Easton Woods Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Easton Woods Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	15% 9% 3 7% 5% 4 5% 4 6% 7% 35%	101421
	Night	15% 10% 3 7% 4 4 4 3 5% 7% 37%	98121
Transparency	Day	45% 5% 11% 23% 14% 2	105138
	Night	45% 5% 10% 22% 15% 2	102543
Seeing	Day	20% 12% 27% 22% 13% 6%	14986
	Night	20% 10% 25% 29% 15% 2	25069

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	18% 19% 6% 9% 5% 4 3 3 5% 6% 22%	2836
	11th to 20th	Day	15% 19% 8% 12% 6% 4 3 2 3 4 23%	2840
	21st to end	Day	25% 19% 7% 8% 6% 3 3 3 3 4 19%	3124
February	1st to 10th	Day	21% 12% 5% 9% 5% 3 3 3 5% 5% 29%	1710
	11th to 20th	Day	18% 13% 5% 10% 4 5% 4 3 5% 6% 27%	1830
	21st to end	Day	20% 13% 4 7% 5% 3 3 3 4 5% 32%	1570
March	1st to 10th	Day	23% 15% 4 8% 4 4 4 3 4 5% 27%	3304
	11th to 20th	Day	14% 7% 3 7% 5% 4 4 4 6% 6% 39%	3480
	21st to end	Day	19% 8% 3 6% 4 3 5% 3 5% 7% 35%	3815
April	1st to 10th	Day	15% 9% 4 8% 4 5% 4 3 5% 6% 35%	2092
	11th to 20th	Day	17% 10% 3 6% 5% 3 4 3 6% 5% 39%	2400
	21st to end	Day	18% 8% 2 7% 6% 4 4 4 4 6% 35%	2470
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 4 5% 7% 29%	4082
	11th to 20th	Day	20% 9% 3 7% 4 4 4 4 5% 7% 32%	4081
	21st to end	Day	10% 7% 2 7% 4 5% 6% 5% 7% 10% 37%	4476
June	1st to 10th	Day	12% 6% 2 6% 6% 5% 4 5% 8% 9% 38%	2470
	11th to 20th	Day	9% 6% 3 6% 6% 6% 6% 4 7% 9% 39%	2470
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 6% 8% 10% 37%	2470
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 4 6% 8% 41%	4120
	11th to 20th	Day	9% 5% 2 6% 4 4 6% 6% 7% 10% 41%	4120
	21st to end	Day	13% 7% 4 8% 6% 6% 6% 5% 6% 8% 31%	4466
August	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 6% 8% 11% 36%	4104
	11th to 20th	Day	8% 6% 2 7% 6% 5% 5% 5% 7% 10% 39%	4044
	21st to end	Day	6% 3 2 5% 4 4 5% 4 5% 8% 54%	4422
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 6% 6% 47%	2090
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 5% 8% 49%	2090
	21st to end	Day	19% 6% 2 5% 4 3 3 3 5% 6% 43%	2090
October	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 6% 7% 47%	2028
	11th to 20th	Day	14% 7% 2 6% 4 4 3 3 5% 6% 45%	1900
	21st to end	Day	19% 9% 2 6% 4 4 3 2 4 6% 41%	2081
November	1st to 10th	Day	12% 6% 3 5% 4 4 4 4 5% 5% 50%	1762
	11th to 20th	Day	16% 12% 3 7% 4 3 3 4 5% 6% 37%	1800
	21st to end	Day	17% 11% 4 6% 5% 5% 4 3 5% 6% 35%	1797
December	1st to 10th	Day	23% 15% 4 9% 5% 3 4 3 4 5% 24%	1710
	11th to 20th	Day	17% 16% 5% 10% 5% 4 4 3 5% 6% 26%	2153
	21st to end	Day	20% 20% 5% 11% 6% 5% 4 3 5% 5% 17%	3124
January	1st to 10th	Night	15% 18% 6% 12% 6% 5% 5% 3 5% 4 21%	4109
	11th to 20th	Night	18% 15% 6% 13% 6% 4 4 3 4 5% 24%	4120
	21st to end	Night	25% 16% 6% 9% 6% 3 4 3 4 4 22%	4532
February	1st to 10th	Night	21% 14% 4 8% 4 4 4 3 5% 5% 29%	2470
	11th to 20th	Night	20% 12% 3 10% 5% 4 4 3 4 5% 30%	2233
