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 2024-12-02.

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 6% 7% 31%	102298
	Night	14% 10% 4 9% 6% 5% 4 4 5% 6% 33%	97343
Transparency	Day	50% 8% 12% 19% 10% 1	105899
	Night	49% 7% 12% 20% 11% 1	101831
Seeing	Day	20% 20% 31% 18% 8% 3	14938
	Night	19% 19% 31% 22% 9% 1	23899

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% 15% 7% 13% 7% 4 5% 4 5% 6% 20%	2517
	11th to 20th	Day	10% 18% 8% 19% 11% 7% 5% 3 4 4 10%	2779
	21st to end	Day	19% 21% 7% 12% 6% 4 5% 3 3 5% 15%	3124
February	1st to 10th	Day	12% 17% 6% 13% 10% 6% 5% 5% 5% 4 18%	1710
	11th to 20th	Day	11% 13% 7% 16% 8% 5% 5% 5% 5% 6% 20%	1663
	21st to end	Day	15% 17% 5% 11% 6% 5% 4 4 5% 5% 24%	1472
March	1st to 10th	Day	16% 13% 6% 10% 6% 5% 4 3 4 5% 29%	3423
	11th to 20th	Day	10% 18% 7% 11% 6% 5% 5% 3 4 5% 28%	3480
	21st to end	Day	13% 13% 4 7% 5% 4 4 3 4 5% 36%	3815
April	1st to 10th	Day	15% 15% 5% 9% 5% 4 4 4 5% 6% 28%	2226
	11th to 20th	Day	18% 10% 3 6% 4 4 3 4 5% 6% 36%	2365
	21st to end	Day	20% 11% 3 8% 4 4 4 3 6% 5% 33%	2470
May	1st to 10th	Day	17% 9% 3 7% 5% 4 4 3 5% 8% 36%	4108
	11th to 20th	Day	17% 9% 3 7% 5% 5% 4 4 6% 7% 32%	4352
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 8% 46%	4837
June	1st to 10th	Day	15% 6% 3 7% 4 4 5% 5% 7% 8% 36%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 9% 41%	2780
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 5% 6% 8% 39%	2850
July	1st to 10th	Day	11% 5% 2 7% 6% 6% 6% 5% 8% 10% 36%	4741
	11th to 20th	Day	11% 5% 2 5% 5% 5% 6% 5% 7% 8% 42%	4440
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 9% 41%	4476
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 5% 7% 8% 37%	4104
	11th to 20th	Day	11% 6% 3 8% 5% 5% 6% 5% 7% 9% 33%	4044
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 4 6% 8% 42%	4405
September	1st to 10th	Day	14% 7% 2 7% 5% 4 5% 3 6% 7% 40%	2280
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 3 6% 8% 42%	2138
	21st to end	Day	15% 6% 3 9% 6% 6% 5% 5% 6% 7% 33%	2070
October	1st to 10th	Day	18% 9% 3 7% 6% 5% 4 4 5% 6% 33%	1900
	11th to 20th	Day	16% 13% 4 10% 6% 5% 5% 4 5% 5% 27%	1900
	21st to end	Day	23% 17% 4 9% 5% 4 4 4 5% 5% 20%	2090
November	1st to 10th	Day	12% 15% 4 11% 5% 5% 5% 4 6% 7% 28%	1673
	11th to 20th	Day	11% 17% 5% 11% 7% 6% 5% 5% 6% 6% 22%	1600
	21st to end	Day	14% 19% 7% 11% 6% 6% 5% 4 5% 7% 17%	1600
December	1st to 10th	Day	13% 19% 8% 15% 9% 5% 4 4 6% 5% 12%	1528
	11th to 20th	Day	16% 17% 6% 13% 7% 5% 5% 5% 5% 5% 16%	1906
	21st to end	Day	23% 21% 8% 16% 6% 4 4 3 4 3 9%	2772
January	1st to 10th	Night	15% 15% 7% 15% 6% 6% 5% 3 4 6% 19%	4429
	11th to 20th	Night	12% 13% 7% 16% 12% 9% 6% 4 5% 5% 11%	4180
	21st to end	Night	19% 13% 8% 12% 10% 6% 5% 4 5% 6% 14%	4532
