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 2024-01-15.

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	13% 7% 3 6% 5% 3 4 4 5% 7% 41%	75121
	Night	11% 7% 3 6% 5% 3 4 4 5% 7% 45%	74568
Transparency	Day	42% 6% 13% 24% 13% 2	74829
	Night	39% 6% 13% 25% 15% 2	74380
Seeing	Day	15% 9% 31% 29% 13% 3	14741
	Night	14% 9% 34% 33% 10% 1	24766

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% 11% 5% 8% 8% 6% 7% 5% 5% 7% 30%	1537
	11th to 20th	Day	8% 12% 6% 10% 8% 4 6% 4 5% 8% 28%	1656
	21st to end	Day	10% 12% 7% 13% 9% 4 5% 6% 5% 6% 23%	1782
February	1st to 10th	Day	10% 9% 5% 12% 8% 5% 8% 5% 5% 7% 26%	1620
	11th to 20th	Day	9% 10% 6% 9% 7% 4 5% 5% 6% 6% 33%	1574
	21st to end	Day	13% 12% 4 6% 7% 5% 5% 4 5% 6% 33%	1472
March	1st to 10th	Day	13% 12% 6% 8% 5% 2 4 4 3 7% 37%	1930
	11th to 20th	Day	13% 11% 5% 9% 5% 3 4 4 5% 7% 34%	1980
	21st to end	Day	18% 13% 4 6% 5% 3 5% 5% 5% 6% 29%	2178
April	1st to 10th	Day	17% 8% 4 7% 6% 3 4 4 5% 7% 37%	2051
	11th to 20th	Day	19% 9% 3 6% 5% 3 4 4 4 6% 37%	2276
	21st to end	Day	22% 7% 3 5% 5% 3 4 4 4 6% 37%	2340
May	1st to 10th	Day	18% 7% 3 5% 6% 3 5% 5% 5% 7% 36%	2340
	11th to 20th	Day	17% 8% 3 5% 5% 3 4 4 5% 8% 38%	2311
	21st to end	Day	18% 6% 2 6% 5% 3 5% 5% 5% 7% 37%	2725
June	1st to 10th	Day	11% 5% 2 5% 4 2 4 5% 6% 8% 48%	2520
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 6% 6% 7% 43%	2677
	21st to end	Day	9% 4 2 5% 5% 2 4 5% 6% 9% 49%	2700
July	1st to 10th	Day	7% 3 2 4 5% 3 5% 5% 6% 11% 51%	2700
	11th to 20th	Day	7% 3 1 4 4 2 4 4 6% 7% 58%	2404
	21st to end	Day	5% 3 2 4 4 3 4 5% 6% 9% 55%	2546
August	1st to 10th	Day	11% 5% 3 5% 5% 4 5% 5% 5% 8% 44%	2328
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 4 6% 9% 46%	2301
	21st to end	Day	8% 4 2 4 4 2 3 4 5% 8% 55%	2558
September	1st to 10th	Day	14% 5% 2 4 3 3 3 4 5% 6% 51%	2116
	11th to 20th	Day	11% 4 2 4 4 3 3 5% 4 6% 54%	1980
	21st to end	Day	11% 4 2 5% 4 3 4 4 5% 6% 52%	1980
October	1st to 10th	Day	15% 6% 3 5% 4 3 3 4 4 6% 46%	1833
	11th to 20th	Day	14% 7% 2 4 3 2 2 4 4 6% 53%	1800
	21st to end	Day	17% 8% 2 6% 4 2 3 4 3 5% 46%	1949
November	1st to 10th	Day	12% 6% 3 4 4 3 5% 4 6% 7% 47%	1673
	11th to 20th	Day	9% 7% 3 10% 6% 4 5% 5% 5% 5% 40%	1549
	21st to end	Day	13% 8% 4 6% 4 3 4 4 5% 8% 41%	1520
December	1st to 10th	Day	9% 7% 5% 10% 7% 4 5% 5% 5% 8% 36%	1520
	11th to 20th	Day	8% 12% 5% 9% 8% 5% 5% 5% 5% 7% 31%	1918
	21st to end	Day	18% 8% 5% 14% 7% 4 5% 4 4 6% 24%	2777
January	1st to 10th	Night	8% 7% 4 7% 8% 4 6% 6% 6% 7% 37%	2537
	11th to 20th	Night	8% 12% 5% 10% 9% 4 5% 5% 6% 7% 31%	2393
	21st to end	Night	11% 12% 5% 8% 7% 3 5% 5% 5% 7% 32%	2574
