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 2024-01-13.

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% 7% 3 5% 5% 3 4 4 5% 7% 40%	74817
	Night	17% 7% 3 5% 5% 3 4 4 5% 6% 40%	74914
Transparency	Day	44% 4 11% 23% 16% 3	74522
	Night	45% 4 11% 23% 15% 2	74743
Seeing	Day	26% 16% 32% 18% 7% 2	13992
	Night	23% 14% 32% 23% 6% 1	23006

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% 9% 3 5% 5% 2 3 5% 3 6% 44%	1517
	11th to 20th	Day	19% 8% 3 5% 4 2 5% 4 5% 6% 39%	1456
	21st to end	Day	15% 7% 3 5% 4 2 4 3 5% 8% 44%	1584
February	1st to 10th	Day	21% 7% 2 5% 4 3 5% 6% 5% 6% 35%	1708
	11th to 20th	Day	15% 7% 3 6% 5% 3 4 5% 4 7% 41%	1748
	21st to end	Day	19% 9% 4 5% 5% 3 4 4 4 6% 37%	1480
March	1st to 10th	Day	17% 9% 3 4 4 3 4 4 6% 7% 40%	1797
	11th to 20th	Day	17% 7% 3 5% 5% 3 4 5% 5% 6% 40%	1903
	21st to end	Day	18% 9% 3 6% 4 3 4 3 4 5% 41%	2283
April	1st to 10th	Day	21% 9% 2 5% 4 2 4 4 5% 7% 38%	2160
	11th to 20th	Day	18% 6% 3 5% 4 4 4 4 6% 7% 40%	2146
	21st to end	Day	22% 5% 3 6% 6% 4 3 4 4 6% 37%	2340
May	1st to 10th	Day	26% 9% 3 5% 4 3 4 4 5% 7% 32%	2340
	11th to 20th	Day	19% 6% 3 4 5% 3 5% 4 4 6% 41%	2304
	21st to end	Day	15% 5% 2 4 5% 3 4 4 5% 7% 44%	2725
June	1st to 10th	Day	19% 8% 3 5% 5% 3 5% 5% 6% 8% 34%	2520
	11th to 20th	Day	18% 5% 3 5% 6% 3 5% 5% 6% 7% 37%	2520
	21st to end	Day	16% 6% 2 4 5% 4 5% 5% 6% 9% 37%	2520
July	1st to 10th	Day	14% 6% 2 5% 5% 3 6% 6% 6% 9% 38%	2520
	11th to 20th	Day	14% 6% 2 5% 6% 4 5% 6% 7% 9% 36%	2520
	21st to end	Day	13% 5% 2 6% 6% 3 6% 6% 7% 9% 38%	2744
August	1st to 10th	Day	11% 7% 2 6% 6% 3 5% 6% 6% 9% 38%	2330
	11th to 20th	Day	13% 6% 3 5% 6% 4 5% 6% 6% 7% 39%	2275
	21st to end	Day	14% 6% 2 4 5% 3 5% 5% 6% 9% 42%	2376
September	1st to 10th	Day	15% 5% 2 5% 4 3 5% 4 5% 6% 44%	2147
	11th to 20th	Day	14% 6% 3 4 3 2 4 4 4 7% 51%	1980
	21st to end	Day	19% 6% 2 4 4 3 3 3 4 7% 45%	1980
October	1st to 10th	Day	25% 5% 2 5% 4 2 3 2 4 5% 42%	1980
	11th to 20th	Day	16% 6% 2 4 4 2 4 3 4 8% 46%	1773
	21st to end	Day	20% 7% 2 5% 4 3 4 4 5% 6% 41%	1782
November	1st to 10th	Day	17% 7% 3 4 3 2 4 3 4 5% 48%	1671
	11th to 20th	Day	20% 6% 2 3 4 2 3 4 4 5% 46%	1710
	21st to end	Day	18% 7% 2 4 4 2 4 3 5% 6% 44%	1710
December	1st to 10th	Day	20% 8% 2 5% 4 3 4 4 4 7% 39%	1590
	11th to 20th	Day	21% 6% 3 4 4 3 3 3 3 5% 44%	1906
	21st to end	Day	21% 10% 4 6% 5% 2 4 4 4 6% 34%	2772
January	1st to 10th	Night	17% 7% 3 5% 4 2 4 3 4 6% 44%	2649
	11th to 20th	Night	20% 11% 3 5% 5% 3 4 4 4 6% 34%	2560
	21st to end	Night	15% 8% 3 6% 5% 3 4 5% 4 6% 42%	2772
