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 2026-03-03.

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% 11% 4 9% 6% 5% 5% 4 5% 7% 31%	132044
	Night	13% 10% 4 10% 6% 5% 5% 4 5% 7% 31%	129105
Transparency	Day	51% 7% 11% 19% 11% 1	135635
	Night	51% 7% 12% 19% 11% 1	133602
Seeing	Day	20% 20% 31% 18% 8% 3	16470
	Night	19% 18% 31% 22% 8% 1	25516

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% 12% 6% 17% 9% 6% 5% 4 5% 5% 17%	4661
	11th to 20th	Day	7% 18% 8% 20% 10% 8% 6% 3 4 5% 9%	5143
	21st to end	Day	12% 16% 6% 16% 8% 8% 7% 4 5% 6% 13%	5780
February	1st to 10th	Day	12% 16% 5% 14% 9% 6% 5% 5% 5% 4 18%	1890
	11th to 20th	Day	11% 13% 7% 16% 9% 6% 5% 5% 5% 5% 19%	1843
	21st to end	Day	15% 16% 5% 11% 6% 5% 5% 4 5% 5% 24%	1622
March	1st to 10th	Day	16% 12% 5% 11% 7% 4 4 3 4 5% 29%	4945
	11th to 20th	Day	9% 14% 5% 10% 6% 4 5% 4 5% 8% 29%	4980
	21st to end	Day	15% 15% 5% 8% 5% 4 4 3 4 5% 32%	5465
April	1st to 10th	Day	15% 15% 5% 9% 5% 4 4 4 5% 6% 29%	2343
	11th to 20th	Day	18% 10% 4 6% 4 4 3 4 5% 6% 36%	2491
	21st to end	Day	19% 11% 3 8% 4 4 4 3 6% 6% 33%	2600
May	1st to 10th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 36%	5888
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 6% 6% 35%	6244
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 7% 39%	6949
June	1st to 10th	Day	15% 6% 3 7% 4 4 5% 5% 7% 8% 36%	2800
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 9% 40%	2927
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 5% 6% 8% 39%	3000
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 9% 37%	6786
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 4 7% 8% 45%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 6% 9% 47%	6434
August	1st to 10th	Day	13% 7% 3 7% 4 5% 6% 5% 7% 8% 37%	4234
	11th to 20th	Day	11% 6% 3 8% 5% 5% 6% 5% 7% 9% 33%	4174
	21st to end	Day	11% 5% 2 6% 4 5% 6% 4 6% 9% 42%	4544
September	1st to 10th	Day	14% 7% 2 6% 5% 4 5% 4 6% 7% 40%	2400
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 3 6% 8% 44%	2250
	21st to end	Day	15% 6% 3 9% 6% 5% 5% 4 6% 7% 34%	2179
October	1st to 10th	Day	18% 9% 3 7% 5% 4 4 4 5% 7% 35%	2000
	11th to 20th	Day	16% 13% 4 9% 6% 5% 5% 4 5% 6% 28%	2000
	21st to end	Day	22% 16% 4 9% 5% 4 4 4 5% 5% 21%	2200
November	1st to 10th	Day	12% 15% 4 11% 5% 5% 5% 4 6% 7% 28%	1758
	11th to 20th	Day	11% 17% 5% 11% 7% 5% 5% 5% 6% 7% 22%	1680
	21st to end	Day	13% 19% 7% 11% 6% 6% 5% 4 6% 7% 16%	1680
December	1st to 10th	Day	16% 20% 8% 13% 8% 6% 4 5% 6% 5% 10%	2680
	11th to 20th	Day	17% 20% 6% 12% 5% 4 4 4 4 4 18%	3066
	21st to end	Day	21% 19% 7% 17% 6% 5% 5% 3 4 4 9%	4048
January	1st to 10th	Night	12% 12% 6% 16% 10% 6% 5% 3 5% 6% 19%	8234
	11th to 20th	Night	8% 15% 8% 18% 12% 8% 6% 4 5% 5% 11%	7727
	21st to end	Night	11% 11% 6% 11% 10% 7% 7% 5% 7% 8% 18%	8401
February	1st to 10th	Night	11% 14% 5% 12% 7% 6% 5% 4 6% 6% 24%	2730
