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 2025-06-10.

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	18% 8% 2 6% 4 4 4 3 5% 7% 39%	119140
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	119079
Transparency	Day	45% 5% 11% 22% 15% 3	122718
	Night	45% 4 11% 21% 16% 3	123649
Seeing	Day	26% 14% 32% 19% 7% 2	15778
	Night	24% 14% 33% 23% 7% 1	24765

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	11% 7% 3 8% 4 4 4 3 5% 6% 44%	3597
	11th to 20th	Day	13% 8% 3 6% 4 3 4 3 4 5% 47%	3570
	21st to end	Day	17% 8% 3 5% 4 3 4 3 5% 4 43%	3960
February	1st to 10th	Day	18% 8% 2 6% 4 5% 5% 5% 5% 6% 36%	1883
	11th to 20th	Day	13% 8% 3 7% 4 5% 5% 4 5% 6% 40%	1948
	21st to end	Day	18% 11% 2 6% 4 5% 3 3 4 5% 39%	1650
March	1st to 10th	Day	21% 11% 2 7% 3 3 4 3 4 5% 37%	4476
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 4 5% 7% 40%	4748
	21st to end	Day	25% 10% 2 5% 4 4 4 4 5% 6% 32%	5801
April	1st to 10th	Day	20% 9% 2 7% 4 4 3 3 4 7% 37%	2400
	11th to 20th	Day	18% 6% 2 6% 4 4 4 4 5% 8% 40%	2374
	21st to end	Day	21% 6% 2 7% 5% 3 5% 4 4 7% 37%	2600
May	1st to 10th	Day	29% 10% 3 7% 4 4 4 3 4 7% 24%	5886
	11th to 20th	Day	21% 9% 3 8% 5% 4 4 3 5% 6% 32%	5966
	21st to end	Day	20% 7% 3 6% 4 4 4 4 6% 7% 35%	6949
June	1st to 10th	Day	19% 6% 2 5% 5% 5% 6% 4 6% 7% 36%	2786
	11th to 20th	Day	17% 6% 2 6% 5% 4 5% 4 6% 8% 37%	2660
	21st to end	Day	17% 6% 2 7% 5% 5% 4 4 6% 8% 36%	2660
July	1st to 10th	Day	13% 6% 3 7% 6% 5% 6% 5% 6% 9% 34%	4440
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 4 6% 9% 39%	4440
	21st to end	Day	18% 8% 2 7% 5% 4 4 4 6% 8% 34%	4814
August	1st to 10th	Day	16% 7% 3 8% 5% 6% 5% 4 6% 9% 32%	4151
	11th to 20th	Day	14% 7% 3 6% 4 5% 5% 4 6% 7% 39%	3956
	21st to end	Day	7% 4 2 5% 5% 5% 5% 5% 7% 10% 45%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 3 4 3 5% 8% 46%	2280
	11th to 20th	Day	15% 6% 2 4 4 3 4 2 4 6% 51%	2091
	21st to end	Day	19% 6% 3 5% 3 4 4 3 5% 6% 43%	2090
October	1st to 10th	Day	23% 5% 3 5% 4 3 3 2 4 5% 42%	2090
	11th to 20th	Day	15% 6% 2 4 3 3 4 3 5% 6% 48%	1858
	21st to end	Day	20% 7% 2 5% 4 4 4 3 4 6% 41%	1881
November	1st to 10th	Day	15% 6% 2 6% 4 3 3 3 4 4 50%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 4 5% 48%	1800
	21st to end	Day	18% 7% 2 5% 4 4 4 3 6% 5% 43%	1800
December	1st to 10th	Day	15% 9% 2 4 4 3 3 3 4 6% 47%	2648
	11th to 20th	Day	19% 7% 2 4 3 2 3 2 4 5% 48%	2986
	21st to end	Day	20% 11% 3 6% 4 3 4 2 4 5% 38%	3960
January	1st to 10th	Night	14% 7% 2 4 3 3 3 3 5% 7% 48%	6349
	11th to 20th	Night	14% 9% 3 8% 5% 3 4 3 4 5% 41%	6315
	21st to end	Night	16% 10% 3 6% 5% 5% 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 9% 3 6% 5% 3 4 4 5% 5% 39%	2513
	11th to 20th	Night	15% 9% 3 7% 4 4 4 4 5% 6% 40%	2338
	21st to end	Night	18% 10% 2 7% 4 4 4 3 4 5% 40%	1980
