Wheaton College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wheaton College Observatory clear sky chart from 2005-11-06 to 2024-11-11.

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	18% 8% 2 6% 4 4 4 3 5% 7% 38%	101240
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	97945
Transparency	Day	45% 4 11% 23% 15% 2	104804
	Night	45% 4 11% 23% 15% 2	102470
Seeing	Day	26% 16% 32% 18% 7% 2	15133
	Night	23% 14% 32% 23% 6% 1	24023

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	14% 10% 3 6% 3 3 3 3 3 5% 45%	2517
	11th to 20th	Day	18% 9% 3 6% 3 4 4 3 5% 4 40%	2520
	21st to end	Day	20% 13% 3 7% 4 4 3 3 5% 6% 33%	2772
February	1st to 10th	Day	20% 7% 2 6% 4 4 5% 5% 5% 6% 35%	1802
	11th to 20th	Day	15% 8% 3 6% 4 4 4 4 4 7% 41%	1848
	21st to end	Day	19% 10% 3 7% 4 4 4 3 4 5% 37%	1570
March	1st to 10th	Day	25% 16% 3 6% 4 4 3 2 4 5% 30%	3116
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 7% 42%	3347
	21st to end	Day	23% 11% 2 6% 4 4 4 3 4 5% 35%	4003
April	1st to 10th	Day	21% 10% 2 6% 3 3 4 3 5% 6% 37%	2280
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 6% 7% 39%	2267
	21st to end	Day	21% 6% 3 7% 5% 4 3 3 5% 6% 38%	2470
May	1st to 10th	Day	25% 13% 3 7% 3 3 3 3 5% 6% 28%	4106
	11th to 20th	Day	24% 10% 3 7% 5% 4 4 3 4 6% 32%	4098
	21st to end	Day	16% 5% 2 5% 4 4 4 3 5% 8% 45%	4837
June	1st to 10th	Day	19% 8% 2 6% 4 4 4 4 6% 8% 35%	2660
	11th to 20th	Day	17% 5% 2 6% 5% 4 5% 4 6% 8% 37%	2660
	21st to end	Day	16% 7% 2 5% 5% 5% 5% 4 6% 8% 37%	2660
July	1st to 10th	Day	14% 6% 2 6% 5% 5% 6% 5% 7% 9% 33%	4440
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 7% 10% 37%	4440
	21st to end	Day	19% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	4814
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 8% 31%	4108
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 4 6% 8% 37%	3988
	21st to end	Day	8% 4 1 5% 4 4 6% 5% 7% 9% 46%	4180
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 3 6% 6% 45%	2266
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 7% 50%	2090
	21st to end	Day	19% 7% 2 6% 3 4 3 3 4 7% 43%	2090
October	1st to 10th	Day	24% 5% 2 5% 3 3 3 2 4 5% 42%	2090
	11th to 20th	Day	15% 6% 2 4 4 3 4 2 4 7% 48%	1871
	21st to end	Day	19% 7% 2 5% 4 4 3 3 5% 6% 42%	1881
November	1st to 10th	Day	16% 7% 2 5% 3 3 3 3 4 5% 49%	1761
	11th to 20th	Day	19% 7% 2 4 4 3 3 3 4 5% 46%	1710
	21st to end	Day	18% 8% 2 4 4 3 4 3 5% 6% 44%	1710
December	1st to 10th	Day	20% 8% 2 6% 3 4 4 3 4 6% 39%	1590
	11th to 20th	Day	21% 7% 2 4 4 3 3 3 3 5% 44%	1906
	21st to end	Day	21% 10% 4 7% 4 3 4 3 5% 5% 34%	2772
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 2 4 6% 48%	4429
	11th to 20th	Night	16% 11% 3 7% 4 4 3 3 4 5% 40%	4440
	21st to end	Night	22% 13% 4 8% 5% 4 3 3 4 4 30%	4884
February	1st to 10th	Night	17% 9% 3 7% 4 4 4 4 4 6% 39%	2372