	11th to 20th	Night	10% 13% 6% 12% 8% 6% 6% 4 5% 6% 24%	2663
	21st to end	Night	11% 13% 5% 12% 7% 7% 5% 4 5% 6% 26%	2181
March	1st to 10th	Night	13% 10% 5% 10% 6% 5% 6% 4 6% 6% 29%	4939
	11th to 20th	Night	8% 10% 4 8% 5% 5% 5% 5% 6% 8% 35%	4979
	21st to end	Night	16% 9% 4 8% 5% 5% 5% 3 5% 6% 35%	5467
April	1st to 10th	Night	16% 14% 3 8% 5% 5% 4 4 5% 6% 30%	2055
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 4 5% 7% 37%	1875
	21st to end	Night	14% 11% 3 8% 5% 4 4 4 5% 7% 35%	1800
May	1st to 10th	Night	18% 8% 2 7% 4 5% 5% 4 5% 6% 37%	4053
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 5% 7% 43%	3649
	21st to end	Night	17% 5% 2 5% 3 3 3 3 5% 7% 46%	3926
June	1st to 10th	Night	13% 5% 2 6% 5% 4 5% 4 6% 8% 43%	1600
	11th to 20th	Night	8% 4 2 6% 5% 4 5% 5% 8% 9% 44%	1467
	21st to end	Night	12% 5% 2 7% 3 3 4 3 6% 8% 47%	1400
July	1st to 10th	Night	7% 5% 2 7% 3 4 4 4 5% 9% 49%	3173
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 6% 7% 52%	3600
	21st to end	Night	7% 5% 1 5% 4 4 4 4 6% 8% 52%	4415
August	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 6% 6% 50%	2915
	11th to 20th	Night	12% 4 2 5% 4 3 4 3 5% 8% 49%	2875
	21st to end	Night	10% 5% 2 5% 3 4 4 3 6% 7% 49%	3320
September	1st to 10th	Night	9% 5% 2 6% 3 5% 4 4 6% 8% 48%	2000
	11th to 20th	Night	10% 5% 1 4 3 3 3 3 5% 6% 57%	2146
	21st to end	Night	16% 6% 2 7% 6% 4 4 3 4 6% 42%	2220
October	1st to 10th	Night	16% 7% 2 7% 5% 4 4 4 4 7% 40%	2400
	11th to 20th	Night	17% 10% 3 7% 5% 5% 5% 3 5% 7% 34%	2400
	21st to end	Night	19% 13% 3 8% 5% 5% 4 5% 5% 6% 27%	2640
November	1st to 10th	Night	14% 10% 3 8% 5% 5% 6% 4 6% 6% 34%	2757
	11th to 20th	Night	13% 15% 4 9% 5% 4 5% 4 6% 8% 27%	2940
	21st to end	Night	15% 15% 6% 11% 6% 5% 5% 4 6% 7% 21%	2940
December	1st to 10th	Night	14% 16% 7% 14% 7% 6% 6% 4 7% 6% 13%	4720
	11th to 20th	Night	12% 17% 7% 13% 7% 6% 5% 4 6% 5% 18%	5348
	21st to end	Night	20% 20% 7% 16% 6% 5% 4 3 4 4 10%	7150

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	65% 8% 12% 10% 5%	4662
	11th to 20th	Day	76% 6% 8% 6% 4	5177
	21st to end	Day	69% 6% 10% 9% 5%	5780
February	1st to 10th	Day	67% 7% 7% 14% 4	1890
	11th to 20th	Day	65% 5% 10% 13% 5% 1	1852
	21st to end	Day	62% 7% 8% 15% 7% 1	1622
March	1st to 10th	Day	56% 6% 13% 13% 12%	4948
	11th to 20th	Day	53% 5% 14% 19% 7% 2	4980
	21st to end	Day	55% 5% 9% 17% 12% 2	5465
April	1st to 10th	Day	56% 5% 10% 17% 10% 1	2343
	11th to 20th	Day	49% 6% 9% 22% 14% 1	2491
	21st to end	Day	51% 6% 10% 18% 12% 2	2600
May	1st to 10th	Day	46% 6% 13% 18% 12% 5%	5897
	11th to 20th	Day	47% 10% 10% 15% 17% 1	6248
	21st to end	Day	44% 7% 11% 21% 15% 2	6949
June	1st to 10th	Day	42% 9% 11% 20% 16% 1	2800
	11th to 20th	Day	39% 8% 12% 25% 16% 1	2927
	21st to end	Day	41% 8% 13% 24% 13% 1	3000
July	1st to 10th	Day	40% 8% 14% 22% 15% 1	6713
	11th to 20th	Day	34% 7% 11% 32% 16% 1	6285
