5% 4 6% 7% 34%	1799
	21st to end	Day	12% 11% 3 7% 7% 6% 4 3 6% 6% 36%	1641
March	1st to 10th	Day	12% 8% 4 7% 4 3 4 3 4 5% 46%	4837
	11th to 20th	Day	9% 9% 2 6% 3 3 3 3 5% 7% 50%	4980
	21st to end	Day	23% 11% 4 10% 5% 4 4 4 5% 5% 24%	5462
April	1st to 10th	Day	16% 8% 3 8% 5% 3 4 3 5% 6% 37%	2279
	11th to 20th	Day	18% 10% 3 6% 4 3 4 3 5% 6% 38%	2532
	21st to end	Day	22% 8% 2 6% 4 3 4 3 4 6% 37%	2600
May	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 45%	5862
	11th to 20th	Day	15% 6% 2 5% 3 3 3 2 4 6% 50%	5899
	21st to end	Day	18% 8% 2 7% 5% 4 4 4 5% 6% 36%	6935
June	1st to 10th	Day	10% 5% 2 6% 4 3 4 4 6% 8% 48%	2800
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 7% 43%	2975
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 9% 47%	3000
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	6800
	11th to 20th	Day	8% 5% 2 6% 4 4 4 3 5% 7% 53%	6044
	21st to end	Day	5% 4 2 6% 4 4 5% 4 7% 10% 49%	6422
August	1st to 10th	Day	9% 6% 2 5% 4 5% 5% 4 6% 9% 45%	4234
	11th to 20th	Day	16% 7% 2 7% 4 4 4 4 6% 8% 37%	4171
	21st to end	Day	9% 5% 2 6% 4 4 4 3 5% 7% 52%	4644
September	1st to 10th	Day	13% 5% 1 5% 2 3 3 3 5% 7% 52%	2350
	11th to 20th	Day	10% 5% 2 5% 3 4 3 4 5% 7% 53%	2200
	21st to end	Day	10% 4 2 5% 3 4 4 3 5% 6% 54%	2200
October	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 5% 5% 49%	2036
	11th to 20th	Day	14% 7% 2 4 2 3 3 3 4 6% 52%	2000
	21st to end	Day	16% 8% 2 6% 4 3 3 3 4 5% 46%	2165
November	1st to 10th	Day	12% 6% 3 5% 4 3 4 4 6% 7% 45%	1853
	11th to 20th	Day	9% 9% 3 11% 4 6% 4 4 5% 6% 40%	1711
	21st to end	Day	13% 10% 4 7% 4 3 4 3 6% 8% 39%	1680
December	1st to 10th	Day	5% 6% 4 10% 7% 6% 4 3 5% 7% 42%	2690
	11th to 20th	Day	10% 12% 4 11% 7% 7% 6% 4 5% 6% 27%	3088
	21st to end	Day	17% 10% 4 16% 6% 5% 4 3 4 7% 24%	4064
January	1st to 10th	Night	7% 12% 4 13% 9% 6% 5% 4 5% 6% 29%	7875
	11th to 20th	Night	3 8% 3 9% 7% 6% 7% 5% 8% 8% 36%	7680
	21st to end	Night	4 9% 4 10% 6% 6% 4 4 6% 7% 40%	8402
February	1st to 10th	Night	8% 11% 5% 9% 6% 4 5% 4 5% 5% 36%	2730
	11th to 20th	Night	9% 12% 4 8% 5% 4 4 3 4 6% 41%	2599
	21st to end	Night	9% 8% 3 9% 4 4 4 3 5% 6% 43%	2005
March	1st to 10th	Night	13% 7% 2 5% 3 2 3 2 4 5% 54%	5020
	11th to 20th	Night	10% 6% 3 5% 3 3 4 3 6% 8% 48%	4980
	21st to end	Night	20% 11% 3 8% 5% 4 3 3 5% 6% 32%	5467
April	1st to 10th	Night	15% 8% 3 6% 5% 5% 5% 3 5% 6% 40%	2120
	11th to 20th	Night	15% 8% 3 6% 5% 4 3 4 5% 5% 42%	1815
	21st to end	Night	18% 8% 2 6% 3 3 4 4 4 6% 41%	1800
May	1st to 10th	Night	13% 5% 2 4 3 3 2 3 5% 7% 53%	4053
	11th to 20th	Night	12% 6% 2 5% 3 2 3 3 5% 7% 53%	4012
	21st to end	Night	18% 7% 2 5% 4 3 4 3 5% 6% 43%	3942