February	1st to 10th	Night	12% 15% 5% 12% 8% 6% 5% 4 6% 6% 22%	2470
	11th to 20th	Night	10% 13% 6% 12% 8% 6% 6% 4 5% 6% 24%	2403
	21st to end	Night	11% 12% 4 11% 7% 6% 4 4 5% 6% 27%	1979
March	1st to 10th	Night	15% 12% 5% 8% 6% 4 5% 4 5% 6% 30%	3416
	11th to 20th	Night	11% 12% 4 8% 5% 4 4 4 5% 6% 36%	3479
	21st to end	Night	13% 9% 4 8% 4 4 4 3 6% 6% 39%	3817
April	1st to 10th	Night	16% 14% 3 8% 5% 5% 4 3 5% 6% 31%	1952
	11th to 20th	Night	16% 9% 3 6% 4 4 4 4 5% 7% 38%	1780
	21st to end	Night	15% 11% 3 8% 5% 4 4 4 5% 7% 35%	1710
May	1st to 10th	Night	16% 8% 2 6% 4 4 4 4 5% 6% 41%	2833
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 5% 7% 41%	2541
	21st to end	Night	9% 4 2 5% 3 3 4 3 5% 8% 53%	2738
June	1st to 10th	Night	13% 5% 2 6% 5% 4 5% 4 6% 8% 42%	1520
	11th to 20th	Night	8% 4 2 6% 5% 4 5% 5% 8% 10% 44%	1394
	21st to end	Night	12% 5% 2 7% 3 3 4 3 6% 7% 48%	1330
July	1st to 10th	Night	5% 5% 2 7% 3 4 4 5% 5% 7% 52%	2223
	11th to 20th	Night	11% 4 2 4 4 3 3 3 6% 7% 53%	2520
	21st to end	Night	6% 5% 1 5% 4 5% 5% 4 6% 8% 51%	3074
August	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 6% 6% 50%	2825
	11th to 20th	Night	12% 4 2 5% 4 3 4 3 5% 8% 49%	2785
	21st to end	Night	11% 5% 2 5% 3 4 4 3 5% 7% 49%	3218
September	1st to 10th	Night	9% 5% 2 6% 3 4 4 3 6% 8% 48%	1900
	11th to 20th	Night	10% 5% 1 4 3 3 3 3 6% 6% 55%	2038
	21st to end	Night	16% 6% 2 7% 6% 4 4 3 4 6% 41%	2109
October	1st to 10th	Night	17% 7% 2 7% 5% 4 4 4 5% 7% 39%	2280
	11th to 20th	Night	18% 10% 3 7% 5% 5% 5% 3 5% 7% 33%	2280
	21st to end	Night	19% 14% 3 8% 5% 5% 4 5% 5% 7% 25%	2508
November	1st to 10th	Night	14% 10% 3 8% 5% 5% 6% 4 6% 6% 34%	2622
	11th to 20th	Night	14% 15% 4 9% 5% 4 5% 4 6% 8% 26%	2800
	21st to end	Night	15% 15% 6% 11% 6% 5% 5% 4 6% 7% 22%	2800
December	1st to 10th	Night	16% 15% 6% 12% 7% 5% 5% 4 6% 6% 16%	2686
	11th to 20th	Night	13% 17% 6% 13% 7% 6% 4 5% 6% 6% 17%	3288
	21st to end	Night	24% 21% 7% 15% 6% 5% 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	63% 8% 15% 11% 4	2518
	11th to 20th	Day	76% 6% 8% 7% 3	2779
	21st to end	Day	72% 6% 11% 7% 3	3124
February	1st to 10th	Day	68% 8% 7% 13% 3 1	1710
	11th to 20th	Day	64% 6% 11% 13% 5% 1	1672
	21st to end	Day	62% 7% 8% 15% 7% 1	1472
March	1st to 10th	Day	57% 8% 11% 15% 9%	3426
	11th to 20th	Day	58% 5% 10% 19% 8% 1	3480
	21st to end	Day	50% 5% 12% 17% 14% 2	3815
April	1st to 10th	Day	56% 6% 10% 17% 10% 1	2226
	11th to 20th	Day	49% 5% 9% 22% 15% 1	2365
	21st to end	Day	52% 6% 10% 18% 12% 2	2470
May	1st to 10th	Day	45% 7% 12% 20% 15% 1	4120
	11th to 20th	Day	47% 12% 12% 20% 8% 1	4356
	21st to end	Day	36% 8% 13% 22% 18% 3	4837
June	1st to 10th	Day	42% 9% 12% 20% 15% 1	2660
	11th to 20th	Day	38% 8% 12% 24% 17% 1	2780
	21st to end	Day	42% 8% 12% 24% 14% 1	2850