	21st to end	Day	33% 8% 13% 31% 15% 1	6249
August	1st to 10th	Day	42% 9% 13% 23% 12% 1	4221
	11th to 20th	Day	43% 9% 16% 23% 10%	4213
	21st to end	Day	37% 8% 12% 32% 10% 1	4544
September	1st to 10th	Day	42% 9% 12% 26% 10% 2	2400
	11th to 20th	Day	38% 7% 12% 30% 13%	2250
	21st to end	Day	48% 8% 12% 23% 9%	2179
October	1st to 10th	Day	44% 6% 12% 25% 11% 1	3260
	11th to 20th	Day	45% 6% 14% 21% 14%	3240
	21st to end	Day	58% 7% 12% 17% 6%	3538
November	1st to 10th	Day	54% 7% 14% 17% 8%	1758
	11th to 20th	Day	59% 8% 13% 12% 7% 1	1680
	21st to end	Day	67% 8% 11% 11% 3	1680
December	1st to 10th	Day	73% 8% 11% 6% 3	2680
	11th to 20th	Day	69% 7% 9% 8% 7%	3066
	21st to end	Day	79% 6% 5% 6% 4	4048
January	1st to 10th	Night	64% 7% 12% 13% 4	8234
	11th to 20th	Night	73% 8% 8% 7% 4	7768
	21st to end	Night	60% 8% 10% 11% 10%	8401
February	1st to 10th	Night	60% 7% 10% 17% 5%	2730
	11th to 20th	Night	60% 7% 12% 15% 7%	2665
	21st to end	Night	57% 7% 12% 15% 8% 1	2181
March	1st to 10th	Night	54% 7% 11% 17% 9% 2	4949
	11th to 20th	Night	44% 6% 20% 19% 9% 1	4980
	21st to end	Night	50% 5% 11% 18% 14% 2	5467
April	1st to 10th	Night	55% 4 10% 17% 13% 1	2055
	11th to 20th	Night	47% 5% 10% 20% 16% 2	1875
	21st to end	Night	48% 6% 11% 17% 16% 2	1800
May	1st to 10th	Night	46% 6% 12% 14% 14% 6%	4060
	11th to 20th	Night	39% 9% 12% 23% 15% 2	3650
	21st to end	Night	38% 7% 12% 26% 15% 2	3927
June	1st to 10th	Night	38% 8% 11% 25% 17% 2	1600
	11th to 20th	Night	34% 8% 13% 25% 19% 1	1467
	21st to end	Night	35% 8% 13% 27% 16% 1	1400
July	1st to 10th	Night	33% 7% 15% 26% 18% 2	3138
	11th to 20th	Night	31% 7% 11% 32% 19% 1	3552
	21st to end	Night	30% 8% 11% 32% 18% 1	4285
August	1st to 10th	Night	34% 6% 13% 30% 16% 2	2904
	11th to 20th	Night	33% 7% 16% 30% 13% 1	2902
	21st to end	Night	33% 7% 14% 33% 13% 1	3320
September	1st to 10th	Night	32% 8% 16% 30% 13% 1	2000
	11th to 20th	Night	27% 7% 13% 35% 18%	2146
	21st to end	Night	43% 7% 12% 27% 11% 1	2220
October	1st to 10th	Night	35% 6% 9% 29% 20% 1	3920
	11th to 20th	Night	43% 6% 14% 23% 14%	3899
	21st to end	Night	46% 7% 15% 22% 9%	4252
November	1st to 10th	Night	49% 8% 13% 20% 9% 1	2757
	11th to 20th	Night	54% 8% 14% 14% 9% 1	2940
	21st to end	Night	61% 9% 14% 13% 2 1	2940
December	1st to 10th	Night	69% 8% 12% 8% 3	4720
	11th to 20th	Night	66% 8% 9% 10% 7%	5348
	21st to end	Night	76% 6% 7% 7% 3	7150

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% 32% 15% 2	220
	11th to 20th	Day	24% 32% 32% 9% 3	311
	21st to end	Day	26% 30% 31% 11% 2	363
February	1st to 10th	Day	29% 33% 26% 10% 3	330
	11th to 20th	Day	25% 35% 35% 5% 1	330
	21st to end	Day	32% 35% 26% 7%	273
March	1st to 10th	Day	23% 27% 33% 12% 4	499
	11th to 20th	Day	23% 27% 34% 12% 4	500
	21st to end	Day	32% 22% 29% 12% 4	550
April	1st to 10th	Day	27% 25% 32% 11% 5%	500
	11th to 20th	Day	32% 23% 25% 10% 8% 2	492