March	1st to 10th	Night	23% 9% 2 4 4 3 4 3 4 6% 38%	5364
	11th to 20th	Night	16% 10% 3 7% 4 4 4 3 4 6% 39%	5209
	21st to end	Night	25% 10% 3 6% 3 3 4 3 5% 5% 34%	5128
April	1st to 10th	Night	19% 8% 2 7% 5% 4 4 4 5% 6% 36%	2000
	11th to 20th	Night	19% 8% 3 6% 5% 5% 3 4 5% 6% 38%	1975
	21st to end	Night	21% 8% 3 6% 4 4 3 3 4 6% 40%	1800
May	1st to 10th	Night	31% 9% 3 7% 4 4 4 3 4 6% 27%	4054
	11th to 20th	Night	19% 8% 3 8% 6% 6% 5% 4 5% 6% 29%	3969
	21st to end	Night	19% 8% 3 7% 5% 4 5% 3 5% 7% 32%	3926
June	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 6% 8% 38%	1598
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 5% 8% 36%	1520
	21st to end	Night	14% 7% 2 6% 3 3 4 4 7% 9% 40%	1520
July	1st to 10th	Night	13% 6% 2 7% 4 5% 6% 4 7% 10% 35%	2520
	11th to 20th	Night	15% 7% 3 5% 4 4 4 4 6% 9% 39%	2520
	21st to end	Night	17% 9% 3 7% 4 4 5% 4 5% 6% 35%	2739
August	1st to 10th	Night	20% 7% 2 8% 5% 5% 5% 4 6% 8% 30%	2748
	11th to 20th	Night	12% 8% 3 8% 5% 6% 6% 4 6% 8% 34%	2868
	21st to end	Night	8% 4 2 5% 3 4 3 3 5% 7% 57%	3476
September	1st to 10th	Night	13% 6% 2 6% 5% 5% 4 3 5% 6% 45%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 7% 51%	2087
	21st to end	Night	18% 7% 2 6% 4 3 3 4 4 5% 43%	2090
October	1st to 10th	Night	22% 5% 2 5% 4 3 4 3 5% 6% 42%	2090
	11th to 20th	Night	16% 5% 2 5% 4 3 4 3 5% 6% 48%	2315
	21st to end	Night	21% 8% 3 6% 4 3 4 3 4 6% 39%	2717
November	1st to 10th	Night	14% 5% 2 6% 4 4 4 2 5% 5% 49%	2537
	11th to 20th	Night	18% 7% 2 5% 3 4 3 3 5% 5% 47%	2600
	21st to end	Night	17% 7% 1 5% 4 4 3 3 5% 7% 45%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 3 3 2 4 4 45%	4514
	11th to 20th	Night	23% 8% 2 5% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	17% 10% 3 6% 4 3 3 3 4 5% 40%	6996

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	40% 3 6% 28% 20% 3	3598
	11th to 20th	Day	40% 4 10% 22% 18% 7%	3600
	21st to end	Day	44% 3 10% 21% 20% 2	3960
February	1st to 10th	Day	48% 4 12% 19% 14% 3	1883
	11th to 20th	Day	45% 5% 10% 21% 16% 3	1957
	21st to end	Day	49% 3 10% 20% 15% 3	1650
March	1st to 10th	Day	50% 3 10% 15% 11% 10%	4479
	11th to 20th	Day	42% 6% 12% 28% 11% 2	4748
	21st to end	Day	54% 3 6% 16% 15% 7%	5801
April	1st to 10th	Day	48% 3 10% 19% 16% 4	2400
	11th to 20th	Day	43% 5% 10% 20% 19% 3	2374
	21st to end	Day	48% 5% 7% 19% 17% 4	2600
May	1st to 10th	Day	60% 4 13% 12% 9% 1	5897
	11th to 20th	Day	53% 5% 14% 16% 11% 1	5970
	21st to end	Day	47% 6% 10% 22% 14% 1	6949
June	1st to 10th	Day	46% 6% 12% 23% 12% 1	2786
	11th to 20th	Day	44% 6% 12% 21% 15% 2	2660
	21st to end	Day	45% 7% 12% 21% 14% 1	2660
July	1st to 10th	Day	46% 7% 10% 21% 14% 2	4370
	11th to 20th	Day	42% 5% 15% 24% 12% 2	4365
	21st to end	Day	49% 5% 11% 20% 13% 2	4661
August	1st to 10th	Day	49% 6% 16% 21% 8% 1	4137
	11th to 20th	Day	43% 5% 13% 27% 10% 1	3995
