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 2025-12-18.

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% 39%	126802
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	124233
Transparency	Day	45% 4 11% 22% 15% 3	130348
	Night	45% 3 10% 22% 16% 3	128774
Seeing	Day	26% 16% 32% 18% 7% 2	16518
	Night	23% 14% 32% 23% 6% 1	25435

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	11% 9% 3 7% 4 3 4 3 4 6% 47%	3597
	11th to 20th	Day	15% 9% 3 5% 3 3 3 3 4 4 48%	3570
	21st to end	Day	17% 10% 3 6% 4 4 4 3 4 5% 42%	3960
February	1st to 10th	Day	20% 7% 2 6% 4 4 5% 5% 5% 6% 35%	1898
	11th to 20th	Day	15% 8% 3 6% 4 4 5% 4 5% 7% 41%	1948
	21st to end	Day	18% 10% 3 7% 4 4 4 3 4 6% 38%	1650
March	1st to 10th	Day	21% 13% 3 5% 3 3 3 2 4 6% 36%	4476
	11th to 20th	Day	16% 9% 2 6% 4 4 4 4 5% 6% 39%	4791
	21st to end	Day	25% 11% 2 6% 4 4 4 3 5% 5% 31%	5737
April	1st to 10th	Day	21% 10% 2 6% 3 3 4 3 5% 6% 37%	2400
	11th to 20th	Day	18% 6% 2 5% 4 4 4 3 6% 7% 39%	2388
	21st to end	Day	20% 6% 3 7% 5% 4 3 3 5% 6% 39%	2600
May	1st to 10th	Day	28% 12% 3 7% 3 4 3 3 5% 6% 26%	5886
	11th to 20th	Day	22% 9% 3 7% 5% 4 4 3 4 6% 32%	5878
	21st to end	Day	21% 6% 2 6% 4 4 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	19% 7% 2 6% 4 4 4 4 6% 8% 34%	2800
	11th to 20th	Day	17% 5% 2 6% 5% 4 5% 4 6% 8% 37%	2800
	21st to end	Day	16% 7% 2 5% 4 5% 5% 4 6% 8% 38%	2800
July	1st to 10th	Day	17% 7% 2 6% 4 5% 5% 5% 7% 10% 32%	6360
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 5% 8% 11% 34%	6360
	21st to end	Day	16% 6% 2 6% 4 4 4 4 5% 8% 41%	6926
August	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 32%	4238
	11th to 20th	Day	12% 6% 3 7% 5% 6% 6% 4 6% 8% 37%	4116
	21st to end	Day	8% 4 1 5% 4 4 6% 5% 7% 9% 47%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 45%	2385
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 4 7% 51%	2200
	21st to end	Day	19% 6% 2 6% 3 4 3 3 4 7% 43%	2200
October	1st to 10th	Day	23% 5% 2 5% 3 3 3 2 4 5% 44%	2200
	11th to 20th	Day	16% 6% 2 4 4 3 4 2 4 8% 47%	1969
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 6% 43%	1980
November	1st to 10th	Day	16% 7% 2 5% 3 3 3 3 4 5% 48%	1851
	11th to 20th	Day	18% 7% 1 4 4 3 3 3 4 5% 48%	1890
	21st to end	Day	18% 8% 2 4 4 3 4 2 5% 6% 43%	1890
December	1st to 10th	Day	15% 9% 2 4 3 3 3 2 4 6% 48%	2795
	11th to 20th	Day	20% 7% 2 4 3 3 3 2 3 5% 49%	3042
	21st to end	Day	20% 11% 3 7% 4 3 4 2 4 5% 37%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 5% 7% 51%	6349
	11th to 20th	Night	14% 10% 3 8% 4 4 4 3 4 5% 41%	6315
	21st to end	Night	16% 10% 3 7% 5% 5% 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 9% 3 7% 3 4 4 4 4 6% 39%	2496
	11th to 20th	Night	17% 8% 3 7% 4 4 3 4 5% 5% 40%	2338
