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 2024-03-23.

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% 8% 4 7% 6% 3 5% 5% 5% 7% 38%	78735
	Night	13% 8% 4 7% 5% 3 4 4 5% 6% 40%	81210
Transparency	Day	44% 7% 13% 22% 13% 2	78440
	Night	43% 6% 12% 22% 14% 2	81026
Seeing	Day	15% 10% 28% 27% 15% 5%	14829
	Night	16% 9% 31% 30% 12% 1	24892

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	11% 11% 5% 14% 10% 4 6% 4 4 6% 25%	2527
	11th to 20th	Day	12% 11% 5% 9% 10% 6% 7% 6% 5% 6% 24%	2635
	21st to end	Day	16% 16% 8% 15% 8% 3 3 3 4 5% 20%	3135
February	1st to 10th	Day	14% 11% 6% 9% 8% 3 5% 4 5% 6% 29%	1710
	11th to 20th	Day	14% 9% 3 6% 6% 4 4 4 5% 6% 37%	1732
	21st to end	Day	13% 12% 4 9% 6% 3 4 4 3 6% 37%	1570
March	1st to 10th	Day	14% 10% 5% 7% 5% 3 4 4 4 6% 37%	1961
	11th to 20th	Day	13% 9% 4 6% 6% 3 5% 4 6% 7% 36%	2090
	21st to end	Day	21% 12% 4 8% 6% 3 4 5% 4 5% 29%	2286
April	1st to 10th	Day	14% 9% 5% 7% 6% 3 4 4 4 7% 36%	2160
	11th to 20th	Day	15% 8% 3 8% 5% 3 4 4 5% 7% 38%	2134
	21st to end	Day	21% 7% 3 6% 6% 3 4 4 5% 7% 33%	2160
May	1st to 10th	Day	17% 7% 4 7% 6% 4 5% 5% 6% 8% 32%	2363
	11th to 20th	Day	17% 9% 2 6% 6% 3 4 5% 5% 7% 35%	2479
	21st to end	Day	14% 6% 3 7% 6% 4 6% 6% 6% 8% 34%	2725
June	1st to 10th	Day	8% 5% 2 6% 5% 3 5% 5% 5% 10% 46%	2520
	11th to 20th	Day	9% 4 3 5% 5% 3 5% 4 6% 8% 48%	2520
	21st to end	Day	10% 5% 3 5% 6% 3 6% 6% 7% 9% 41%	2520
July	1st to 10th	Day	8% 4 2 3 4 3 5% 5% 6% 10% 50%	2520
	11th to 20th	Day	8% 4 2 4 5% 4 6% 6% 7% 8% 48%	2520
	21st to end	Day	6% 4 2 5% 5% 4 4 5% 7% 11% 47%	2670
August	1st to 10th	Day	6% 4 2 6% 6% 4 6% 6% 7% 11% 40%	2328
	11th to 20th	Day	9% 4 2 6% 5% 3 5% 5% 6% 9% 45%	2220
	21st to end	Day	8% 4 2 4 5% 3 5% 4 5% 8% 52%	2376
September	1st to 10th	Day	12% 4 2 5% 4 3 5% 5% 6% 7% 48%	2160
	11th to 20th	Day	10% 3 1 3 3 3 4 4 5% 9% 54%	2160
	21st to end	Day	9% 5% 2 3 4 3 5% 6% 6% 7% 50%	1921
October	1st to 10th	Day	13% 6% 2 5% 4 3 4 3 5% 5% 49%	1800
	11th to 20th	Day	15% 8% 4 5% 4 3 3 4 4 6% 44%	1800
	21st to end	Day	18% 8% 3 6% 3 3 4 4 4 6% 41%	1949
November	1st to 10th	Day	10% 6% 3 4 6% 3 4 5% 5% 7% 47%	1672
	11th to 20th	Day	15% 9% 4 6% 6% 3 4 4 5% 8% 38%	1671
	21st to end	Day	16% 10% 4 6% 4 3 3 4 5% 6% 39%	1520
December	1st to 10th	Day	14% 11% 5% 9% 5% 3 5% 3 4 6% 34%	1520
	11th to 20th	Day	14% 11% 4 7% 5% 3 5% 5% 5% 7% 34%	1918
	21st to end	Day	21% 17% 8% 11% 6% 3 3 4 4 4 18%	2783
January	1st to 10th	Night	13% 10% 6% 13% 7% 3 5% 4 3 5% 30%	4428
	11th to 20th	Night	12% 15% 7% 10% 7% 3 6% 4 7% 6% 24%	4333
	21st to end	Night	17% 15% 6% 12% 6% 3 4 4 4 5% 24%	4532
February	1st to 10th	Night	14% 11% 4 8% 6% 4 5% 4 5% 5% 35%	2470
	11th to 20th	Night	12% 9% 4 7% 6% 3 4 5% 5% 6% 40%	2333
	21st to end	Night	14% 9% 3 8% 6% 3 3 4 4 5% 42%	1884
