John H. Witte Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John H. Witte Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	11% 8% 3 8% 5% 4 5% 4 6% 8% 39%	149859
	Night	12% 8% 3 8% 5% 4 4 3 5% 7% 41%	142243
Transparency	Day	42% 5% 12% 24% 15% 2	153424
	Night	42% 5% 12% 23% 16% 2	146758
Seeing	Day	15% 10% 29% 27% 15% 4	18497
	Night	16% 10% 32% 30% 12% 1	28233

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% 11% 4 15% 7% 6% 6% 4 5% 6% 26%	4691
	11th to 20th	Day	7% 8% 5% 10% 7% 7% 7% 4 6% 7% 34%	4927
	21st to end	Day	10% 13% 6% 13% 6% 4 4 3 4 6% 30%	5812
February	1st to 10th	Day	13% 12% 4 11% 7% 5% 4 3 6% 6% 28%	1890
	11th to 20th	Day	14% 10% 3 9% 4 5% 4 3 5% 6% 36%	1920
	21st to end	Day	12% 11% 3 10% 5% 4 3 4 3 6% 38%	1730
March	1st to 10th	Day	17% 11% 3 5% 3 2 3 2 3 5% 45%	4738
	11th to 20th	Day	8% 6% 3 6% 4 4 4 4 6% 8% 47%	5090
	21st to end	Day	18% 11% 3 10% 5% 4 4 4 5% 7% 29%	5813
April	1st to 10th	Day	14% 12% 4 8% 5% 4 4 4 4 7% 35%	2520
	11th to 20th	Day	14% 8% 3 9% 4 4 4 3 5% 7% 37%	2494
	21st to end	Day	19% 8% 3 7% 5% 4 4 3 6% 7% 33%	2520
May	1st to 10th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 8% 36%	7710
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 44%	8222
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	9047
June	1st to 10th	Day	8% 5% 2 6% 4 4 4 5% 5% 9% 45%	2940
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 8% 47%	2940
	21st to end	Day	10% 6% 2 7% 5% 5% 4 5% 7% 8% 41%	2940
July	1st to 10th	Day	7% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	8280
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 6% 8% 49%	8280
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 10% 38%	8738
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	6014
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 4 7% 8% 35%	5722
	21st to end	Day	5% 4 2 5% 4 4 4 3 6% 10% 53%	6080
September	1st to 10th	Day	11% 4 2 5% 3 4 5% 4 6% 7% 51%	2520
	11th to 20th	Day	10% 5% 2 4 3 4 4 3 5% 9% 51%	2520
	21st to end	Day	11% 5% 3 5% 4 3 6% 4 5% 6% 48%	2240
October	1st to 10th	Day	12% 6% 2 6% 3 4 3 2 4 5% 52%	2100
	11th to 20th	Day	13% 8% 3 6% 4 4 3 3 4 6% 46%	2030
	21st to end	Day	17% 8% 2 7% 3 4 4 3 4 6% 42%	2165
November	1st to 10th	Day	10% 7% 2 5% 5% 4 4 4 6% 7% 46%	1852
	11th to 20th	Day	14% 10% 3 8% 5% 4 3 3 5% 8% 37%	1846
	21st to end	Day	15% 11% 4 7% 4 4 3 3 5% 6% 38%	1680
December	1st to 10th	Day	8% 8% 3 9% 4 4 4 3 4 6% 47%	2690
	11th to 20th	Day	14% 10% 4 14% 6% 5% 5% 4 5% 6% 27%	3088
	21st to end	Day	21% 21% 7% 12% 5% 3 3 2 4 4 16%	4070
January	1st to 10th	Night	11% 11% 5% 14% 5% 4 4 3 4 6% 33%	8232
	11th to 20th	Night	8% 10% 5% 11% 8% 6% 6% 4 7% 6% 30%	8061
	21st to end	Night	12% 12% 5% 11% 5% 5% 4 3 4 5% 35%	8402
February	1st to 10th	Night	13% 11% 4 9% 5% 5% 4 3 5% 5% 35%	2730
	11th to 20th	Night	12% 10% 4 8% 5% 4 4 3 5% 6% 39%	2585