July	1st to 10th	Day	41% 8% 15% 20% 15% 1	4667
	11th to 20th	Day	35% 8% 12% 33% 11% 1	4365
	21st to end	Day	38% 9% 13% 23% 17% 1	4333
August	1st to 10th	Day	43% 9% 13% 23% 12% 1	4091
	11th to 20th	Day	43% 9% 16% 22% 10%	4083
	21st to end	Day	37% 8% 12% 32% 10% 1	4405
September	1st to 10th	Day	43% 8% 12% 25% 10% 2	2280
	11th to 20th	Day	39% 7% 12% 28% 13%	2138
	21st to end	Day	48% 8% 12% 23% 9%	2070
October	1st to 10th	Day	45% 7% 13% 24% 11% 1	3160
	11th to 20th	Day	45% 7% 14% 20% 14%	3140
	21st to end	Day	59% 7% 12% 17% 5%	3428
November	1st to 10th	Day	53% 8% 14% 16% 8%	1673
	11th to 20th	Day	59% 8% 12% 12% 7% 1	1600
	21st to end	Day	67% 8% 11% 11% 3	1600
December	1st to 10th	Day	72% 7% 11% 7% 2	1528
	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	65% 8% 13% 11% 3	4429
	11th to 20th	Night	72% 9% 10% 6% 3	4180
	21st to end	Night	70% 8% 10% 8% 3 1	4532
February	1st to 10th	Night	61% 7% 10% 17% 4	2470
	11th to 20th	Night	59% 7% 12% 15% 7%	2405
	21st to end	Night	56% 7% 12% 16% 9% 1	1979
March	1st to 10th	Night	54% 7% 11% 21% 7% 1	3426
	11th to 20th	Night	46% 7% 16% 20% 9% 2	3480
	21st to end	Night	45% 5% 12% 22% 16% 2	3817
April	1st to 10th	Night	54% 4 10% 18% 13% 1	1952
	11th to 20th	Night	46% 6% 11% 20% 16% 2	1780
	21st to end	Night	48% 6% 12% 17% 15% 2	1710
May	1st to 10th	Night	43% 7% 12% 18% 17% 3	2840
	11th to 20th	Night	40% 10% 13% 23% 11% 2	2542
	21st to end	Night	29% 9% 13% 26% 21% 2	2739
June	1st to 10th	Night	39% 8% 11% 24% 17% 2	1520
	11th to 20th	Night	33% 8% 13% 25% 19% 1	1394
	21st to end	Night	35% 9% 12% 27% 16% 1	1330
July	1st to 10th	Night	30% 7% 15% 26% 19% 3	2188
	11th to 20th	Night	30% 7% 11% 35% 17% 1	2472
	21st to end	Night	30% 10% 14% 29% 16% 2	2976
August	1st to 10th	Night	35% 6% 12% 30% 15% 2	2814
	11th to 20th	Night	33% 7% 16% 30% 13% 1	2812
	21st to end	Night	33% 7% 13% 32% 14% 1	3218
September	1st to 10th	Night	33% 8% 16% 29% 13% 1	1900
	11th to 20th	Night	28% 7% 12% 33% 18%	2038
	21st to end	Night	43% 7% 12% 27% 10% 1	2109
October	1st to 10th	Night	36% 6% 9% 28% 20% 1	3800
	11th to 20th	Night	43% 6% 14% 23% 14%	3779
	21st to end	Night	47% 7% 15% 21% 9%	4120
November	1st to 10th	Night	49% 8% 14% 20% 9% 1	2622
	11th to 20th	Night	54% 8% 14% 15% 8% 1	2800
	21st to end	Night	61% 9% 14% 13% 2 1	2800
December	1st to 10th	Night	66% 9% 12% 10% 3	2686
	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% 31% 32% 9% 4	254
	21st to end	Day	26% 29% 33% 10% 2	297
February	1st to 10th	Day	29% 34% 26% 8% 3	270
	11th to 20th	Day	22% 37% 36% 4 1	270
	21st to end	Day	32% 37% 25% 5%	225
March	1st to 10th	Day	23% 28% 33% 13% 4	451
	11th to 20th	Day	22% 26% 36% 12% 4	460
	21st to end	Day	30% 24% 29% 12% 4	506
April	1st to 10th	Day	25% 25% 33% 12% 5%	460
