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 2024-02-20.

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% 10% 4 7% 6% 3 5% 5% 5% 6% 32%	78176
	Night	18% 11% 4 8% 6% 3 5% 4 5% 6% 30%	80995
Transparency	Day	52% 6% 11% 20% 10% 1	77881
	Night	55% 5% 11% 18% 9% 1	80824
Seeing	Day	24% 17% 30% 19% 8% 2	13526
	Night	24% 14% 29% 23% 8% 1	22732

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	17% 16% 5% 12% 7% 4 4 4 5% 4 22%	2517
	11th to 20th	Day	14% 14% 8% 11% 7% 4 5% 5% 5% 4 23%	2520
	21st to end	Day	15% 18% 7% 9% 6% 5% 5% 5% 6% 6% 18%	2786
February	1st to 10th	Day	16% 15% 5% 10% 8% 3 4 4 4 5% 24%	1710
	11th to 20th	Day	15% 14% 5% 10% 7% 4 4 4 5% 5% 27%	1714
	21st to end	Day	20% 14% 7% 8% 5% 4 5% 4 5% 4 25%	1480
March	1st to 10th	Day	15% 13% 6% 9% 8% 3 4 4 3 5% 29%	1797
	11th to 20th	Day	18% 12% 4 7% 7% 4 4 4 4 5% 31%	1913
	21st to end	Day	21% 10% 3 6% 5% 2 4 5% 4 5% 35%	2340
April	1st to 10th	Day	22% 11% 4 6% 6% 3 3 3 3 5% 33%	2160
	11th to 20th	Day	21% 9% 4 6% 6% 2 3 4 4 6% 35%	2134
	21st to end	Day	24% 9% 4 7% 6% 3 4 4 5% 5% 31%	2266
May	1st to 10th	Day	21% 7% 3 6% 6% 3 5% 5% 4 6% 34%	2520
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 4 4 6% 39%	2478
	21st to end	Day	17% 6% 3 6% 6% 4 4 5% 5% 8% 36%	2725
June	1st to 10th	Day	24% 7% 3 6% 6% 3 5% 5% 5% 7% 29%	2520
	11th to 20th	Day	16% 6% 2 6% 6% 4 5% 5% 6% 8% 38%	2520
	21st to end	Day	19% 7% 3 6% 5% 3 5% 5% 6% 9% 31%	2520
July	1st to 10th	Day	13% 5% 3 6% 5% 4 5% 5% 5% 8% 41%	2520
	11th to 20th	Day	13% 6% 3 5% 5% 4 6% 5% 7% 10% 37%	2520
	21st to end	Day	12% 5% 2 6% 6% 3 6% 5% 7% 10% 37%	2744
August	1st to 10th	Day	13% 6% 3 6% 6% 3 4 6% 6% 9% 38%	2506
	11th to 20th	Day	11% 5% 2 6% 6% 4 5% 6% 6% 9% 41%	2255
	21st to end	Day	14% 5% 2 6% 5% 4 5% 5% 6% 8% 40%	2376
September	1st to 10th	Day	14% 6% 3 5% 5% 2 5% 5% 6% 7% 43%	2160
	11th to 20th	Day	14% 4 2 6% 4 3 4 4 5% 7% 48%	2118
	21st to end	Day	17% 7% 3 5% 4 3 4 3 6% 7% 40%	1980
October	1st to 10th	Day	20% 8% 3 6% 5% 2 5% 5% 4 6% 35%	1810
	11th to 20th	Day	21% 10% 4 8% 7% 4 4 4 5% 5% 29%	1800
	21st to end	Day	25% 11% 4 6% 5% 3 5% 4 4 5% 27%	1785
November	1st to 10th	Day	16% 9% 3 7% 6% 4 5% 6% 4 7% 33%	1671
	11th to 20th	Day	18% 10% 5% 8% 6% 4 6% 4 5% 7% 28%	1593
	21st to end	Day	21% 13% 5% 8% 8% 5% 4 5% 5% 6% 22%	1520
December	1st to 10th	Day	24% 14% 5% 10% 7% 4 5% 5% 4 5% 18%	1520
	11th to 20th	Day	22% 14% 6% 9% 5% 2 5% 4 4 5% 23%	1906
	21st to end	Day	28% 18% 5% 8% 6% 2 4 3 3 2 20%	2772
January	1st to 10th	Night	17% 16% 6% 11% 7% 4 5% 4 4 5% 21%	4429
	11th to 20th	Night	13% 14% 7% 13% 9% 4 6% 5% 6% 5% 18%	4440
	21st to end	Night	17% 19% 7% 9% 7% 4 4 3 3 5% 22%	4856
February	1st to 10th	Night	15% 13% 5% 11% 8% 5% 6% 5% 5% 5% 22%	2470
	11th to 20th	Night	14% 12% 5% 10% 7% 4 5% 5% 5% 4 28%	2337