June	1st to 10th	Night	8% 5% 1 5% 4 3 3 3 6% 8% 55%	1600
	11th to 20th	Night	8% 5% 2 5% 2 3 5% 4 6% 9% 51%	1418
	21st to end	Night	7% 3 2 4 4 4 3 3 6% 8% 55%	1400
July	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 6% 9% 51%	3178
	11th to 20th	Night	8% 4 2 4 3 3 3 2 5% 8% 58%	3925
	21st to end	Night	6% 4 1 4 3 4 4 4 6% 10% 54%	4449
August	1st to 10th	Night	7% 6% 2 6% 4 4 5% 4 5% 9% 49%	2926
	11th to 20th	Night	10% 5% 1 5% 3 3 3 3 5% 8% 52%	2888
	21st to end	Night	7% 4 2 5% 4 4 4 3 5% 8% 55%	3263
September	1st to 10th	Night	9% 4 2 5% 3 3 3 3 5% 6% 59%	2046
	11th to 20th	Night	9% 4 2 4 3 3 4 3 6% 6% 56%	2200
	21st to end	Night	8% 5% 1 4 3 3 3 2 4 6% 59%	2200
October	1st to 10th	Night	14% 5% 2 5% 3 2 3 3 5% 5% 53%	2343
	11th to 20th	Night	14% 7% 2 5% 3 3 3 2 4 6% 52%	2400
	21st to end	Night	14% 8% 2 6% 3 3 4 3 4 5% 48%	2672
November	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 6% 6% 51%	2665
	11th to 20th	Night	10% 7% 3 8% 4 4 4 4 5% 7% 46%	2908
	21st to end	Night	10% 8% 3 7% 4 3 3 3 4 7% 47%	2940
December	1st to 10th	Night	5% 7% 4 10% 5% 5% 5% 4 5% 6% 43%	4720
	11th to 20th	Night	10% 8% 4 10% 8% 7% 5% 4 5% 7% 34%	5339
	21st to end	Night	15% 13% 5% 14% 6% 4 4 4 5% 5% 26%	7150

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	63% 4 12% 13% 6% 2	4783
	11th to 20th	Day	51% 3 9% 21% 15% 1	5310
	21st to end	Day	50% 4 12% 22% 9% 2	5812
February	1st to 10th	Day	59% 5% 13% 14% 8% 1	1890
	11th to 20th	Day	48% 6% 13% 21% 11% 1	1808
	21st to end	Day	48% 6% 11% 21% 12% 2	1641
March	1st to 10th	Day	41% 3 11% 30% 13% 2	4837
	11th to 20th	Day	34% 4 10% 32% 17% 3	4980
	21st to end	Day	61% 6% 12% 16% 4 1	5464
April	1st to 10th	Day	46% 6% 13% 24% 11% 1	2279
	11th to 20th	Day	47% 4 12% 22% 13% 1	2532
	21st to end	Day	49% 5% 10% 23% 12% 1	2600
May	1st to 10th	Day	36% 6% 10% 27% 20% 1	5900
	11th to 20th	Day	35% 5% 11% 23% 19% 7%	5903
	21st to end	Day	48% 5% 10% 22% 15% 1	6949
June	1st to 10th	Day	33% 7% 16% 27% 14% 2	2800
	11th to 20th	Day	38% 7% 14% 24% 14% 3	2975
	21st to end	Day	32% 8% 15% 29% 14% 3	3000
July	1st to 10th	Day	35% 7% 17% 29% 11% 1	6726
	11th to 20th	Day	31% 6% 12% 31% 17% 2	5970
	21st to end	Day	29% 7% 18% 28% 16% 1	6240
August	1st to 10th	Day	35% 7% 16% 28% 14% 1	4220
	11th to 20th	Day	44% 6% 14% 27% 9% 1	4210
	21st to end	Day	32% 6% 15% 28% 16% 3	4643
September	1st to 10th	Day	33% 6% 11% 28% 20% 2	2350
	11th to 20th	Day	31% 6% 14% 32% 15% 2	2200
	21st to end	Day	30% 6% 10% 30% 22% 2	2200
October	1st to 10th	Day	24% 3 7% 30% 32% 3	3309
	11th to 20th	Day	29% 4 16% 25% 22% 4	3240
	21st to end	Day	38% 4 12% 27% 17% 3	3481
November	1st to 10th	Day	37% 6% 15% 25% 15% 2	1853
