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 2024-11-30.

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 7% 5% 4 4 3 5% 7% 41%	102110
	Night	11% 8% 3 7% 5% 4 4 3 5% 7% 44%	97413
Transparency	Day	41% 6% 13% 25% 14% 2	105737
	Night	39% 5% 13% 25% 16% 2	101915
Seeing	Day	15% 9% 31% 29% 13% 3	15997
	Night	14% 9% 34% 33% 10% 1	25906

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	10% 15% 5% 14% 8% 7% 6% 3 5% 6% 21%	2552
	11th to 20th	Day	6% 14% 5% 11% 6% 6% 9% 3 7% 8% 24%	2850
	21st to end	Day	7% 13% 6% 17% 10% 6% 5% 4 4 5% 22%	3135
February	1st to 10th	Day	9% 11% 5% 13% 7% 7% 7% 4 5% 6% 26%	1710
	11th to 20th	Day	8% 11% 5% 9% 6% 5% 5% 4 6% 7% 33%	1664
	21st to end	Day	13% 12% 3 7% 7% 6% 4 3 6% 6% 34%	1561
March	1st to 10th	Day	12% 10% 4 8% 4 3 3 3 4 6% 42%	3363
	11th to 20th	Day	10% 10% 3 8% 3 3 4 3 5% 6% 45%	3480
	21st to end	Day	25% 12% 5% 10% 4 4 4 3 5% 5% 23%	3814
April	1st to 10th	Day	16% 8% 3 8% 5% 3 4 3 5% 6% 37%	2165
	11th to 20th	Day	19% 10% 2 6% 4 3 4 3 5% 6% 38%	2404
	21st to end	Day	22% 8% 2 6% 5% 3 4 3 4 6% 37%	2470
May	1st to 10th	Day	17% 7% 2 6% 5% 5% 5% 4 7% 8% 33%	4082
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 49%	4105
	21st to end	Day	17% 7% 2 6% 4 4 4 4 5% 7% 40%	4824
June	1st to 10th	Day	10% 5% 2 5% 4 3 4 4 6% 8% 48%	2660
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 7% 43%	2826
	21st to end	Day	9% 4 2 5% 4 4 4 4 6% 9% 48%	2850
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 4 7% 10% 41%	4750
	11th to 20th	Day	7% 3 1 5% 3 3 4 3 5% 7% 58%	4224
	21st to end	Day	3 3 1 5% 4 4 5% 5% 7% 10% 54%	4465
August	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 4 6% 9% 46%	4104
	11th to 20th	Day	16% 7% 3 7% 4 4 4 4 6% 8% 37%	4041
	21st to end	Day	9% 5% 2 6% 4 4 4 3 5% 8% 51%	4502
September	1st to 10th	Day	14% 5% 1 5% 2 3 3 3 5% 7% 52%	2233
	11th to 20th	Day	10% 5% 2 4 3 4 3 4 5% 6% 54%	2090
	21st to end	Day	10% 4 2 6% 3 4 4 3 5% 6% 54%	2090
October	1st to 10th	Day	14% 7% 2 5% 3 4 3 3 5% 5% 48%	1934
	11th to 20th	Day	13% 7% 2 4 2 3 3 3 4 5% 53%	1900
	21st to end	Day	16% 7% 2 6% 4 3 3 3 4 5% 46%	2057
November	1st to 10th	Day	13% 6% 3 5% 4 4 4 4 6% 7% 45%	1763
	11th to 20th	Day	9% 9% 3 11% 4 6% 4 4 5% 5% 40%	1630
	21st to end	Day	12% 9% 4 6% 4 3 4 3 6% 8% 40%	1597
December	1st to 10th	Day	9% 8% 4 11% 7% 6% 4 4 5% 8% 35%	1520
	11th to 20th	Day	8% 13% 4 10% 7% 6% 6% 4 5% 6% 31%	1918
	21st to end	Day	18% 9% 4 15% 6% 5% 5% 3 4 6% 24%	2777
January	1st to 10th	Night	9% 13% 4 14% 11% 6% 5% 4 5% 5% 24%	4239
	11th to 20th	Night	5% 11% 4 10% 7% 5% 8% 5% 9% 8% 27%	4120
	21st to end	Night	7% 13% 7% 15% 7% 6% 5% 4 5% 6% 26%	4532
February	1st to 10th	Night	9% 11% 5% 9% 6% 4 5% 4 5% 5% 37%	2470
	11th to 20th	Night	9% 11% 4 8% 5% 4 4 4 5% 6% 41%	2403
	21st to end	Night	9% 9% 4 9% 4 4 5% 3 5% 6% 43%	1908