March	1st to 10th	Night	14% 8% 4 6% 5% 3 5% 4 4 5% 42%	2236
	11th to 20th	Night	14% 8% 3 6% 5% 3 5% 4 6% 6% 40%	2090
	21st to end	Night	19% 10% 4 9% 6% 4 5% 4 4 6% 27%	2113
April	1st to 10th	Night	15% 8% 3 6% 5% 3 4 4 6% 6% 41%	1800
	11th to 20th	Night	15% 10% 2 6% 4 3 3 4 4 6% 43%	1775
	21st to end	Night	18% 6% 2 6% 5% 3 4 4 4 7% 39%	1800
May	1st to 10th	Night	16% 6% 3 6% 6% 3 4 5% 5% 8% 39%	1592
	11th to 20th	Night	17% 5% 2 6% 5% 3 4 5% 5% 7% 41%	1416
	21st to end	Night	9% 6% 3 8% 4 3 6% 5% 7% 8% 41%	1551
June	1st to 10th	Night	8% 4 2 4 5% 3 5% 5% 6% 8% 50%	1440
	11th to 20th	Night	8% 4 3 4 5% 3 5% 5% 6% 10% 48%	1440
	21st to end	Night	9% 4 2 5% 4 3 5% 6% 6% 9% 47%	1440
July	1st to 10th	Night	8% 4 2 3 4 2 4 5% 6% 11% 51%	1440
	11th to 20th	Night	5% 3 2 3 3 2 3 5% 7% 11% 56%	1440
	21st to end	Night	7% 3 1 4 5% 4 4 5% 6% 9% 52%	1645
August	1st to 10th	Night	5% 3 3 5% 4 3 4 5% 6% 10% 52%	1611
	11th to 20th	Night	8% 4 2 4 4 2 4 5% 6% 6% 54%	1679
	21st to end	Night	7% 3 2 4 4 2 4 4 4 7% 60%	1980
September	1st to 10th	Night	12% 3 2 3 4 3 3 4 4 6% 56%	1800
	11th to 20th	Night	10% 4 2 4 4 3 4 5% 5% 7% 53%	1800
	21st to end	Night	7% 4 2 3 6% 3 4 4 5% 9% 53%	2018
October	1st to 10th	Night	11% 6% 2 5% 4 2 3 4 4 6% 53%	2160
	11th to 20th	Night	14% 5% 2 5% 5% 2 4 4 5% 6% 48%	2160
	21st to end	Night	17% 8% 3 5% 5% 3 4 4 4 6% 41%	2407
November	1st to 10th	Night	11% 6% 1 4 3 2 3 4 4 7% 54%	2406
	11th to 20th	Night	14% 7% 3 5% 5% 2 4 5% 5% 7% 43%	2508
	21st to end	Night	15% 9% 3 7% 6% 3 3 4 4 5% 40%	2660
December	1st to 10th	Night	12% 10% 5% 8% 6% 3 4 4 5% 6% 37%	2660
	11th to 20th	Night	14% 9% 4 7% 6% 3 4 3 5% 5% 38%	3279
	21st to end	Night	20% 17% 7% 9% 7% 4 4 4 4 4 21%	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	57% 10% 12% 14% 6%	2528
	11th to 20th	Day	60% 6% 10% 17% 7%	2635
	21st to end	Day	68% 6% 9% 11% 5%	3135
February	1st to 10th	Day	56% 5% 14% 17% 7% 1	1710
	11th to 20th	Day	47% 6% 14% 22% 10% 1	1741
	21st to end	Day	50% 5% 13% 20% 10% 2	1570
March	1st to 10th	Day	49% 6% 13% 20% 11% 1	1961
	11th to 20th	Day	46% 6% 14% 22% 11% 1	2090
	21st to end	Day	56% 6% 12% 18% 7% 1	2286
April	1st to 10th	Day	48% 6% 12% 22% 11%	2160
	11th to 20th	Day	45% 7% 10% 25% 11% 2	2134
	21st to end	Day	50% 7% 10% 18% 13% 1	2160
May	1st to 10th	Day	49% 7% 12% 21% 10% 1	2363
	11th to 20th	Day	48% 7% 12% 21% 12%	2483
	21st to end	Day	46% 9% 13% 17% 13% 2	2725
June	1st to 10th	Day	34% 9% 15% 25% 15% 2	2520
	11th to 20th	Day	34% 8% 11% 22% 22% 3	2520
	21st to end	Day	38% 9% 15% 22% 15% 2	2520
July	1st to 10th	Day	29% 8% 16% 29% 15% 3	2451
	11th to 20th	Day	31% 9% 15% 27% 14% 3	2445
	21st to end	Day	29% 9% 18% 28% 15% 1	2522
August	1st to 10th	Day	33% 11% 18% 26% 11% 1	2314
	11th to 20th	Day	34% 8% 13% 28% 15% 1	2220
	21st to end	Day	30% 7% 12% 28% 18% 4	2376