	21st to end	Night	13% 9% 3 9% 5% 3 3 3 4 5% 43%	2076
March	1st to 10th	Night	17% 9% 4 6% 3 4 3 2 3 5% 42%	5366
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 7% 50%	5090
	21st to end	Night	15% 12% 5% 10% 5% 5% 5% 4 5% 6% 29%	5357
April	1st to 10th	Night	16% 8% 3 6% 5% 4 4 3 5% 6% 40%	2100
	11th to 20th	Night	14% 9% 2 6% 4 4 3 3 5% 6% 43%	2075
	21st to end	Night	17% 7% 3 7% 5% 4 4 4 5% 7% 39%	2100
May	1st to 10th	Night	15% 7% 3 6% 4 4 4 4 5% 6% 42%	5145
	11th to 20th	Night	12% 5% 2 5% 5% 4 5% 4 7% 9% 42%	4670
	21st to end	Night	12% 8% 3 8% 4 4 4 3 6% 8% 40%	5139
June	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	1680
	11th to 20th	Night	7% 5% 2 4 5% 4 5% 4 7% 9% 48%	1680
	21st to end	Night	9% 6% 2 5% 4 4 5% 4 6% 9% 46%	1680
July	1st to 10th	Night	9% 4 2 6% 4 4 5% 4 7% 10% 46%	4708
	11th to 20th	Night	6% 4 2 5% 3 4 4 3 6% 8% 54%	4707
	21st to end	Night	9% 6% 2 7% 4 4 4 4 6% 8% 45%	5352
August	1st to 10th	Night	7% 6% 2 6% 4 3 4 3 5% 9% 51%	4156
	11th to 20th	Night	8% 5% 3 6% 5% 4 4 4 5% 8% 47%	4332
	21st to end	Night	4 3 1 5% 4 3 4 4 5% 9% 57%	5046
September	1st to 10th	Night	10% 3 2 4 3 3 3 3 5% 7% 57%	2100
	11th to 20th	Night	10% 5% 2 5% 3 4 4 4 5% 7% 51%	2100
	21st to end	Night	9% 5% 2 5% 5% 4 4 3 5% 8% 51%	2355
October	1st to 10th	Night	10% 6% 2 6% 3 2 3 3 4 5% 55%	2520
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 49%	2437
	21st to end	Night	16% 8% 3 7% 4 4 4 3 4 6% 42%	2675
November	1st to 10th	Night	11% 7% 1 5% 3 3 3 3 4 7% 54%	2666
	11th to 20th	Night	14% 9% 3 6% 4 4 4 4 5% 7% 42%	2772
	21st to end	Night	15% 10% 3 8% 5% 4 4 3 4 5% 39%	2940
December	1st to 10th	Night	7% 9% 3 7% 5% 4 4 3 5% 6% 47%	4720
	11th to 20th	Night	14% 9% 4 11% 6% 5% 5% 3 5% 5% 33%	5339
	21st to end	Night	21% 19% 7% 11% 5% 5% 4 3 4 4 18%	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	56% 6% 14% 15% 8% 1	4692
	11th to 20th	Day	49% 4 8% 21% 15% 3	4927
	21st to end	Day	57% 4 10% 22% 7% 1	5812
February	1st to 10th	Day	56% 4 15% 17% 7% 1	1890
	11th to 20th	Day	48% 5% 13% 22% 10% 2	1929
	21st to end	Day	49% 5% 12% 22% 11% 2	1730
March	1st to 10th	Day	43% 5% 8% 23% 18% 2	4738
	11th to 20th	Day	34% 5% 11% 31% 18% 1	5090
	21st to end	Day	55% 5% 15% 19% 6% 1	5815
April	1st to 10th	Day	50% 5% 12% 21% 11%	2520
	11th to 20th	Day	45% 7% 11% 24% 12% 2	2494
	21st to end	Day	48% 7% 11% 19% 13% 1	2520
May	1st to 10th	Day	44% 5% 10% 22% 18% 1	7746
	11th to 20th	Day	39% 4 12% 28% 14% 3	8226
	21st to end	Day	45% 5% 10% 21% 17% 1	9061
June	1st to 10th	Day	34% 8% 15% 25% 15% 2	2940
	11th to 20th	Day	34% 7% 11% 23% 21% 3	2940
	21st to end	Day	38% 8% 15% 23% 15% 2	2940
July	1st to 10th	Day	39% 4 14% 27% 15% 1	8211
	11th to 20th	Day	34% 4 14% 29% 17% 1	8205
	21st to end	Day	41% 4 20% 26% 10%	8546
August	1st to 10th	Day	36% 5% 16% 29% 13% 1	6000
