White Mills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the White Mills Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	13% 7% 3 6% 5% 3 5% 5% 5% 7% 40%	75022
	Night	14% 7% 3 6% 5% 3 4 4 5% 7% 41%	74694
Transparency	Day	43% 6% 11% 22% 15% 3	74735
	Night	42% 6% 10% 22% 17% 3	74501
Seeing	Day	16% 9% 22% 25% 19% 9%	14137
	Night	17% 7% 25% 31% 18% 2	23975

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	17% 9% 4 7% 6% 3 4 4 5% 5% 36%	1517
	11th to 20th	Day	20% 10% 5% 10% 7% 3 3 4 4 5% 28%	1502
	21st to end	Day	14% 10% 4 7% 5% 3 5% 3 5% 7% 38%	1850
February	1st to 10th	Day	18% 11% 6% 8% 4 2 4 4 4 5% 34%	1800
	11th to 20th	Day	20% 8% 4 7% 6% 3 4 4 5% 6% 32%	1748
	21st to end	Day	18% 10% 4 7% 6% 3 4 5% 4 5% 33%	1480
March	1st to 10th	Day	17% 9% 3 6% 4 2 4 4 5% 5% 40%	1800
	11th to 20th	Day	20% 11% 3 6% 6% 3 5% 5% 4 5% 34%	1923
	21st to end	Day	17% 9% 4 8% 6% 4 4 5% 4 6% 33%	2245
April	1st to 10th	Day	11% 6% 3 5% 5% 3 5% 6% 6% 8% 41%	2160
	11th to 20th	Day	15% 7% 3 7% 5% 3 4 5% 4 6% 41%	2134
	21st to end	Day	17% 6% 3 7% 6% 3 4 5% 5% 7% 38%	2272
May	1st to 10th	Day	17% 7% 2 5% 5% 3 5% 4 5% 8% 38%	2340
	11th to 20th	Day	15% 8% 3 7% 5% 4 5% 5% 5% 7% 36%	2301
	21st to end	Day	8% 5% 3 6% 7% 4 7% 6% 7% 10% 37%	2531
June	1st to 10th	Day	9% 5% 2 5% 6% 3 6% 5% 7% 8% 43%	2380
	11th to 20th	Day	6% 4 2 4 6% 3 6% 7% 7% 10% 45%	2520
	21st to end	Day	7% 4 3 5% 5% 4 6% 7% 7% 11% 43%	2520
July	1st to 10th	Day	10% 5% 2 6% 7% 4 6% 7% 7% 10% 34%	2520
	11th to 20th	Day	8% 5% 2 6% 6% 4 6% 6% 7% 9% 41%	2489
	21st to end	Day	9% 4 2 5% 6% 4 6% 6% 8% 10% 39%	2547
August	1st to 10th	Day	8% 5% 2 6% 6% 4 6% 6% 7% 11% 39%	2327
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 6% 6% 10% 39%	2138
	21st to end	Day	8% 5% 2 5% 6% 4 5% 5% 6% 9% 46%	2376
September	1st to 10th	Day	13% 5% 2 6% 7% 4 5% 5% 5% 7% 41%	2108
	11th to 20th	Day	12% 6% 3 4 5% 3 4 4 6% 8% 45%	1980
	21st to end	Day	12% 4 2 5% 4 3 4 3 5% 8% 50%	1980
October	1st to 10th	Day	9% 5% 2 5% 4 2 4 4 4 5% 55%	1980
	11th to 20th	Day	13% 5% 3 5% 4 3 4 4 5% 6% 50%	1919
	21st to end	Day	20% 6% 2 6% 5% 2 3 3 4 5% 43%	1782
November	1st to 10th	Day	12% 5% 3 4 4 2 3 3 3 6% 55%	1672
	11th to 20th	Day	15% 7% 3 6% 4 2 4 3 4 5% 46%	1710
	21st to end	Day	17% 8% 3 5% 4 2 3 4 4 7% 42%	1710
December	1st to 10th	Day	22% 10% 3 5% 5% 3 3 4 5% 5% 34%	1710
	11th to 20th	Day	16% 10% 5% 8% 6% 4 4 4 4 5% 35%	2153
	21st to end	Day	18% 15% 5% 6% 6% 3 5% 4 5% 6% 26%	2898
January	1st to 10th	Night	15% 11% 4 7% 6% 4 4 4 3 6% 34%	2648
	11th to 20th	Night	21% 11% 5% 8% 7% 3 4 4 5% 6% 28%	2493
	21st to end	Night	16% 11% 4 7% 6% 3 4 3 5% 6% 36%	2502
February	1st to 10th	Night	22% 11% 4 8% 5% 3 4 4 5% 6% 28%	2160
	11th to 20th	Night	20% 8% 4 7% 5% 2 5% 4 4 5% 35%	2098
	21st to end	Night	22% 11% 4 7% 5% 3 4 4 4 6% 31%	1776