	21st to end	Night	16% 14% 5% 9% 6% 3 5% 4 4 5% 30%	1776
March	1st to 10th	Night	16% 12% 6% 9% 7% 4 5% 4 4 4 28%	2150
	11th to 20th	Night	17% 13% 4 7% 6% 3 4 4 4 5% 32%	2043
	21st to end	Night	17% 11% 4 7% 5% 3 4 3 3 5% 37%	2016
April	1st to 10th	Night	23% 9% 4 6% 6% 3 4 3 5% 5% 32%	1800
	11th to 20th	Night	22% 8% 4 7% 5% 3 3 5% 4 5% 35%	1775
	21st to end	Night	21% 9% 3 6% 6% 3 4 3 5% 7% 32%	1700
May	1st to 10th	Night	22% 8% 3 6% 4 3 5% 4 6% 6% 34%	1448
	11th to 20th	Night	18% 8% 3 6% 5% 3 3 4 4 6% 40%	1415
	21st to end	Night	14% 6% 3 5% 5% 3 5% 5% 5% 8% 41%	1550
June	1st to 10th	Night	20% 5% 3 6% 6% 3 5% 6% 6% 9% 30%	1440
	11th to 20th	Night	18% 6% 2 5% 5% 3 4 4 5% 8% 40%	1440
	21st to end	Night	16% 6% 4 6% 5% 4 6% 4 6% 9% 35%	1440
July	1st to 10th	Night	9% 5% 2 5% 6% 3 6% 5% 6% 8% 45%	1440
	11th to 20th	Night	12% 5% 3 4 6% 3 5% 5% 8% 9% 41%	1440
	21st to end	Night	11% 6% 2 4 5% 3 5% 6% 6% 9% 42%	1574
August	1st to 10th	Night	11% 5% 2 5% 5% 3 4 5% 8% 10% 42%	1432
	11th to 20th	Night	12% 5% 3 5% 6% 3 5% 4 4 8% 43%	1622
	21st to end	Night	13% 6% 3 5% 5% 3 5% 5% 5% 8% 43%	1980
September	1st to 10th	Night	11% 6% 3 7% 7% 3 5% 4 5% 7% 42%	1800
	11th to 20th	Night	15% 5% 2 4 5% 2 3 3 5% 8% 47%	1841
	21st to end	Night	17% 5% 2 4 5% 3 4 5% 5% 7% 42%	1980
October	1st to 10th	Night	21% 7% 3 6% 5% 3 4 5% 5% 5% 37%	2149
	11th to 20th	Night	19% 9% 3 7% 5% 3 5% 5% 4 6% 32%	2160
	21st to end	Night	24% 9% 4 7% 6% 4 4 4 4 5% 28%	2569
November	1st to 10th	Night	19% 10% 4 6% 5% 3 4 4 4 7% 33%	2407
	11th to 20th	Night	19% 9% 3 7% 6% 3 5% 5% 5% 6% 31%	2584
	21st to end	Night	21% 12% 5% 9% 6% 4 5% 4 5% 5% 25%	2660
December	1st to 10th	Night	20% 17% 6% 9% 7% 4 5% 5% 4 5% 18%	2660
	11th to 20th	Night	23% 15% 5% 8% 6% 3 4 4 3 4 24%	3288
	21st to end	Night	31% 16% 4 9% 5% 3 3 3 3 4 20%	4884

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	65% 6% 13% 11% 5% 1	2518
	11th to 20th	Day	63% 6% 11% 13% 7% 1	2520
	21st to end	Day	66% 7% 13% 9% 5% 1	2786
February	1st to 10th	Day	63% 4 8% 19% 4 2	1710
	11th to 20th	Day	59% 4 11% 17% 7% 1	1723
	21st to end	Day	63% 4 10% 14% 9% 1	1480
March	1st to 10th	Day	59% 4 10% 16% 9% 2	1800
	11th to 20th	Day	56% 4 12% 15% 12% 3	1913
	21st to end	Day	53% 4 8% 17% 15% 4	2340
April	1st to 10th	Day	55% 3 7% 19% 12% 3	2160
	11th to 20th	Day	51% 4 6% 20% 16% 2	2134
	21st to end	Day	55% 5% 8% 15% 13% 4	2266
May	1st to 10th	Day	51% 5% 7% 19% 15% 3	2520
	11th to 20th	Day	46% 4 9% 22% 16% 2	2482
	21st to end	Day	46% 6% 12% 20% 14% 2	2725
June	1st to 10th	Day	53% 5% 11% 18% 12% 1	2520
	11th to 20th	Day	44% 6% 11% 24% 13% 2	2520
	21st to end	Day	50% 7% 13% 19% 10% 1	2520
July	1st to 10th	Day	39% 7% 12% 28% 14% 1	2450
	11th to 20th	Day	40% 7% 15% 29% 9%	2445
	21st to end	Day	40% 8% 16% 25% 10%	2591
August	1st to 10th	Day	41% 7% 17% 26% 9%	2492