February	1st to 10th	Night	9% 10% 5% 8% 7% 3 5% 5% 5% 5% 37%	2340
	11th to 20th	Night	9% 10% 4 7% 6% 3 5% 5% 5% 6% 40%	2273
	21st to end	Night	10% 8% 4 9% 5% 3 5% 4 5% 6% 41%	1798
March	1st to 10th	Night	11% 8% 4 6% 6% 2 4 5% 6% 7% 43%	2029
	11th to 20th	Night	14% 8% 4 6% 5% 3 4 4 5% 6% 41%	1980
	21st to end	Night	16% 11% 4 7% 5% 3 4 4 5% 7% 33%	2178
April	1st to 10th	Night	15% 7% 3 5% 5% 3 6% 5% 5% 6% 40%	1908
	11th to 20th	Night	16% 9% 3 6% 5% 3 4 4 4 6% 41%	1632
	21st to end	Night	18% 7% 2 6% 4 2 4 5% 3 6% 42%	1620
May	1st to 10th	Night	15% 6% 3 5% 5% 2 3 5% 5% 8% 42%	1620
	11th to 20th	Night	15% 7% 2 4 4 2 4 4 5% 7% 46%	1580
	21st to end	Night	14% 5% 2 6% 5% 4 4 6% 5% 7% 41%	1551
June	1st to 10th	Night	8% 4 1 4 4 2 3 4 6% 9% 55%	1440
	11th to 20th	Night	8% 4 2 3 3 2 4 4 6% 9% 53%	1277
	21st to end	Night	5% 3 2 4 4 3 3 3 6% 9% 57%	1260
July	1st to 10th	Night	5% 3 1 3 4 2 4 4 5% 9% 60%	1260
	11th to 20th	Night	3 2 1 3 3 2 3 3 5% 9% 66%	1548
	21st to end	Night	3 3 2 2 3 2 5% 5% 6% 10% 60%	1772
August	1st to 10th	Night	7% 5% 2 5% 5% 3 4 5% 5% 8% 51%	1611
	11th to 20th	Night	8% 3 1 4 3 2 3 4 5% 8% 59%	1600
	21st to end	Night	5% 3 1 3 4 3 4 4 5% 8% 59%	1797
September	1st to 10th	Night	10% 4 1 4 4 2 3 3 5% 7% 58%	1840
	11th to 20th	Night	9% 4 2 3 3 2 4 3 5% 6% 57%	1980
	21st to end	Night	9% 4 2 3 4 2 3 3 5% 6% 59%	1980
October	1st to 10th	Night	15% 5% 2 4 4 2 3 4 4 5% 51%	2108
	11th to 20th	Night	14% 7% 2 4 4 2 3 3 4 6% 53%	2160
	21st to end	Night	15% 7% 3 5% 4 2 4 3 4 5% 49%	2404
November	1st to 10th	Night	9% 5% 2 4 5% 3 4 4 6% 6% 52%	2405
	11th to 20th	Night	10% 6% 3 8% 5% 3 4 4 5% 7% 45%	2630
	21st to end	Night	11% 7% 3 6% 5% 3 3 4 4 7% 48%	2660
December	1st to 10th	Night	8% 9% 4 9% 6% 4 5% 5% 5% 6% 39%	2660
	11th to 20th	Night	10% 8% 4 7% 7% 4 5% 5% 5% 7% 39%	3279
	21st to end	Night	17% 10% 5% 13% 8% 3 4 4 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	54% 6% 16% 16% 8% 1	1538
	11th to 20th	Day	55% 5% 12% 20% 8%	1656
	21st to end	Day	61% 6% 12% 16% 5%	1782
February	1st to 10th	Day	58% 5% 13% 15% 9% 1	1620
	11th to 20th	Day	49% 6% 14% 20% 10% 1	1583
	21st to end	Day	52% 7% 10% 20% 10% 2	1472
March	1st to 10th	Day	48% 5% 14% 22% 9% 1	1930
	11th to 20th	Day	51% 6% 12% 23% 9%	1980
	21st to end	Day	56% 7% 14% 16% 6% 1	2178
April	1st to 10th	Day	47% 6% 13% 24% 10% 1	2051
	11th to 20th	Day	49% 4 12% 21% 13% 1	2276
	21st to end	Day	49% 5% 10% 24% 11%	2340
May	1st to 10th	Day	48% 7% 12% 21% 12% 1	2340
	11th to 20th	Day	44% 6% 14% 24% 11% 1	2315
	21st to end	Day	46% 7% 11% 22% 13% 1	2725
June	1st to 10th	Day	33% 8% 16% 26% 15% 2	2520
	11th to 20th	Day	37% 8% 14% 24% 15% 3	2677
	21st to end	Day	30% 8% 15% 30% 14% 3	2700