	11th to 20th	Day	46% 5% 15% 25% 10% 1	5761
	21st to end	Day	28% 3 15% 33% 18% 2	6079
September	1st to 10th	Day	31% 7% 11% 26% 21% 4	2520
	11th to 20th	Day	31% 5% 13% 28% 19% 3	2520
	21st to end	Day	34% 6% 11% 27% 20% 2	2240
October	1st to 10th	Day	23% 3 6% 22% 44% 2	3360
	11th to 20th	Day	28% 3 10% 23% 31% 6%	3270
	21st to end	Day	40% 3 8% 27% 18% 3	3479
November	1st to 10th	Day	37% 5% 15% 25% 15% 3	1852
	11th to 20th	Day	46% 5% 12% 22% 14% 2	1846
	21st to end	Day	48% 4 15% 23% 10%	1680
December	1st to 10th	Day	39% 3 14% 33% 9% 1	2690
	11th to 20th	Day	57% 4 11% 16% 9% 2	3085
	21st to end	Day	72% 5% 9% 10% 3	4070
January	1st to 10th	Night	53% 4 15% 20% 7% 1	8233
	11th to 20th	Night	53% 3 11% 17% 15% 1	8061
	21st to end	Night	53% 3 12% 20% 11% 1	8402
February	1st to 10th	Night	50% 4 13% 21% 11% 1	2730
	11th to 20th	Night	45% 5% 12% 24% 12% 1	2586
	21st to end	Night	44% 4 10% 24% 15% 2	2076
March	1st to 10th	Night	45% 5% 7% 25% 16% 2	5366
	11th to 20th	Night	34% 5% 8% 30% 18% 5%	5090
	21st to end	Night	55% 7% 14% 16% 7% 1	5357
April	1st to 10th	Night	44% 6% 11% 24% 13% 2	2100
	11th to 20th	Night	41% 5% 11% 22% 16% 4	2075
	21st to end	Night	45% 6% 12% 20% 14% 3	2100
May	1st to 10th	Night	41% 6% 10% 21% 19% 3	5174
	11th to 20th	Night	36% 7% 14% 22% 16% 4	4671
	21st to end	Night	41% 5% 11% 21% 20% 2	5149
June	1st to 10th	Night	30% 8% 14% 26% 18% 3	1680
	11th to 20th	Night	30% 7% 12% 23% 24% 4	1680
	21st to end	Night	34% 8% 15% 21% 19% 3	1680
July	1st to 10th	Night	33% 3 17% 28% 17% 2	4670
	11th to 20th	Night	28% 3 14% 29% 23% 2	4662
	21st to end	Night	36% 3 18% 29% 12% 1	5234
August	1st to 10th	Night	32% 4 16% 28% 18% 1	4146
	11th to 20th	Night	36% 5% 18% 27% 13% 1	4359
	21st to end	Night	26% 4 15% 35% 17% 3	5046
September	1st to 10th	Night	27% 5% 11% 27% 26% 4	2100
	11th to 20th	Night	31% 6% 12% 27% 19% 5%	2100
	21st to end	Night	31% 6% 10% 26% 24% 2	2355
October	1st to 10th	Night	22% 4 6% 24% 41% 3	4040
	11th to 20th	Night	26% 4 9% 22% 33% 6%	3936
	21st to end	Night	44% 4 7% 21% 20% 4	4311
November	1st to 10th	Night	31% 6% 14% 26% 19% 4	2666
	11th to 20th	Night	41% 6% 12% 25% 15% 2	2772
	21st to end	Night	48% 4 11% 23% 13% 1	2940
December	1st to 10th	Night	38% 4 12% 34% 11% 1	4720
	11th to 20th	Night	53% 4 12% 22% 8% 1	5341
	21st to end	Night	70% 5% 9% 12% 4	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	20% 13% 34% 25% 7%	330
	11th to 20th	Day	20% 19% 42% 15% 3	330
	21st to end	Day	18% 18% 39% 21% 4	363
February	1st to 10th	Day	18% 19% 39% 19% 4	330
	11th to 20th	Day	24% 17% 35% 19% 4 1	330
	21st to end	Day	19% 17% 46% 15% 2 1	273
March	1st to 10th	Day	20% 19% 38% 19% 3	330
	11th to 20th	Day	21% 19% 33% 20% 5% 1	330
	21st to end	Day	30% 19% 31% 15% 4 1	464
April	1st to 10th	Day	23% 17% 35% 20% 4	540
	11th to 20th	Day	20% 17% 34% 18% 9% 1	532
