Windham Hill Observatory
Clear Sky Chart
History

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

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	12% 6% 2 5% 4 4 4 4 6% 8% 45%	101182
	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	98053
Transparency	Day	36% 6% 12% 21% 19% 7%	104899
	Night	33% 5% 11% 21% 21% 10%	102463
Seeing	Day	13% 7% 21% 27% 24% 8%	15010
	Night	14% 8% 26% 33% 17% 2	24853

By Month

Percentage Cloud Cover SE (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	15% 7% 2 4 3 2 3 3 5% 7% 49%	2847
	11th to 20th	Day	18% 6% 2 4 2 2 2 2 3 7% 51%	2850
	21st to end	Day	22% 9% 3 6% 3 2 2 3 3 5% 41%	3135
February	1st to 10th	Day	21% 8% 3 6% 3 3 3 2 4 5% 41%	1805
	11th to 20th	Day	20% 9% 2 6% 4 4 3 3 3 6% 41%	1849
	21st to end	Day	22% 6% 2 5% 3 3 3 2 4 5% 45%	1677
March	1st to 10th	Day	18% 10% 3 7% 4 4 4 3 5% 6% 35%	3436
	11th to 20th	Day	20% 8% 3 6% 4 4 5% 4 6% 6% 34%	3480
	21st to end	Day	16% 8% 3 7% 5% 4 4 3 6% 6% 39%	3814
April	1st to 10th	Day	14% 8% 3 7% 4 4 4 4 5% 6% 40%	2090
	11th to 20th	Day	15% 8% 2 5% 5% 5% 4 4 5% 7% 40%	2192
	21st to end	Day	12% 6% 2 5% 4 5% 4 4 5% 6% 45%	2280
May	1st to 10th	Day	20% 9% 3 7% 5% 4 5% 4 5% 8% 29%	4050
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 6% 43%	4082
	21st to end	Day	12% 6% 2 7% 5% 5% 6% 5% 8% 10% 33%	4477
June	1st to 10th	Day	6% 4 2 6% 6% 5% 5% 5% 7% 10% 46%	2470
	11th to 20th	Day	5% 4 2 4 5% 5% 5% 5% 7% 10% 48%	2470
	21st to end	Day	5% 5% 2 5% 6% 6% 7% 5% 8% 11% 40%	2470
July	1st to 10th	Day	8% 4 2 4 4 4 5% 4 8% 11% 46%	4120
	11th to 20th	Day	4 4 2 6% 4 5% 5% 4 7% 11% 48%	4120
	21st to end	Day	12% 8% 2 8% 6% 5% 6% 5% 6% 8% 35%	4465
August	1st to 10th	Day	2 1 1 2 2 2 3 3 5% 8% 71%	4104
	11th to 20th	Day	3 2 1 3 3 2 4 4 7% 11% 60%	4009
	21st to end	Day	5% 3 2 5% 5% 5% 6% 5% 8% 11% 45%	4037
September	1st to 10th	Day	8% 6% 2 6% 6% 5% 5% 4 5% 8% 47%	2090
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 4 6% 8% 45%	2090
	21st to end	Day	6% 4 1 4 4 4 4 4 7% 7% 54%	2090
October	1st to 10th	Day	6% 4 2 5% 4 3 4 3 5% 6% 61%	2000
	11th to 20th	Day	12% 4 2 3 3 4 3 3 4 4 57%	1900
	21st to end	Day	14% 5% 1 3 3 3 2 2 3 4 59%	2090
November	1st to 10th	Day	12% 6% 3 5% 4 3 4 3 4 6% 50%	1958
	11th to 20th	Day	12% 6% 2 4 3 4 3 3 5% 5% 53%	1900
