Wetzel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wetzel Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	17% 8% 2 6% 4 4 4 4 6% 7% 38%	108608
	Night	17% 8% 3 6% 4 4 4 4 5% 7% 37%	110574
Transparency	Day	44% 6% 11% 22% 15% 2	112163
	Night	45% 5% 11% 22% 14% 2	115183
Seeing	Day	23% 16% 30% 20% 8% 2	15343
	Night	22% 12% 31% 26% 8% 1	24300

By Month

Percentage Cloud Cover NE (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	13% 7% 3 7% 6% 4 5% 4 6% 7% 38%	3597
	11th to 20th	Day	12% 8% 3 7% 5% 4 4 3 5% 7% 42%	3570
	21st to end	Day	12% 10% 3 5% 4 3 4 3 4 4 47%	3960
February	1st to 10th	Day	16% 10% 2 7% 4 4 4 4 6% 6% 37%	1761
	11th to 20th	Day	12% 9% 3 7% 5% 5% 5% 4 6% 6% 38%	1948
	21st to end	Day	15% 10% 4 7% 5% 4 6% 4 6% 5% 36%	1650
March	1st to 10th	Day	23% 13% 2 8% 5% 4 4 3 4 5% 29%	3216
	11th to 20th	Day	16% 8% 2 6% 5% 4 5% 4 6% 6% 38%	3489
	21st to end	Day	23% 10% 2 5% 4 3 4 3 5% 6% 35%	4026
April	1st to 10th	Day	19% 10% 3 6% 4 4 4 3 4 7% 35%	2280
	11th to 20th	Day	18% 6% 2 5% 5% 5% 4 4 6% 7% 38%	2333
	21st to end	Day	22% 8% 3 7% 4 4 4 3 5% 4 34%	2470
May	1st to 10th	Day	22% 11% 3 7% 5% 5% 5% 4 6% 6% 27%	4106
	11th to 20th	Day	19% 8% 3 8% 6% 5% 4 4 6% 7% 31%	4197
	21st to end	Day	15% 6% 2 6% 5% 4 5% 5% 7% 9% 37%	4837
June	1st to 10th	Day	22% 8% 3 6% 4 5% 4 3 6% 8% 30%	2660
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 5% 5% 7% 39%	2660
	21st to end	Day	21% 8% 3 6% 5% 5% 5% 4 6% 7% 31%	2660
July	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 9% 35%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 8% 10% 38%	4440
	21st to end	Day	18% 7% 3 6% 4 4 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 9% 36%	4174
	11th to 20th	Day	16% 9% 2 8% 5% 4 5% 4 6% 7% 35%	4045
	21st to end	Day	10% 4 2 5% 4 4 5% 4 7% 9% 46%	4291
September	1st to 10th	Day	12% 4 2 4 4 4 4 4 6% 7% 49%	2252
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 6% 53%	2090
	21st to end	Day	16% 5% 2 5% 4 4 4 3 6% 7% 46%	2090
October	1st to 10th	Day	19% 8% 2 6% 4 4 3 3 4 6% 40%	2090
	11th to 20th	Day	17% 8% 2 5% 3 3 5% 2 4 6% 44%	1825
	21st to end	Day	23% 7% 2 6% 4 4 3 3 4 5% 38%	1881
November	1st to 10th	Day	16% 5% 2 6% 3 3 4 3 6% 7% 46%	1761
	11th to 20th	Day	20% 4 1 4 4 4 3 4 6% 6% 44%	1800
	21st to end	Day	22% 8% 2 5% 3 3 3 3 5% 7% 40%	1799
December	1st to 10th	Day	16% 9% 3 6% 5% 4 4 3 5% 8% 38%	2482
	11th to 20th	Day	19% 8% 2 4 3 3 3 3 5% 6% 43%	2954
	21st to end	Day	21% 10% 3 6% 4 4 5% 3 5% 5% 35%	3960
January	1st to 10th	Night	12% 8% 3 7% 5% 5% 5% 4 5% 6% 40%	6349
	11th to 20th	Night	12% 10% 4 9% 6% 5% 5% 4 6% 7% 33%	6315
	21st to end	Night	14% 9% 3 7% 4 5% 5% 4 5% 6% 39%	6996
February	1st to 10th	Night	15% 11% 3 8% 4 5% 4 4 5% 7% 34%	2634
	11th to 20th	Night	12% 10% 2 6% 6% 4 5% 4 5% 6% 41%	2338
	21st to end	Night	14% 9% 2 8% 5% 4 4 4 5% 7% 38%	1980
