Willowbrook Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willowbrook Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	14% 10% 4 8% 7% 4 5% 5% 6% 7% 31%	75136
	Night	14% 10% 4 8% 6% 4 5% 5% 5% 7% 32%	74628
Transparency	Day	52% 8% 11% 18% 10% 1	74848
	Night	51% 7% 12% 19% 10% 1	74451
Seeing	Day	19% 20% 31% 18% 8% 3	13727
	Night	19% 19% 31% 22% 8% 1	22816

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	12% 13% 9% 13% 12% 4 7% 6% 5% 5% 16%	1517
	11th to 20th	Day	14% 16% 8% 12% 9% 5% 6% 5% 5% 6% 14%	1621
	21st to end	Day	9% 17% 8% 12% 10% 4 7% 5% 4 6% 16%	1782
February	1st to 10th	Day	13% 16% 6% 11% 11% 6% 6% 6% 5% 5% 16%	1620
	11th to 20th	Day	11% 12% 7% 13% 10% 4 5% 6% 5% 6% 21%	1573
	21st to end	Day	16% 16% 5% 9% 8% 4 4 4 5% 5% 24%	1387
March	1st to 10th	Day	12% 11% 6% 10% 9% 4 5% 4 5% 5% 28%	1968
	11th to 20th	Day	11% 13% 6% 9% 7% 4 5% 4 4 6% 31%	1980
	21st to end	Day	15% 13% 5% 6% 7% 3 5% 4 5% 4 33%	2178
April	1st to 10th	Day	14% 13% 7% 8% 5% 3 5% 5% 5% 7% 28%	2108
	11th to 20th	Day	18% 9% 3 6% 5% 3 3 5% 5% 6% 36%	2239
	21st to end	Day	20% 10% 4 7% 5% 3 5% 4 5% 6% 33%	2340
May	1st to 10th	Day	16% 9% 4 7% 6% 3 5% 4 5% 7% 35%	2340
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 5% 5% 6% 39%	2446
	21st to end	Day	12% 5% 2 6% 5% 3 5% 5% 6% 8% 42%	2725
June	1st to 10th	Day	15% 5% 3 6% 5% 3 6% 6% 6% 9% 37%	2520
	11th to 20th	Day	12% 6% 2 5% 6% 3 5% 6% 6% 9% 41%	2633
	21st to end	Day	14% 5% 2 5% 5% 4 5% 6% 6% 9% 40%	2700
July	1st to 10th	Day	9% 4 2 5% 5% 4 5% 6% 7% 10% 43%	2695
	11th to 20th	Day	12% 5% 2 5% 7% 4 6% 6% 6% 9% 39%	2520
	21st to end	Day	10% 5% 3 7% 7% 4 5% 7% 7% 9% 35%	2557
August	1st to 10th	Day	10% 5% 2 6% 6% 4 6% 6% 7% 9% 39%	2327
	11th to 20th	Day	8% 5% 3 6% 8% 4 6% 7% 7% 9% 36%	2303
	21st to end	Day	13% 6% 2 5% 5% 4 6% 6% 6% 8% 38%	2503
September	1st to 10th	Day	15% 7% 2 6% 6% 3 5% 4 6% 7% 38%	2160
	11th to 20th	Day	13% 5% 3 5% 6% 4 5% 4 6% 8% 41%	2026
	21st to end	Day	15% 6% 3 8% 7% 4 5% 6% 6% 7% 34%	1962
October	1st to 10th	Day	19% 10% 3 5% 7% 3 4 4 5% 6% 32%	1800
	11th to 20th	Day	17% 13% 4 9% 7% 4 5% 5% 5% 5% 26%	1800
	21st to end	Day	24% 17% 4 8% 6% 3 4 5% 5% 5% 20%	1980
November	1st to 10th	Day	12% 13% 4 9% 6% 4 5% 5% 6% 7% 28%	1588
	11th to 20th	Day	11% 15% 5% 10% 8% 5% 5% 7% 6% 7% 22%	1520
	21st to end	Day	15% 17% 8% 10% 7% 4 5% 5% 5% 7% 17%	1520
December	1st to 10th	Day	14% 17% 9% 14% 10% 3 5% 5% 6% 6% 12%	1520
	11th to 20th	Day	16% 15% 7% 12% 8% 3 6% 5% 5% 5% 16%	1906
	21st to end	Day	23% 21% 8% 15% 7% 3 4 4 4 3 9%	2772
January	1st to 10th	Night	14% 13% 6% 13% 10% 5% 6% 5% 5% 6% 18%	2649
	11th to 20th	Night	15% 13% 6% 12% 11% 5% 7% 5% 6% 5% 15%	2433
	21st to end	Night	10% 10% 7% 12% 12% 6% 6% 6% 5% 9% 18%	2574
February	1st to 10th	Night	13% 13% 6% 10% 10% 4 6% 5% 6% 6% 21%	2340