	11th to 20th	Night	17% 8% 3 7% 5% 4 3 4 5% 5% 40%	2218
	21st to end	Night	19% 9% 2 6% 4 3 4 3 4 5% 41%	1884
March	1st to 10th	Night	29% 11% 2 5% 3 3 3 3 4 5% 32%	3724
	11th to 20th	Night	10% 10% 3 6% 5% 4 4 4 5% 6% 42%	3612
	21st to end	Night	25% 9% 2 6% 4 4 5% 3 5% 3 34%	3626
April	1st to 10th	Night	19% 8% 3 7% 4 4 5% 4 4 6% 37%	1900
	11th to 20th	Night	19% 9% 2 5% 4 4 4 3 6% 6% 38%	1861
	21st to end	Night	20% 8% 3 6% 4 5% 4 3 3 5% 40%	1710
May	1st to 10th	Night	25% 10% 3 8% 4 4 4 3 6% 7% 26%	2834
	11th to 20th	Night	23% 11% 2 8% 5% 5% 5% 4 7% 6% 25%	2813
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 5% 8% 38%	2749
June	1st to 10th	Night	16% 7% 3 7% 4 5% 5% 3 6% 7% 37%	1520
	11th to 20th	Night	19% 7% 3 5% 5% 4 4 3 5% 8% 37%	1520
	21st to end	Night	16% 8% 3 6% 3 4 4 4 6% 8% 38%	1520
July	1st to 10th	Night	15% 7% 3 7% 4 4 6% 4 8% 10% 33%	2520
	11th to 20th	Night	15% 6% 3 6% 4 4 4 4 7% 9% 37%	2520
	21st to end	Night	17% 9% 3 8% 5% 5% 5% 4 5% 8% 31%	2739
August	1st to 10th	Night	22% 7% 2 7% 4 5% 5% 5% 6% 7% 29%	2789
	11th to 20th	Night	10% 7% 3 8% 5% 5% 6% 5% 7% 8% 36%	2836
	21st to end	Night	9% 4 2 4 4 3 4 3 5% 8% 55%	3476
September	1st to 10th	Night	13% 7% 2 7% 4 4 5% 4 5% 6% 43%	1914
	11th to 20th	Night	15% 6% 2 5% 4 3 3 2 4 5% 52%	2090
	21st to end	Night	19% 7% 3 6% 4 3 4 3 5% 5% 41%	2090
October	1st to 10th	Night	20% 8% 3 6% 4 3 3 3 4 6% 41%	2090
	11th to 20th	Night	15% 5% 2 5% 3 3 4 3 5% 7% 47%	2304
	21st to end	Night	20% 8% 2 6% 4 3 4 3 4 6% 40%	2717
November	1st to 10th	Night	15% 5% 2 6% 3 4 3 4 4 6% 48%	2537
	11th to 20th	Night	19% 6% 2 5% 4 4 3 3 4 4 46%	2477
	21st to end	Night	16% 6% 2 5% 4 3 4 3 5% 6% 45%	2470
December	1st to 10th	Night	21% 7% 3 5% 5% 4 4 3 4 6% 39%	2588
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 5% 6% 41%	3288
	21st to end	Night	20% 12% 3 8% 3 3 3 3 4 6% 35%	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	42% 3 7% 31% 15% 2	2518
	11th to 20th	Day	48% 4 9% 25% 13% 2	2520
	21st to end	Day	54% 2 8% 23% 11% 2	2772
February	1st to 10th	Day	48% 4 12% 19% 14% 3	1802
	11th to 20th	Day	43% 5% 10% 23% 16% 3	1857
	21st to end	Day	49% 3 10% 20% 15% 3	1570
March	1st to 10th	Day	58% 3 10% 14% 12% 3	3119
	11th to 20th	Day	38% 5% 11% 29% 14% 3	3347
	21st to end	Day	53% 2 5% 17% 16% 7%	4003
April	1st to 10th	Day	48% 4 9% 20% 16% 4	2280
	11th to 20th	Day	44% 4 9% 22% 18% 3	2267
	21st to end	Day	48% 3 8% 20% 17% 4	2470
May	1st to 10th	Day	58% 3 12% 15% 11% 1	4120
	11th to 20th	Day	55% 3 14% 16% 11% 2	4102
	21st to end	Day	39% 4 11% 25% 19% 2	4837
June	1st to 10th	Day	46% 5% 12% 24% 12% 1	2660
	11th to 20th	Day	44% 5% 12% 22% 14% 2	2660
	21st to end	Day	44% 6% 13% 23% 14% 1	2660
July	1st to 10th	Day	45% 7% 11% 21% 14% 2	4370
	11th to 20th	Day	42% 4 16% 23% 13% 2	4365
	21st to end	Day	50% 5% 11% 20% 12% 2	4661