March	1st to 10th	Night	26% 12% 2 6% 3 3 4 2 4 4 33%	3844
	11th to 20th	Night	13% 8% 3 8% 5% 6% 6% 5% 6% 7% 33%	3689
	21st to end	Night	24% 9% 3 6% 4 3 4 4 6% 7% 31%	3742
April	1st to 10th	Night	20% 10% 3 6% 4 4 4 3 4 6% 36%	1900
	11th to 20th	Night	18% 7% 3 7% 5% 4 4 3 6% 6% 37%	1796
	21st to end	Night	22% 8% 3 6% 3 4 4 4 5% 6% 35%	1710
May	1st to 10th	Night	22% 10% 3 7% 4 5% 5% 4 6% 7% 27%	2834
	11th to 20th	Night	18% 8% 3 9% 5% 5% 6% 5% 7% 8% 26%	2725
	21st to end	Night	15% 7% 3 7% 4 4 4 4 6% 7% 39%	2738
June	1st to 10th	Night	21% 8% 2 7% 5% 3 4 4 6% 6% 34%	1520
	11th to 20th	Night	17% 6% 3 4 5% 4 4 4 5% 8% 40%	1520
	21st to end	Night	22% 6% 3 6% 5% 3 5% 3 6% 8% 34%	1520
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 6% 5% 8% 10% 35%	2520
	11th to 20th	Night	14% 6% 2 6% 5% 4 4 4 7% 9% 39%	2520
	21st to end	Night	15% 5% 2 5% 4 5% 4 4 6% 8% 41%	2739
August	1st to 10th	Night	21% 8% 3 7% 4 4 5% 4 6% 7% 33%	2725
	11th to 20th	Night	15% 9% 3 7% 4 4 5% 4 5% 8% 36%	2782
	21st to end	Night	11% 5% 2 5% 4 5% 5% 4 6% 8% 44%	3348
September	1st to 10th	Night	13% 5% 2 4 4 4 4 3 5% 8% 49%	1928
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 4 6% 52%	2090
	21st to end	Night	16% 6% 2 5% 3 4 3 3 5% 7% 47%	2090
October	1st to 10th	Night	19% 6% 2 6% 5% 5% 3 4 6% 6% 38%	2090
	11th to 20th	Night	17% 6% 2 5% 4 4 4 5% 6% 7% 41%	2354
	21st to end	Night	25% 7% 2 6% 3 3 3 3 5% 6% 36%	2717
November	1st to 10th	Night	16% 6% 3 6% 3 4 4 3 4 7% 44%	2537
	11th to 20th	Night	17% 7% 2 5% 3 4 4 4 5% 6% 44%	2600
	21st to end	Night	17% 8% 2 6% 4 4 4 2 6% 6% 41%	2600
December	1st to 10th	Night	16% 11% 3 7% 5% 4 4 3 6% 7% 34%	4580
	11th to 20th	Night	24% 7% 2 5% 4 3 3 3 4 5% 39%	5208
	21st to end	Night	20% 11% 3 7% 4 4 3 3 4 5% 37%	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	44% 5% 9% 27% 15% 1	3598
	11th to 20th	Day	40% 6% 12% 26% 13% 3	3600
	21st to end	Day	40% 4 9% 21% 19% 6%	3960
February	1st to 10th	Day	46% 5% 11% 20% 15% 3	1761
	11th to 20th	Day	46% 4 12% 19% 15% 4	1957
	21st to end	Day	48% 5% 10% 19% 15% 4	1650
March	1st to 10th	Day	58% 4 9% 11% 14% 4	3219
	11th to 20th	Day	43% 6% 12% 19% 15% 4	3489
	21st to end	Day	50% 3 7% 15% 20% 6%	4026
April	1st to 10th	Day	49% 5% 8% 18% 16% 4	2280
	11th to 20th	Day	44% 6% 10% 21% 16% 3	2333
	21st to end	Day	51% 6% 8% 16% 15% 4	2470
May	1st to 10th	Day	55% 8% 10% 16% 9% 1	4120
	11th to 20th	Day	50% 8% 14% 17% 10% 2	4201
	21st to end	Day	39% 8% 16% 20% 15% 2	4837
June	1st to 10th	Day	51% 7% 11% 21% 10%	2660
	11th to 20th	Day	42% 6% 12% 20% 16% 3	2660
	21st to end	Day	51% 7% 11% 18% 11% 1	2660
July	1st to 10th	Day	43% 7% 12% 25% 13% 1	4370
	11th to 20th	Day	39% 7% 16% 27% 11% 1	4365
	21st to end	Day	45% 6% 12% 20% 16% 1	4661
August	1st to 10th	Day	45% 7% 15% 24% 9%	4160
	11th to 20th	Day	48% 6% 14% 23% 8%	4084