	11th to 20th	Night	11% 12% 6% 10% 9% 5% 6% 6% 6% 6% 24%	2273
	21st to end	Night	12% 12% 5% 10% 8% 5% 5% 5% 5% 6% 27%	1866
March	1st to 10th	Night	11% 10% 6% 10% 7% 3 6% 5% 5% 6% 32%	1978
	11th to 20th	Night	13% 10% 5% 9% 7% 4 4 4 4 6% 36%	1979
	21st to end	Night	14% 10% 4 6% 5% 4 4 4 6% 6% 38%	2178
April	1st to 10th	Night	16% 12% 5% 7% 6% 4 4 4 5% 6% 31%	1850
	11th to 20th	Night	16% 8% 3 5% 5% 3 4 5% 5% 7% 38%	1686
	21st to end	Night	15% 10% 3 7% 6% 2 4 5% 5% 6% 35%	1620
May	1st to 10th	Night	15% 6% 2 5% 5% 3 6% 5% 6% 8% 39%	1620
	11th to 20th	Night	15% 5% 2 4 4 3 5% 4 5% 7% 47%	1447
	21st to end	Night	9% 4 2 5% 4 2 4 4 5% 9% 53%	1550
June	1st to 10th	Night	13% 4 3 5% 6% 4 5% 4 6% 8% 43%	1440
	11th to 20th	Night	9% 4 2 5% 5% 2 5% 6% 7% 10% 44%	1321
	21st to end	Night	12% 4 2 6% 4 3 4 4 5% 8% 49%	1260
July	1st to 10th	Night	6% 4 2 4 4 2 4 4 4 7% 59%	1260
	11th to 20th	Night	9% 4 2 5% 5% 3 4 4 6% 7% 50%	1440
	21st to end	Night	8% 4 1 4 4 4 5% 5% 5% 8% 52%	1758
August	1st to 10th	Night	8% 4 1 4 4 3 4 4 6% 9% 52%	1611
	11th to 20th	Night	6% 4 2 5% 5% 2 4 6% 6% 10% 51%	1598
	21st to end	Night	11% 5% 2 4 4 3 5% 5% 6% 8% 48%	1834
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 5% 6% 9% 47%	1800
	11th to 20th	Night	11% 4 2 4 3 2 3 3 5% 7% 54%	1930
	21st to end	Night	15% 6% 3 6% 7% 3 4 4 4 6% 42%	1998
October	1st to 10th	Night	17% 7% 2 6% 7% 3 4 5% 4 7% 37%	2160
	11th to 20th	Night	18% 9% 3 7% 6% 3 6% 4 5% 7% 32%	2160
	21st to end	Night	20% 13% 3 7% 7% 4 4 5% 5% 7% 24%	2376
November	1st to 10th	Night	15% 9% 4 6% 5% 3 6% 6% 6% 6% 34%	2487
	11th to 20th	Night	14% 13% 5% 7% 6% 3 6% 5% 6% 8% 26%	2660
	21st to end	Night	15% 14% 6% 10% 7% 4 5% 5% 6% 7% 23%	2660
December	1st to 10th	Night	16% 14% 7% 10% 8% 4 5% 5% 6% 7% 16%	2660
	11th to 20th	Night	14% 16% 7% 11% 9% 4 5% 5% 6% 6% 17%	3288
	21st to end	Night	24% 20% 8% 14% 6% 3 4 3 4 4 9%	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	69% 6% 12% 9% 3	1518
	11th to 20th	Day	71% 7% 9% 9% 5% 1	1621
	21st to end	Day	68% 5% 11% 8% 6% 1	1782
February	1st to 10th	Day	69% 8% 7% 12% 3 1	1620
	11th to 20th	Day	64% 6% 10% 13% 6% 1	1582
	21st to end	Day	64% 6% 8% 15% 7% 1	1387
March	1st to 10th	Day	57% 8% 11% 14% 9% 1	1971
	11th to 20th	Day	56% 6% 8% 17% 11% 2	1980
	21st to end	Day	55% 5% 8% 16% 13% 2	2178
April	1st to 10th	Day	56% 6% 11% 17% 10% 1	2108
	11th to 20th	Day	48% 5% 9% 22% 15% 1	2239
	21st to end	Day	52% 6% 10% 18% 11% 2	2340
May	1st to 10th	Day	48% 7% 8% 21% 15% 1	2340
	11th to 20th	Day	45% 7% 10% 23% 14% 1	2450
	21st to end	Day	39% 9% 14% 22% 14% 2	2725
June	1st to 10th	Day	41% 9% 12% 21% 16% 1	2520
	11th to 20th	Day	39% 8% 11% 24% 17% 1	2633
	21st to end	Day	40% 8% 13% 24% 14% 1	2700
July	1st to 10th	Day	34% 9% 14% 22% 18% 2	2621
	11th to 20th	Day	39% 8% 14% 27% 11% 1	2445
	21st to end	Day	43% 10% 14% 20% 12% 1	2414