March	1st to 10th	Night	17% 9% 3 7% 4 3 5% 4 4 5% 40%	2160
	11th to 20th	Night	18% 9% 3 7% 5% 3 5% 3 5% 6% 35%	2036
	21st to end	Night	16% 9% 3 7% 6% 3 4 4 5% 7% 35%	2124
April	1st to 10th	Night	11% 5% 3 5% 5% 3 5% 5% 6% 8% 45%	1800
	11th to 20th	Night	17% 7% 3 5% 5% 2 4 3 4 6% 44%	1775
	21st to end	Night	15% 8% 2 7% 6% 2 5% 5% 5% 7% 38%	1686
May	1st to 10th	Night	16% 6% 3 6% 6% 3 5% 4 6% 7% 38%	1620
	11th to 20th	Night	12% 5% 3 6% 5% 3 5% 5% 6% 8% 40%	1592
	21st to end	Night	5% 4 2 6% 6% 4 6% 7% 7% 11% 42%	1744
June	1st to 10th	Night	9% 5% 2 5% 4 2 4 5% 7% 9% 48%	1575
	11th to 20th	Night	5% 3 2 5% 4 3 4 5% 7% 9% 53%	1440
	21st to end	Night	3 3 2 4 5% 4 5% 7% 8% 13% 47%	1440
July	1st to 10th	Night	9% 4 3 5% 5% 4 5% 5% 6% 10% 43%	1440
	11th to 20th	Night	8% 4 2 4 3 3 4 5% 7% 11% 50%	1466
	21st to end	Night	7% 3 2 5% 5% 4 6% 5% 7% 10% 45%	1771
August	1st to 10th	Night	7% 4 2 4 4 3 5% 4 6% 9% 51%	1611
	11th to 20th	Night	6% 4 2 6% 5% 3 5% 7% 8% 10% 45%	1760
	21st to end	Night	6% 4 2 4 3 2 4 5% 7% 10% 53%	1980
September	1st to 10th	Night	12% 5% 2 4 4 3 4 4 6% 7% 49%	1832
	11th to 20th	Night	10% 5% 2 5% 4 3 3 4 5% 8% 49%	1980
	21st to end	Night	10% 4 2 4 4 3 4 5% 4 7% 54%	1980
October	1st to 10th	Night	10% 4 1 4 3 2 3 4 4 6% 59%	1980
	11th to 20th	Night	15% 5% 2 4 5% 3 4 4 4 6% 49%	2037
	21st to end	Night	17% 7% 2 5% 4 2 4 3 4 6% 46%	2574
November	1st to 10th	Night	11% 6% 2 4 5% 2 4 3 4 4 54%	2406
	11th to 20th	Night	13% 7% 2 5% 4 2 5% 4 5% 6% 46%	2470
	21st to end	Night	17% 7% 4 5% 4 2 4 4 4 6% 42%	2470
December	1st to 10th	Night	19% 11% 5% 6% 6% 3 4 4 4 5% 33%	2470
	11th to 20th	Night	19% 9% 4 8% 5% 2 4 4 4 6% 35%	3042
	21st to end	Night	17% 15% 4 9% 7% 4 5% 4 4 5% 25%	4756

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	51% 4 8% 23% 12% 2	1518
	11th to 20th	Day	59% 4 10% 17% 10% 1	1502
	21st to end	Day	47% 4 10% 26% 11% 2	1850
February	1st to 10th	Day	54% 4 8% 21% 12% 1	1800
	11th to 20th	Day	53% 4 8% 21% 11% 3	1758
	21st to end	Day	53% 5% 9% 16% 12% 5%	1480
March	1st to 10th	Day	45% 5% 8% 20% 19% 2	1800
	11th to 20th	Day	52% 5% 10% 18% 12% 2	1923
	21st to end	Day	52% 6% 10% 21% 10% 2	2245
April	1st to 10th	Day	39% 7% 11% 24% 17% 2	2160
	11th to 20th	Day	44% 5% 10% 20% 18% 3	2134
	21st to end	Day	46% 5% 10% 25% 12% 2	2272
May	1st to 10th	Day	45% 7% 13% 25% 11% 1	2340
	11th to 20th	Day	48% 7% 10% 21% 13% 1	2305
	21st to end	Day	40% 10% 15% 21% 12% 2	2531
June	1st to 10th	Day	39% 7% 10% 24% 18% 2	2380
	11th to 20th	Day	30% 10% 15% 25% 18% 2	2520
	21st to end	Day	34% 8% 16% 20% 18% 4	2520
July	1st to 10th	Day	42% 10% 13% 20% 12% 3	2451
	11th to 20th	Day	37% 11% 14% 20% 13% 4	2414
	21st to end	Day	37% 10% 15% 24% 12% 2	2405
August	1st to 10th	Day	37% 9% 16% 23% 14% 1	2314
	11th to 20th	Day	38% 9% 13% 23% 16% 2	2138
	21st to end	Day	34% 7% 13% 25% 16% 5%	2376
