Mill Pond Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mill Pond Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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% 10% 4 8% 5% 5% 4 4 5% 6% 31%	108808
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	110177
Transparency	Day	51% 6% 12% 21% 9% 1	112297
	Night	54% 5% 11% 19% 10% 1	114852
Seeing	Day	24% 18% 30% 19% 8% 2	14873
	Night	24% 15% 30% 23% 8% 1	24059

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	15% 20% 6% 15% 5% 4 3 2 4 4 20%	3597
	11th to 20th	Day	13% 16% 7% 14% 6% 5% 5% 3 5% 4 22%	3570
	21st to end	Day	12% 18% 7% 12% 5% 6% 4 4 5% 6% 21%	3988
February	1st to 10th	Day	16% 16% 4 11% 7% 4 4 4 4 5% 24%	1800
	11th to 20th	Day	15% 14% 4 11% 7% 5% 4 3 6% 5% 26%	1818
	21st to end	Day	19% 15% 6% 9% 5% 5% 4 3 5% 4 25%	1650
March	1st to 10th	Day	20% 13% 4 10% 5% 4 4 3 4 4 27%	3216
	11th to 20th	Day	14% 17% 5% 11% 6% 4 4 3 4 5% 26%	3381
	21st to end	Day	15% 9% 3 8% 6% 4 4 4 4 5% 38%	4102
April	1st to 10th	Day	22% 13% 3 7% 5% 3 3 3 3 5% 33%	2280
	11th to 20th	Day	22% 10% 3 8% 5% 3 3 3 4 6% 34%	2254
	21st to end	Day	23% 10% 3 8% 5% 4 3 4 4 5% 31%	2393
May	1st to 10th	Day	22% 9% 3 8% 5% 4 5% 4 5% 6% 29%	4426
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 5% 7% 33%	4398
	21st to end	Day	16% 7% 2 7% 5% 5% 4 4 6% 8% 36%	4837
June	1st to 10th	Day	23% 8% 3 7% 5% 4 4 4 5% 7% 29%	2660
	11th to 20th	Day	15% 6% 2 6% 6% 5% 5% 4 6% 7% 37%	2660
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 8% 30%	2660
July	1st to 10th	Day	12% 6% 2 7% 6% 6% 6% 4 7% 8% 35%	4440
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	4440
	21st to end	Day	12% 5% 2 5% 4 4 4 4 7% 9% 43%	4814
August	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 9% 33%	4423
	11th to 20th	Day	13% 7% 2 7% 4 4 5% 4 6% 8% 39%	3939
	21st to end	Day	16% 5% 2 6% 4 5% 5% 4 6% 9% 38%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 43%	2280
	11th to 20th	Day	13% 4 2 6% 4 4 4 3 5% 7% 48%	2235
	21st to end	Day	17% 7% 3 6% 4 4 4 3 6% 7% 41%	2090
October	1st to 10th	Day	20% 8% 2 7% 4 4 5% 4 5% 6% 35%	1910
	11th to 20th	Day	20% 11% 3 9% 6% 4 4 3 5% 5% 30%	1900
	21st to end	Day	24% 12% 4 7% 4 4 5% 3 4 6% 28%	1884
November	1st to 10th	Day	16% 10% 3 7% 6% 6% 5% 4 5% 6% 33%	1761
	11th to 20th	Day	17% 11% 4 8% 5% 6% 5% 3 4 7% 29%	1676
	21st to end	Day	21% 15% 4 9% 7% 6% 4 4 5% 6% 21%	1600
December	1st to 10th	Day	20% 20% 6% 10% 5% 5% 5% 4 5% 5% 15%	2600
	11th to 20th	Day	19% 16% 5% 11% 5% 4 5% 3 4 4 23%	2986
	21st to end	Day	23% 16% 4 10% 6% 4 4 3 4 4 22%	3960
January	1st to 10th	Night	15% 23% 8% 13% 5% 4 4 3 4 4 19%	6349
	11th to 20th	Night	11% 17% 8% 17% 8% 6% 5% 3 5% 4 17%	6315
	21st to end	Night	14% 17% 6% 12% 6% 6% 5% 3 4 6% 22%	6954
February	1st to 10th	Night	15% 14% 5% 12% 7% 6% 5% 4 5% 5% 22%	2600
	11th to 20th	Night	13% 14% 4 11% 6% 5% 5% 4 5% 4 27%	2468