September	1st to 10th	Day	33% 8% 12% 24% 19% 4	2160
	11th to 20th	Day	28% 6% 13% 29% 20% 4	2160
	21st to end	Day	31% 7% 12% 29% 19% 2	1921
October	1st to 10th	Day	37% 5% 10% 24% 22% 1	1800
	11th to 20th	Day	41% 4 9% 26% 17% 2	1800
	21st to end	Day	45% 5% 12% 22% 14% 2	1949
November	1st to 10th	Day	36% 6% 15% 24% 16% 3	1672
	11th to 20th	Day	45% 5% 13% 21% 14% 1	1671
	21st to end	Day	47% 5% 15% 23% 10%	1520
December	1st to 10th	Day	52% 5% 13% 20% 8% 1	1520
	11th to 20th	Day	49% 6% 12% 22% 9% 2	1915
	21st to end	Day	68% 7% 8% 11% 5%	2783
January	1st to 10th	Night	56% 7% 11% 17% 8% 1	4428
	11th to 20th	Night	59% 4 12% 14% 10% 1	4333
	21st to end	Night	63% 5% 11% 15% 5% 1	4532
February	1st to 10th	Night	51% 4 13% 20% 11% 1	2470
	11th to 20th	Night	44% 6% 12% 24% 13% 1	2334
	21st to end	Night	45% 5% 9% 24% 14% 3	1884
March	1st to 10th	Night	44% 6% 12% 25% 12% 1	2236
	11th to 20th	Night	42% 7% 12% 23% 12% 3	2090
	21st to end	Night	57% 6% 11% 17% 8% 1	2113
April	1st to 10th	Night	43% 6% 11% 24% 14% 2	1800
	11th to 20th	Night	42% 5% 9% 23% 17% 4	1775
	21st to end	Night	45% 6% 12% 19% 14% 4	1800
May	1st to 10th	Night	42% 9% 14% 21% 13% 1	1592
	11th to 20th	Night	41% 8% 12% 20% 17% 1	1417
	21st to end	Night	37% 9% 14% 20% 16% 3	1552
June	1st to 10th	Night	30% 9% 15% 25% 18% 3	1440
	11th to 20th	Night	30% 8% 13% 22% 23% 5%	1440
	21st to end	Night	32% 8% 16% 19% 20% 3	1440
July	1st to 10th	Night	28% 7% 16% 30% 16% 4	1400
	11th to 20th	Night	21% 7% 18% 31% 20% 3	1392
	21st to end	Night	26% 7% 16% 27% 20% 3	1554
August	1st to 10th	Night	25% 9% 19% 29% 15% 2	1601
	11th to 20th	Night	27% 9% 12% 29% 20% 2	1679
	21st to end	Night	24% 7% 13% 28% 21% 7%	1980
September	1st to 10th	Night	29% 5% 11% 25% 25% 5%	1800
	11th to 20th	Night	29% 7% 12% 27% 20% 6%	1800
	21st to end	Night	28% 6% 10% 29% 24% 3	2018
October	1st to 10th	Night	32% 6% 11% 23% 24% 4	2160
	11th to 20th	Night	35% 6% 11% 25% 19% 3	2160
	21st to end	Night	44% 6% 11% 20% 17% 2	2407
November	1st to 10th	Night	30% 6% 15% 26% 19% 4	2406
	11th to 20th	Night	41% 6% 12% 25% 15% 2	2508
	21st to end	Night	46% 5% 12% 24% 13% 1	2660
December	1st to 10th	Night	48% 6% 14% 22% 10% 1	2660
	11th to 20th	Night	48% 6% 13% 24% 9% 1	3281
	21st to end	Night	66% 7% 10% 12% 6%	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	21% 14% 33% 23% 8%	270
	11th to 20th	Day	24% 18% 38% 17% 3	270
	21st to end	Day	20% 18% 34% 23% 5%	297
February	1st to 10th	Day	19% 19% 39% 19% 4	270
	11th to 20th	Day	24% 17% 31% 22% 5% 1	270
	21st to end	Day	19% 17% 44% 16% 2 1	225
March	1st to 10th	Day	17% 21% 37% 21% 4	270
	11th to 20th	Day	21% 18% 34% 20% 7% 1	270
	21st to end	Day	33% 18% 28% 15% 5% 1	356
April	1st to 10th	Day	20% 16% 36% 22% 5%	420
	11th to 20th	Day	20% 15% 33% 19% 12% 1	412
	21st to end	Day	21% 12% 34% 22% 9% 2	420