August	1st to 10th	Day	38% 10% 15% 23% 12% 2	2314
	11th to 20th	Day	40% 11% 16% 22% 11%	2303
	21st to end	Day	42% 10% 13% 23% 11% 1	2503
September	1st to 10th	Day	43% 9% 12% 25% 10% 2	2160
	11th to 20th	Day	41% 7% 12% 28% 12%	2026
	21st to end	Day	48% 8% 12% 23% 8% 1	1962
October	1st to 10th	Day	52% 7% 13% 17% 11% 1	1800
	11th to 20th	Day	58% 6% 13% 16% 7% 1	1800
	21st to end	Day	65% 7% 11% 11% 6%	1980
November	1st to 10th	Day	53% 8% 15% 16% 8% 1	1588
	11th to 20th	Day	59% 8% 13% 12% 7% 1	1520
	21st to end	Day	67% 8% 11% 11% 3	1520
December	1st to 10th	Day	72% 7% 11% 7% 3	1520
	11th to 20th	Day	69% 8% 11% 9% 3	1906
	21st to end	Day	81% 6% 5% 7% 2	2772
January	1st to 10th	Night	66% 7% 11% 11% 5%	2649
	11th to 20th	Night	69% 8% 10% 8% 4	2433
	21st to end	Night	63% 7% 12% 12% 6% 1	2574
February	1st to 10th	Night	62% 7% 11% 16% 4	2340
	11th to 20th	Night	60% 7% 11% 15% 7%	2275
	21st to end	Night	57% 7% 12% 15% 9% 1	1866
March	1st to 10th	Night	53% 6% 12% 18% 10% 1	1988
	11th to 20th	Night	51% 6% 9% 20% 12% 2	1980
	21st to end	Night	47% 5% 9% 21% 14% 2	2178
April	1st to 10th	Night	54% 4 10% 18% 13% 1	1850
	11th to 20th	Night	45% 5% 11% 20% 16% 2	1686
	21st to end	Night	48% 6% 12% 17% 15% 2	1620
May	1st to 10th	Night	43% 7% 9% 21% 17% 3	1620
	11th to 20th	Night	38% 6% 11% 25% 18% 3	1448
	21st to end	Night	30% 8% 15% 26% 17% 4	1551
June	1st to 10th	Night	39% 8% 10% 25% 17% 2	1440
	11th to 20th	Night	33% 8% 12% 25% 20% 1	1321
	21st to end	Night	34% 9% 13% 28% 16% 1	1260
July	1st to 10th	Night	26% 7% 11% 28% 23% 4	1225
	11th to 20th	Night	31% 8% 12% 32% 16% 1	1392
	21st to end	Night	29% 9% 15% 29% 15% 3	1660
August	1st to 10th	Night	30% 8% 16% 31% 14% 1	1600
	11th to 20th	Night	27% 9% 15% 33% 15% 2	1598
	21st to end	Night	34% 9% 13% 28% 15% 1	1834
September	1st to 10th	Night	34% 8% 16% 27% 13% 1	1800
	11th to 20th	Night	29% 8% 12% 33% 17% 1	1930
	21st to end	Night	43% 7% 12% 28% 10% 1	1998
October	1st to 10th	Night	47% 8% 10% 20% 14% 1	2160
	11th to 20th	Night	51% 8% 13% 20% 7%	2160
	21st to end	Night	60% 7% 11% 13% 9%	2376
November	1st to 10th	Night	48% 8% 14% 20% 9% 1	2487
	11th to 20th	Night	54% 8% 14% 15% 8% 1	2660
	21st to end	Night	61% 9% 15% 13% 2 1	2660
December	1st to 10th	Night	66% 9% 12% 10% 3	2660
	11th to 20th	Night	66% 9% 12% 11% 3	3288
	21st to end	Night	79% 6% 7% 6% 2	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	22% 28% 31% 17% 3 1	180
	11th to 20th	Day	24% 29% 33% 9% 4	236
	21st to end	Day	23% 31% 34% 9% 2	264
February	1st to 10th	Day	32% 36% 25% 7% 1	240
	11th to 20th	Day	23% 39% 34% 4 1	240
	21st to end	Day	32% 40% 23% 5%	198
March	1st to 10th	Day	22% 29% 32% 14% 3	411
	11th to 20th	Day	21% 25% 37% 12% 5%	420
	21st to end	Day	30% 24% 29% 13% 5%	462
April	1st to 10th	Day	24% 26% 33% 11% 5%	420
	11th to 20th	Day	30% 23% 26% 11% 8% 3	412
