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 2025-06-10.

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% 39%	119832
	Night	12% 9% 3 8% 4 4 4 3 5% 6% 41%	118624
Transparency	Day	42% 6% 12% 23% 15% 2	123442
	Night	42% 5% 12% 23% 16% 2	123158
Seeing	Day	15% 10% 29% 27% 15% 4	16561
	Night	16% 10% 32% 30% 12% 1	26483

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% 12% 5% 16% 7% 6% 6% 3 4 5% 26%	3617
	11th to 20th	Day	8% 8% 4 10% 6% 7% 6% 4 5% 6% 33%	3781
	21st to end	Day	13% 14% 6% 14% 6% 4 4 3 4 5% 28%	4488
February	1st to 10th	Day	14% 12% 4 11% 7% 5% 5% 3 6% 6% 28%	1800
	11th to 20th	Day	14% 10% 3 8% 5% 5% 4 3 5% 6% 36%	1826
	21st to end	Day	12% 12% 3 10% 5% 4 3 4 3 6% 38%	1650
March	1st to 10th	Day	17% 11% 3 5% 3 2 3 2 3 5% 45%	4635
	11th to 20th	Day	8% 6% 3 6% 4 4 5% 4 6% 8% 47%	4980
	21st to end	Day	19% 11% 3 10% 5% 4 4 4 5% 7% 28%	5687
April	1st to 10th	Day	14% 11% 3 8% 6% 4 4 4 4 7% 35%	2400
	11th to 20th	Day	15% 8% 3 9% 4 4 4 3 5% 7% 38%	2374
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 6% 7% 33%	2400
May	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 9% 36%	5943
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 7% 44%	6319
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	6935
June	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 5% 9% 46%	2787
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 3 6% 8% 47%	2660
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 8% 41%	2660
July	1st to 10th	Day	8% 8% 2 7% 6% 6% 6% 5% 8% 10% 35%	4440
	11th to 20th	Day	6% 4 1 4 4 5% 5% 4 6% 8% 51%	4440
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 6% 8% 11% 41%	4673
August	1st to 10th	Day	5% 5% 2 6% 5% 5% 6% 5% 8% 11% 43%	4104
	11th to 20th	Day	10% 5% 3 7% 4 5% 5% 4 7% 9% 40%	3890
	21st to end	Day	7% 5% 2 6% 4 4 4 3 6% 9% 51%	4180
September	1st to 10th	Day	11% 4 2 5% 3 4 4 4 6% 7% 50%	2280
	11th to 20th	Day	10% 4 1 4 3 4 4 3 6% 9% 53%	2280
	21st to end	Day	10% 5% 2 4 4 3 6% 5% 5% 6% 50%	2026
October	1st to 10th	Day	12% 6% 2 6% 3 4 4 2 4 5% 51%	1900
	11th to 20th	Day	14% 8% 3 6% 4 3 3 3 4 6% 46%	1900
	21st to end	Day	17% 8% 2 7% 3 4 4 3 4 6% 42%	2057
November	1st to 10th	Day	11% 7% 2 5% 5% 4 4 4 5% 7% 45%	1762
	11th to 20th	Day	14% 10% 3 7% 5% 4 3 3 5% 8% 37%	1758
	21st to end	Day	15% 11% 4 7% 4 4 3 3 5% 6% 38%	1600
December	1st to 10th	Day	8% 8% 3 8% 4 4 4 3 4 6% 48%	2610
	11th to 20th	Day	14% 10% 4 14% 6% 5% 5% 4 5% 6% 27%	3008
	21st to end	Day	21% 21% 8% 12% 5% 3 3 2 4 4 16%	3982
January	1st to 10th	Night	12% 11% 5% 14% 5% 4 4 3 4 6% 31%	6348
	11th to 20th	Night	10% 12% 5% 12% 7% 5% 6% 3 7% 6% 27%	6197
	21st to end	Night	14% 13% 5% 11% 5% 4 4 3 4 5% 32%	6490
February	1st to 10th	Night	13% 12% 4 9% 5% 5% 4 3 5% 5% 34%	2600
	11th to 20th	Night	12% 10% 4 8% 5% 4 4 3 5% 6% 39%	2459