	21st to end	Night	15% 16% 4 10% 5% 5% 4 3 4 5% 29%	1980
March	1st to 10th	Night	20% 11% 4 8% 6% 4 4 3 4 4 32%	3844
	11th to 20th	Night	18% 16% 4 10% 6% 4 3 3 4 5% 26%	3604
	21st to end	Night	14% 12% 3 8% 5% 4 4 3 4 5% 38%	3530
April	1st to 10th	Night	23% 11% 3 7% 5% 4 4 3 5% 5% 31%	1900
	11th to 20th	Night	22% 9% 3 8% 5% 3 3 4 4 5% 34%	1875
	21st to end	Night	21% 9% 3 7% 5% 3 4 3 5% 7% 33%	1794
May	1st to 10th	Night	21% 10% 3 7% 4 5% 5% 3 6% 7% 29%	2522
	11th to 20th	Night	22% 8% 3 7% 4 3 3 3 5% 6% 36%	2495
	21st to end	Night	15% 7% 3 7% 4 4 4 4 6% 8% 38%	2738
June	1st to 10th	Night	20% 6% 3 7% 6% 5% 5% 5% 6% 9% 30%	1520
	11th to 20th	Night	17% 7% 2 6% 4 3 4 4 5% 8% 40%	1520
	21st to end	Night	18% 7% 3 7% 4 5% 5% 3 6% 7% 34%	1520
July	1st to 10th	Night	12% 8% 3 8% 6% 5% 5% 4 6% 7% 35%	2520
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 3 7% 8% 41%	2520
	21st to end	Night	12% 7% 2 6% 5% 4 4 5% 5% 8% 42%	2739
August	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 6% 8% 38%	2506
	11th to 20th	Night	14% 8% 3 7% 4 5% 5% 3 5% 8% 40%	2852
	21st to end	Night	13% 7% 2 7% 5% 4 5% 3 5% 8% 40%	3476
September	1st to 10th	Night	10% 6% 3 8% 5% 5% 5% 3 5% 7% 43%	1900
	11th to 20th	Night	15% 6% 2 5% 4 3 3 2 5% 7% 49%	1944
	21st to end	Night	17% 5% 2 5% 5% 4 4 3 5% 7% 42%	2090
October	1st to 10th	Night	20% 7% 3 7% 4 4 4 4 5% 6% 37%	2269
	11th to 20th	Night	19% 9% 3 8% 5% 4 5% 4 4 6% 33%	2280
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 4 5% 29%	2712
November	1st to 10th	Night	19% 11% 4 7% 4 4 4 3 4 7% 33%	2537
	11th to 20th	Night	18% 10% 3 9% 5% 5% 5% 4 5% 5% 32%	2720
	21st to end	Night	21% 13% 4 10% 6% 5% 4 3 5% 5% 24%	2800
December	1st to 10th	Night	17% 18% 6% 12% 7% 5% 5% 4 4 5% 16%	4580
	11th to 20th	Night	23% 16% 5% 10% 5% 4 4 3 3 4 24%	5208
	21st to end	Night	26% 17% 4 10% 4 4 3 3 4 5% 20%	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	68% 5% 10% 13% 3 1	3598
	11th to 20th	Day	64% 5% 10% 13% 8% 1	3600
	21st to end	Day	63% 6% 12% 10% 8% 1	3988
February	1st to 10th	Day	63% 4 8% 19% 5% 2	1800
	11th to 20th	Day	60% 5% 11% 16% 7% 1	1827
	21st to end	Day	61% 4 10% 14% 9% 1	1650
March	1st to 10th	Day	60% 5% 8% 18% 8% 1	3219
	11th to 20th	Day	60% 5% 11% 14% 9% 2	3381
	21st to end	Day	48% 4 8% 20% 18% 2	4102
April	1st to 10th	Day	55% 3 7% 19% 12% 3	2280
	11th to 20th	Day	52% 4 6% 20% 16% 2	2254
	21st to end	Day	55% 5% 8% 16% 13% 4	2393
May	1st to 10th	Day	53% 7% 8% 20% 10% 1	4440
	11th to 20th	Day	51% 6% 14% 19% 9% 1	4402
	21st to end	Day	43% 6% 12% 24% 13% 1	4837
June	1st to 10th	Day	53% 6% 11% 18% 12% 1	2660
	11th to 20th	Day	44% 7% 11% 23% 13% 2	2660
	21st to end	Day	50% 7% 13% 19% 9% 1	2660
July	1st to 10th	Day	42% 8% 13% 26% 11% 1	4370
	11th to 20th	Day	40% 8% 15% 30% 7%	4365
	21st to end	Day	36% 8% 14% 33% 9%	4661