February	1st to 10th	Night	18% 9% 3 7% 5% 3 4 5% 4 6% 38%	2246
	11th to 20th	Night	17% 8% 3 7% 5% 3 4 4 5% 5% 39%	2098
	21st to end	Night	18% 8% 3 6% 4 3 4 4 4 5% 41%	1776
March	1st to 10th	Night	16% 7% 2 6% 4 3 4 5% 5% 7% 42%	2150
	11th to 20th	Night	15% 9% 4 6% 5% 3 4 5% 5% 6% 39%	2056
	21st to end	Night	19% 9% 3 6% 6% 3 4 3 3 5% 41%	2070
April	1st to 10th	Night	19% 7% 3 5% 5% 3 5% 5% 4 6% 38%	1800
	11th to 20th	Night	19% 8% 2 4 5% 3 5% 3 6% 6% 39%	1762
	21st to end	Night	22% 7% 3 5% 4 3 4 4 3 5% 39%	1620
May	1st to 10th	Night	24% 10% 3 6% 5% 3 4 4 5% 6% 31%	1620
	11th to 20th	Night	19% 6% 3 6% 6% 3 4 5% 7% 7% 35%	1593
	21st to end	Night	18% 7% 2 6% 5% 3 4 5% 4 7% 38%	1561
June	1st to 10th	Night	17% 6% 3 7% 5% 3 5% 4 6% 7% 37%	1440
	11th to 20th	Night	20% 6% 3 5% 6% 3 4 4 5% 7% 36%	1440
	21st to end	Night	16% 7% 3 4 4 3 4 5% 5% 9% 40%	1440
July	1st to 10th	Night	12% 6% 3 6% 5% 3 6% 5% 7% 10% 38%	1440
	11th to 20th	Night	14% 6% 3 6% 6% 4 5% 5% 7% 9% 36%	1440
	21st to end	Night	13% 6% 3 6% 5% 3 6% 6% 6% 9% 38%	1574
August	1st to 10th	Night	9% 6% 2 7% 4 4 6% 6% 6% 8% 42%	1589
	11th to 20th	Night	11% 6% 2 6% 6% 4 5% 6% 6% 8% 39%	1622
	21st to end	Night	14% 5% 3 4 5% 2 4 4 5% 8% 47%	1980
September	1st to 10th	Night	13% 7% 2 5% 5% 3 4 6% 5% 6% 42%	1813
	11th to 20th	Night	14% 5% 2 4 4 3 3 4 3 5% 52%	1980
	21st to end	Night	18% 6% 2 5% 4 2 4 4 5% 6% 43%	1980
October	1st to 10th	Night	21% 7% 3 5% 5% 2 3 4 3 5% 42%	1980
	11th to 20th	Night	16% 5% 2 5% 4 2 3 4 5% 7% 46%	2182
	21st to end	Night	21% 8% 3 5% 5% 3 4 4 4 6% 38%	2574
November	1st to 10th	Night	16% 5% 2 5% 4 3 3 5% 5% 6% 47%	2407
	11th to 20th	Night	19% 6% 2 5% 3 3 3 4 4 4 46%	2470
	21st to end	Night	17% 5% 3 5% 4 2 4 4 5% 6% 46%	2470
December	1st to 10th	Night	21% 6% 3 5% 5% 3 4 4 4 6% 39%	2588
	11th to 20th	Night	20% 7% 2 5% 4 3 4 4 5% 6% 41%	3288
	21st to end	Night	20% 12% 4 7% 4 2 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	43% 3 8% 26% 16% 4	1518
	11th to 20th	Day	47% 5% 12% 21% 12% 3	1456
	21st to end	Day	41% 3 10% 26% 18% 3	1584
February	1st to 10th	Day	49% 4 12% 18% 14% 3	1708
	11th to 20th	Day	44% 5% 10% 23% 15% 4	1757
	21st to end	Day	49% 3 9% 20% 15% 3	1480
March	1st to 10th	Day	43% 4 12% 21% 15% 5%	1800
	11th to 20th	Day	44% 4 8% 24% 16% 4	1903
	21st to end	Day	48% 3 7% 18% 18% 6%	2283
April	1st to 10th	Day	47% 4 9% 20% 17% 4	2160
	11th to 20th	Day	43% 4 9% 23% 18% 3	2146
	21st to end	Day	49% 3 8% 19% 16% 4	2340
May	1st to 10th	Day	52% 4 8% 20% 14% 2	2340
	11th to 20th	Day	45% 4 9% 19% 19% 3	2308
	21st to end	Day	40% 4 14% 24% 14% 2	2725