	11th to 20th	Day	32% 23% 25% 10% 7% 3	452
	21st to end	Day	27% 21% 32% 13% 5% 2	460
May	1st to 10th	Day	21% 19% 33% 17% 8% 2	463
	11th to 20th	Day	20% 17% 32% 19% 9% 3	487
	21st to end	Day	15% 17% 31% 24% 10% 4	529
June	1st to 10th	Day	18% 17% 33% 23% 7% 2	490
	11th to 20th	Day	13% 16% 34% 26% 9% 2	610
	21st to end	Day	16% 16% 37% 21% 7% 3	680
July	1st to 10th	Day	12% 10% 34% 29% 12% 3	665
	11th to 20th	Day	12% 10% 32% 26% 12% 7%	484
	21st to end	Day	10% 11% 37% 25% 12% 5%	532
August	1st to 10th	Day	10% 10% 29% 28% 14% 8%	486
	11th to 20th	Day	7% 10% 31% 26% 18% 8%	479
	21st to end	Day	14% 11% 30% 24% 15% 5%	539
September	1st to 10th	Day	14% 13% 30% 23% 12% 7%	490
	11th to 20th	Day	10% 12% 24% 24% 18% 12%	490
	21st to end	Day	15% 17% 28% 23% 13% 4	490
October	1st to 10th	Day	23% 19% 24% 19% 12% 3	490
	11th to 20th	Day	21% 25% 32% 15% 5% 2	490
	21st to end	Day	33% 23% 28% 13% 3	539
November	1st to 10th	Day	22% 29% 30% 15% 3 1	260
	11th to 20th	Day	26% 26% 32% 12% 4 1	200
	21st to end	Day	28% 25% 37% 10% 1	200
December	1st to 10th	Day	26% 31% 32% 10% 1	182
	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	24% 25% 31% 16% 3	760
	21st to end	Night	18% 22% 36% 19% 5%	836
February	1st to 10th	Night	22% 30% 32% 14% 1	760
	11th to 20th	Night	23% 25% 36% 14% 2	743
	21st to end	Night	22% 27% 34% 15% 2	628
March	1st to 10th	Night	21% 26% 34% 17% 3	760
	11th to 20th	Night	22% 26% 31% 16% 5%	760
	21st to end	Night	21% 21% 33% 20% 5%	836
April	1st to 10th	Night	24% 23% 33% 18% 2	760
	11th to 20th	Night	22% 19% 32% 17% 8% 1	656
	21st to end	Night	21% 17% 33% 22% 8%	570
May	1st to 10th	Night	19% 9% 31% 30% 9% 1	570
	11th to 20th	Night	17% 11% 34% 28% 10% 1	407
	21st to end	Night	10% 11% 35% 34% 11%	411
June	1st to 10th	Night	15% 9% 34% 33% 9%	380
	11th to 20th	Night	10% 10% 31% 37% 12%	380
	21st to end	Night	12% 9% 35% 30% 13% 1	380
July	1st to 10th	Night	5% 6% 31% 38% 19% 1	370
	11th to 20th	Night	10% 10% 27% 32% 19% 1	368
	21st to end	Night	8% 6% 30% 37% 18% 1	611
August	1st to 10th	Night	9% 4 27% 37% 22% 1	567
	11th to 20th	Night	6% 7% 24% 40% 21% 2	564
	21st to end	Night	14% 6% 29% 29% 21% 1	682
September	1st to 10th	Night	11% 10% 28% 29% 17% 5%	760
	11th to 20th	Night	12% 8% 26% 27% 21% 5%	760
	21st to end	Night	18% 12% 23% 27% 16% 4	760
October	1st to 10th	Night	19% 13% 33% 20% 13% 2	760
	11th to 20th	Night	23% 18% 31% 20% 7% 1	760
	21st to end	Night	27% 22% 30% 18% 4	836
November	1st to 10th	Night	23% 20% 29% 20% 7% 1	780
	11th to 20th	Night	20% 28% 28% 18% 5% 1	800
	21st to end	Night	25% 23% 32% 18% 3	800
December	1st to 10th	Night	26% 26% 33% 13% 2 1	768
	11th to 20th	Night	21% 30% 33% 13% 3	760
	21st to end	Night	30% 24% 31% 13% 2	836

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