August	1st to 10th	Day	51% 5% 15% 21% 8% 1	4094
	11th to 20th	Day	45% 4 12% 28% 10% 1	4027
	21st to end	Day	33% 4 14% 28% 19% 2	4180
September	1st to 10th	Day	39% 5% 9% 24% 22% 2	2266
	11th to 20th	Day	35% 3 9% 26% 24% 3	2090
	21st to end	Day	44% 2 7% 22% 22% 2	2090
October	1st to 10th	Day	41% 3 8% 24% 21% 4	3480
	11th to 20th	Day	32% 2 6% 29% 28% 3	3087
	21st to end	Day	32% 3 13% 29% 21% 3	3081
November	1st to 10th	Day	39% 3 12% 24% 19% 2	1761
	11th to 20th	Day	41% 3 9% 23% 21% 3	1710
	21st to end	Day	42% 3 11% 27% 13% 3	1710
December	1st to 10th	Day	47% 3 13% 17% 18% 2	1590
	11th to 20th	Day	44% 3 12% 18% 16% 6%	1906
	21st to end	Day	53% 3 10% 23% 11% 1	2772
January	1st to 10th	Night	39% 2 10% 32% 13% 3	4429
	11th to 20th	Night	48% 3 10% 23% 12% 3	4440
	21st to end	Night	59% 3 8% 17% 11% 2	4884
February	1st to 10th	Night	47% 3 12% 22% 14% 2	2372
	11th to 20th	Night	47% 4 8% 20% 17% 4	2220
	21st to end	Night	46% 2 7% 24% 16% 4	1884
March	1st to 10th	Night	55% 2 8% 20% 12% 3	3734
	11th to 20th	Night	41% 4 15% 21% 17% 2	3613
	21st to end	Night	55% 2 5% 15% 17% 7%	3626
April	1st to 10th	Night	49% 4 9% 20% 16% 3	1900
	11th to 20th	Night	47% 3 8% 21% 18% 3	1861
	21st to end	Night	48% 3 9% 20% 15% 4	1710
May	1st to 10th	Night	58% 4 12% 17% 7% 2	2840
	11th to 20th	Night	58% 5% 13% 15% 7% 2	2814
	21st to end	Night	44% 5% 14% 23% 13% 1	2750
June	1st to 10th	Night	46% 5% 13% 23% 13% 1	1520
	11th to 20th	Night	46% 4 11% 24% 14% 1	1520
	21st to end	Night	42% 6% 15% 23% 13% 1	1520
July	1st to 10th	Night	45% 7% 12% 21% 15%	2480
	11th to 20th	Night	43% 4 16% 25% 11% 1	2472
	21st to end	Night	51% 6% 10% 17% 13% 2	2651
August	1st to 10th	Night	54% 5% 12% 18% 8% 1	2779
	11th to 20th	Night	44% 6% 14% 27% 9%	2863
	21st to end	Night	28% 4 15% 30% 22% 1	3476
September	1st to 10th	Night	41% 5% 10% 25% 18% 2	1914
	11th to 20th	Night	36% 3 10% 24% 26% 2	2090
	21st to end	Night	46% 3 7% 21% 20% 3	2090
October	1st to 10th	Night	42% 4 11% 26% 16% 3	3480
	11th to 20th	Night	27% 2 8% 29% 31% 3	3827
	21st to end	Night	35% 2 11% 29% 21% 2	4467
November	1st to 10th	Night	38% 4 12% 24% 19% 4	2537
	11th to 20th	Night	43% 2 9% 23% 20% 3	2477
	21st to end	Night	39% 4 13% 28% 14% 2	2470
December	1st to 10th	Night	48% 3 12% 22% 15% 1	2588
	11th to 20th	Night	44% 4 14% 18% 18% 2	3288
	21st to end	Night	52% 4 7% 25% 12% 1	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	36% 22% 27% 12% 4	180
	11th to 20th	Day	31% 23% 36% 8% 1 1	180
	21st to end	Day	29% 23% 34% 11% 2 1	198
February	1st to 10th	Day	36% 20% 31% 13% 2	400
	11th to 20th	Day	35% 27% 30% 6% 2	456
	21st to end	Day	34% 20% 32% 11% 3	382
March	1st to 10th	Day	33% 18% 32% 13% 3	460
	11th to 20th	Day	34% 22% 29% 10% 4	460
	21st to end	Day	32% 21% 32% 8% 5% 2	506
April	1st to 10th	Day	37% 18% 27% 11% 5% 1	460
	11th to 20th	Day	33% 21% 30% 13% 3 1	452