August	1st to 10th	Day	46% 7% 17% 23% 8%	4409
	11th to 20th	Day	41% 7% 17% 29% 7%	3979
	21st to end	Day	41% 8% 16% 23% 12%	4180
September	1st to 10th	Day	38% 6% 14% 29% 13%	2280
	11th to 20th	Day	36% 5% 13% 31% 15% 1	2235
	21st to end	Day	43% 6% 14% 27% 10%	2090
October	1st to 10th	Day	45% 6% 13% 29% 8%	3170
	11th to 20th	Day	47% 5% 16% 25% 7%	3140
	21st to end	Day	47% 5% 18% 23% 7%	3084
November	1st to 10th	Day	51% 6% 13% 22% 8% 1	1761
	11th to 20th	Day	55% 6% 10% 17% 11% 1	1676
	21st to end	Day	64% 6% 13% 12% 5%	1600
December	1st to 10th	Day	70% 7% 11% 10% 2	2600
	11th to 20th	Day	64% 6% 9% 15% 6%	2986
	21st to end	Day	65% 5% 8% 16% 6%	3960
January	1st to 10th	Night	70% 5% 11% 11% 4	6349
	11th to 20th	Night	69% 6% 10% 10% 5%	6360
	21st to end	Night	64% 4 10% 10% 10% 1	6954
February	1st to 10th	Night	63% 5% 9% 16% 6% 1	2600
	11th to 20th	Night	59% 5% 10% 19% 8%	2470
	21st to end	Night	58% 4 8% 18% 11% 1	1980
March	1st to 10th	Night	55% 4 9% 20% 10% 3	3854
	11th to 20th	Night	60% 5% 10% 15% 8% 2	3605
	21st to end	Night	49% 4 5% 23% 16% 3	3530
April	1st to 10th	Night	55% 4 7% 18% 13% 2	1900
	11th to 20th	Night	52% 5% 8% 20% 14% 2	1875
	21st to end	Night	51% 4 8% 18% 14% 4	1794
May	1st to 10th	Night	52% 7% 10% 19% 9% 3	2528
	11th to 20th	Night	48% 7% 11% 19% 13% 2	2496
	21st to end	Night	42% 7% 10% 22% 17% 2	2739
June	1st to 10th	Night	49% 8% 13% 19% 11% 1	1520
	11th to 20th	Night	42% 6% 11% 22% 18% 2	1520
	21st to end	Night	48% 6% 13% 19% 13% 1	1520
July	1st to 10th	Night	45% 7% 13% 20% 14% 1	2480
	11th to 20th	Night	38% 7% 16% 31% 8%	2472
	21st to end	Night	38% 8% 16% 23% 15%	2651
August	1st to 10th	Night	43% 6% 16% 26% 9%	2496
	11th to 20th	Night	43% 6% 17% 27% 8%	2878
	21st to end	Night	40% 8% 15% 23% 13% 1	3476
September	1st to 10th	Night	41% 6% 10% 27% 16% 1	1900
	11th to 20th	Night	35% 5% 12% 34% 13% 1	1944
	21st to end	Night	42% 5% 12% 28% 13%	2090
October	1st to 10th	Night	42% 7% 15% 28% 9%	3789
	11th to 20th	Night	44% 6% 14% 27% 9%	3779
	21st to end	Night	48% 5% 17% 19% 10% 1	4462
November	1st to 10th	Night	52% 5% 13% 21% 9% 1	2537
	11th to 20th	Night	52% 6% 12% 19% 10% 1	2720
	21st to end	Night	62% 5% 12% 16% 5%	2800
December	1st to 10th	Night	69% 5% 12% 11% 3	4580
	11th to 20th	Night	66% 4 6% 15% 9% 1	5208
	21st to end	Night	66% 4 7% 13% 10%	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	30% 20% 33% 15% 3	200
	11th to 20th	Day	28% 30% 30% 8% 4 1	200
	21st to end	Day	18% 26% 37% 16% 3	227
February	1st to 10th	Day	33% 31% 25% 10% 2	300
	11th to 20th	Day	30% 27% 35% 8%	365
	21st to end	Day	31% 21% 33% 14% 1	414
March	1st to 10th	Day	30% 22% 30% 15% 4	500
	11th to 20th	Day	29% 23% 32% 13% 2 1	464
	21st to end	Day	33% 23% 24% 13% 6% 1	506
April	1st to 10th	Day	30% 21% 30% 14% 4	460
	11th to 20th	Day	34% 20% 25% 13% 6% 1	452
	21st to end	Day	32% 20% 29% 12% 7% 1	460