	21st to end	Day	17% 8% 2 7% 4 4 4 2 5% 4 44%	1729
December	1st to 10th	Day	15% 8% 3 4 3 3 3 3 5% 6% 47%	1710
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 3 5% 44%	2161
	21st to end	Day	15% 8% 3 6% 4 3 4 3 4 5% 45%	3135
January	1st to 10th	Night	16% 6% 2 5% 2 2 2 2 3 5% 54%	4108
	11th to 20th	Night	17% 7% 1 4 2 2 2 2 3 4 56%	4120
	21st to end	Night	21% 8% 3 6% 3 3 3 2 4 6% 42%	4532
February	1st to 10th	Night	18% 8% 3 6% 4 3 3 3 3 4 45%	2390
	11th to 20th	Night	20% 8% 3 5% 3 3 3 2 3 5% 46%	2218
	21st to end	Night	21% 6% 2 5% 3 3 3 2 4 5% 47%	1775
March	1st to 10th	Night	20% 10% 3 6% 5% 5% 4 3 5% 5% 36%	3419
	11th to 20th	Night	19% 8% 3 7% 5% 3 5% 3 5% 6% 36%	3480
	21st to end	Night	14% 7% 3 6% 4 4 3 3 5% 6% 45%	3817
April	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 5% 8% 47%	2090
	11th to 20th	Night	13% 6% 2 5% 3 3 3 4 5% 8% 47%	1934
	21st to end	Night	13% 6% 2 4 4 3 3 3 6% 5% 51%	1900
May	1st to 10th	Night	18% 10% 3 7% 3 3 4 3 6% 8% 36%	2837
	11th to 20th	Night	12% 6% 3 7% 4 4 4 4 5% 7% 45%	2823
	21st to end	Night	10% 5% 2 6% 4 4 4 4 7% 10% 43%	3090
June	1st to 10th	Night	6% 3 1 5% 3 4 3 3 6% 11% 57%	1710
	11th to 20th	Night	4 2 1 2 3 2 3 3 6% 10% 63%	1710
	21st to end	Night	5% 4 2 4 4 4 3 4 6% 10% 54%	1710
July	1st to 10th	Night	4 4 2 5% 4 4 5% 4 8% 13% 48%	2850
	11th to 20th	Night	3 3 1 4 3 4 4 4 7% 11% 57%	2850
	21st to end	Night	9% 8% 3 7% 4 4 5% 4 7% 9% 40%	3098
August	1st to 10th	Night	1 1 2 1 1 2 2 4 9% 76%	2836
	11th to 20th	Night	2 2 1 3 3 3 5% 3 6% 11% 60%	2829
	21st to end	Night	4 3 1 4 3 4 5% 5% 7% 10% 53%	3600
September	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 7% 8% 51%	2090
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 49%	2090
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 8% 60%	2090
October	1st to 10th	Night	5% 3 1 3 2 3 3 2 4 6% 67%	2158
	11th to 20th	Night	12% 4 1 3 2 2 2 2 4 5% 61%	2280
	21st to end	Night	13% 4 2 3 3 3 2 2 3 4 61%	2508
November	1st to 10th	Night	12% 5% 2 4 4 3 3 3 4 6% 55%	2340
	11th to 20th	Night	9% 7% 2 5% 3 4 4 3 5% 6% 53%	2287
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 5% 47%	2446
December	1st to 10th	Night	15% 7% 2 6% 3 3 3 3 5% 6% 48%	2470
	11th to 20th	Night	15% 9% 3 6% 4 3 3 3 4 5% 46%	3036
	21st to end	Night	16% 6% 3 5% 3 3 3 2 4 4 52%	4532