June	1st to 10th	Day	48% 5% 12% 23% 12% 1	2520
	11th to 20th	Day	46% 5% 11% 22% 15% 2	2520
	21st to end	Day	44% 6% 13% 23% 13% 1	2520
July	1st to 10th	Day	41% 6% 10% 25% 15% 3	2450
	11th to 20th	Day	43% 4 13% 25% 12% 2	2445
	21st to end	Day	40% 6% 13% 24% 13% 4	2591
August	1st to 10th	Day	42% 6% 14% 24% 13% 1	2316
	11th to 20th	Day	43% 6% 12% 24% 14% 1	2275
	21st to end	Day	39% 5% 11% 23% 19% 2	2376
September	1st to 10th	Day	40% 5% 10% 23% 20% 1	2147
	11th to 20th	Day	36% 3 9% 25% 24% 3	1980
	21st to end	Day	42% 2 8% 23% 23% 2	1980
October	1st to 10th	Day	47% 4 8% 22% 16% 4	1980
	11th to 20th	Day	38% 3 9% 31% 17% 2	1773
	21st to end	Day	44% 4 13% 20% 16% 4	1782
November	1st to 10th	Day	41% 3 12% 24% 18% 2	1671
	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	43% 2 11% 24% 17% 3	2649
	11th to 20th	Night	52% 4 12% 20% 10% 2	2560
	21st to end	Night	45% 3 9% 20% 19% 3	2772
February	1st to 10th	Night	48% 3 12% 21% 13% 2	2246
	11th to 20th	Night	48% 4 8% 21% 15% 4	2100
	21st to end	Night	47% 2 7% 24% 16% 4	1776
March	1st to 10th	Night	43% 4 9% 24% 16% 4	2160
	11th to 20th	Night	47% 3 11% 17% 19% 4	2057
	21st to end	Night	48% 2 7% 19% 18% 6%	2070
April	1st to 10th	Night	48% 4 9% 20% 16% 4	1800
	11th to 20th	Night	46% 4 8% 21% 18% 3	1762
	21st to end	Night	49% 3 9% 20% 14% 4	1620
May	1st to 10th	Night	54% 4 10% 17% 12% 3	1620
	11th to 20th	Night	46% 5% 12% 20% 13% 4	1594
	21st to end	Night	47% 4 16% 19% 12% 1	1562
June	1st to 10th	Night	47% 5% 13% 22% 13% 1	1440
	11th to 20th	Night	47% 4 11% 23% 14% 1	1440
	21st to end	Night	41% 6% 15% 23% 13% 1	1440
July	1st to 10th	Night	41% 6% 13% 24% 15% 1	1400
	11th to 20th	Night	43% 5% 14% 28% 9% 1	1392
	21st to end	Night	41% 6% 13% 26% 12% 2	1486
August	1st to 10th	Night	39% 5% 16% 26% 12% 2	1579
	11th to 20th	Night	41% 6% 15% 25% 12% 1	1622
	21st to end	Night	36% 4 11% 28% 19% 1	1980
September	1st to 10th	Night	42% 5% 10% 24% 17% 2	1813
	11th to 20th	Night	36% 3 10% 23% 26% 2	1980
	21st to end	Night	44% 3 7% 22% 21% 3	1980
October	1st to 10th	Night	46% 4 10% 22% 14% 4	1980
	11th to 20th	Night	39% 3 12% 31% 14% 1	2182
	21st to end	Night	48% 4 11% 20% 15% 2	2574
November	1st to 10th	Night	39% 4 12% 23% 18% 4	2407
	11th to 20th	Night	43% 2 9% 23% 20% 3	2470
	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	30% 23% 36% 9% 1 1	164
	21st to end	Day	28% 26% 34% 10% 1 1	176
February	1st to 10th	Day	38% 20% 29% 11% 2	366
	11th to 20th	Day	34% 26% 31% 6% 2	416
	21st to end	Day	34% 22% 32% 10% 2	346
March	1st to 10th	Day	31% 19% 34% 12% 3	420
	11th to 20th	Day	33% 23% 29% 10% 5%	420