September	1st to 10th	Day	41% 8% 11% 20% 16% 4	2108
	11th to 20th	Day	37% 6% 10% 23% 22% 3	1980
	21st to end	Day	33% 5% 9% 27% 23% 3	1980
October	1st to 10th	Day	31% 4 9% 23% 29% 4	1980
	11th to 20th	Day	35% 5% 8% 20% 26% 5%	1919
	21st to end	Day	43% 4 7% 24% 18% 3	1782
November	1st to 10th	Day	33% 3 9% 28% 22% 5%	1672
	11th to 20th	Day	41% 3 7% 23% 20% 5%	1710
	21st to end	Day	44% 3 8% 24% 19% 2	1710
December	1st to 10th	Day	52% 4 10% 22% 10% 2	1710
	11th to 20th	Day	53% 4 8% 19% 15% 2	2150
	21st to end	Day	57% 5% 11% 18% 8%	2898
January	1st to 10th	Night	52% 4 8% 20% 14% 2	2648
	11th to 20th	Night	59% 4 9% 16% 11% 1	2493
	21st to end	Night	50% 3 11% 23% 12% 2	2502
February	1st to 10th	Night	57% 5% 9% 18% 10% 1	2160
	11th to 20th	Night	52% 4 7% 22% 13% 2	2099
	21st to end	Night	56% 4 10% 15% 13% 3	1776
March	1st to 10th	Night	46% 4 7% 18% 20% 4	2160
	11th to 20th	Night	51% 5% 9% 21% 13% 2	2036
	21st to end	Night	49% 6% 11% 22% 10% 2	2124
April	1st to 10th	Night	36% 6% 10% 25% 20% 3	1800
	11th to 20th	Night	42% 4 8% 18% 22% 5%	1775
	21st to end	Night	44% 7% 8% 24% 15% 2	1686
May	1st to 10th	Night	45% 6% 9% 25% 11% 3	1620
	11th to 20th	Night	41% 9% 12% 21% 15% 1	1593
	21st to end	Night	33% 11% 15% 23% 15% 2	1745
June	1st to 10th	Night	32% 7% 13% 26% 19% 3	1575
	11th to 20th	Night	26% 7% 15% 25% 22% 4	1440
	21st to end	Night	27% 10% 18% 21% 18% 6%	1440
July	1st to 10th	Night	35% 8% 13% 26% 14% 3	1400
	11th to 20th	Night	29% 8% 17% 24% 18% 5%	1418
	21st to end	Night	30% 9% 15% 29% 15% 1	1672
August	1st to 10th	Night	29% 8% 16% 28% 18% 2	1600
	11th to 20th	Night	29% 10% 16% 26% 17% 3	1760
	21st to end	Night	26% 7% 15% 27% 20% 4	1980
September	1st to 10th	Night	33% 7% 11% 23% 22% 5%	1832
	11th to 20th	Night	33% 5% 9% 26% 23% 4	1980
	21st to end	Night	30% 6% 7% 25% 26% 6%	1980
October	1st to 10th	Night	27% 4 9% 24% 30% 6%	1980
	11th to 20th	Night	37% 4 8% 23% 24% 5%	2037
	21st to end	Night	41% 4 7% 22% 22% 5%	2574
November	1st to 10th	Night	34% 4 7% 25% 25% 5%	2406
	11th to 20th	Night	39% 4 7% 22% 21% 7%	2470
	21st to end	Night	44% 4 7% 24% 18% 3	2470
December	1st to 10th	Night	53% 5% 10% 20% 11% 2	2470
	11th to 20th	Night	50% 4 10% 18% 16% 3	3044
	21st to end	Night	62% 4 9% 15% 8% 1	4756

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	22% 17% 33% 18% 9%	270
	11th to 20th	Day	25% 20% 33% 16% 6%	245
	21st to end	Day	21% 19% 41% 15% 5%	264
February	1st to 10th	Day	24% 21% 37% 16% 1 1	240
	11th to 20th	Day	28% 20% 35% 15% 3	240
	21st to end	Day	29% 17% 37% 14% 3	198
March	1st to 10th	Day	21% 20% 38% 18% 4	240
	11th to 20th	Day	23% 16% 36% 17% 8% 1	363
	21st to end	Day	22% 20% 31% 20% 5% 2	462
April	1st to 10th	Day	16% 17% 27% 27% 11% 2	420
	11th to 20th	Day	18% 15% 26% 23% 15% 3	412
	21st to end	Day	21% 11% 28% 27% 9% 4	420
