Milford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Milford clear sky chart from 2005-11-06 to 2026-10-14.

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	17% 7% 2 6% 4 4 4 3 5% 7% 39%	149006
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	142730
Transparency	Day	44% 4 11% 23% 15% 3	152541
	Night	44% 4 11% 22% 16% 3	147271
Seeing	Day	26% 15% 32% 19% 7% 2	17568
	Night	23% 14% 33% 23% 6% 1	26408

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	10% 8% 3 8% 4 4 4 4 5% 6% 42%	4661
	11th to 20th	Day	14% 9% 3 6% 3 3 3 3 4 5% 46%	4650
	21st to end	Day	15% 9% 2 5% 3 3 4 3 5% 5% 46%	5124
February	1st to 10th	Day	17% 8% 3 6% 4 5% 5% 5% 5% 7% 37%	1977
	11th to 20th	Day	13% 8% 3 7% 4 5% 5% 4 5% 6% 40%	2048
	21st to end	Day	18% 12% 2 6% 4 5% 3 2 4 4 39%	1730
March	1st to 10th	Day	21% 11% 2 7% 3 3 4 3 4 5% 37%	4576
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 5% 7% 40%	4853
	21st to end	Day	25% 10% 2 5% 4 4 4 4 5% 6% 32%	5929
April	1st to 10th	Day	20% 9% 2 6% 4 4 3 3 4 7% 37%	2520
	11th to 20th	Day	18% 6% 2 6% 4 4 4 4 5% 8% 39%	2494
	21st to end	Day	21% 6% 2 7% 5% 3 5% 4 4 6% 37%	2730
May	1st to 10th	Day	26% 10% 3 6% 4 4 4 3 5% 7% 28%	7639
	11th to 20th	Day	20% 8% 3 7% 4 4 4 3 5% 6% 36%	7758
	21st to end	Day	22% 8% 3 7% 4 4 5% 4 6% 7% 31%	9060
June	1st to 10th	Day	18% 6% 2 5% 4 5% 5% 4 6% 7% 36%	2940
	11th to 20th	Day	17% 6% 2 6% 5% 4 4 4 6% 8% 38%	2940
	21st to end	Day	17% 5% 2 6% 5% 5% 4 4 6% 8% 37%	2940
July	1st to 10th	Day	19% 6% 2 6% 5% 4 5% 4 6% 9% 35%	8280
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 43%	8280
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 44%	8993
August	1st to 10th	Day	13% 6% 3 8% 5% 6% 5% 4 7% 10% 33%	6090
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 6% 8% 43%	5819
	21st to end	Day	11% 4 2 5% 4 4 5% 5% 7% 9% 43%	6080
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 8% 46%	2520
	11th to 20th	Day	15% 5% 2 4 4 3 4 3 4 6% 51%	2311
	21st to end	Day	20% 6% 3 5% 3 4 4 3 5% 7% 41%	2310
October	1st to 10th	Day	21% 5% 3 5% 4 3 3 2 4 5% 45%	2310
	11th to 20th	Day	17% 6% 2 5% 3 3 4 3 4 7% 47%	1989
	21st to end	Day	20% 6% 2 5% 4 4 4 3 4 6% 42%	1980
November	1st to 10th	Day	15% 7% 2 6% 4 3 3 3 4 4 50%	1851
	11th to 20th	Day	17% 6% 2 4 3 3 3 3 4 5% 48%	1890
	21st to end	Day	18% 7% 2 5% 4 4 4 3 6% 5% 43%	1890
December	1st to 10th	Day	14% 9% 2 4 4 3 3 3 4 6% 47%	2730
	11th to 20th	Day	19% 7% 2 4 3 2 3 2 4 5% 48%	3066
	21st to end	Day	20% 11% 3 6% 4 3 4 2 4 6% 38%	4048
January	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 5% 7% 46%	8234
	11th to 20th	Night	15% 10% 3 8% 5% 4 4 3 4 5% 39%	8235
	21st to end	Night	15% 9% 3 6% 4 5% 3 3 4 5% 42%	9057
February	1st to 10th	Night	16% 9% 3 6% 5% 3 4 4 5% 6% 40%	2639
	11th to 20th	Night	15% 8% 3 7% 4 4 4 4 5% 6% 40%	2458
