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 2024-11-03.

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 6% 7% 39%	101415
	Night	12% 8% 3 7% 4 4 4 3 5% 7% 41%	97660
Transparency	Day	43% 7% 13% 23% 14% 1	101208
	Night	42% 6% 13% 23% 15% 2	97539
Seeing	Day	15% 10% 28% 27% 15% 5%	15791
	Night	16% 9% 31% 30% 12% 1	25610

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% 16% 8% 5% 5% 3 4 6% 25%	2527
	11th to 20th	Day	12% 11% 4 11% 8% 7% 7% 4 5% 6% 24%	2635
	21st to end	Day	15% 17% 8% 16% 6% 4 3 3 4 5% 19%	3135
February	1st to 10th	Day	14% 12% 4 11% 7% 5% 4 3 5% 6% 28%	1710
	11th to 20th	Day	14% 10% 3 8% 4 5% 4 4 6% 6% 36%	1732
	21st to end	Day	13% 12% 3 10% 5% 4 3 4 3 6% 36%	1570
March	1st to 10th	Day	16% 12% 4 6% 4 3 4 2 4 5% 40%	3233
	11th to 20th	Day	9% 7% 3 5% 4 4 4 4 6% 7% 47%	3480
	21st to end	Day	20% 13% 3 11% 5% 4 4 3 4 6% 27%	3969
April	1st to 10th	Day	14% 11% 3 8% 6% 4 4 4 4 7% 36%	2280
	11th to 20th	Day	15% 9% 3 9% 4 4 4 3 5% 7% 38%	2254
	21st to end	Day	21% 8% 3 7% 5% 4 4 3 5% 7% 33%	2280
May	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 8% 35%	4135
	11th to 20th	Day	16% 8% 3 7% 4 4 4 3 5% 7% 40%	4399
	21st to end	Day	12% 6% 2 6% 4 4 4 4 5% 6% 45%	4824
June	1st to 10th	Day	8% 5% 2 6% 5% 5% 4 5% 5% 10% 46%	2660
	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	10% 7% 2 5% 5% 4 4 4 5% 7% 46%	1690
	11th to 20th	Day	14% 10% 3 7% 5% 4 3 3 5% 8% 37%	1671
	21st to end	Day	16% 11% 3 7% 4 4 3 3 5% 6% 39%	1520
December	1st to 10th	Day	14% 11% 5% 10% 5% 4 4 3 4 5% 34%	1520
	11th to 20th	Day	13% 11% 4 8% 4 4 5% 4 6% 7% 34%	1918
	21st to end	Day	21% 17% 8% 12% 5% 4 4 3 4 4 18%	2783
January	1st to 10th	Night	13% 11% 6% 14% 6% 5% 5% 3 3 5% 29%	4428
	11th to 20th	Night	11% 16% 6% 12% 6% 4 5% 3 8% 6% 24%	4333
	21st to end	Night	17% 16% 6% 13% 5% 4 4 3 4 5% 24%	4532
February	1st to 10th	Night	13% 12% 4 9% 5% 5% 4 3 6% 5% 34%	2470
	11th to 20th	Night	12% 10% 3 8% 5% 4 4 4 5% 6% 40%	2333
	21st to end	Night	14% 10% 3 9% 5% 3 3 3 4 4 42%	1884
March	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 4 5% 41%	3650
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 6% 51%	3480
	21st to end	Night	19% 12% 5% 11% 5% 5% 4 3 5% 6% 24%	3661
April	1st to 10th	Night	15% 9% 3 6% 5% 4 4 3 5% 6% 41%	1900
	11th to 20th	Night	14% 10% 2 6% 4 4 3 3 4 6% 44%	1875
	21st to end	Night	18% 6% 2 6% 5% 4 4 3 5% 7% 39%	1900
May	1st to 10th	Night	19% 8% 3 6% 4 4 4 4 5% 7% 36%	2762
	11th to 20th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 43%	2506
	21st to end	Night	7% 5% 2 6% 3 4 5% 4 7% 8% 49%	2743
June	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 8% 50%	1520
	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% 9% 3 6% 4 4 4 3 4 6% 41%	2541
November	1st to 10th	Night	10% 6% 1 5% 3 3 3 3 5% 7% 54%	2433