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	36% 3 6% 19% 28% 7%	2848
	11th to 20th	Day	37% 3 5% 23% 27% 5%	2850
	21st to end	Day	48% 2 4 16% 20% 11%	3135
February	1st to 10th	Day	47% 3 6% 15% 21% 9%	1805
	11th to 20th	Day	47% 4 6% 15% 19% 10%	1859
	21st to end	Day	43% 4 6% 16% 17% 14%	1677
March	1st to 10th	Day	49% 6% 8% 13% 20% 5%	3436
	11th to 20th	Day	49% 6% 10% 14% 16% 5%	3480
	21st to end	Day	45% 5% 9% 16% 20% 4	3814
April	1st to 10th	Day	45% 5% 8% 18% 14% 10%	2090
	11th to 20th	Day	44% 5% 9% 16% 18% 9%	2192
	21st to end	Day	38% 6% 9% 19% 23% 5%	2280
May	1st to 10th	Day	51% 7% 12% 17% 10% 4	4088
	11th to 20th	Day	39% 7% 9% 18% 25% 2	4086
	21st to end	Day	41% 10% 14% 22% 11% 1	4489
June	1st to 10th	Day	31% 8% 15% 30% 14% 2	2470
	11th to 20th	Day	27% 8% 15% 29% 17% 4	2470
	21st to end	Day	34% 10% 14% 25% 16% 2	2470
July	1st to 10th	Day	30% 9% 26% 28% 7%	4056
	11th to 20th	Day	29% 8% 18% 30% 15% 1	4051
	21st to end	Day	44% 9% 16% 24% 7%	4325
August	1st to 10th	Day	12% 5% 14% 40% 29% 1	4090
	11th to 20th	Day	17% 6% 26% 39% 10% 2	4048
	21st to end	Day	29% 12% 20% 25% 12% 2	4036
September	1st to 10th	Day	33% 7% 13% 21% 16% 9%	2090
	11th to 20th	Day	37% 7% 14% 19% 16% 8%	2090
	21st to end	Day	27% 7% 11% 22% 19% 14%	2090
October	1st to 10th	Day	15% 3 6% 13% 22% 41%	3325
	11th to 20th	Day	19% 3 5% 19% 33% 21%	3140
	21st to end	Day	25% 4 5% 20% 34% 12%	3428
November	1st to 10th	Day	36% 5% 8% 19% 22% 10%	1958
	11th to 20th	Day	34% 4 8% 19% 23% 13%	1900
	21st to end	Day	44% 4 6% 17% 19% 10%	1729
December	1st to 10th	Day	38% 4 11% 19% 19% 9%	1710
	11th to 20th	Day	43% 4 6% 15% 21% 10%	2159
	21st to end	Day	43% 2 6% 13% 26% 9%	3135
January	1st to 10th	Night	35% 3 6% 17% 27% 12%	4108
	11th to 20th	Night	34% 3 6% 16% 29% 12%	4120
	21st to end	Night	46% 3 5% 15% 21% 11%	4532
February	1st to 10th	Night	44% 4 5% 14% 18% 16%	2390
	11th to 20th	Night	43% 3 7% 15% 20% 13%	2218
	21st to end	Night	42% 4 7% 14% 16% 17%	1775
March	1st to 10th	Night	50% 5% 7% 13% 18% 7%	3419
	11th to 20th	Night	47% 7% 9% 16% 16% 6%	3480
	21st to end	Night	40% 5% 10% 15% 22% 8%	3817
April	1st to 10th	Night	36% 6% 9% 19% 18% 12%	2090
	11th to 20th	Night	35% 6% 11% 19% 17% 13%	1934
	21st to end	Night	34% 5% 9% 19% 25% 7%	1900
May	1st to 10th	Night	46% 6% 11% 21% 12% 4	2864
	11th to 20th	Night	38% 6% 10% 19% 23% 3	2824
	21st to end	Night	34% 9% 15% 28% 12% 3	3099
June	1st to 10th	Night	23% 8% 18% 30% 20% 2	1710
	11th to 20th	Night	17% 6% 18% 34% 20% 6%	1710
	21st to end	Night	27% 8% 15% 28% 19% 3	1710
July	1st to 10th	Night	27% 8% 28% 32% 6%	2805
	11th to 20th	Night	20% 8% 23% 35% 14% 1	2796
	21st to end	Night	38% 8% 21% 25% 7%	2999
August	1st to 10th	Night	7% 4 15% 36% 34% 3	2826
	11th to 20th	Night	19% 6% 22% 41% 11% 2	2856
	21st to end	Night	25% 11% 21% 28% 13% 2	3600
September	1st to 10th	Night	30% 7% 16% 22% 16% 9%	2090
	11th to 20th	Night	34% 7% 15% 19% 17% 8%	2090
	21st to end	Night	23% 6% 12% 22% 23% 14%	2090
October	1st to 10th	Night	12% 3 7% 16% 19% 44%	3600
	11th to 20th	Night	17% 2 3 19% 33% 26%	3779
	21st to end	Night	22% 3 5% 19% 33% 17%	4120
November	1st to 10th	Night	32% 4 9% 19% 23% 13%	2340
	11th to 20th	Night	32% 4 8% 19% 23% 13%	2287
	21st to end	Night	40% 4 8% 15% 21% 12%	2446
December	1st to 10th	Night	38% 4 8% 20% 19% 10%	2470
	11th to 20th	Night	42% 3 6% 13% 22% 14%	3037
	21st to end	Night	37% 3 6% 13% 30% 11%	4532