	11th to 20th	Day	39% 7% 17% 25% 11%	2255
	21st to end	Day	41% 6% 13% 26% 13% 1	2376
September	1st to 10th	Day	39% 6% 14% 28% 13%	2160
	11th to 20th	Day	36% 5% 12% 31% 15% 1	2118
	21st to end	Day	43% 6% 14% 27% 9%	1980
October	1st to 10th	Day	50% 5% 10% 24% 11%	1810
	11th to 20th	Day	57% 6% 13% 18% 7%	1800
	21st to end	Day	60% 5% 10% 17% 7% 1	1785
November	1st to 10th	Day	52% 5% 13% 21% 8% 1	1671
	11th to 20th	Day	56% 6% 11% 16% 10% 1	1593
	21st to end	Day	63% 6% 13% 12% 5%	1520
December	1st to 10th	Day	69% 6% 10% 11% 3	1520
	11th to 20th	Day	64% 6% 12% 14% 4	1906
	21st to end	Day	71% 4 8% 13% 4	2772
January	1st to 10th	Night	65% 6% 14% 11% 4	4429
	11th to 20th	Night	66% 7% 13% 10% 5%	4440
	21st to end	Night	67% 5% 12% 10% 6% 1	4856
February	1st to 10th	Night	64% 5% 9% 16% 6% 1	2470
	11th to 20th	Night	58% 5% 10% 19% 8%	2339
	21st to end	Night	57% 4 8% 18% 12% 1	1776
March	1st to 10th	Night	59% 4 9% 15% 10% 2	2160
	11th to 20th	Night	55% 4 9% 19% 10% 3	2044
	21st to end	Night	52% 3 6% 20% 14% 5%	2016
April	1st to 10th	Night	55% 4 7% 18% 13% 2	1800
	11th to 20th	Night	51% 5% 8% 20% 15% 2	1775
	21st to end	Night	52% 4 8% 18% 14% 4	1700
May	1st to 10th	Night	50% 4 7% 18% 16% 5%	1448
	11th to 20th	Night	46% 4 8% 19% 20% 3	1416
	21st to end	Night	41% 7% 12% 22% 16% 3	1551
June	1st to 10th	Night	49% 8% 13% 19% 11% 1	1440
	11th to 20th	Night	42% 6% 11% 22% 18% 2	1440
	21st to end	Night	47% 6% 14% 20% 13%	1440
July	1st to 10th	Night	34% 7% 13% 26% 18% 1	1400
	11th to 20th	Night	37% 7% 12% 32% 12%	1392
	21st to end	Night	37% 8% 17% 24% 14% 1	1486
August	1st to 10th	Night	36% 7% 20% 25% 12%	1422
	11th to 20th	Night	40% 5% 15% 27% 12% 1	1622
	21st to end	Night	39% 7% 14% 27% 12% 1	1980
September	1st to 10th	Night	42% 5% 10% 26% 15% 1	1800
	11th to 20th	Night	36% 5% 12% 34% 12% 1	1841
	21st to end	Night	42% 5% 12% 29% 12%	1980
October	1st to 10th	Night	48% 6% 10% 23% 12% 1	2149
	11th to 20th	Night	53% 6% 11% 21% 8%	2160
	21st to end	Night	58% 6% 12% 15% 8% 1	2569
November	1st to 10th	Night	52% 5% 13% 21% 9% 1	2407
	11th to 20th	Night	53% 6% 12% 19% 9% 1	2584
	21st to end	Night	62% 5% 12% 16% 5%	2660
December	1st to 10th	Night	69% 4 10% 14% 3	2660
	11th to 20th	Night	64% 4 8% 16% 6% 1	3288
	21st to end	Night	70% 4 8% 13% 4	4884

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	29% 19% 32% 17% 3	180
	11th to 20th	Day	29% 31% 27% 8% 4 1	180
	21st to end	Day	19% 23% 37% 17% 3	204
February	1st to 10th	Day	31% 31% 26% 10% 2	270
	11th to 20th	Day	29% 27% 36% 9%	327
	21st to end	Day	33% 20% 32% 14% 1	346
March	1st to 10th	Day	27% 22% 31% 16% 4	420
	11th to 20th	Day	29% 23% 31% 13% 2 1	420
	21st to end	Day	33% 21% 24% 15% 6% 1	462
April	1st to 10th	Day	30% 21% 31% 14% 5%	420
	11th to 20th	Day	32% 19% 26% 14% 7% 1	412