	21st to end	Night	18% 9% 2 7% 4 4 4 3 4 5% 40%	2076
March	1st to 10th	Night	23% 9% 2 4 4 3 3 3 4 6% 39%	5484
	11th to 20th	Night	16% 10% 3 7% 4 4 4 3 4 6% 39%	5324
	21st to end	Night	25% 10% 3 6% 3 3 4 3 5% 5% 33%	5242
April	1st to 10th	Night	19% 8% 2 7% 5% 4 4 4 5% 6% 36%	2100
	11th to 20th	Night	19% 8% 3 6% 4 5% 3 4 5% 5% 38%	2075
	21st to end	Night	20% 8% 3 6% 4 4 3 3 4 6% 40%	1890
May	1st to 10th	Night	28% 9% 3 6% 4 4 4 3 4 6% 30%	5259
	11th to 20th	Night	18% 8% 3 7% 6% 5% 5% 4 5% 6% 33%	5157
	21st to end	Night	21% 8% 3 7% 5% 4 5% 3 5% 7% 31%	5114
June	1st to 10th	Night	16% 7% 2 6% 4 4 4 4 6% 7% 39%	1680
	11th to 20th	Night	18% 7% 2 5% 5% 4 4 4 5% 8% 36%	1680
	21st to end	Night	14% 7% 2 6% 4 3 4 3 7% 9% 41%	1680
July	1st to 10th	Night	17% 7% 2 6% 4 4 5% 4 6% 10% 36%	4680
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 6% 9% 45%	4680
	21st to end	Night	15% 7% 3 6% 4 4 4 3 4 6% 44%	5084
August	1st to 10th	Night	15% 6% 2 8% 5% 5% 5% 4 7% 9% 33%	4013
	11th to 20th	Night	13% 7% 2 7% 4 5% 6% 4 6% 9% 37%	4209
	21st to end	Night	12% 5% 2 5% 3 4 4 4 5% 7% 49%	5045
September	1st to 10th	Night	12% 6% 2 7% 5% 4 4 3 5% 5% 45%	2100
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 7% 52%	2307
	21st to end	Night	19% 8% 3 6% 3 3 3 4 4 5% 42%	2310
October	1st to 10th	Night	20% 5% 2 4 4 3 3 3 4 6% 45%	2310
	11th to 20th	Night	16% 5% 2 4 4 3 4 3 5% 7% 47%	2471
	21st to end	Night	21% 8% 3 6% 4 3 4 3 4 6% 40%	2860
November	1st to 10th	Night	15% 5% 2 5% 4 4 4 3 5% 5% 49%	2667
	11th to 20th	Night	17% 7% 2 4 3 4 3 3 5% 5% 47%	2730
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 7% 44%	2730
December	1st to 10th	Night	18% 8% 2 5% 4 3 3 2 4 4 45%	4652
	11th to 20th	Night	22% 8% 2 5% 3 3 3 3 4 5% 41%	5348
	21st to end	Night	17% 10% 3 6% 4 4 3 3 4 5% 40%	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	42% 3 8% 25% 18% 2	4662
	11th to 20th	Day	42% 3 9% 21% 18% 7%	4680
	21st to end	Day	40% 3 10% 23% 22% 2	5124
February	1st to 10th	Day	47% 4 12% 19% 15% 3	1977
	11th to 20th	Day	44% 5% 9% 22% 16% 3	2057
	21st to end	Day	49% 3 9% 21% 15% 3	1730
March	1st to 10th	Day	50% 3 9% 15% 12% 10%	4579
	11th to 20th	Day	42% 6% 12% 28% 11% 2	4853
	21st to end	Day	54% 3 6% 16% 15% 7%	5929
April	1st to 10th	Day	48% 3 10% 19% 16% 4	2520
	11th to 20th	Day	43% 5% 10% 20% 18% 3	2494
	21st to end	Day	47% 5% 7% 19% 18% 4	2730
May	1st to 10th	Day	56% 4 12% 15% 12% 1	7651
	11th to 20th	Day	50% 4 13% 19% 12% 1	7762
	21st to end	Day	51% 5% 10% 22% 12% 1	9061
June	1st to 10th	Day	45% 6% 12% 24% 13% 1	2940
	11th to 20th	Day	44% 6% 12% 22% 15% 2	2940
	21st to end	Day	44% 7% 12% 22% 15% 1	2940
July	1st to 10th	Day	47% 6% 13% 21% 12% 1	8210
	11th to 20th	Day	36% 5% 16% 29% 14% 1	8205
	21st to end	Day	42% 4 10% 26% 18% 2	8795
August	1st to 10th	Day	47% 6% 18% 20% 9% 1	6076