	21st to end	Day	29% 18% 32% 17% 2 2	460
May	1st to 10th	Day	36% 20% 28% 13% 3	463
	11th to 20th	Day	25% 16% 41% 13% 4 1	487
	21st to end	Day	21% 14% 36% 22% 5% 2	529
June	1st to 10th	Day	25% 11% 34% 21% 8% 1	490
	11th to 20th	Day	17% 17% 38% 20% 6% 2	490
	21st to end	Day	18% 13% 39% 21% 7% 1	490
July	1st to 10th	Day	18% 10% 33% 25% 12% 2	485
	11th to 20th	Day	18% 7% 34% 27% 12% 2	484
	21st to end	Day	14% 10% 39% 27% 9% 1	532
August	1st to 10th	Day	15% 9% 34% 30% 9% 2	486
	11th to 20th	Day	16% 9% 33% 28% 13% 3	479
	21st to end	Day	14% 10% 32% 25% 13% 5%	539
September	1st to 10th	Day	20% 10% 29% 24% 13% 4	490
	11th to 20th	Day	19% 11% 28% 22% 13% 8%	490
	21st to end	Day	25% 10% 37% 17% 8% 4	490
October	1st to 10th	Day	28% 12% 27% 19% 9% 5%	490
	11th to 20th	Day	19% 15% 36% 18% 10% 3	433
	21st to end	Day	28% 14% 30% 17% 9% 2	429
November	1st to 10th	Day	24% 14% 33% 17% 9% 2	395
	11th to 20th	Day	31% 18% 28% 17% 5% 1	370
	21st to end	Day	28% 19% 29% 16% 7% 1	370
December	1st to 10th	Day	28% 25% 32% 11% 4	250
	11th to 20th	Day	37% 23% 34% 6% 1	180
	21st to end	Day	31% 20% 32% 13% 3 2	198
January	1st to 10th	Night	24% 19% 35% 18% 3	760
	11th to 20th	Night	31% 19% 33% 14% 2	760
	21st to end	Night	25% 19% 33% 19% 3	836
February	1st to 10th	Night	25% 19% 34% 20% 1	760
	11th to 20th	Night	27% 23% 34% 14% 2	743
	21st to end	Night	28% 20% 32% 17% 3	628
March	1st to 10th	Night	27% 17% 35% 18% 3	760
	11th to 20th	Night	22% 24% 34% 18% 3	760
	21st to end	Night	28% 19% 32% 16% 4	836
April	1st to 10th	Night	27% 20% 32% 17% 4	760
	11th to 20th	Night	25% 20% 33% 18% 4	737
	21st to end	Night	26% 16% 37% 18% 3	570
May	1st to 10th	Night	32% 12% 33% 19% 4	570
	11th to 20th	Night	24% 16% 35% 22% 2	562
	21st to end	Night	22% 7% 37% 27% 6%	422
June	1st to 10th	Night	18% 8% 33% 30% 10% 1	380
	11th to 20th	Night	22% 6% 36% 30% 6%	380
	21st to end	Night	18% 4 38% 34% 6% 1	380
July	1st to 10th	Night	16% 4 35% 37% 8% 1	370
	11th to 20th	Night	16% 2 32% 40% 9% 1	368
	21st to end	Night	17% 4 30% 40% 10%	416
August	1st to 10th	Night	14% 6% 34% 37% 9%	544
	11th to 20th	Night	14% 5% 29% 39% 11% 1	590
	21st to end	Night	16% 5% 25% 37% 16% 1	836
September	1st to 10th	Night	17% 6% 31% 35% 11%	760
	11th to 20th	Night	18% 8% 28% 28% 17% 2	760
	21st to end	Night	25% 7% 28% 27% 12% 2	760
October	1st to 10th	Night	25% 10% 25% 27% 11% 2	760
	11th to 20th	Night	20% 11% 33% 23% 12% 1	760
	21st to end	Night	25% 13% 32% 21% 7%	836
November	1st to 10th	Night	19% 14% 33% 24% 9% 1	780
	11th to 20th	Night	25% 18% 32% 18% 7%	763
	21st to end	Night	23% 20% 31% 21% 5% 1	760
December	1st to 10th	Night	28% 17% 33% 18% 5%	760
	11th to 20th	Night	27% 19% 31% 20% 3	760
	21st to end	Night	29% 18% 32% 18% 2	836

Page generated at 2024-11-08 06:44:46. Version ac07

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