	21st to end	Day	30% 22% 32% 9% 5% 2	462
April	1st to 10th	Day	37% 19% 26% 12% 5% 1	420
	11th to 20th	Day	31% 21% 30% 13% 3 1	412
	21st to end	Day	30% 19% 30% 16% 2 2	420
May	1st to 10th	Day	37% 20% 27% 13% 3	423
	11th to 20th	Day	25% 16% 41% 13% 4 1	447
	21st to end	Day	21% 15% 34% 22% 6% 2	485
June	1st to 10th	Day	25% 11% 34% 20% 8% 1	450
	11th to 20th	Day	19% 16% 39% 19% 5% 2	450
	21st to end	Day	18% 12% 39% 22% 7% 2	450
July	1st to 10th	Day	18% 10% 32% 26% 12% 2	445
	11th to 20th	Day	19% 6% 33% 27% 12% 2	444
	21st to end	Day	13% 10% 39% 28% 9% 1	488
August	1st to 10th	Day	14% 9% 33% 31% 10% 2	446
	11th to 20th	Day	16% 9% 33% 28% 12% 3	439
	21st to end	Day	15% 11% 31% 26% 13% 5%	495
September	1st to 10th	Day	21% 10% 28% 24% 14% 4	450
	11th to 20th	Day	19% 12% 29% 22% 11% 7%	450
	21st to end	Day	24% 10% 36% 17% 8% 4	450
October	1st to 10th	Day	30% 12% 26% 19% 8% 5%	450
	11th to 20th	Day	20% 15% 35% 18% 10% 2	399
	21st to end	Day	29% 15% 30% 16% 8% 2	396
November	1st to 10th	Day	24% 14% 32% 18% 9% 2	365
	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	732
	21st to end	Night	24% 20% 33% 19% 3	792
February	1st to 10th	Night	26% 19% 34% 20% 1	720
	11th to 20th	Night	28% 22% 34% 15% 2	703
	21st to end	Night	27% 21% 32% 16% 3	592
March	1st to 10th	Night	25% 18% 36% 18% 3	720
	11th to 20th	Night	22% 24% 33% 18% 3	720
	21st to end	Night	27% 20% 33% 17% 4	792
April	1st to 10th	Night	27% 21% 31% 17% 4	720
	11th to 20th	Night	24% 19% 34% 18% 5%	698
	21st to end	Night	27% 16% 36% 18% 3	540
May	1st to 10th	Night	32% 12% 33% 19% 4	540
	11th to 20th	Night	24% 17% 35% 22% 2	532
	21st to end	Night	23% 7% 37% 28% 6%	400
June	1st to 10th	Night	19% 7% 33% 30% 11% 1	360
	11th to 20th	Night	23% 6% 35% 29% 6%	360
	21st to end	Night	18% 4 38% 34% 6% 1	360
July	1st to 10th	Night	16% 3 35% 37% 9% 1	350
	11th to 20th	Night	16% 2 32% 40% 10% 1	348
	21st to end	Night	16% 3 31% 40% 10%	394
August	1st to 10th	Night	12% 6% 35% 38% 9%	515
	11th to 20th	Night	15% 5% 30% 40% 10% 1	558
	21st to end	Night	16% 5% 25% 37% 15% 2	792
September	1st to 10th	Night	17% 6% 30% 36% 11%	720
	11th to 20th	Night	18% 8% 29% 28% 16% 2	720
	21st to end	Night	24% 7% 28% 27% 12% 2	720
October	1st to 10th	Night	26% 10% 25% 26% 11% 2	720
	11th to 20th	Night	20% 11% 33% 23% 11% 1	720
	21st to end	Night	27% 14% 32% 20% 7% 1	792
November	1st to 10th	Night	19% 14% 32% 24% 10% 1	740
	11th to 20th	Night	25% 18% 33% 18% 7%	760
	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 2024-01-10 10:16:09. Version ac07

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