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	24% 16% 33% 20% 6% 2	270
	11th to 20th	Day	24% 14% 37% 19% 6% 1	270
	21st to end	Day	23% 17% 37% 15% 7%	297
February	1st to 10th	Day	27% 19% 30% 19% 4	270
	11th to 20th	Day	31% 20% 30% 15% 3	270
	21st to end	Day	30% 22% 38% 8% 2	225
March	1st to 10th	Day	24% 15% 36% 18% 7%	270
	11th to 20th	Day	19% 23% 37% 17% 4	270
	21st to end	Day	20% 15% 39% 19% 7%	297
April	1st to 10th	Day	21% 14% 40% 19% 4	270
	11th to 20th	Day	22% 16% 31% 19% 11% 1	390
	21st to end	Day	16% 13% 33% 30% 8% 2	460
May	1st to 10th	Day	16% 11% 34% 29% 8% 1	634
	11th to 20th	Day	15% 6% 26% 32% 17% 3	675
	21st to end	Day	12% 7% 25% 33% 19% 4	734
June	1st to 10th	Day	6% 2 16% 38% 29% 10%	680
	11th to 20th	Day	6% 2 15% 41% 28% 8%	680
	21st to end	Day	7% 2 12% 32% 35% 13%	680
July	1st to 10th	Day	7% 1 8% 31% 37% 16%	671
	11th to 20th	Day	3 1 5% 32% 42% 16%	668
	21st to end	Day	6% 1 4 28% 48% 12%	739
August	1st to 10th	Day	4 1 4 25% 52% 14%	676
	11th to 20th	Day	6% 1 3 28% 46% 15%	646
	21st to end	Day	8% 2 3 20% 43% 25%	461
September	1st to 10th	Day	5% 8% 28% 35% 23%	300
	11th to 20th	Day	6% 2 8% 25% 38% 20%	300
	21st to end	Day	9% 2 9% 30% 33% 18%	300
October	1st to 10th	Day	4 3 17% 36% 32% 7%	300
	11th to 20th	Day	9% 7% 26% 35% 18% 5%	300
	21st to end	Day	22% 12% 25% 22% 14% 5%	330
November	1st to 10th	Day	17% 8% 24% 33% 15% 3	300
	11th to 20th	Day	17% 11% 28% 29% 11% 3	270
	21st to end	Day	20% 15% 31% 23% 10% 1	270
December	1st to 10th	Day	17% 9% 36% 21% 12% 5%	270
	11th to 20th	Day	27% 17% 30% 20% 6%	270
	21st to end	Day	21% 14% 36% 22% 5% 1	297
January	1st to 10th	Night	19% 12% 35% 27% 7%	950
	11th to 20th	Night	23% 12% 31% 26% 7%	950
	21st to end	Night	21% 11% 34% 29% 5%	1045
February	1st to 10th	Night	24% 14% 33% 27% 3	870
	11th to 20th	Night	25% 13% 37% 22% 3	744
	21st to end	Night	25% 16% 37% 20% 2	519
March	1st to 10th	Night	22% 12% 37% 25% 5%	570
	11th to 20th	Night	17% 11% 41% 25% 6%	570
	21st to end	Night	13% 14% 41% 26% 5%	627
April	1st to 10th	Night	14% 11% 44% 26% 6%	570
	11th to 20th	Night	13% 14% 38% 26% 8%	563
	21st to end	Night	14% 10% 43% 24% 9% 1	570
May	1st to 10th	Night	11% 11% 41% 29% 7% 1	570
	11th to 20th	Night	13% 6% 32% 33% 15%	561
	21st to end	Night	7% 5% 29% 40% 18% 2	617
June	1st to 10th	Night	6% 2 13% 44% 32% 3	570
	11th to 20th	Night	4 1 21% 50% 22% 2	570
	21st to end	Night	5% 1 14% 44% 30% 6%	570
July	1st to 10th	Night	4 1 12% 44% 34% 6%	555
	11th to 20th	Night	3 1 8% 51% 33% 5%	552
	21st to end	Night	4 2 8% 46% 36% 4	624
August	1st to 10th	Night	3 6% 46% 40% 5%	567
	11th to 20th	Night	5% 1 4 47% 37% 5%	564
	21st to end	Night	4 2 7% 39% 42% 7%	627
September	1st to 10th	Night	7% 1 8% 41% 38% 5%	570
	11th to 20th	Night	11% 2 6% 37% 39% 4	570
	21st to end	Night	8% 1 11% 38% 38% 5%	570
October	1st to 10th	Night	6% 2 14% 45% 28% 5%	638
	11th to 20th	Night	12% 5% 23% 39% 18% 2	760
	21st to end	Night	14% 8% 28% 30% 18% 2	836
November	1st to 10th	Night	14% 5% 26% 34% 19% 2	780
	11th to 20th	Night	12% 10% 29% 37% 13%	763
	21st to end	Night	19% 11% 29% 26% 14% 1	926
December	1st to 10th	Night	19% 8% 32% 29% 11%	950
	11th to 20th	Night	23% 11% 30% 29% 6%	950
	21st to end	Night	20% 11% 31% 30% 7%	1045

Page generated at 2024-11-08 05:44:49. Version ac07

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