	21st to end	Day	32% 19% 29% 12% 8% 1	420
May	1st to 10th	Day	25% 19% 34% 14% 6% 2	423
	11th to 20th	Day	19% 20% 33% 18% 9% 1	447
	21st to end	Day	19% 20% 33% 20% 7% 1	485
June	1st to 10th	Day	29% 12% 30% 20% 7% 2	450
	11th to 20th	Day	18% 16% 30% 25% 8% 2	450
	21st to end	Day	24% 10% 32% 19% 11% 3	450
July	1st to 10th	Day	16% 9% 31% 28% 13% 3	445
	11th to 20th	Day	14% 11% 33% 29% 11% 3	444
	21st to end	Day	16% 10% 34% 28% 10% 2	488
August	1st to 10th	Day	15% 8% 28% 30% 14% 4	446
	11th to 20th	Day	15% 7% 29% 29% 13% 7%	439
	21st to end	Day	16% 12% 31% 21% 15% 6%	495
September	1st to 10th	Day	16% 12% 30% 22% 14% 6%	450
	11th to 20th	Day	14% 12% 27% 22% 17% 9%	450
	21st to end	Day	19% 14% 30% 19% 13% 5%	450
October	1st to 10th	Day	23% 17% 26% 18% 13% 3	450
	11th to 20th	Day	25% 22% 26% 18% 7% 1	450
	21st to end	Day	34% 16% 29% 16% 5%	397
November	1st to 10th	Day	27% 19% 27% 19% 6% 1	365
	11th to 20th	Day	24% 25% 31% 16% 4	253
	21st to end	Day	28% 23% 29% 15% 4 1	180
December	1st to 10th	Day	30% 18% 34% 15% 3	180
	11th to 20th	Day	33% 26% 30% 10% 1 1	180
	21st to end	Day	33% 28% 24% 14% 2	198
January	1st to 10th	Night	27% 19% 35% 15% 4	760
	11th to 20th	Night	29% 19% 28% 19% 4 1	760
	21st to end	Night	22% 18% 35% 21% 4	836
February	1st to 10th	Night	23% 20% 34% 20% 3	760
	11th to 20th	Night	23% 22% 34% 18% 3	742
	21st to end	Night	25% 17% 32% 23% 2	592
March	1st to 10th	Night	24% 20% 32% 20% 4	720
	11th to 20th	Night	26% 19% 32% 19% 4	720
	21st to end	Night	24% 17% 26% 27% 6%	792
April	1st to 10th	Night	30% 16% 29% 21% 4	720
	11th to 20th	Night	24% 19% 30% 21% 6%	711
	21st to end	Night	30% 14% 29% 20% 8%	620
May	1st to 10th	Night	27% 11% 29% 23% 8% 2	368
	11th to 20th	Night	23% 13% 30% 25% 9% 1	354
	21st to end	Night	16% 10% 38% 26% 9%	389
June	1st to 10th	Night	21% 9% 29% 32% 8% 1	360
	11th to 20th	Night	19% 11% 26% 31% 11% 1	360
	21st to end	Night	17% 8% 30% 28% 16% 1	360
July	1st to 10th	Night	11% 8% 30% 35% 15% 1	350
	11th to 20th	Night	13% 5% 30% 35% 17%	348
	21st to end	Night	14% 4 31% 37% 12% 1	394
August	1st to 10th	Night	14% 5% 25% 36% 20% 1	358
	11th to 20th	Night	15% 4 24% 35% 21% 2	558
	21st to end	Night	16% 6% 29% 29% 17% 2	792
September	1st to 10th	Night	14% 7% 31% 32% 14% 1	720
	11th to 20th	Night	19% 8% 23% 30% 18% 3	720
	21st to end	Night	19% 10% 28% 23% 18% 2	720
October	1st to 10th	Night	24% 10% 29% 24% 12% 1	720
	11th to 20th	Night	25% 13% 29% 23% 10%	720
	21st to end	Night	29% 16% 27% 24% 4	792
November	1st to 10th	Night	27% 16% 25% 23% 7% 1	740
	11th to 20th	Night	24% 19% 33% 15% 9%	760
	21st to end	Night	28% 18% 27% 20% 7%	760
December	1st to 10th	Night	31% 18% 28% 18% 4	760
	11th to 20th	Night	32% 20% 27% 15% 5% 1	760
	21st to end	Night	37% 19% 26% 14% 3	836

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