	21st to end	Night	14% 9% 3 9% 5% 3 3 3 4 4 43%	1980
March	1st to 10th	Night	17% 9% 4 6% 3 4 3 2 3 5% 42%	5248
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 7% 51%	4980
	21st to end	Night	15% 12% 5% 10% 5% 5% 5% 4 5% 6% 29%	5241
April	1st to 10th	Night	15% 8% 3 6% 5% 4 4 3 5% 6% 42%	2000
	11th to 20th	Night	14% 9% 2 6% 4 4 3 3 4 6% 43%	1975
	21st to end	Night	18% 6% 2 6% 5% 4 4 3 5% 7% 39%	2000
May	1st to 10th	Night	17% 8% 3 6% 4 4 4 4 5% 7% 39%	3961
	11th to 20th	Night	12% 4 2 5% 4 4 5% 4 6% 9% 44%	3596
	21st to end	Night	11% 7% 3 7% 3 4 4 3 6% 8% 43%	3942
June	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 8% 50%	1597
	11th to 20th	Night	8% 5% 2 4 5% 4 5% 4 7% 9% 49%	1520
	21st to end	Night	9% 6% 2 5% 4 4 5% 4 6% 9% 46%	1520
July	1st to 10th	Night	13% 5% 2 6% 4 3 5% 4 6% 9% 42%	2530
	11th to 20th	Night	4 3 2 4 3 3 3 4 6% 9% 59%	2530
	21st to end	Night	7% 4 2 5% 4 4 5% 4 7% 9% 49%	2873
August	1st to 10th	Night	4 3 2 5% 4 3 4 4 6% 10% 56%	2836
	11th to 20th	Night	7% 4 2 5% 4 4 3 4 6% 7% 55%	2942
	21st to end	Night	6% 3 2 6% 4 3 4 3 5% 9% 55%	3476
September	1st to 10th	Night	11% 3 2 4 3 3 3 3 5% 6% 58%	1900
	11th to 20th	Night	10% 5% 2 5% 3 4 4 4 5% 7% 52%	1900
	21st to end	Night	8% 5% 2 5% 5% 4 4 3 5% 8% 52%	2131
October	1st to 10th	Night	11% 6% 2 6% 3 3 3 3 4 6% 54%	2280
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 50%	2280
	21st to end	Night	16% 8% 3 6% 4 4 4 3 4 6% 41%	2541
November	1st to 10th	Night	11% 7% 1 5% 3 3 3 3 5% 7% 53%	2536
	11th to 20th	Night	14% 9% 3 6% 4 4 4 4 5% 6% 42%	2640
	21st to end	Night	15% 10% 3 8% 5% 4 3 3 4 5% 39%	2800
December	1st to 10th	Night	7% 9% 3 6% 4 4 3 4 5% 6% 48%	4580
	11th to 20th	Night	14% 9% 4 11% 6% 5% 5% 3 5% 5% 33%	5199
	21st to end	Night	21% 19% 7% 11% 5% 5% 4 3 4 4 18%	6996

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	58% 7% 11% 15% 8% 1	3618
	11th to 20th	Day	51% 5% 9% 17% 15% 4	3781
	21st to end	Day	59% 4 10% 19% 7% 1	4488
February	1st to 10th	Day	57% 4 15% 17% 7% 1	1800
	11th to 20th	Day	48% 6% 14% 21% 10% 1	1835
	21st to end	Day	48% 5% 13% 21% 11% 2	1650
March	1st to 10th	Day	43% 5% 8% 23% 18% 2	4635
	11th to 20th	Day	34% 5% 11% 31% 18% 1	4980
	21st to end	Day	55% 5% 15% 18% 5% 1	5689
April	1st to 10th	Day	49% 6% 12% 22% 12%	2400
	11th to 20th	Day	45% 7% 11% 24% 11% 2	2374
	21st to end	Day	49% 7% 11% 19% 13% 1	2400
May	1st to 10th	Day	44% 7% 11% 20% 17% 1	5979
	11th to 20th	Day	40% 5% 11% 27% 13% 3	6323
	21st to end	Day	44% 6% 11% 23% 15% 1	6949
June	1st to 10th	Day	34% 9% 15% 25% 15% 2	2787
	11th to 20th	Day	34% 8% 11% 22% 22% 3	2660
	21st to end	Day	37% 9% 15% 23% 15% 2	2660
July	1st to 10th	Day	43% 6% 13% 25% 12% 2	4371
	11th to 20th	Day	29% 6% 15% 30% 18% 2	4365
	21st to end	Day	35% 6% 21% 22% 15% 1	4526
August	1st to 10th	Day	32% 7% 18% 30% 12% 1	4090
	11th to 20th	Day	39% 5% 14% 27% 14% 1	3929