	21st to end	Night	22% 15% 4 7% 4 4 4 2 4 5% 29%	1884
March	1st to 10th	Night	22% 11% 3 7% 4 3 4 3 4 6% 32%	3547
	11th to 20th	Night	13% 10% 3 6% 4 3 4 3 6% 8% 40%	3480
	21st to end	Night	17% 8% 3 6% 5% 4 4 3 4 6% 39%	3817
April	1st to 10th	Night	15% 10% 3 6% 5% 4 5% 3 6% 7% 38%	2090
	11th to 20th	Night	13% 8% 2 7% 5% 4 4 4 7% 8% 40%	1741
	21st to end	Night	15% 8% 3 7% 5% 4 5% 4 5% 6% 38%	1710
May	1st to 10th	Night	25% 12% 3 6% 4 4 4 4 5% 6% 28%	2813
	11th to 20th	Night	20% 8% 3 6% 4 2 4 3 5% 7% 39%	2812
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 7% 10% 42%	3079
June	1st to 10th	Night	9% 5% 2 5% 5% 5% 5% 4 8% 9% 44%	1710
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 5% 9% 10% 44%	1710
	21st to end	Night	8% 5% 2 5% 5% 4 5% 5% 7% 10% 45%	1710
July	1st to 10th	Night	8% 7% 2 6% 4 4 4 4 7% 9% 45%	2840
	11th to 20th	Night	7% 5% 2 6% 4 5% 4 4 7% 9% 46%	2840
	21st to end	Night	9% 8% 3 8% 5% 6% 5% 4 7% 8% 37%	3087
August	1st to 10th	Night	7% 6% 3 7% 5% 5% 5% 5% 8% 9% 40%	2825
	11th to 20th	Night	9% 7% 3 8% 5% 5% 5% 4 5% 7% 43%	2785
	21st to end	Night	5% 3 1 5% 4 4 5% 4 7% 10% 52%	3183
September	1st to 10th	Night	11% 4 1 5% 3 3 4 4 6% 6% 52%	2090
	11th to 20th	Night	9% 5% 2 6% 3 4 4 3 5% 7% 54%	2090
	21st to end	Night	14% 7% 2 6% 4 4 3 3 5% 6% 47%	2090
October	1st to 10th	Night	11% 5% 2 5% 4 4 2 3 5% 6% 53%	2152
	11th to 20th	Night	13% 7% 3 5% 3 3 4 4 6% 7% 45%	2280
	21st to end	Night	18% 7% 2 6% 4 3 4 3 5% 5% 42%	2513
November	1st to 10th	Night	14% 7% 2 5% 3 3 4 3 4 6% 49%	2536
	11th to 20th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 38%	2600
	21st to end	Night	19% 12% 3 7% 5% 3 3 3 4 6% 35%	2599
December	1st to 10th	Night	21% 14% 4 8% 5% 4 4 3 5% 5% 27%	2470
	11th to 20th	Night	17% 14% 6% 9% 5% 4 3 3 4 7% 28%	3042
	21st to end	Night	19% 17% 5% 10% 6% 5% 5% 3 5% 6% 19%	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	63% 4 11% 15% 8%	2837
	11th to 20th	Day	67% 3 8% 15% 7%	2840
	21st to end	Day	70% 4 6% 15% 4 1	3124
February	1st to 10th	Day	57% 4 11% 17% 9% 1	1710
	11th to 20th	Day	58% 5% 10% 17% 10% 1	1840
	21st to end	Day	55% 4 8% 22% 10% 2	1570
March	1st to 10th	Day	60% 3 8% 21% 7% 1	3304
	11th to 20th	Day	43% 5% 9% 26% 16% 1	3480
	21st to end	Day	47% 5% 10% 23% 14% 1	3815
April	1st to 10th	Day	48% 6% 10% 23% 11% 3	2092
	11th to 20th	Day	45% 5% 10% 20% 16% 4	2400
	21st to end	Day	49% 6% 10% 20% 12% 3	2470
May	1st to 10th	Day	54% 5% 10% 22% 8% 1	4120
	11th to 20th	Day	50% 6% 10% 20% 13% 1	4085
	21st to end	Day	39% 8% 14% 19% 16% 3	4489
June	1st to 10th	Day	39% 8% 13% 24% 14% 2	2470
	11th to 20th	Day	39% 7% 14% 21% 17% 2	2470
	21st to end	Day	39% 8% 14% 22% 14% 3	2470
July	1st to 10th	Day	39% 7% 12% 24% 15% 2	4055
	11th to 20th	Day	33% 8% 13% 30% 15% 1	4051
	21st to end	Day	48% 7% 10% 17% 15% 3	4324