	21st to end	Night	18% 9% 2 6% 4 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	24% 9% 2 5% 3 4 4 3 5% 5% 37%	5364
	11th to 20th	Night	17% 11% 3 6% 4 3 3 3 4 6% 39%	5168
	21st to end	Night	25% 10% 2 6% 4 4 4 3 5% 4 33%	5192
April	1st to 10th	Night	20% 8% 3 7% 4 4 4 4 4 6% 36%	2000
	11th to 20th	Night	20% 9% 2 5% 4 4 4 3 6% 6% 38%	1960
	21st to end	Night	20% 8% 3 6% 4 5% 4 3 3 5% 40%	1800
May	1st to 10th	Night	29% 9% 3 7% 4 4 4 3 5% 7% 26%	4054
	11th to 20th	Night	19% 10% 2 9% 6% 5% 5% 4 7% 7% 28%	4033
	21st to end	Night	20% 8% 3 8% 5% 4 5% 4 5% 7% 30%	3937
June	1st to 10th	Night	16% 7% 3 8% 4 5% 5% 3 7% 7% 36%	1600
	11th to 20th	Night	19% 7% 3 6% 5% 4 4 3 5% 8% 36%	1600
	21st to end	Night	16% 7% 3 6% 3 4 4 4 6% 9% 39%	1600
July	1st to 10th	Night	16% 9% 3 6% 4 3 5% 4 7% 10% 34%	3600
	11th to 20th	Night	15% 7% 3 7% 5% 5% 4 4 6% 9% 35%	3600
	21st to end	Night	13% 7% 3 7% 4 4 5% 4 5% 7% 41%	3926
August	1st to 10th	Night	22% 7% 2 7% 4 5% 5% 5% 6% 7% 30%	2878
	11th to 20th	Night	9% 7% 3 8% 5% 5% 6% 5% 7% 8% 37%	2928
	21st to end	Night	9% 4 2 4 4 3 4 3 5% 8% 55%	3586
September	1st to 10th	Night	13% 7% 2 7% 4 4 5% 5% 5% 6% 43%	2015
	11th to 20th	Night	14% 6% 2 5% 4 3 3 2 4 5% 52%	2200
	21st to end	Night	20% 8% 3 6% 4 3 4 3 4 5% 41%	2200
October	1st to 10th	Night	19% 7% 3 6% 4 3 3 3 3 6% 44%	2200
	11th to 20th	Night	16% 5% 2 5% 3 3 4 3 5% 7% 47%	2426
	21st to end	Night	20% 8% 2 6% 4 4 4 3 4 5% 40%	2860
November	1st to 10th	Night	15% 5% 2 6% 3 4 3 4 5% 6% 48%	2667
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 4 47%	2730
	21st to end	Night	16% 6% 2 5% 4 3 4 3 5% 7% 45%	2730
December	1st to 10th	Night	19% 8% 2 4 4 3 3 3 4 5% 45%	4602
	11th to 20th	Night	22% 8% 2 5% 3 3 3 3 4 5% 41%	5307
	21st to end	Night	17% 11% 3 7% 3 3 3 3 4 6% 40%	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	39% 2 5% 28% 22% 3	3598
	11th to 20th	Day	41% 3 10% 22% 18% 7%	3600
	21st to end	Day	47% 2 8% 21% 19% 3	3960
February	1st to 10th	Day	48% 4 12% 18% 14% 3	1898
	11th to 20th	Day	44% 4 10% 23% 16% 4	1957
	21st to end	Day	48% 3 10% 20% 16% 3	1650
March	1st to 10th	Day	50% 3 9% 14% 13% 11%	4479
	11th to 20th	Day	44% 4 10% 29% 11% 2	4791
	21st to end	Day	56% 3 4 16% 14% 7%	5737
April	1st to 10th	Day	49% 3 9% 19% 16% 4	2400
	11th to 20th	Day	44% 4 9% 21% 19% 3	2388
	21st to end	Day	47% 3 8% 20% 17% 4	2600
May	1st to 10th	Day	60% 3 14% 14% 8% 1	5897
	11th to 20th	Day	55% 3 14% 17% 11% 2	5882
	21st to end	Day	48% 3 11% 23% 13% 1	6949
June	1st to 10th	Day	47% 5% 12% 24% 13% 1	2800
	11th to 20th	Day	45% 5% 12% 22% 14% 2	2800
	21st to end	Day	43% 6% 12% 23% 15% 1	2800
July	1st to 10th	Day	47% 6% 11% 21% 13% 2	6290
	11th to 20th	Day	43% 5% 17% 24% 11% 1	6285
	21st to end	Day	42% 4 10% 27% 16% 2	6728
August	1st to 10th	Day	50% 5% 14% 21% 8% 1	4224
	11th to 20th	Day	45% 4 12% 28% 11% 1	4155