	11th to 20th	Day	40% 5% 14% 32% 9%	5858
	21st to end	Day	36% 5% 13% 28% 16% 2	6080
September	1st to 10th	Day	38% 6% 11% 26% 19% 2	2520
	11th to 20th	Day	35% 4 8% 25% 24% 3	2311
	21st to end	Day	44% 4 8% 21% 21% 2	2310
October	1st to 10th	Day	40% 4 8% 23% 21% 4	3700
	11th to 20th	Day	33% 3 7% 25% 28% 4	3192
	21st to end	Day	32% 4 13% 29% 20% 3	3180
November	1st to 10th	Day	39% 3 12% 24% 19% 2	1851
	11th to 20th	Day	38% 4 9% 23% 23% 3	1890
	21st to end	Day	42% 4 11% 28% 12% 2	1890
December	1st to 10th	Day	40% 3 14% 27% 15% 1	2730
	11th to 20th	Day	40% 3 13% 18% 20% 6%	3066
	21st to end	Day	50% 4 7% 20% 15% 4	4048
January	1st to 10th	Night	39% 3 10% 30% 16% 2	8234
	11th to 20th	Night	48% 3 10% 19% 15% 5%	8280
	21st to end	Night	46% 3 8% 18% 21% 4	9057
February	1st to 10th	Night	45% 4 12% 22% 15% 2	2639
	11th to 20th	Night	45% 4 9% 21% 17% 4	2460
	21st to end	Night	47% 4 7% 24% 16% 4	2076
March	1st to 10th	Night	49% 3 8% 17% 16% 7%	5494
	11th to 20th	Night	46% 5% 15% 19% 12% 3	5325
	21st to end	Night	54% 2 5% 13% 20% 6%	5242
April	1st to 10th	Night	49% 4 9% 19% 16% 3	2100
	11th to 20th	Night	47% 4 9% 19% 18% 2	2075
	21st to end	Night	47% 4 9% 20% 16% 4	1890
May	1st to 10th	Night	55% 5% 13% 14% 12% 1	5265
	11th to 20th	Night	51% 5% 13% 19% 9% 2	5158
	21st to end	Night	54% 5% 11% 21% 8%	5115
June	1st to 10th	Night	45% 6% 13% 22% 14% 1	1680
	11th to 20th	Night	46% 6% 12% 21% 14% 1	1680
	21st to end	Night	39% 7% 14% 25% 13% 1	1680
July	1st to 10th	Night	46% 5% 14% 21% 14%	4640
	11th to 20th	Night	38% 5% 16% 25% 16% 1	4632
	21st to end	Night	41% 5% 11% 26% 17% 1	4967
August	1st to 10th	Night	48% 6% 18% 17% 11% 1	4003
	11th to 20th	Night	43% 6% 15% 26% 9%	4236
	21st to end	Night	34% 5% 14% 29% 17% 1	5045
September	1st to 10th	Night	41% 6% 9% 24% 18% 1	2100
	11th to 20th	Night	34% 4 10% 23% 26% 3	2307
	21st to end	Night	46% 4 8% 21% 19% 2	2310
October	1st to 10th	Night	39% 4 13% 24% 17% 3	3700
	11th to 20th	Night	28% 3 9% 28% 30% 3	3994
	21st to end	Night	35% 3 11% 29% 20% 2	4610
November	1st to 10th	Night	37% 5% 11% 23% 20% 3	2667
	11th to 20th	Night	40% 3 9% 22% 22% 4	2730
	21st to end	Night	40% 4 13% 28% 14% 2	2730
December	1st to 10th	Night	43% 3 11% 29% 13% 1	4652
	11th to 20th	Night	46% 4 14% 15% 20% 2	5348
	21st to end	Night	47% 4 7% 22% 17% 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	31% 25% 27% 14% 3	220
	11th to 20th	Day	36% 22% 31% 8% 3	220
	21st to end	Day	33% 21% 32% 11% 2 1	242
February	1st to 10th	Day	35% 19% 30% 14% 1	447
	11th to 20th	Day	34% 24% 32% 9% 2	536
	21st to end	Day	33% 20% 31% 13% 3	446
March	1st to 10th	Day	34% 19% 31% 12% 4	540
	11th to 20th	Day	32% 22% 30% 11% 3 1	540
	21st to end	Day	34% 20% 31% 9% 4 2	594
April	1st to 10th	Day	38% 20% 26% 10% 5% 1	540
	11th to 20th	Day	33% 20% 31% 11% 3 1	532