August	1st to 10th	Day	36% 8% 15% 28% 11% 1	4091
	11th to 20th	Day	36% 7% 11% 30% 16% 1	4083
	21st to end	Day	27% 6% 14% 29% 21% 3	4422
September	1st to 10th	Day	37% 5% 10% 21% 21% 6%	2090
	11th to 20th	Day	33% 6% 10% 27% 22% 3	2090
	21st to end	Day	42% 5% 8% 25% 19% 1	2090
October	1st to 10th	Day	24% 4 8% 30% 32% 2	3366
	11th to 20th	Day	31% 3 9% 31% 24% 2	3140
	21st to end	Day	33% 4 11% 29% 22% 2	3407
November	1st to 10th	Day	36% 4 10% 29% 19% 3	1762
	11th to 20th	Day	48% 4 7% 21% 17% 3	1800
	21st to end	Day	50% 4 9% 22% 14% 1	1797
December	1st to 10th	Day	63% 3 9% 18% 6% 1	1710
	11th to 20th	Day	58% 6% 9% 18% 9%	2150
	21st to end	Day	69% 5% 9% 12% 5%	3124
January	1st to 10th	Night	65% 6% 8% 15% 7%	4109
	11th to 20th	Night	64% 4 10% 15% 6% 1	4120
	21st to end	Night	66% 5% 7% 16% 4 1	4532
February	1st to 10th	Night	58% 4 10% 19% 9% 1	2470
	11th to 20th	Night	57% 4 8% 18% 13% 1	2234
	21st to end	Night	57% 4 10% 17% 9% 3	1884
March	1st to 10th	Night	52% 5% 7% 23% 12% 1	3547
	11th to 20th	Night	41% 5% 9% 30% 14% 1	3480
	21st to end	Night	46% 5% 8% 27% 13% 1	3817
April	1st to 10th	Night	45% 6% 8% 22% 16% 3	2090
	11th to 20th	Night	41% 4 11% 20% 17% 6%	1741
	21st to end	Night	45% 6% 10% 21% 16% 2	1710
May	1st to 10th	Night	56% 5% 10% 17% 10% 2	2840
	11th to 20th	Night	44% 7% 12% 24% 12% 1	2813
	21st to end	Night	33% 9% 15% 26% 13% 5%	3087
June	1st to 10th	Night	34% 7% 12% 25% 19% 3	1710
	11th to 20th	Night	31% 8% 17% 21% 20% 3	1710
	21st to end	Night	32% 8% 15% 24% 18% 3	1710
July	1st to 10th	Night	34% 7% 12% 24% 18% 4	2795
	11th to 20th	Night	32% 8% 14% 28% 17% 1	2786
	21st to end	Night	42% 8% 11% 21% 12% 6%	2988
August	1st to 10th	Night	36% 9% 15% 27% 11% 1	2814
	11th to 20th	Night	38% 7% 10% 24% 21%	2812
	21st to end	Night	26% 6% 14% 29% 22% 3	3183
September	1st to 10th	Night	31% 5% 14% 24% 19% 7%	2090
	11th to 20th	Night	30% 5% 10% 25% 26% 3	2090
	21st to end	Night	39% 4 9% 21% 25% 1	2090
October	1st to 10th	Night	26% 4 7% 27% 33% 4	3594
	11th to 20th	Night	28% 4 8% 27% 26% 7%	3779
	21st to end	Night	31% 4 8% 26% 26% 5%	4137
November	1st to 10th	Night	37% 4 10% 25% 21% 4	2536
	11th to 20th	Night	46% 4 7% 22% 17% 3	2600
	21st to end	Night	51% 4 9% 22% 13% 1	2599
December	1st to 10th	Night	59% 5% 9% 19% 7%	2470
	11th to 20th	Night	57% 4 10% 21% 8% 1	3044
	21st to end	Night	66% 5% 9% 15% 5%	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	30% 20% 35% 10% 4	270
	11th to 20th	Day	36% 19% 32% 12% 1	270
	21st to end	Day	29% 26% 32% 10% 3	297
February	1st to 10th	Day	33% 21% 27% 16% 3	270
	11th to 20th	Day	39% 27% 27% 5% 1 1	270
	21st to end	Day	33% 21% 36% 8%	225
March	1st to 10th	Day	28% 20% 33% 16% 3	376
	11th to 20th	Day	28% 19% 33% 14% 5% 1	460
	21st to end	Day	32% 16% 30% 18% 4	506
April	1st to 10th	Day	25% 18% 30% 17% 9% 1	460
	11th to 20th	Day	26% 15% 30% 19% 8% 2	452