July	1st to 10th	Day	27% 8% 18% 31% 15% 2	2626
	11th to 20th	Day	25% 7% 12% 30% 19% 6%	2330
	21st to end	Day	25% 7% 16% 32% 17% 3	2405
August	1st to 10th	Day	37% 8% 16% 25% 12% 2	2314
	11th to 20th	Day	33% 7% 13% 33% 12% 1	2301
	21st to end	Day	28% 6% 12% 30% 19% 4	2558
September	1st to 10th	Day	34% 6% 11% 26% 21% 2	2116
	11th to 20th	Day	31% 6% 14% 33% 15% 2	1980
	21st to end	Day	32% 6% 11% 30% 20% 1	1980
October	1st to 10th	Day	38% 5% 12% 28% 16% 1	1833
	11th to 20th	Day	34% 4 11% 29% 19% 3	1800
	21st to end	Day	42% 5% 11% 24% 17% 2	1949
November	1st to 10th	Day	35% 6% 15% 25% 16% 2	1673
	11th to 20th	Day	44% 6% 14% 22% 13% 1	1549
	21st to end	Day	43% 5% 15% 23% 14% 1	1520
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	45% 6% 13% 24% 13%	2537
	11th to 20th	Night	52% 6% 15% 20% 6%	2393
	21st to end	Night	51% 7% 13% 19% 9% 1	2574
February	1st to 10th	Night	48% 5% 11% 25% 10% 1	2340
	11th to 20th	Night	45% 5% 12% 22% 12% 4	2274
	21st to end	Night	44% 6% 12% 22% 14% 3	1798
March	1st to 10th	Night	41% 5% 14% 28% 10% 2	2029
	11th to 20th	Night	44% 6% 10% 25% 14% 2	1980
	21st to end	Night	51% 7% 12% 20% 8% 2	2178
April	1st to 10th	Night	43% 7% 12% 23% 14% 1	1908
	11th to 20th	Night	44% 5% 11% 21% 17% 2	1632
	21st to end	Night	44% 5% 12% 23% 14% 2	1620
May	1st to 10th	Night	39% 6% 14% 24% 14% 2	1620
	11th to 20th	Night	38% 8% 14% 24% 16% 1	1581
	21st to end	Night	40% 7% 15% 20% 15% 2	1552
June	1st to 10th	Night	27% 7% 17% 29% 18% 3	1440
	11th to 20th	Night	27% 7% 16% 25% 21% 4	1277
	21st to end	Night	24% 6% 18% 30% 20% 3	1260
July	1st to 10th	Night	21% 6% 16% 34% 20% 3	1225
	11th to 20th	Night	16% 8% 14% 33% 22% 7%	1499
	21st to end	Night	19% 7% 16% 31% 21% 5%	1673
August	1st to 10th	Night	31% 7% 13% 30% 16% 3	1601
	11th to 20th	Night	24% 7% 16% 34% 17% 3	1600
	21st to end	Night	24% 6% 14% 28% 23% 5%	1797
September	1st to 10th	Night	28% 5% 13% 28% 22% 5%	1840
	11th to 20th	Night	27% 6% 13% 33% 19% 2	1980
	21st to end	Night	27% 4 11% 33% 23% 2	1980
October	1st to 10th	Night	35% 4 12% 26% 21% 1	2108
	11th to 20th	Night	35% 4 9% 28% 20% 3	2160
	21st to end	Night	40% 4 12% 24% 20% 1	2404
November	1st to 10th	Night	32% 5% 15% 23% 21% 4	2405
	11th to 20th	Night	39% 5% 14% 25% 16% 2	2630
	21st to end	Night	38% 5% 13% 28% 15% 1	2660
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% 28% 5% 2	252
	21st to end	Day	18% 9% 38% 29% 5%	264
February	1st to 10th	Day	13% 16% 41% 29% 2	240
	11th to 20th	Day	13% 11% 48% 20% 7%	240
	21st to end	Day	21% 15% 38% 21% 5%	198
March	1st to 10th	Day	18% 16% 38% 23% 5% 1	240
	11th to 20th	Day	23% 9% 40% 21% 7%	240
	21st to end	Day	27% 14% 34% 19% 5% 2	264