March	1st to 10th	Night	10% 7% 3 6% 4 3 3 3 4 6% 51%	3492
	11th to 20th	Night	10% 7% 3 5% 3 3 4 4 5% 8% 48%	3480
	21st to end	Night	24% 12% 4 9% 6% 4 3 3 4 6% 24%	3817
April	1st to 10th	Night	15% 8% 3 6% 5% 5% 5% 3 5% 6% 40%	2014
	11th to 20th	Night	16% 9% 3 6% 4 4 3 3 4 5% 41%	1723
	21st to end	Night	18% 8% 2 6% 3 3 4 4 4 6% 41%	1710
May	1st to 10th	Night	18% 6% 2 5% 4 3 2 4 4 6% 45%	2823
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 6% 7% 51%	2796
	21st to end	Night	14% 6% 2 5% 5% 4 4 4 5% 6% 45%	2743
June	1st to 10th	Night	8% 5% 1 4 3 3 3 3 6% 9% 55%	1520
	11th to 20th	Night	8% 5% 2 4 3 3 5% 3 6% 9% 52%	1347
	21st to end	Night	6% 3 2 4 4 4 3 3 6% 9% 56%	1330
July	1st to 10th	Night	8% 5% 2 4 4 4 4 3 6% 8% 51%	2220
	11th to 20th	Night	4 3 1 3 2 2 3 2 5% 9% 65%	2738
	21st to end	Night	2 3 1 4 3 4 4 4 6% 10% 59%	3098
August	1st to 10th	Night	7% 6% 2 6% 4 4 4 4 5% 9% 49%	2836
	11th to 20th	Night	11% 5% 1 5% 3 3 3 3 5% 8% 52%	2798
	21st to end	Night	7% 4 1 5% 4 4 4 3 5% 8% 54%	3163
September	1st to 10th	Night	9% 3 1 4 3 3 2 3 5% 6% 59%	1943
	11th to 20th	Night	9% 4 2 4 3 3 4 3 6% 6% 57%	2090
	21st to end	Night	8% 5% 1 4 3 3 3 2 5% 6% 59%	2090
October	1st to 10th	Night	14% 5% 2 5% 3 2 3 3 4 5% 53%	2226
	11th to 20th	Night	13% 7% 2 5% 3 2 3 2 4 6% 53%	2280
	21st to end	Night	14% 8% 2 6% 3 3 4 3 4 5% 48%	2538
November	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 6% 6% 51%	2535
	11th to 20th	Night	10% 7% 3 8% 4 4 4 4 5% 6% 46%	2769
	21st to end	Night	10% 8% 3 7% 4 3 3 3 4 7% 47%	2799
December	1st to 10th	Night	8% 9% 4 10% 5% 5% 4 4 5% 7% 39%	2660
	11th to 20th	Night	9% 9% 4 8% 6% 6% 4 4 5% 7% 39%	3279
	21st to end	Night	17% 10% 5% 15% 6% 4 4 3 4 5% 27%	4884

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% 6% 13% 13% 5%	2553
	11th to 20th	Day	57% 4 8% 25% 6%	2850
	21st to end	Day	63% 6% 12% 13% 5%	3135
February	1st to 10th	Day	59% 5% 13% 14% 9% 1	1710
	11th to 20th	Day	48% 6% 14% 21% 10% 1	1673
	21st to end	Day	50% 6% 10% 20% 11% 2	1561
March	1st to 10th	Day	44% 4 11% 28% 12% 1	3363
	11th to 20th	Day	40% 5% 9% 30% 12% 4	3480
	21st to end	Day	63% 7% 13% 13% 4 1	3814
April	1st to 10th	Day	46% 6% 13% 23% 11% 1	2165
	11th to 20th	Day	48% 4 12% 21% 13% 1	2404
	21st to end	Day	49% 5% 10% 24% 12%	2470
May	1st to 10th	Day	45% 8% 12% 22% 13%	4120
	11th to 20th	Day	35% 6% 12% 28% 17% 2	4109
	21st to end	Day	43% 6% 11% 23% 16% 1	4837
June	1st to 10th	Day	33% 8% 16% 27% 14% 2	2660
	11th to 20th	Day	38% 7% 13% 24% 15% 3	2826
	21st to end	Day	31% 8% 15% 30% 14% 3	2850
July	1st to 10th	Day	37% 7% 18% 25% 12% 1	4676
	11th to 20th	Day	26% 6% 12% 33% 20% 3	4150
	21st to end	Day	24% 8% 17% 28% 21% 2	4325
August	1st to 10th	Day	34% 7% 15% 28% 14% 1	4090
	11th to 20th	Day	45% 6% 14% 27% 9% 1	4080