	21st to end	Day	25% 15% 35% 17% 6% 2	460
May	1st to 10th	Day	26% 14% 31% 17% 10% 2	463
	11th to 20th	Day	21% 11% 27% 24% 10% 6%	487
	21st to end	Day	13% 7% 26% 28% 18% 8%	529
June	1st to 10th	Day	16% 7% 23% 29% 19% 7%	490
	11th to 20th	Day	14% 8% 25% 30% 17% 7%	490
	21st to end	Day	13% 6% 32% 24% 17% 9%	490
July	1st to 10th	Day	13% 3 21% 24% 18% 21%	485
	11th to 20th	Day	10% 5% 20% 32% 22% 11%	484
	21st to end	Day	12% 5% 18% 30% 22% 13%	532
August	1st to 10th	Day	12% 4 18% 29% 26% 12%	486
	11th to 20th	Day	12% 3 22% 34% 18% 11%	479
	21st to end	Day	9% 3 17% 32% 25% 14%	539
September	1st to 10th	Day	13% 4 17% 27% 26% 12%	490
	11th to 20th	Day	10% 3 15% 28% 32% 13%	490
	21st to end	Day	22% 9% 19% 24% 20% 6%	490
October	1st to 10th	Day	13% 7% 26% 28% 19% 7%	490
	11th to 20th	Day	18% 13% 31% 22% 13% 4	490
	21st to end	Day	25% 12% 31% 24% 8%	530
November	1st to 10th	Day	16% 16% 36% 24% 7% 1	300
	11th to 20th	Day	28% 14% 33% 18% 5% 2	300
	21st to end	Day	22% 19% 36% 17% 5%	299
December	1st to 10th	Day	32% 18% 30% 14% 4 1	270
	11th to 20th	Day	26% 20% 37% 12% 4 1	270
	21st to end	Day	32% 18% 32% 13% 5% 1	297
January	1st to 10th	Night	28% 17% 31% 19% 5%	760
	11th to 20th	Night	30% 17% 29% 20% 3	760
	21st to end	Night	26% 16% 33% 22% 3	836
February	1st to 10th	Night	30% 14% 29% 25% 2	760
	11th to 20th	Night	29% 18% 33% 19% 1	743
	21st to end	Night	30% 16% 33% 19% 2	628
March	1st to 10th	Night	26% 13% 30% 27% 4	760
	11th to 20th	Night	25% 15% 32% 20% 8% 1	760
	21st to end	Night	25% 13% 31% 24% 7%	836
April	1st to 10th	Night	21% 14% 34% 24% 7%	760
	11th to 20th	Night	17% 12% 34% 24% 12% 1	577
	21st to end	Night	18% 8% 38% 28% 7% 1	570
May	1st to 10th	Night	21% 8% 27% 30% 11% 2	570
	11th to 20th	Night	20% 5% 25% 30% 18% 1	561
	21st to end	Night	10% 5% 19% 35% 28% 4	617
June	1st to 10th	Night	9% 3 22% 40% 25% 2	570
	11th to 20th	Night	9% 3 18% 42% 27% 2	570
	21st to end	Night	9% 2 22% 36% 27% 3	570
July	1st to 10th	Night	12% 3 14% 33% 29% 8%	555
	11th to 20th	Night	7% 3 15% 36% 34% 4	552
	21st to end	Night	10% 2 15% 33% 37% 4	624
August	1st to 10th	Night	8% 2 11% 38% 37% 4	567
	11th to 20th	Night	11% 2 12% 38% 31% 7%	564
	21st to end	Night	9% 2 15% 30% 36% 8%	627
September	1st to 10th	Night	13% 3 12% 34% 33% 6%	760
	11th to 20th	Night	11% 3 12% 35% 32% 6%	760
	21st to end	Night	18% 6% 19% 32% 22% 3	760
October	1st to 10th	Night	14% 5% 20% 34% 24% 4	760
	11th to 20th	Night	18% 10% 24% 33% 14% 1	760
	21st to end	Night	22% 10% 26% 30% 12% 1	836
November	1st to 10th	Night	17% 10% 26% 30% 16% 2	780
	11th to 20th	Night	22% 16% 27% 26% 9% 1	800
	21st to end	Night	25% 12% 25% 29% 8% 1	800
December	1st to 10th	Night	32% 13% 28% 23% 5%	760
	11th to 20th	Night	24% 13% 32% 25% 6%	760
	21st to end	Night	27% 16% 28% 27% 3	836

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