	21st to end	Day	20% 14% 35% 22% 8% 1	540
May	1st to 10th	Day	18% 12% 35% 25% 8% 2	687
	11th to 20th	Day	20% 12% 29% 24% 13% 2	775
	21st to end	Day	16% 8% 29% 27% 14% 7%	844
June	1st to 10th	Day	9% 6% 25% 33% 21% 6%	780
	11th to 20th	Day	9% 6% 29% 32% 19% 4	780
	21st to end	Day	10% 9% 32% 32% 14% 3	780
July	1st to 10th	Day	10% 2 18% 33% 24% 13%	771
	11th to 20th	Day	9% 4 21% 36% 22% 8%	768
	21st to end	Day	9% 1 19% 39% 26% 6%	849
August	1st to 10th	Day	8% 2 21% 36% 24% 9%	776
	11th to 20th	Day	11% 5% 18% 29% 28% 10%	669
	21st to end	Day	6% 3 18% 33% 29% 10%	627
September	1st to 10th	Day	12% 5% 18% 33% 24% 8%	570
	11th to 20th	Day	9% 2 18% 36% 26% 9%	570
	21st to end	Day	16% 6% 24% 23% 23% 8%	484
October	1st to 10th	Day	15% 5% 31% 27% 16% 7%	360
	11th to 20th	Day	16% 17% 35% 22% 9%	339
	21st to end	Day	24% 15% 35% 21% 4 1	363
November	1st to 10th	Day	12% 12% 40% 25% 8% 2	330
	11th to 20th	Day	22% 15% 33% 22% 7% 2	330
	21st to end	Day	24% 13% 32% 25% 5%	330
December	1st to 10th	Day	15% 14% 42% 20% 7% 2	330
	11th to 20th	Day	19% 16% 37% 22% 6%	330
	21st to end	Day	25% 19% 36% 17% 3	363
January	1st to 10th	Night	17% 13% 39% 24% 6%	1050
	11th to 20th	Night	17% 16% 40% 21% 5%	1050
	21st to end	Night	19% 14% 38% 25% 4	1155
February	1st to 10th	Night	21% 12% 39% 24% 4	1050
	11th to 20th	Night	18% 15% 36% 28% 3	951
	21st to end	Night	18% 14% 42% 23% 2 1	692
March	1st to 10th	Night	19% 14% 39% 23% 5%	791
	11th to 20th	Night	19% 11% 36% 28% 6%	630
	21st to end	Night	23% 14% 36% 22% 5% 1	693
April	1st to 10th	Night	20% 13% 39% 23% 3	630
	11th to 20th	Night	18% 13% 34% 28% 7%	623
	21st to end	Night	21% 10% 33% 29% 7%	630
May	1st to 10th	Night	17% 8% 38% 29% 7% 1	630
	11th to 20th	Night	17% 8% 30% 33% 11%	621
	21st to end	Night	11% 7% 29% 36% 17% 1	683
June	1st to 10th	Night	10% 4 23% 45% 17% 1	630
	11th to 20th	Night	9% 4 28% 40% 19%	630
	21st to end	Night	11% 3 29% 40% 17% 1	630
July	1st to 10th	Night	9% 1 14% 40% 32% 3	615
	11th to 20th	Night	6% 2 19% 43% 27% 3	612
	21st to end	Night	9% 1 14% 44% 29% 1	690
August	1st to 10th	Night	7% 3 13% 44% 30% 3	627
	11th to 20th	Night	9% 2 13% 42% 29% 4	624
	21st to end	Night	8% 1 14% 46% 29% 3	693
September	1st to 10th	Night	10% 4 18% 39% 26% 3	630
	11th to 20th	Night	11% 3 19% 35% 28% 5%	630
	21st to end	Night	9% 6% 24% 35% 24% 2	803
October	1st to 10th	Night	13% 6% 29% 31% 20% 1	840
	11th to 20th	Night	16% 11% 31% 32% 9%	813
	21st to end	Night	20% 12% 35% 26% 7%	880
November	1st to 10th	Night	15% 10% 41% 26% 8%	820
	11th to 20th	Night	18% 13% 39% 23% 6% 1	882
	21st to end	Night	21% 14% 36% 23% 6%	1050
December	1st to 10th	Night	17% 13% 39% 26% 4 1	1050
	11th to 20th	Night	21% 12% 35% 26% 6%	1050
	21st to end	Night	26% 15% 33% 22% 3	1155

Page generated at 2026-10-11 04:06:30. Version ac07

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