	21st to end	Day	31% 5% 15% 32% 15% 3	4179
September	1st to 10th	Day	32% 7% 11% 24% 21% 4	2280
	11th to 20th	Day	29% 6% 13% 29% 20% 4	2280
	21st to end	Day	32% 7% 12% 28% 19% 2	2026
October	1st to 10th	Day	23% 3 6% 21% 44% 2	3160
	11th to 20th	Day	27% 3 10% 23% 31% 6%	3140
	21st to end	Day	40% 3 9% 27% 18% 3	3371
November	1st to 10th	Day	38% 5% 15% 24% 15% 3	1762
	11th to 20th	Day	46% 5% 13% 21% 14% 2	1758
	21st to end	Day	47% 4 15% 23% 9%	1600
December	1st to 10th	Day	39% 3 14% 34% 10%	2610
	11th to 20th	Day	57% 4 11% 16% 9% 2	3005
	21st to end	Day	72% 5% 9% 10% 3	3982
January	1st to 10th	Night	55% 5% 11% 21% 7% 1	6348
	11th to 20th	Night	56% 3 12% 13% 14% 2	6197
	21st to end	Night	56% 4 9% 19% 12% 1	6490
February	1st to 10th	Night	51% 4 13% 20% 11% 1	2600
	11th to 20th	Night	46% 5% 12% 23% 13% 1	2460
	21st to end	Night	44% 5% 10% 24% 15% 3	1980
March	1st to 10th	Night	45% 5% 7% 25% 16% 2	5248
	11th to 20th	Night	33% 5% 9% 30% 18% 5%	4980
	21st to end	Night	55% 7% 14% 16% 7% 1	5241
April	1st to 10th	Night	42% 6% 11% 25% 14% 2	2000
	11th to 20th	Night	41% 5% 10% 23% 17% 4	1975
	21st to end	Night	45% 6% 12% 20% 14% 4	2000
May	1st to 10th	Night	43% 7% 12% 19% 17% 2	3990
	11th to 20th	Night	35% 8% 14% 24% 15% 3	3597
	21st to end	Night	37% 7% 11% 25% 18% 1	3950
June	1st to 10th	Night	29% 8% 14% 26% 18% 3	1597
	11th to 20th	Night	29% 7% 13% 23% 23% 4	1520
	21st to end	Night	33% 8% 16% 20% 20% 4	1520
July	1st to 10th	Night	39% 5% 15% 25% 14% 2	2490
	11th to 20th	Night	21% 5% 16% 34% 22% 2	2482
	21st to end	Night	30% 5% 20% 23% 19% 3	2782
August	1st to 10th	Night	24% 6% 19% 31% 18% 2	2826
	11th to 20th	Night	29% 6% 15% 32% 18% 1	2969
	21st to end	Night	28% 5% 15% 31% 16% 5%	3476
September	1st to 10th	Night	27% 5% 11% 26% 26% 5%	1900
	11th to 20th	Night	30% 7% 12% 26% 20% 6%	1900
	21st to end	Night	29% 6% 10% 28% 24% 3	2131
October	1st to 10th	Night	22% 4 7% 23% 41% 3	3800
	11th to 20th	Night	26% 4 9% 22% 33% 6%	3779
	21st to end	Night	44% 4 7% 21% 20% 4	4177
November	1st to 10th	Night	31% 6% 15% 25% 19% 4	2536
	11th to 20th	Night	41% 6% 12% 24% 15% 2	2640
	21st to end	Night	47% 4 11% 24% 13% 1	2800
December	1st to 10th	Night	37% 4 13% 34% 11% 1	4580
	11th to 20th	Night	53% 4 12% 22% 8% 1	5201
	21st to end	Night	70% 5% 9% 12% 4	6996

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	21% 14% 33% 24% 8%	300
	11th to 20th	Day	22% 18% 41% 16% 3	300
	21st to end	Day	19% 17% 38% 21% 4	330
February	1st to 10th	Day	18% 19% 39% 20% 4	300
	11th to 20th	Day	23% 17% 34% 20% 4 1	300
	21st to end	Day	20% 16% 44% 17% 2 1	249
March	1st to 10th	Day	18% 20% 38% 20% 3	300
	11th to 20th	Day	20% 19% 34% 20% 6% 1	300
	21st to end	Day	31% 19% 30% 15% 4 1	426
April	1st to 10th	Day	22% 17% 35% 21% 5%	500
	11th to 20th	Day	20% 17% 33% 19% 10% 1	492
	21st to end	Day	20% 14% 35% 21% 8% 2	500