	21st to end	Day	33% 7% 15% 25% 19% 2	4291
September	1st to 10th	Day	32% 8% 13% 28% 19% 1	2252
	11th to 20th	Day	32% 4 9% 30% 22% 3	2090
	21st to end	Day	37% 5% 10% 28% 18% 2	2090
October	1st to 10th	Day	45% 5% 9% 24% 16% 1	3480
	11th to 20th	Day	35% 4 10% 29% 21% 1	3002
	21st to end	Day	37% 3 9% 26% 21% 4	3081
November	1st to 10th	Day	38% 4 13% 26% 16% 3	1761
	11th to 20th	Day	39% 5% 10% 20% 22% 4	1800
	21st to end	Day	45% 4 16% 22% 13% 1	1799
December	1st to 10th	Day	45% 4 12% 27% 10% 1	2482
	11th to 20th	Day	40% 4 11% 19% 18% 8%	2954
	21st to end	Day	50% 5% 9% 18% 12% 6%	3960
January	1st to 10th	Night	42% 5% 11% 29% 12% 2	6349
	11th to 20th	Night	50% 5% 12% 20% 11% 2	6360
	21st to end	Night	45% 4 8% 19% 18% 6%	6996
February	1st to 10th	Night	49% 5% 11% 17% 15% 3	2634
	11th to 20th	Night	45% 4 10% 22% 16% 3	2340
	21st to end	Night	44% 5% 10% 22% 15% 5%	1980
March	1st to 10th	Night	56% 4 8% 17% 11% 4	3854
	11th to 20th	Night	45% 8% 12% 20% 12% 3	3690
	21st to end	Night	52% 3 7% 15% 18% 5%	3742
April	1st to 10th	Night	49% 5% 7% 19% 16% 4	1900
	11th to 20th	Night	46% 5% 9% 21% 19% 1	1796
	21st to end	Night	48% 6% 10% 17% 15% 4	1710
May	1st to 10th	Night	53% 8% 12% 17% 8% 1	2840
	11th to 20th	Night	52% 8% 14% 18% 7% 2	2726
	21st to end	Night	43% 7% 16% 20% 13% 1	2739
June	1st to 10th	Night	50% 7% 10% 21% 11% 1	1520
	11th to 20th	Night	43% 6% 14% 20% 15% 2	1520
	21st to end	Night	47% 9% 13% 21% 9% 2	1520
July	1st to 10th	Night	41% 7% 18% 21% 13% 1	2480
	11th to 20th	Night	42% 6% 14% 25% 13%	2472
	21st to end	Night	41% 8% 11% 27% 12% 1	2651
August	1st to 10th	Night	51% 7% 13% 21% 8%	2715
	11th to 20th	Night	46% 6% 15% 21% 11%	2809
	21st to end	Night	35% 7% 15% 25% 17% 1	3348
September	1st to 10th	Night	34% 6% 12% 31% 16%	1928
	11th to 20th	Night	34% 4 10% 29% 19% 2	2090
	21st to end	Night	37% 5% 10% 30% 16% 2	2090
October	1st to 10th	Night	45% 5% 12% 22% 15% 1	3480
	11th to 20th	Night	33% 5% 10% 27% 24% 2	3916
	21st to end	Night	40% 4 9% 27% 17% 3	4467
November	1st to 10th	Night	40% 4 11% 27% 16% 2	2537
	11th to 20th	Night	40% 4 11% 24% 18% 3	2600
	21st to end	Night	43% 5% 14% 22% 14% 1	2600
December	1st to 10th	Night	47% 5% 9% 28% 10%	4580
	11th to 20th	Night	48% 4 10% 16% 19% 3	5208
	21st to end	Night	51% 4 8% 19% 16% 2	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	28% 22% 25% 21% 4 1	200
	11th to 20th	Day	24% 23% 30% 21% 2 2	200
	21st to end	Day	18% 23% 35% 20% 5%	220
February	1st to 10th	Day	27% 19% 35% 17% 2	349
	11th to 20th	Day	27% 23% 35% 14% 2	496
	21st to end	Day	26% 18% 35% 17% 5%	414
March	1st to 10th	Day	27% 19% 36% 15% 4	500
	11th to 20th	Day	26% 24% 31% 14% 3 1	500
	21st to end	Day	29% 19% 31% 12% 6% 2	530
April	1st to 10th	Day	30% 22% 27% 14% 6% 1	460
	11th to 20th	Day	27% 22% 29% 15% 5% 2	452
	21st to end	Day	32% 18% 28% 13% 5% 2	460