	21st to end	Day	33% 4 14% 28% 20% 2	4312
September	1st to 10th	Day	39% 5% 9% 24% 21% 1	2385
	11th to 20th	Day	35% 3 9% 26% 24% 3	2200
	21st to end	Day	44% 2 7% 22% 22% 2	2200
October	1st to 10th	Day	40% 3 8% 24% 21% 4	3590
	11th to 20th	Day	33% 2 6% 29% 27% 3	3185
	21st to end	Day	32% 3 12% 29% 21% 3	3180
November	1st to 10th	Day	40% 3 12% 24% 19% 2	1851
	11th to 20th	Day	39% 3 9% 23% 22% 4	1890
	21st to end	Day	43% 3 10% 28% 13% 3	1890
December	1st to 10th	Day	40% 2 14% 28% 16% 1	2795
	11th to 20th	Day	40% 2 13% 18% 19% 7%	3042
	21st to end	Day	51% 3 7% 21% 14% 4	3960
January	1st to 10th	Night	35% 2 8% 35% 18% 3	6349
	11th to 20th	Night	46% 3 10% 21% 15% 5%	6360
	21st to end	Night	50% 2 9% 17% 17% 5%	6996
February	1st to 10th	Night	47% 3 12% 22% 14% 2	2496
	11th to 20th	Night	47% 4 8% 21% 17% 4	2340
	21st to end	Night	47% 2 7% 24% 16% 4	1980
March	1st to 10th	Night	49% 3 9% 17% 15% 7%	5374
	11th to 20th	Night	47% 3 15% 19% 13% 2	5169
	21st to end	Night	55% 2 4 13% 19% 6%	5192
April	1st to 10th	Night	50% 4 8% 20% 15% 4	2000
	11th to 20th	Night	48% 3 8% 20% 18% 3	1960
	21st to end	Night	48% 3 9% 20% 16% 4	1800
May	1st to 10th	Night	60% 3 14% 14% 7% 2	4060
	11th to 20th	Night	56% 4 14% 16% 8% 2	4034
	21st to end	Night	53% 4 12% 20% 10% 1	3938
June	1st to 10th	Night	46% 5% 12% 23% 13% 1	1600
	11th to 20th	Night	47% 4 11% 23% 14% 1	1600
	21st to end	Night	42% 6% 15% 23% 14% 1	1600
July	1st to 10th	Night	46% 5% 11% 22% 14%	3560
	11th to 20th	Night	46% 4 16% 23% 10% 1	3552
	21st to end	Night	43% 5% 10% 26% 15% 1	3809
August	1st to 10th	Night	54% 5% 12% 19% 9% 1	2868
	11th to 20th	Night	43% 6% 14% 28% 9%	2955
	21st to end	Night	29% 4 15% 30% 22% 1	3586
September	1st to 10th	Night	40% 5% 10% 25% 18% 2	2015
	11th to 20th	Night	36% 3 9% 24% 26% 2	2200
	21st to end	Night	47% 3 8% 21% 19% 2	2200
October	1st to 10th	Night	41% 4 10% 26% 17% 3	3590
	11th to 20th	Night	27% 2 8% 29% 31% 3	3949
	21st to end	Night	35% 2 11% 29% 21% 2	4610
November	1st to 10th	Night	38% 4 11% 24% 19% 3	2667
	11th to 20th	Night	41% 2 8% 24% 21% 4	2730
	21st to end	Night	39% 4 13% 28% 14% 2	2730
December	1st to 10th	Night	44% 2 11% 28% 15% 1	4602
	11th to 20th	Night	47% 3 13% 15% 20% 2	5307
	21st to end	Night	48% 3 6% 23% 17% 3	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	36% 22% 29% 11% 4	200
	11th to 20th	Day	31% 24% 37% 8% 1 1	200
	21st to end	Day	31% 22% 34% 11% 1 1	220
February	1st to 10th	Day	35% 20% 31% 12% 2	436
	11th to 20th	Day	37% 26% 29% 6% 2	496
	21st to end	Day	32% 22% 32% 11% 3	414
March	1st to 10th	Day	34% 20% 32% 12% 3	500
	11th to 20th	Day	34% 22% 29% 11% 4	500
	21st to end	Day	33% 21% 31% 8% 5% 2	550
April	1st to 10th	Day	38% 19% 27% 11% 4 1	500
	11th to 20th	Day	34% 21% 29% 12% 2 1	492