May	1st to 10th	Day	18% 11% 35% 25% 9% 2	640
	11th to 20th	Day	20% 11% 29% 24% 14% 2	725
	21st to end	Day	16% 8% 29% 27% 14% 6%	789
June	1st to 10th	Day	8% 7% 25% 33% 21% 6%	726
	11th to 20th	Day	8% 5% 29% 34% 21% 4	680
	21st to end	Day	10% 10% 31% 31% 15% 3	680
July	1st to 10th	Day	11% 2 18% 32% 24% 13%	671
	11th to 20th	Day	9% 4 21% 36% 23% 7%	668
	21st to end	Day	7% 1 19% 39% 27% 7%	739
August	1st to 10th	Day	8% 2 21% 36% 24% 9%	676
	11th to 20th	Day	10% 5% 18% 29% 29% 9%	579
	21st to end	Day	7% 3 17% 32% 31% 11%	539
September	1st to 10th	Day	12% 5% 18% 32% 24% 9%	490
	11th to 20th	Day	8% 3 19% 36% 26% 9%	490
	21st to end	Day	13% 8% 26% 23% 23% 7%	412
October	1st to 10th	Day	16% 6% 31% 25% 15% 7%	300
	11th to 20th	Day	17% 18% 34% 21% 10%	300
	21st to end	Day	25% 14% 35% 20% 5% 1	330
November	1st to 10th	Day	13% 12% 38% 26% 8% 3	300
	11th to 20th	Day	23% 15% 30% 23% 7% 2	300
	21st to end	Day	23% 13% 34% 25% 5%	300
December	1st to 10th	Day	15% 15% 41% 19% 7% 3	300
	11th to 20th	Day	18% 17% 37% 22% 7%	300
	21st to end	Day	25% 20% 34% 18% 3	330
January	1st to 10th	Night	17% 13% 39% 24% 6%	1000
	11th to 20th	Night	18% 16% 39% 22% 6%	1000
	21st to end	Night	19% 13% 37% 26% 4	1100
February	1st to 10th	Night	21% 12% 39% 23% 4	1000
	11th to 20th	Night	18% 16% 35% 28% 3	905
	21st to end	Night	18% 14% 41% 24% 2 1	660
March	1st to 10th	Night	19% 14% 40% 23% 5%	753
	11th to 20th	Night	18% 11% 36% 29% 6%	600
	21st to end	Night	23% 13% 36% 22% 5% 1	660
April	1st to 10th	Night	19% 13% 39% 25% 4	600
	11th to 20th	Night	18% 13% 34% 28% 7%	593
	21st to end	Night	21% 10% 33% 29% 7% 1	600
May	1st to 10th	Night	17% 9% 36% 30% 7% 1	600
	11th to 20th	Night	17% 7% 29% 35% 12%	591
	21st to end	Night	11% 6% 29% 35% 17% 1	650
June	1st to 10th	Night	9% 4 23% 46% 17% 1	599
	11th to 20th	Night	9% 3 27% 41% 19% 1	570
	21st to end	Night	10% 3 29% 39% 18% 1	570
July	1st to 10th	Night	9% 1 14% 40% 32% 4	555
	11th to 20th	Night	5% 2 21% 43% 26% 2	552
	21st to end	Night	9% 1 13% 45% 30% 1	624
August	1st to 10th	Night	6% 3 14% 45% 31% 2	567
	11th to 20th	Night	9% 2 13% 41% 29% 5%	564
	21st to end	Night	9% 1 13% 45% 30% 3	627
September	1st to 10th	Night	11% 4 18% 38% 26% 3	570
	11th to 20th	Night	11% 3 19% 35% 27% 5%	570
	21st to end	Night	8% 6% 26% 34% 23% 2	727
October	1st to 10th	Night	14% 6% 29% 29% 21% 1	760
	11th to 20th	Night	16% 12% 30% 32% 10%	760
	21st to end	Night	20% 12% 35% 26% 7%	836
November	1st to 10th	Night	16% 9% 40% 26% 9% 1	780
	11th to 20th	Night	18% 13% 39% 23% 6% 1	840
	21st to end	Night	20% 14% 37% 24% 6%	1000
December	1st to 10th	Night	17% 13% 39% 26% 4 1	1000
	11th to 20th	Night	21% 12% 35% 26% 6%	1000
	21st to end	Night	26% 15% 32% 23% 3	1100

Page generated at 2025-06-07 09:28:15. Version ac07

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