May	1st to 10th	Day	25% 19% 34% 15% 6% 2	463
	11th to 20th	Day	18% 19% 32% 19% 10% 2	487
	21st to end	Day	19% 19% 32% 22% 7% 1	529
June	1st to 10th	Day	29% 12% 31% 20% 7% 2	490
	11th to 20th	Day	17% 17% 31% 25% 8% 2	490
	21st to end	Day	26% 11% 32% 18% 10% 3	490
July	1st to 10th	Day	15% 10% 32% 27% 13% 2	485
	11th to 20th	Day	14% 12% 33% 27% 11% 2	484
	21st to end	Day	16% 10% 33% 29% 10% 2	532
August	1st to 10th	Day	16% 9% 27% 31% 13% 3	486
	11th to 20th	Day	15% 7% 28% 30% 13% 7%	479
	21st to end	Day	16% 11% 30% 23% 14% 5%	539
September	1st to 10th	Day	17% 12% 30% 22% 14% 6%	490
	11th to 20th	Day	13% 11% 25% 23% 18% 11%	490
	21st to end	Day	19% 13% 31% 19% 14% 4	490
October	1st to 10th	Day	22% 17% 27% 18% 12% 3	490
	11th to 20th	Day	25% 23% 27% 18% 6% 1	490
	21st to end	Day	33% 16% 30% 17% 5%	430
November	1st to 10th	Day	28% 20% 26% 18% 6% 1	395
	11th to 20th	Day	24% 24% 32% 16% 4	276
	21st to end	Day	31% 23% 29% 14% 4 1	200
December	1st to 10th	Day	31% 18% 33% 16% 3	200
	11th to 20th	Day	32% 29% 29% 10% 1 1	200
	21st to end	Day	30% 27% 26% 15% 2	220
January	1st to 10th	Night	27% 20% 35% 14% 4	800
	11th to 20th	Night	29% 20% 28% 19% 4 1	800
	21st to end	Night	21% 17% 35% 21% 4	880
February	1st to 10th	Night	23% 20% 34% 20% 3	800
	11th to 20th	Night	23% 23% 34% 17% 3	783
	21st to end	Night	26% 17% 33% 22% 2	660
March	1st to 10th	Night	24% 20% 31% 20% 4	800
	11th to 20th	Night	27% 19% 32% 19% 3	767
	21st to end	Night	25% 17% 27% 26% 6%	836
April	1st to 10th	Night	30% 16% 29% 21% 4	760
	11th to 20th	Night	25% 19% 30% 20% 5%	751
	21st to end	Night	30% 14% 29% 20% 8%	654
May	1st to 10th	Night	27% 10% 30% 24% 7% 2	388
	11th to 20th	Night	24% 12% 29% 25% 9% 1	374
	21st to end	Night	15% 10% 40% 26% 9%	411
June	1st to 10th	Night	21% 9% 29% 32% 9% 1	380
	11th to 20th	Night	19% 11% 26% 31% 11% 1	380
	21st to end	Night	19% 8% 30% 27% 15% 1	380
July	1st to 10th	Night	11% 9% 30% 35% 14% 1	370
	11th to 20th	Night	14% 5% 29% 35% 17%	368
	21st to end	Night	14% 5% 31% 36% 13% 1	416
August	1st to 10th	Night	15% 4 25% 36% 19% 1	378
	11th to 20th	Night	15% 4 23% 35% 21% 2	591
	21st to end	Night	16% 7% 28% 30% 17% 2	836
September	1st to 10th	Night	14% 7% 32% 32% 14% 1	760
	11th to 20th	Night	18% 7% 21% 30% 20% 4	760
	21st to end	Night	19% 9% 29% 23% 18% 2	760
October	1st to 10th	Night	24% 10% 29% 26% 12% 1	760
	11th to 20th	Night	25% 13% 29% 23% 10%	760
	21st to end	Night	28% 17% 28% 24% 4	836
November	1st to 10th	Night	28% 17% 25% 22% 7% 1	780
	11th to 20th	Night	23% 18% 34% 16% 8%	800
	21st to end	Night	28% 19% 26% 19% 7%	800
December	1st to 10th	Night	31% 18% 29% 18% 4	800
	11th to 20th	Night	33% 20% 27% 15% 5% 1	800
	21st to end	Night	36% 19% 27% 15% 3	880

Page generated at 2025-03-09 08:08:49. Version ac07

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