	21st to end	Day	32% 6% 15% 28% 16% 3	4501
September	1st to 10th	Day	33% 6% 10% 28% 21% 2	2233
	11th to 20th	Day	30% 6% 14% 32% 16% 2	2090
	21st to end	Day	31% 6% 11% 30% 22% 1	2090
October	1st to 10th	Day	24% 3 8% 29% 32% 3	3207
	11th to 20th	Day	28% 4 16% 26% 22% 4	3140
	21st to end	Day	37% 4 12% 27% 17% 3	3373
November	1st to 10th	Day	37% 6% 15% 24% 16% 2	1763
	11th to 20th	Day	44% 6% 14% 22% 13% 1	1630
	21st to end	Day	42% 5% 15% 23% 13% 1	1597
December	1st to 10th	Day	48% 5% 16% 20% 10% 2	1520
	11th to 20th	Day	53% 5% 16% 20% 5%	1915
	21st to end	Day	61% 5% 11% 15% 7% 1	2777
January	1st to 10th	Night	61% 5% 9% 17% 8%	4239
	11th to 20th	Night	51% 5% 14% 20% 8% 2	4120
	21st to end	Night	59% 6% 12% 15% 8% 1	4532
February	1st to 10th	Night	48% 5% 11% 24% 10% 1	2470
	11th to 20th	Night	44% 5% 12% 23% 12% 3	2404
	21st to end	Night	42% 5% 13% 23% 14% 3	1908
March	1st to 10th	Night	35% 4 10% 29% 21% 2	3492
	11th to 20th	Night	34% 5% 10% 29% 18% 3	3480
	21st to end	Night	60% 8% 13% 13% 5% 1	3817
April	1st to 10th	Night	43% 6% 13% 23% 14% 1	2014
	11th to 20th	Night	43% 5% 11% 21% 18% 2	1723
	21st to end	Night	45% 5% 12% 22% 14% 2	1710
May	1st to 10th	Night	39% 5% 14% 21% 20% 1	2850
	11th to 20th	Night	31% 7% 12% 25% 20% 5%	2797
	21st to end	Night	38% 7% 10% 24% 17% 2	2751
June	1st to 10th	Night	26% 7% 16% 29% 18% 4	1520
	11th to 20th	Night	28% 7% 16% 25% 21% 4	1347
	21st to end	Night	25% 6% 18% 30% 19% 2	1330
July	1st to 10th	Night	31% 6% 20% 29% 13% 2	2185
	11th to 20th	Night	17% 6% 14% 38% 21% 4	2689
	21st to end	Night	20% 7% 17% 31% 21% 3	2999
August	1st to 10th	Night	32% 6% 12% 31% 16% 3	2826
	11th to 20th	Night	31% 7% 18% 30% 13% 2	2825
	21st to end	Night	29% 6% 16% 24% 21% 3	3163
September	1st to 10th	Night	27% 5% 12% 30% 23% 4	1943
	11th to 20th	Night	26% 6% 13% 33% 20% 2	2090
	21st to end	Night	26% 4 11% 32% 25% 2	2090
October	1st to 10th	Night	21% 2 7% 23% 44% 3	3720
	11th to 20th	Night	27% 3 12% 30% 25% 4	3779
	21st to end	Night	40% 3 10% 30% 15% 1	4174
November	1st to 10th	Night	33% 5% 15% 23% 21% 4	2535
	11th to 20th	Night	38% 5% 14% 25% 16% 2	2769
	21st to end	Night	38% 5% 13% 28% 15% 1	2799
December	1st to 10th	Night	44% 5% 15% 25% 9% 1	2660
	11th to 20th	Night	44% 5% 20% 24% 6%	3281
	21st to end	Night	59% 5% 10% 18% 7% 1	4884

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	16% 12% 34% 23% 13% 3	270
	11th to 20th	Day	19% 9% 37% 29% 4 2	270
	21st to end	Day	16% 8% 35% 30% 10%	297
February	1st to 10th	Day	13% 15% 41% 28% 3	270
	11th to 20th	Day	13% 10% 48% 21% 7% 1	270
	21st to end	Day	20% 14% 41% 21% 4	225
March	1st to 10th	Day	17% 17% 39% 22% 5% 1	270
	11th to 20th	Day	21% 11% 39% 22% 7%	270
	21st to end	Day	28% 13% 35% 19% 4 1	297
April	1st to 10th	Day	23% 17% 30% 23% 5%	345
	11th to 20th	Day	25% 13% 35% 21% 5%	602