	11th to 20th	Day	44% 6% 14% 22% 13% 1	1711
	21st to end	Day	43% 5% 15% 23% 13% 1	1680
December	1st to 10th	Day	41% 5% 15% 29% 10% 1	2690
	11th to 20th	Day	57% 5% 15% 17% 6%	3085
	21st to end	Day	60% 4 13% 16% 6%	4064
January	1st to 10th	Night	55% 5% 14% 18% 8% 1	7876
	11th to 20th	Night	42% 4 15% 20% 17% 2	7680
	21st to end	Night	43% 4 13% 25% 14%	8402
February	1st to 10th	Night	48% 5% 12% 23% 10% 1	2730
	11th to 20th	Night	45% 5% 12% 23% 12% 3	2600
	21st to end	Night	40% 5% 13% 23% 15% 3	2005
March	1st to 10th	Night	34% 3 8% 34% 20% 1	5020
	11th to 20th	Night	33% 4 9% 30% 20% 3	4980
	21st to end	Night	52% 7% 14% 20% 6% 2	5467
April	1st to 10th	Night	43% 6% 12% 23% 14% 1	2120
	11th to 20th	Night	42% 5% 11% 22% 18% 2	1815
	21st to end	Night	45% 5% 12% 22% 14% 2	1800
May	1st to 10th	Night	32% 4 11% 28% 24% 1	4080
	11th to 20th	Night	32% 5% 11% 22% 21% 10%	4013
	21st to end	Night	41% 6% 9% 28% 15% 1	3950
June	1st to 10th	Night	27% 6% 16% 29% 18% 4	1600
	11th to 20th	Night	29% 7% 16% 25% 21% 4	1418
	21st to end	Night	26% 6% 17% 29% 19% 2	1400
July	1st to 10th	Night	30% 6% 18% 31% 14% 2	3145
	11th to 20th	Night	26% 5% 13% 37% 16% 3	3879
	21st to end	Night	26% 7% 20% 29% 16% 2	4320
August	1st to 10th	Night	32% 6% 13% 31% 15% 3	2916
	11th to 20th	Night	30% 7% 18% 30% 13% 2	2915
	21st to end	Night	28% 6% 16% 25% 21% 3	3263
September	1st to 10th	Night	27% 5% 13% 30% 22% 4	2046
	11th to 20th	Night	27% 6% 14% 33% 19% 2	2200
	21st to end	Night	26% 4 10% 31% 26% 2	2200
October	1st to 10th	Night	21% 3 7% 23% 43% 3	3837
	11th to 20th	Night	27% 3 11% 30% 25% 4	3899
	21st to end	Night	40% 3 11% 30% 15% 1	4308
November	1st to 10th	Night	33% 5% 15% 24% 20% 3	2665
	11th to 20th	Night	38% 5% 14% 25% 16% 2	2908
	21st to end	Night	39% 4 13% 29% 14% 1	2940
December	1st to 10th	Night	40% 5% 14% 32% 9%	4720
	11th to 20th	Night	50% 5% 17% 22% 6%	5341
	21st to end	Night	59% 5% 10% 19% 7%	7150

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	15% 11% 36% 23% 12% 3	330
	11th to 20th	Day	17% 12% 39% 27% 4 2	330
	21st to end	Day	15% 10% 39% 27% 9%	363
February	1st to 10th	Day	14% 16% 42% 25% 3	330
	11th to 20th	Day	12% 10% 47% 23% 8% 1	315
	21st to end	Day	20% 14% 42% 21% 4	249
March	1st to 10th	Day	17% 16% 41% 22% 4 1	300
	11th to 20th	Day	21% 11% 39% 22% 8%	300
	21st to end	Day	28% 15% 34% 18% 5% 1	330
April	1st to 10th	Day	23% 17% 31% 24% 4	379
	11th to 20th	Day	25% 14% 34% 21% 6%	650
	21st to end	Day	26% 14% 29% 21% 9% 2	700
May	1st to 10th	Day	20% 12% 29% 26% 10% 2	703
	11th to 20th	Day	21% 10% 29% 24% 14% 3	725
	21st to end	Day	20% 8% 30% 29% 11% 2	789
June	1st to 10th	Day	8% 8% 23% 36% 22% 4	730