May	1st to 10th	Day	19% 11% 29% 24% 12% 5%	423
	11th to 20th	Day	15% 7% 25% 30% 15% 7%	447
	21st to end	Day	13% 3 17% 29% 22% 16%	485
June	1st to 10th	Day	11% 4 18% 27% 25% 16%	490
	11th to 20th	Day	6% 3 16% 34% 29% 12%	630
	21st to end	Day	8% 3 20% 30% 24% 14%	630
July	1st to 10th	Day	10% 3 12% 23% 30% 22%	621
	11th to 20th	Day	9% 3 13% 29% 29% 18%	587
	21st to end	Day	10% 2 10% 26% 32% 19%	488
August	1st to 10th	Day	11% 2 11% 22% 34% 20%	446
	11th to 20th	Day	13% 2 10% 28% 34% 12%	439
	21st to end	Day	11% 2 7% 23% 35% 22%	495
September	1st to 10th	Day	13% 3 10% 21% 38% 15%	450
	11th to 20th	Day	12% 2 11% 24% 34% 18%	450
	21st to end	Day	16% 3 18% 25% 25% 13%	450
October	1st to 10th	Day	11% 4 18% 32% 26% 9%	450
	11th to 20th	Day	13% 11% 31% 27% 13% 5%	438
	21st to end	Day	23% 14% 24% 25% 13% 1	297
November	1st to 10th	Day	17% 7% 26% 33% 14% 3	270
	11th to 20th	Day	19% 14% 27% 31% 8% 1	270
	21st to end	Day	23% 10% 30% 23% 12%	270
December	1st to 10th	Day	20% 20% 29% 21% 9% 1	270
	11th to 20th	Day	19% 15% 33% 23% 10%	270
	21st to end	Day	26% 20% 30% 20% 5%	297
January	1st to 10th	Night	21% 12% 35% 27% 4	950
	11th to 20th	Night	27% 12% 34% 23% 4	856
	21st to end	Night	21% 13% 39% 26% 1	792
February	1st to 10th	Night	27% 10% 36% 25% 3	720
	11th to 20th	Night	24% 12% 41% 22% 2	703
	21st to end	Night	27% 11% 39% 21% 2	592
March	1st to 10th	Night	20% 11% 37% 28% 5%	720
	11th to 20th	Night	24% 10% 33% 27% 6% 1	720
	21st to end	Night	20% 12% 29% 31% 7%	738
April	1st to 10th	Night	13% 8% 36% 33% 10%	540
	11th to 20th	Night	20% 8% 30% 28% 13% 1	533
	21st to end	Night	17% 7% 32% 31% 11% 1	540
May	1st to 10th	Night	21% 3 29% 31% 14% 1	540
	11th to 20th	Night	14% 3 21% 39% 20% 3	531
	21st to end	Night	6% 2 15% 39% 31% 6%	584
June	1st to 10th	Night	11% 1 16% 37% 31% 4	540
	11th to 20th	Night	6% 2 12% 41% 34% 3	540
	21st to end	Night	4 1 15% 42% 33% 5%	540
July	1st to 10th	Night	10% 2 8% 26% 44% 10%	525
	11th to 20th	Night	9% 2 7% 31% 45% 7%	522
	21st to end	Night	9% 2 7% 36% 39% 7%	591
August	1st to 10th	Night	8% 1 6% 32% 47% 6%	537
	11th to 20th	Night	7% 8% 35% 43% 7%	534
	21st to end	Night	7% 1 6% 32% 46% 7%	594
September	1st to 10th	Night	14% 1 8% 31% 40% 6%	572
	11th to 20th	Night	11% 3 10% 32% 38% 6%	720
	21st to end	Night	11% 4 18% 30% 32% 5%	720
October	1st to 10th	Night	11% 3 16% 35% 29% 6%	720
	11th to 20th	Night	17% 5% 23% 35% 18% 2	720
	21st to end	Night	19% 6% 32% 29% 13% 1	792
November	1st to 10th	Night	13% 7% 28% 35% 16% 2	740
	11th to 20th	Night	15% 9% 32% 31% 13%	760
	21st to end	Night	21% 7% 28% 32% 11% 1	760
December	1st to 10th	Night	25% 9% 30% 30% 6% 1	760
	11th to 20th	Night	24% 8% 32% 28% 7% 1	760
	21st to end	Night	27% 12% 28% 29% 3	969

Page generated at 2024-01-09 16:24:28. Version ac07

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