	21st to end	Day	26% 22% 33% 13% 6% 1	420
May	1st to 10th	Day	21% 20% 31% 17% 9% 2	423
	11th to 20th	Day	20% 19% 32% 17% 9% 3	447
	21st to end	Day	16% 16% 30% 24% 10% 4	485
June	1st to 10th	Day	16% 18% 33% 24% 8% 2	450
	11th to 20th	Day	14% 17% 33% 25% 10% 2	563
	21st to end	Day	15% 16% 37% 21% 8% 3	630
July	1st to 10th	Day	11% 10% 34% 28% 13% 4	616
	11th to 20th	Day	12% 10% 33% 27% 12% 7%	444
	21st to end	Day	10% 11% 38% 26% 11% 4	488
August	1st to 10th	Day	9% 10% 29% 29% 15% 8%	446
	11th to 20th	Day	6% 10% 32% 27% 18% 7%	439
	21st to end	Day	15% 11% 31% 23% 14% 5%	495
September	1st to 10th	Day	14% 13% 30% 24% 12% 7%	450
	11th to 20th	Day	11% 13% 26% 24% 17% 9%	450
	21st to end	Day	14% 18% 28% 22% 14% 4	450
October	1st to 10th	Day	24% 19% 23% 18% 12% 4	450
	11th to 20th	Day	22% 26% 32% 15% 4 1	450
	21st to end	Day	34% 24% 27% 13% 3	495
November	1st to 10th	Day	22% 28% 30% 16% 3 1	235
	11th to 20th	Day	25% 27% 31% 12% 4 1	180
	21st to end	Day	27% 24% 39% 8% 1	180
December	1st to 10th	Day	26% 31% 32% 11% 1	180
	11th to 20th	Day	27% 33% 26% 12% 1 1	180
	21st to end	Day	27% 34% 32% 6% 2	198
January	1st to 10th	Night	23% 29% 30% 13% 4	760
	11th to 20th	Night	25% 25% 30% 17% 3	739
	21st to end	Night	17% 22% 36% 20% 5%	792
February	1st to 10th	Night	23% 31% 32% 13% 1	720
	11th to 20th	Night	22% 25% 36% 14% 2	703
	21st to end	Night	21% 28% 34% 14% 2	592
March	1st to 10th	Night	21% 26% 33% 17% 3	720
	11th to 20th	Night	22% 26% 31% 16% 5%	720
	21st to end	Night	21% 20% 33% 20% 6%	792
April	1st to 10th	Night	24% 24% 32% 18% 2	720
	11th to 20th	Night	22% 19% 32% 18% 9% 1	622
	21st to end	Night	21% 16% 33% 22% 8%	540
May	1st to 10th	Night	19% 9% 31% 29% 10% 1	540
	11th to 20th	Night	16% 11% 35% 28% 10% 1	386
	21st to end	Night	10% 11% 35% 32% 12%	389
June	1st to 10th	Night	14% 9% 34% 33% 9%	360
	11th to 20th	Night	11% 10% 31% 35% 12%	360
	21st to end	Night	11% 9% 35% 31% 14% 1	360
July	1st to 10th	Night	5% 6% 31% 37% 20% 1	350
	11th to 20th	Night	9% 10% 28% 32% 19% 1	348
	21st to end	Night	8% 6% 31% 37% 17% 1	578
August	1st to 10th	Night	9% 4 28% 36% 22% 1	537
	11th to 20th	Night	6% 7% 24% 41% 21% 2	534
	21st to end	Night	14% 6% 30% 28% 21% 2	646
September	1st to 10th	Night	12% 9% 28% 28% 17% 5%	720
	11th to 20th	Night	13% 9% 28% 28% 19% 5%	720
	21st to end	Night	17% 13% 24% 27% 16% 4	720
October	1st to 10th	Night	19% 13% 33% 19% 13% 2	720
	11th to 20th	Night	24% 18% 32% 19% 7% 1	720
	21st to end	Night	28% 21% 30% 17% 4	792
November	1st to 10th	Night	22% 20% 30% 20% 7% 1	740
	11th to 20th	Night	20% 28% 28% 17% 5% 1	760
	21st to end	Night	24% 23% 32% 18% 3	760
December	1st to 10th	Night	27% 26% 33% 13% 2 1	760
	11th to 20th	Night	21% 30% 33% 13% 3	760
	21st to end	Night	30% 24% 31% 13% 2	836

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