	11th to 20th	Day	12% 7% 34% 30% 14% 3	730
	21st to end	Day	12% 7% 33% 30% 14% 2	730
July	1st to 10th	Day	6% 4 21% 38% 25% 6%	721
	11th to 20th	Day	7% 5% 27% 39% 17% 5%	718
	21st to end	Day	5% 3 25% 40% 21% 6%	794
August	1st to 10th	Day	9% 4 23% 39% 22% 3	726
	11th to 20th	Day	10% 5% 22% 36% 21% 6%	715
	21st to end	Day	9% 4 25% 36% 23% 4	785
September	1st to 10th	Day	15% 8% 25% 32% 16% 4	480
	11th to 20th	Day	8% 4 39% 30% 15% 5%	330
	21st to end	Day	13% 11% 32% 25% 14% 6%	330
October	1st to 10th	Day	17% 8% 28% 28% 14% 5%	330
	11th to 20th	Day	16% 15% 38% 22% 6% 1	330
	21st to end	Day	20% 17% 36% 20% 6% 1	363
November	1st to 10th	Day	20% 13% 32% 25% 7% 2	330
	11th to 20th	Day	14% 18% 42% 24% 2	330
	21st to end	Day	20% 13% 36% 21% 8% 1	330
December	1st to 10th	Day	11% 12% 42% 26% 8% 1	330
	11th to 20th	Day	16% 17% 40% 23% 5%	330
	21st to end	Day	25% 11% 40% 21% 2 1	363
January	1st to 10th	Night	13% 10% 38% 31% 7% 1	1050
	11th to 20th	Night	12% 14% 37% 31% 5% 1	1050
	21st to end	Night	16% 9% 41% 28% 6%	1155
February	1st to 10th	Night	13% 13% 42% 27% 4 1	1050
	11th to 20th	Night	14% 11% 43% 26% 5% 1	1005
	21st to end	Night	13% 9% 41% 29% 7%	685
March	1st to 10th	Night	14% 9% 40% 29% 7%	653
	11th to 20th	Night	17% 11% 39% 29% 4	600
	21st to end	Night	21% 11% 34% 27% 7%	660
April	1st to 10th	Night	18% 15% 39% 25% 4	600
	11th to 20th	Night	20% 15% 35% 24% 5% 1	593
	21st to end	Night	21% 12% 32% 26% 8% 1	600
May	1st to 10th	Night	17% 9% 33% 31% 9% 1	600
	11th to 20th	Night	17% 10% 28% 34% 10% 1	591
	21st to end	Night	16% 7% 32% 34% 10% 1	650
June	1st to 10th	Night	9% 6% 25% 45% 15% 1	600
	11th to 20th	Night	9% 5% 32% 40% 14%	600
	21st to end	Night	8% 6% 27% 45% 15%	600
July	1st to 10th	Night	5% 3 21% 44% 27% 1	585
	11th to 20th	Night	5% 2 24% 51% 17% 2	582
	21st to end	Night	4 2 22% 52% 20%	657
August	1st to 10th	Night	8% 3 23% 47% 18% 1	597
	11th to 20th	Night	8% 3 21% 41% 25% 1	594
	21st to end	Night	7% 2 27% 43% 18% 2	660
September	1st to 10th	Night	11% 5% 25% 41% 17% 2	600
	11th to 20th	Night	9% 4 33% 40% 13% 1	600
	21st to end	Night	11% 6% 35% 34% 13% 1	600
October	1st to 10th	Night	16% 9% 31% 31% 11% 1	743
	11th to 20th	Night	18% 11% 35% 30% 6% 1	800
	21st to end	Night	18% 12% 33% 27% 9%	912
November	1st to 10th	Night	12% 11% 40% 29% 8% 1	1025
	11th to 20th	Night	14% 12% 41% 28% 5%	1050
	21st to end	Night	14% 11% 38% 29% 8%	1050
December	1st to 10th	Night	13% 11% 41% 30% 5%	1050
	11th to 20th	Night	17% 9% 38% 30% 6% 1	1050
	21st to end	Night	20% 11% 37% 26% 5% 1	1155

Page generated at 2026-02-13 00:42:15. Version ac07

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