	11th to 20th	Night	13% 8% 3 6% 4 4 4 4 5% 7% 42%	2508
	21st to end	Night	15% 10% 3 7% 5% 4 3 3 4 5% 40%	2660
December	1st to 10th	Night	12% 11% 4 9% 5% 4 3 3 5% 6% 37%	2660
	11th to 20th	Night	14% 9% 3 9% 6% 4 4 3 5% 5% 37%	3279
	21st to end	Night	20% 18% 7% 11% 6% 5% 3 3 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	46% 7% 11% 20% 15% 1	3233
	11th to 20th	Day	35% 7% 15% 25% 18% 1	3480
	21st to end	Day	56% 7% 15% 17% 4 1	3969
April	1st to 10th	Day	48% 6% 12% 22% 11%	2280
	11th to 20th	Day	45% 7% 10% 24% 12% 2	2254
	21st to end	Day	49% 8% 10% 18% 14% 1	2280
May	1st to 10th	Day	44% 9% 12% 19% 16% 1	4171
	11th to 20th	Day	43% 7% 11% 28% 11%	4403
	21st to end	Day	37% 8% 11% 23% 20% 1	4837
June	1st to 10th	Day	33% 9% 15% 25% 15% 2	2660
	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	36% 5% 10% 24% 24% 1	1900
	11th to 20th	Day	40% 4 9% 25% 19% 3	1900
	21st to end	Day	44% 5% 12% 22% 15% 2	2057
November	1st to 10th	Day	36% 6% 15% 24% 15% 3	1690
	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	45% 7% 10% 24% 14% 1	3650
	11th to 20th	Night	33% 7% 11% 30% 15% 5%	3480
	21st to end	Night	59% 9% 14% 13% 5%	3661
April	1st to 10th	Night	43% 6% 11% 24% 14% 2	1900
	11th to 20th	Night	41% 6% 10% 23% 17% 4	1875
	21st to end	Night	44% 6% 12% 20% 14% 4	1900
May	1st to 10th	Night	44% 10% 13% 17% 15% 1	2791
	11th to 20th	Night	36% 11% 14% 25% 13% 1	2507
	21st to end	Night	28% 10% 13% 24% 23% 2	2751
June	1st to 10th	Night	30% 9% 15% 25% 18% 3	1520
	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	31% 6% 10% 23% 26% 4	2280
	11th to 20th	Night	34% 6% 11% 24% 21% 4	2280
	21st to end	Night	44% 6% 10% 20% 18% 2	2541
November	1st to 10th	Night	29% 6% 15% 26% 19% 4	2433
	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	32% 18% 29% 15% 4 1	388
April	1st to 10th	Day	21% 16% 36% 21% 5%	460
	11th to 20th	Day	20% 15% 33% 19% 11% 1	452
	21st to end	Day	21% 13% 35% 21% 9% 2	460
May	1st to 10th	Day	19% 12% 34% 26% 8% 2	593
	11th to 20th	Day	20% 11% 29% 24% 14% 2	675
	21st to end	Day	16% 8% 28% 26% 15% 7%	734
June	1st to 10th	Day	8% 6% 25% 33% 21% 7%	680
	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	11% 11% 41% 26% 9% 3	276
	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% 22% 5% 1	627
April	1st to 10th	Night	19% 13% 40% 24% 4	570
	11th to 20th	Night	18% 13% 34% 28% 7%	563
	21st to end	Night	21% 10% 33% 29% 7% 1	570
May	1st to 10th	Night	18% 9% 36% 29% 7% 1	570
	11th to 20th	Night	17% 7% 30% 35% 11%	561
	21st to end	Night	10% 6% 29% 36% 18% 1	617
June	1st to 10th	Night	9% 4 23% 46% 17% 1	570
	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	15% 9% 41% 26% 9% 1	749
	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

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