May	1st to 10th	Day	31% 20% 24% 18% 5% 2	463
	11th to 20th	Day	20% 19% 34% 18% 7% 2	487
	21st to end	Day	22% 16% 32% 23% 8% 1	529
June	1st to 10th	Day	21% 13% 36% 19% 9% 2	490
	11th to 20th	Day	18% 16% 33% 21% 10% 2	490
	21st to end	Day	23% 14% 32% 21% 9% 1	490
July	1st to 10th	Day	17% 9% 32% 27% 11% 3	485
	11th to 20th	Day	15% 5% 37% 28% 11% 4	484
	21st to end	Day	15% 8% 35% 27% 13% 2	532
August	1st to 10th	Day	12% 10% 29% 29% 16% 3	486
	11th to 20th	Day	17% 10% 25% 28% 14% 5%	479
	21st to end	Day	12% 10% 28% 29% 14% 6%	539
September	1st to 10th	Day	14% 9% 30% 23% 18% 5%	490
	11th to 20th	Day	17% 13% 26% 21% 13% 10%	490
	21st to end	Day	18% 11% 31% 23% 10% 6%	490
October	1st to 10th	Day	24% 13% 24% 23% 13% 3	490
	11th to 20th	Day	20% 16% 30% 20% 11% 3	405
	21st to end	Day	31% 16% 26% 20% 8%	429
November	1st to 10th	Day	23% 16% 29% 21% 8% 2	395
	11th to 20th	Day	28% 17% 31% 19% 4 1	400
	21st to end	Day	31% 16% 26% 20% 7% 1	399
December	1st to 10th	Day	27% 24% 27% 17% 7%	200
	11th to 20th	Day	33% 16% 32% 15% 5%	200
	21st to end	Day	30% 19% 26% 20% 5%	220
January	1st to 10th	Night	21% 18% 36% 21% 4	800
	11th to 20th	Night	26% 16% 34% 21% 4	800
	21st to end	Night	19% 17% 38% 22% 3	880
February	1st to 10th	Night	24% 15% 35% 23% 3	800
	11th to 20th	Night	21% 18% 38% 21% 2	783
	21st to end	Night	22% 15% 35% 24% 5%	660
March	1st to 10th	Night	26% 16% 35% 21% 2	800
	11th to 20th	Night	23% 16% 34% 20% 6%	800
	21st to end	Night	26% 15% 33% 20% 5% 1	863
April	1st to 10th	Night	28% 14% 31% 24% 4	760
	11th to 20th	Night	22% 14% 36% 22% 5%	672
	21st to end	Night	26% 15% 32% 22% 5%	570
May	1st to 10th	Night	25% 11% 32% 27% 5% 1	570
	11th to 20th	Night	24% 11% 32% 27% 6%	513
	21st to end	Night	22% 8% 24% 36% 9% 1	411
June	1st to 10th	Night	24% 8% 29% 29% 9%	380
	11th to 20th	Night	20% 5% 26% 34% 14%	380
	21st to end	Night	24% 3 25% 37% 11%	380
July	1st to 10th	Night	16% 3 26% 39% 16%	370
	11th to 20th	Night	15% 4 24% 36% 21% 1	368
	21st to end	Night	15% 3 22% 42% 17% 1	416
August	1st to 10th	Night	15% 2 22% 43% 17%	506
	11th to 20th	Night	16% 4 27% 35% 17% 2	562
	21st to end	Night	14% 5% 25% 37% 17% 2	760
September	1st to 10th	Night	15% 3 30% 33% 17% 1	760
	11th to 20th	Night	18% 8% 26% 26% 20% 4	760
	21st to end	Night	19% 8% 28% 28% 16% 1	760
October	1st to 10th	Night	22% 10% 22% 30% 15% 1	760
	11th to 20th	Night	21% 10% 31% 25% 11% 1	760
	21st to end	Night	28% 14% 28% 24% 6%	836
November	1st to 10th	Night	21% 12% 32% 27% 6% 1	780
	11th to 20th	Night	23% 15% 32% 24% 6% 1	800
	21st to end	Night	22% 20% 31% 22% 5% 1	800
December	1st to 10th	Night	26% 11% 37% 20% 5%	800
	11th to 20th	Night	25% 18% 32% 21% 4	800
	21st to end	Night	27% 15% 32% 23% 3	880

Page generated at 2025-03-24 09:04:26. Version ac07

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