	21st to end	Day	27% 21% 32% 13% 6% 1	500
May	1st to 10th	Day	21% 19% 33% 17% 8% 2	503
	11th to 20th	Day	20% 17% 33% 18% 9% 3	527
	21st to end	Day	17% 16% 31% 23% 9% 3	573
June	1st to 10th	Day	18% 18% 34% 22% 7% 2	530
	11th to 20th	Day	13% 16% 35% 25% 9% 2	657
	21st to end	Day	16% 16% 37% 21% 7% 3	730
July	1st to 10th	Day	12% 10% 34% 30% 12% 3	714
	11th to 20th	Day	12% 10% 32% 26% 13% 6%	524
	21st to end	Day	9% 10% 38% 26% 12% 5%	576
August	1st to 10th	Day	9% 10% 28% 30% 15% 8%	526
	11th to 20th	Day	8% 10% 30% 26% 18% 8%	519
	21st to end	Day	14% 11% 29% 26% 14% 5%	583
September	1st to 10th	Day	14% 13% 30% 23% 13% 8%	530
	11th to 20th	Day	9% 11% 23% 24% 18% 14%	530
	21st to end	Day	15% 16% 27% 24% 14% 4	530
October	1st to 10th	Day	22% 18% 24% 20% 13% 3	530
	11th to 20th	Day	21% 24% 33% 15% 5% 2	530
	21st to end	Day	31% 22% 29% 14% 3	583
November	1st to 10th	Day	25% 27% 30% 14% 3 1	285
	11th to 20th	Day	27% 25% 32% 11% 4 1	220
	21st to end	Day	27% 25% 37% 10% 1	220
December	1st to 10th	Day	28% 29% 30% 13%	220
	11th to 20th	Day	31% 30% 25% 11% 1 1	220
	21st to end	Day	28% 31% 31% 6% 2 1	242
January	1st to 10th	Night	22% 29% 31% 14% 4	840
	11th to 20th	Night	24% 26% 31% 16% 3	840
	21st to end	Night	18% 22% 36% 19% 5%	924
February	1st to 10th	Night	21% 29% 32% 16% 1	840
	11th to 20th	Night	23% 25% 35% 15% 2	823
	21st to end	Night	23% 26% 34% 15% 2	692
March	1st to 10th	Night	22% 26% 33% 17% 2	809
	11th to 20th	Night	21% 26% 31% 16% 5%	800
	21st to end	Night	22% 20% 33% 20% 5%	880
April	1st to 10th	Night	25% 23% 33% 18% 2	800
	11th to 20th	Night	23% 19% 32% 17% 8% 1	691
	21st to end	Night	21% 17% 33% 23% 8%	600
May	1st to 10th	Night	19% 9% 32% 30% 9% 1	600
	11th to 20th	Night	17% 11% 34% 28% 10% 1	429
	21st to end	Night	11% 10% 34% 34% 11%	433
June	1st to 10th	Night	15% 9% 34% 33% 9%	400
	11th to 20th	Night	10% 10% 30% 37% 12%	400
	21st to end	Night	12% 9% 34% 31% 13% 1	400
July	1st to 10th	Night	5% 6% 30% 39% 18% 1	390
	11th to 20th	Night	10% 11% 26% 34% 19% 1	388
	21st to end	Night	8% 6% 29% 38% 18% 1	644
August	1st to 10th	Night	9% 4 26% 37% 23% 1	597
	11th to 20th	Night	7% 7% 22% 41% 21% 2	594
	21st to end	Night	13% 6% 29% 29% 21% 1	718
September	1st to 10th	Night	11% 10% 28% 29% 18% 5%	800
	11th to 20th	Night	11% 8% 25% 28% 22% 6%	800
	21st to end	Night	19% 12% 23% 27% 16% 3	800
October	1st to 10th	Night	18% 13% 32% 21% 15% 2	800
	11th to 20th	Night	23% 18% 30% 21% 7% 1	800
	21st to end	Night	26% 21% 30% 19% 5%	880
November	1st to 10th	Night	23% 21% 29% 20% 7% 1	820
	11th to 20th	Night	21% 28% 27% 17% 6% 1	840
	21st to end	Night	25% 23% 32% 17% 3	840
December	1st to 10th	Night	26% 25% 33% 14% 2 1	840
	11th to 20th	Night	21% 30% 34% 13% 3	840
	21st to end	Night	30% 23% 32% 13% 2	924

Page generated at 2026-02-28 01:40:22. Version ac07

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