	21st to end	Day	33% 6% 12% 29% 18% 1	4180
September	1st to 10th	Day	38% 6% 11% 25% 20% 2	2280
	11th to 20th	Day	36% 4 8% 25% 24% 3	2091
	21st to end	Day	43% 3 9% 21% 22% 2	2090
October	1st to 10th	Day	41% 4 8% 23% 20% 4	3480
	11th to 20th	Day	31% 3 7% 26% 29% 4	3061
	21st to end	Day	32% 4 13% 29% 20% 3	3081
November	1st to 10th	Day	39% 3 12% 24% 19% 2	1761
	11th to 20th	Day	38% 4 9% 22% 23% 3	1800
	21st to end	Day	41% 5% 11% 28% 12% 3	1800
December	1st to 10th	Day	40% 3 14% 27% 14% 1	2648
	11th to 20th	Day	40% 3 14% 18% 20% 6%	2986
	21st to end	Day	50% 4 7% 20% 15% 5%	3960
January	1st to 10th	Night	36% 3 9% 33% 17% 2	6349
	11th to 20th	Night	46% 4 11% 20% 14% 5%	6360
	21st to end	Night	49% 3 9% 16% 18% 5%	6996
February	1st to 10th	Night	46% 5% 12% 21% 14% 2	2513
	11th to 20th	Night	45% 4 9% 19% 18% 4	2340
	21st to end	Night	46% 4 7% 23% 16% 4	1980
March	1st to 10th	Night	49% 3 8% 17% 15% 7%	5374
	11th to 20th	Night	46% 5% 16% 19% 12% 3	5210
	21st to end	Night	53% 2 5% 13% 20% 6%	5128
April	1st to 10th	Night	49% 4 8% 19% 15% 4	2000
	11th to 20th	Night	47% 4 9% 19% 18% 2	1975
	21st to end	Night	47% 4 9% 19% 16% 4	1800
May	1st to 10th	Night	59% 5% 14% 12% 9% 1	4060
	11th to 20th	Night	54% 7% 13% 16% 8% 2	3970
	21st to end	Night	51% 6% 12% 19% 10% 1	3927
June	1st to 10th	Night	46% 6% 13% 21% 13% 1	1598
	11th to 20th	Night	46% 6% 12% 22% 14% 1	1520
	21st to end	Night	39% 8% 15% 24% 13% 1	1520
July	1st to 10th	Night	44% 7% 14% 20% 14% 1	2480
	11th to 20th	Night	41% 6% 17% 23% 12% 1	2472
	21st to end	Night	50% 6% 11% 17% 14% 2	2651
August	1st to 10th	Night	52% 6% 14% 18% 9% 1	2738
	11th to 20th	Night	46% 7% 13% 24% 9%	2895
	21st to end	Night	27% 5% 14% 30% 22% 1	3476
September	1st to 10th	Night	40% 7% 10% 23% 19% 1	1900
	11th to 20th	Night	35% 5% 10% 23% 26% 2	2087
	21st to end	Night	44% 4 8% 22% 19% 2	2090
October	1st to 10th	Night	40% 5% 13% 24% 16% 2	3480
	11th to 20th	Night	27% 3 9% 27% 31% 3	3838
	21st to end	Night	35% 3 11% 29% 19% 2	4467
November	1st to 10th	Night	38% 5% 11% 23% 20% 4	2537
	11th to 20th	Night	41% 3 9% 22% 22% 4	2600
	21st to end	Night	40% 4 13% 27% 14% 2	2600
December	1st to 10th	Night	43% 3 11% 29% 13% 1	4514
	11th to 20th	Night	47% 4 14% 14% 20% 2	5208
	21st to end	Night	47% 4 7% 22% 16% 3	6996

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	33% 24% 26% 15% 4	200
	11th to 20th	Day	35% 22% 33% 9% 3	200
	21st to end	Day	31% 20% 34% 12% 2 1	220
February	1st to 10th	Day	37% 19% 30% 13% 1	413
	11th to 20th	Day	35% 25% 30% 9% 2	496
	21st to end	Day	33% 20% 30% 13% 3	414
March	1st to 10th	Day	35% 18% 31% 11% 4	500
	11th to 20th	Day	31% 22% 30% 12% 4 1	500
	21st to end	Day	33% 18% 33% 10% 4 2	550
April	1st to 10th	Day	38% 20% 26% 11% 5% 1	500
	11th to 20th	Day	33% 20% 32% 11% 4 1	492