	21st to end	Day	29% 18% 33% 16% 2 2	500
May	1st to 10th	Day	36% 21% 28% 13% 3	503
	11th to 20th	Day	26% 16% 40% 13% 4 1	527
	21st to end	Day	22% 14% 36% 21% 5% 2	573
June	1st to 10th	Day	25% 11% 35% 20% 7% 1	530
	11th to 20th	Day	18% 16% 38% 20% 6% 2	530
	21st to end	Day	18% 13% 38% 22% 7% 1	530
July	1st to 10th	Day	19% 10% 33% 25% 11% 2	525
	11th to 20th	Day	17% 7% 33% 27% 13% 2	524
	21st to end	Day	13% 10% 39% 27% 9% 1	576
August	1st to 10th	Day	14% 9% 34% 30% 10% 3	526
	11th to 20th	Day	15% 8% 33% 29% 13% 3	519
	21st to end	Day	13% 11% 34% 24% 13% 5%	583
September	1st to 10th	Day	20% 11% 29% 24% 12% 4	530
	11th to 20th	Day	18% 11% 28% 23% 13% 7%	530
	21st to end	Day	25% 9% 36% 17% 8% 4	530
October	1st to 10th	Day	27% 12% 27% 20% 9% 5%	530
	11th to 20th	Day	21% 15% 34% 17% 10% 3	467
	21st to end	Day	28% 14% 30% 18% 8% 2	462
November	1st to 10th	Day	25% 15% 32% 17% 9% 2	425
	11th to 20th	Day	29% 20% 30% 16% 5%	430
	21st to end	Day	31% 19% 28% 15% 6% 1	430
December	1st to 10th	Day	27% 25% 31% 12% 4	296
	11th to 20th	Day	37% 23% 31% 7% 1	214
	21st to end	Day	30% 19% 31% 15% 3 2	220
January	1st to 10th	Night	24% 20% 35% 18% 3	800
	11th to 20th	Night	31% 19% 33% 16% 2	800
	21st to end	Night	24% 20% 34% 19% 3	880
February	1st to 10th	Night	25% 19% 34% 20% 1	800
	11th to 20th	Night	28% 22% 34% 14% 2	783
	21st to end	Night	27% 20% 32% 17% 3	660
March	1st to 10th	Night	27% 18% 34% 17% 3	800
	11th to 20th	Night	22% 23% 33% 18% 3	800
	21st to end	Night	29% 19% 33% 16% 4	880
April	1st to 10th	Night	28% 20% 32% 17% 3	800
	11th to 20th	Night	26% 20% 33% 18% 4	776
	21st to end	Night	26% 16% 38% 17% 3	600
May	1st to 10th	Night	32% 12% 33% 19% 4	600
	11th to 20th	Night	23% 17% 35% 22% 3	592
	21st to end	Night	23% 7% 37% 28% 5%	444
June	1st to 10th	Night	18% 8% 34% 30% 10% 1	400
	11th to 20th	Night	22% 7% 35% 30% 6%	400
	21st to end	Night	18% 4 37% 35% 6% 1	400
July	1st to 10th	Night	16% 4 34% 37% 8% 1	390
	11th to 20th	Night	16% 2 32% 39% 10% 1	388
	21st to end	Night	16% 3 31% 40% 9%	438
August	1st to 10th	Night	14% 6% 33% 38% 9%	573
	11th to 20th	Night	14% 5% 29% 40% 11% 1	622
	21st to end	Night	15% 5% 26% 37% 15% 1	880
September	1st to 10th	Night	17% 6% 31% 35% 12% 1	800
	11th to 20th	Night	17% 7% 27% 29% 17% 2	800
	21st to end	Night	25% 7% 27% 28% 12% 2	800
October	1st to 10th	Night	24% 10% 25% 27% 12% 2	800
	11th to 20th	Night	21% 11% 33% 23% 11% 1	800
	21st to end	Night	25% 13% 32% 22% 7%	880
November	1st to 10th	Night	20% 15% 32% 24% 9% 1	820
	11th to 20th	Night	24% 19% 33% 18% 7%	840
	21st to end	Night	23% 20% 31% 21% 4 1	840
December	1st to 10th	Night	27% 16% 34% 18% 5%	840
	11th to 20th	Night	28% 18% 31% 20% 3	829
	21st to end	Night	28% 18% 32% 19% 3	880

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