	21st to end	Day	30% 16% 34% 16% 3 2	540
May	1st to 10th	Day	35% 20% 28% 12% 4 1	543
	11th to 20th	Day	26% 16% 38% 14% 4 1	567
	21st to end	Day	22% 16% 35% 20% 6% 1	617
June	1st to 10th	Day	20% 12% 34% 24% 8% 2	570
	11th to 20th	Day	21% 13% 39% 20% 6% 2	570
	21st to end	Day	19% 12% 35% 25% 8% 2	570
July	1st to 10th	Day	19% 8% 32% 28% 11% 2	565
	11th to 20th	Day	16% 7% 31% 32% 11% 3	564
	21st to end	Day	14% 7% 39% 30% 9% 2	620
August	1st to 10th	Day	12% 7% 32% 34% 13% 2	566
	11th to 20th	Day	16% 7% 32% 28% 13% 4	559
	21st to end	Day	12% 10% 33% 27% 13% 5%	627
September	1st to 10th	Day	19% 10% 30% 23% 14% 4	570
	11th to 20th	Day	19% 8% 29% 23% 12% 9%	570
	21st to end	Day	26% 9% 31% 21% 9% 4	570
October	1st to 10th	Day	26% 11% 27% 20% 12% 4	570
	11th to 20th	Day	23% 15% 32% 19% 9% 2	487
	21st to end	Day	28% 13% 32% 17% 8% 2	462
November	1st to 10th	Day	26% 16% 31% 17% 8% 3	425
	11th to 20th	Day	29% 17% 32% 17% 4	430
	21st to end	Day	31% 20% 28% 15% 6%	430
December	1st to 10th	Day	28% 25% 28% 14% 4	257
	11th to 20th	Day	40% 22% 29% 7% 2	220
	21st to end	Day	33% 20% 30% 12% 3 2	242
January	1st to 10th	Night	26% 19% 37% 16% 3	840
	11th to 20th	Night	31% 17% 34% 16% 2	840
	21st to end	Night	24% 20% 33% 19% 3	924
February	1st to 10th	Night	25% 17% 37% 19% 3	840
	11th to 20th	Night	26% 22% 35% 15% 1	823
	21st to end	Night	30% 19% 33% 15% 3	692
March	1st to 10th	Night	28% 18% 33% 17% 3	840
	11th to 20th	Night	25% 22% 33% 16% 3	840
	21st to end	Night	30% 18% 32% 16% 4	924
April	1st to 10th	Night	28% 19% 33% 17% 3	840
	11th to 20th	Night	26% 18% 34% 18% 4	831
	21st to end	Night	24% 19% 35% 19% 3	630
May	1st to 10th	Night	31% 13% 30% 22% 3	630
	11th to 20th	Night	21% 15% 39% 21% 4	596
	21st to end	Night	20% 7% 40% 28% 5%	455
June	1st to 10th	Night	20% 6% 34% 31% 9%	420
	11th to 20th	Night	21% 7% 38% 28% 5% 1	420
	21st to end	Night	15% 6% 37% 35% 8%	420
July	1st to 10th	Night	16% 6% 30% 37% 11%	410
	11th to 20th	Night	14% 2 31% 39% 13%	408
	21st to end	Night	16% 4 34% 37% 9%	460
August	1st to 10th	Night	12% 5% 34% 40% 10%	570
	11th to 20th	Night	14% 4 29% 41% 13%	674
	21st to end	Night	16% 5% 26% 37% 14% 2	924
September	1st to 10th	Night	16% 6% 32% 34% 12% 1	840
	11th to 20th	Night	17% 6% 28% 29% 18% 3	840
	21st to end	Night	24% 8% 26% 29% 11% 2	840
October	1st to 10th	Night	24% 8% 29% 26% 12% 1	840
	11th to 20th	Night	20% 12% 31% 23% 13% 1	813
	21st to end	Night	28% 13% 30% 21% 7%	880
November	1st to 10th	Night	19% 14% 34% 24% 9% 1	820
	11th to 20th	Night	25% 17% 34% 17% 7% 1	840
	21st to end	Night	25% 18% 31% 20% 5% 1	840
December	1st to 10th	Night	28% 16% 34% 17% 4	840
	11th to 20th	Night	29% 17% 33% 19% 2	840
	21st to end	Night	28% 17% 34% 18% 3	924

Page generated at 2026-10-11 05:06:29. Version ac07

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