April	1st to 10th	Day	23% 17% 31% 23% 5%	311
	11th to 20th	Day	26% 12% 35% 21% 6% 1	554
	21st to end	Day	25% 13% 28% 22% 10% 2	600
May	1st to 10th	Day	22% 13% 30% 25% 8% 2	603
	11th to 20th	Day	20% 11% 29% 24% 13% 3	625
	21st to end	Day	20% 8% 28% 31% 11% 3	679
June	1st to 10th	Day	8% 7% 22% 36% 23% 3	630
	11th to 20th	Day	12% 6% 34% 31% 14% 3	630
	21st to end	Day	11% 7% 31% 32% 16% 3	630
July	1st to 10th	Day	6% 3 19% 39% 26% 6%	621
	11th to 20th	Day	7% 5% 27% 38% 18% 5%	618
	21st to end	Day	5% 4 25% 40% 22% 5%	684
August	1st to 10th	Day	9% 4 20% 40% 24% 4	626
	11th to 20th	Day	9% 5% 22% 37% 22% 6%	615
	21st to end	Day	9% 3 25% 34% 23% 5%	677
September	1st to 10th	Day	18% 7% 25% 32% 15% 4	406
	11th to 20th	Day	8% 3 38% 30% 16% 5%	270
	21st to end	Day	15% 12% 35% 24% 10% 3	270
October	1st to 10th	Day	20% 8% 27% 27% 13% 6%	270
	11th to 20th	Day	15% 16% 36% 23% 7% 1	270
	21st to end	Day	20% 16% 37% 19% 6% 1	297
November	1st to 10th	Day	19% 13% 30% 27% 8% 3	270
	11th to 20th	Day	13% 17% 44% 23% 3	270
	21st to end	Day	21% 11% 36% 23% 8% 1	270
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	14% 12% 34% 33% 6% 1	921
	21st to end	Night	17% 9% 39% 27% 6%	990
February	1st to 10th	Night	14% 11% 41% 28% 4 1	900
	11th to 20th	Night	14% 11% 42% 26% 6% 1	879
	21st to end	Night	15% 9% 40% 28% 8%	614
March	1st to 10th	Night	14% 10% 39% 29% 8% 1	589
	11th to 20th	Night	18% 9% 39% 30% 5%	540
	21st to end	Night	21% 10% 33% 28% 8% 1	594
April	1st to 10th	Night	18% 14% 39% 26% 3	540
	11th to 20th	Night	20% 15% 35% 24% 6% 1	533
	21st to end	Night	21% 11% 32% 26% 8% 1	540
May	1st to 10th	Night	17% 9% 34% 31% 8% 1	540
	11th to 20th	Night	17% 9% 30% 33% 10% 1	531
	21st to end	Night	16% 7% 31% 35% 10% 1	584
June	1st to 10th	Night	9% 6% 24% 45% 15% 1	540
	11th to 20th	Night	9% 4 31% 41% 14%	540
	21st to end	Night	6% 5% 26% 46% 16%	540
July	1st to 10th	Night	5% 2 20% 44% 28% 1	525
	11th to 20th	Night	3 2 25% 52% 17% 2	522
	21st to end	Night	4 2 22% 52% 20%	591
August	1st to 10th	Night	8% 3 22% 47% 19% 1	537
	11th to 20th	Night	7% 3 21% 41% 26% 1	534
	21st to end	Night	7% 2 28% 43% 18% 3	594
September	1st to 10th	Night	12% 4 25% 41% 16% 2	540
	11th to 20th	Night	10% 4 33% 39% 13% 1	540
	21st to end	Night	12% 6% 36% 34% 11% 1	540
October	1st to 10th	Night	18% 9% 32% 30% 12% 1	668
	11th to 20th	Night	18% 11% 35% 30% 6% 1	720
	21st to end	Night	18% 12% 33% 28% 10%	820
November	1st to 10th	Night	12% 11% 39% 30% 8% 1	925
	11th to 20th	Night	13% 12% 41% 28% 5%	950
	21st to end	Night	14% 10% 39% 29% 8%	950
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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