May	1st to 10th	Day	20% 12% 33% 26% 8% 2	546
	11th to 20th	Day	21% 11% 29% 23% 14% 2	625
	21st to end	Day	16% 8% 28% 25% 16% 7%	679
June	1st to 10th	Day	8% 5% 23% 33% 23% 7%	630
	11th to 20th	Day	8% 5% 28% 34% 21% 4	630
	21st to end	Day	9% 10% 29% 32% 15% 4	630
July	1st to 10th	Day	10% 2 17% 32% 25% 14%	621
	11th to 20th	Day	9% 5% 21% 35% 23% 7%	618
	21st to end	Day	7% 1 19% 39% 27% 7%	684
August	1st to 10th	Day	7% 2 20% 36% 25% 10%	626
	11th to 20th	Day	9% 5% 17% 29% 30% 10%	534
	21st to end	Day	7% 3 18% 32% 30% 11%	495
September	1st to 10th	Day	13% 5% 18% 31% 24% 8%	450
	11th to 20th	Day	7% 3 20% 36% 25% 10%	450
	21st to end	Day	12% 8% 26% 23% 25% 7%	377
October	1st to 10th	Day	17% 6% 31% 26% 13% 6%	270
	11th to 20th	Day	18% 19% 36% 19% 9%	270
	21st to end	Day	26% 14% 34% 20% 4 1	297
November	1st to 10th	Day	11% 11% 41% 26% 9% 3	270
	11th to 20th	Day	22% 15% 30% 24% 7% 2	270
	21st to end	Day	24% 12% 35% 24% 5%	270
December	1st to 10th	Day	16% 14% 39% 21% 7% 3	270
	11th to 20th	Day	18% 16% 37% 22% 8%	270
	21st to end	Day	23% 22% 35% 18% 3	297
January	1st to 10th	Night	17% 13% 40% 24% 7%	950
	11th to 20th	Night	18% 15% 39% 22% 6%	950
	21st to end	Night	19% 13% 37% 27% 4	1045
February	1st to 10th	Night	22% 11% 39% 24% 4	950
	11th to 20th	Night	17% 15% 36% 29% 3	859
	21st to end	Night	19% 14% 41% 23% 3 1	628
March	1st to 10th	Night	18% 13% 40% 23% 6%	716
	11th to 20th	Night	18% 11% 35% 30% 6%	570
	21st to end	Night	24% 13% 35% 23% 6% 1	600
April	1st to 10th	Night	19% 12% 41% 24% 4	540
	11th to 20th	Night	19% 12% 33% 28% 8%	533
	21st to end	Night	21% 9% 33% 29% 8% 1	540
May	1st to 10th	Night	18% 8% 36% 29% 7% 1	540
	11th to 20th	Night	17% 7% 31% 34% 11%	531
	21st to end	Night	10% 6% 28% 35% 19% 1	584
June	1st to 10th	Night	9% 4 23% 46% 17% 1	540
	11th to 20th	Night	9% 3 26% 41% 20% 1	540
	21st to end	Night	9% 3 29% 41% 18% 1	540
July	1st to 10th	Night	8% 2 13% 40% 34% 4	525
	11th to 20th	Night	5% 2 20% 43% 27% 2	522
	21st to end	Night	8% 2 14% 46% 30% 1	591
August	1st to 10th	Night	6% 3 12% 46% 31% 3	537
	11th to 20th	Night	9% 3 13% 40% 31% 5%	534
	21st to end	Night	9% 1 13% 44% 30% 3	594
September	1st to 10th	Night	12% 4 18% 37% 26% 3	540
	11th to 20th	Night	11% 3 19% 35% 27% 5%	540
	21st to end	Night	8% 6% 25% 34% 24% 3	688
October	1st to 10th	Night	14% 6% 29% 29% 20% 1	720
	11th to 20th	Night	17% 12% 31% 31% 9%	720
	21st to end	Night	21% 12% 35% 25% 7%	792
November	1st to 10th	Night	15% 9% 41% 26% 9% 1	740
	11th to 20th	Night	17% 13% 39% 24% 6% 1	798
	21st to end	Night	20% 13% 37% 24% 6%	950
December	1st to 10th	Night	18% 13% 38% 26% 5% 1	950
	11th to 20th	Night	21% 12% 34% 26% 6%	950
	21st to end	Night	25% 16% 33% 23% 3	1045

Page generated at 2024-03-19 22:31:05. Version ac07

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