	21st to end	Day	31% 15% 34% 16% 3 2	500
May	1st to 10th	Day	36% 19% 27% 13% 4 1	503
	11th to 20th	Day	26% 15% 38% 15% 4 2	527
	21st to end	Day	21% 14% 35% 21% 7% 1	573
June	1st to 10th	Day	22% 11% 35% 23% 8% 2	526
	11th to 20th	Day	19% 12% 40% 21% 7% 2	490
	21st to end	Day	19% 11% 36% 24% 8% 2	490
July	1st to 10th	Day	19% 7% 32% 28% 12% 3	485
	11th to 20th	Day	18% 6% 30% 32% 11% 3	484
	21st to end	Day	14% 7% 40% 30% 8% 2	532
August	1st to 10th	Day	13% 7% 33% 34% 10% 2	486
	11th to 20th	Day	18% 7% 32% 27% 13% 4	479
	21st to end	Day	12% 8% 33% 28% 14% 6%	539
September	1st to 10th	Day	19% 11% 27% 23% 16% 4	490
	11th to 20th	Day	20% 8% 28% 21% 12% 10%	490
	21st to end	Day	24% 8% 33% 22% 9% 4	490
October	1st to 10th	Day	28% 10% 26% 20% 11% 4	490
	11th to 20th	Day	22% 14% 33% 19% 9% 3	440
	21st to end	Day	27% 13% 34% 17% 8% 2	429
November	1st to 10th	Day	25% 15% 31% 18% 9% 3	395
	11th to 20th	Day	30% 17% 31% 18% 5%	400
	21st to end	Day	30% 18% 30% 15% 7%	400
December	1st to 10th	Day	27% 25% 29% 14% 4	235
	11th to 20th	Day	40% 20% 30% 8% 2 1	200
	21st to end	Day	31% 20% 30% 13% 3 3	220
January	1st to 10th	Night	26% 19% 36% 16% 3	800
	11th to 20th	Night	31% 17% 34% 16% 2	800
	21st to end	Night	24% 20% 33% 19% 3	880
February	1st to 10th	Night	26% 17% 36% 19% 3	800
	11th to 20th	Night	26% 23% 35% 14% 1	783
	21st to end	Night	30% 19% 32% 16% 3	660
March	1st to 10th	Night	28% 19% 32% 18% 3	800
	11th to 20th	Night	24% 22% 34% 17% 3	800
	21st to end	Night	29% 17% 32% 17% 4	880
April	1st to 10th	Night	28% 19% 33% 18% 3	800
	11th to 20th	Night	26% 18% 33% 18% 4	791
	21st to end	Night	24% 19% 35% 18% 3	600
May	1st to 10th	Night	32% 12% 30% 22% 4 1	600
	11th to 20th	Night	22% 15% 38% 21% 4	567
	21st to end	Night	19% 6% 41% 28% 5%	433
June	1st to 10th	Night	21% 6% 35% 30% 9%	400
	11th to 20th	Night	21% 6% 38% 29% 6% 1	380
	21st to end	Night	15% 6% 38% 33% 8%	380
July	1st to 10th	Night	15% 5% 31% 36% 13%	370
	11th to 20th	Night	14% 2 30% 40% 14%	368
	21st to end	Night	16% 4 33% 38% 9%	416
August	1st to 10th	Night	12% 4 35% 38% 10%	516
	11th to 20th	Night	14% 4 29% 40% 13%	609
	21st to end	Night	16% 4 24% 38% 15% 3	836
September	1st to 10th	Night	16% 5% 33% 33% 12% 1	760
	11th to 20th	Night	18% 6% 28% 27% 19% 3	760
	21st to end	Night	22% 7% 27% 30% 11% 3	760
October	1st to 10th	Night	25% 8% 28% 26% 11% 1	760
	11th to 20th	Night	20% 11% 31% 23% 13% 1	760
	21st to end	Night	28% 13% 31% 21% 7%	836
November	1st to 10th	Night	18% 13% 34% 24% 9% 1	780
	11th to 20th	Night	25% 17% 33% 17% 7% 1	800
	21st to end	Night	25% 18% 32% 19% 5% 1	800
December	1st to 10th	Night	28% 16% 33% 17% 5%	800
	11th to 20th	Night	28% 17% 32% 20% 2	800
	21st to end	Night	27% 16% 35% 19% 3	880

Page generated at 2025-06-07 10:28:17. Version ac07

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