	21st to end	Day	26% 14% 27% 22% 10% 2	650
May	1st to 10th	Day	21% 13% 31% 25% 8% 2	653
	11th to 20th	Day	19% 11% 30% 24% 13% 3	675
	21st to end	Day	20% 9% 29% 30% 11% 2	734
June	1st to 10th	Day	8% 7% 23% 36% 22% 3	680
	11th to 20th	Day	12% 7% 34% 30% 13% 3	680
	21st to end	Day	11% 7% 33% 31% 15% 3	680
July	1st to 10th	Day	7% 3 20% 38% 25% 6%	671
	11th to 20th	Day	7% 5% 26% 39% 18% 5%	668
	21st to end	Day	4 3 24% 40% 22% 6%	739
August	1st to 10th	Day	9% 4 22% 39% 23% 3	676
	11th to 20th	Day	11% 5% 23% 36% 21% 5%	665
	21st to end	Day	9% 4 25% 35% 22% 5%	731
September	1st to 10th	Day	16% 7% 26% 33% 15% 4	443
	11th to 20th	Day	7% 3 39% 31% 15% 5%	300
	21st to end	Day	14% 12% 34% 24% 12% 5%	300
October	1st to 10th	Day	18% 8% 28% 27% 14% 6%	300
	11th to 20th	Day	14% 16% 38% 24% 7% 1	300
	21st to end	Day	18% 17% 37% 20% 6% 1	330
November	1st to 10th	Day	22% 13% 30% 26% 8% 2	300
	11th to 20th	Day	15% 17% 43% 22% 3	300
	21st to end	Day	19% 13% 37% 22% 8% 1	299
December	1st to 10th	Day	10% 11% 44% 25% 9% 1	270
	11th to 20th	Day	15% 14% 40% 25% 6%	270
	21st to end	Day	26% 12% 38% 21% 2 1	297
January	1st to 10th	Night	13% 11% 37% 30% 7% 1	950
	11th to 20th	Night	13% 12% 35% 33% 6% 1	950
	21st to end	Night	17% 9% 38% 28% 7%	1045
February	1st to 10th	Night	14% 11% 42% 28% 5% 1	950
	11th to 20th	Night	14% 11% 42% 26% 6% 1	929
	21st to end	Night	14% 9% 40% 29% 7%	652
March	1st to 10th	Night	14% 9% 40% 29% 8%	621
	11th to 20th	Night	17% 10% 39% 29% 4	570
	21st to end	Night	22% 11% 33% 27% 7%	627
April	1st to 10th	Night	18% 15% 38% 25% 4	570
	11th to 20th	Night	20% 15% 35% 24% 5% 1	563
	21st to end	Night	21% 12% 32% 26% 8% 1	570
May	1st to 10th	Night	17% 10% 34% 30% 8% 1	570
	11th to 20th	Night	16% 10% 29% 34% 10% 1	561
	21st to end	Night	16% 7% 32% 35% 10% 1	617
June	1st to 10th	Night	9% 6% 25% 45% 15% 1	570
	11th to 20th	Night	9% 5% 32% 41% 14%	570
	21st to end	Night	6% 5% 26% 46% 16%	570
July	1st to 10th	Night	5% 2 21% 44% 28% 1	555
	11th to 20th	Night	3 1 24% 52% 18% 2	552
	21st to end	Night	3 2 22% 52% 21%	624
August	1st to 10th	Night	8% 3 23% 47% 18% 1	567
	11th to 20th	Night	8% 3 21% 41% 25% 1	564
	21st to end	Night	7% 2 28% 43% 17% 3	627
September	1st to 10th	Night	11% 4 25% 41% 16% 2	570
	11th to 20th	Night	9% 4 33% 39% 13% 1	570
	21st to end	Night	11% 6% 35% 34% 12% 1	570
October	1st to 10th	Night	17% 9% 31% 31% 11% 1	706
	11th to 20th	Night	17% 11% 36% 31% 6% 1	760
	21st to end	Night	18% 12% 33% 27% 9%	866
November	1st to 10th	Night	13% 11% 38% 30% 8% 1	975
	11th to 20th	Night	14% 12% 41% 28% 5%	1000
	21st to end	Night	14% 11% 39% 29% 8%	1000
December	1st to 10th	Night	13% 11% 41% 30% 5%	950
	11th to 20th	Night	17% 7% 38% 31% 7% 1	950
	21st to end	Night	21% 11% 37% 25% 6%	1045

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