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 2024-11-08.

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% 8% 3 7% 5% 4 5% 4 6% 7% 39%	101156
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 41%	97919
Transparency	Day	42% 6% 10% 24% 16% 2	104864
	Night	41% 5% 10% 23% 18% 3	102350
Seeing	Day	15% 9% 23% 25% 19% 9%	15299
	Night	17% 7% 25% 31% 18% 2	25019

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	13% 13% 6% 9% 6% 5% 4 4 5% 5% 30%	2527
	11th to 20th	Day	17% 10% 4 14% 6% 5% 5% 2 4 4 29%	2605
	21st to end	Day	25% 12% 8% 7% 3 3 3 2 4 6% 26%	3234
February	1st to 10th	Day	17% 12% 5% 9% 4 3 3 4 4 5% 35%	1900
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 3 5% 6% 34%	1848
	21st to end	Day	18% 11% 3 8% 6% 4 4 4 4 5% 33%	1570
March	1st to 10th	Day	24% 14% 3 6% 4 3 3 3 4 5% 31%	3120
	11th to 20th	Day	14% 8% 2 5% 4 3 4 3 4 7% 46%	3381
	21st to end	Day	12% 8% 3 7% 5% 5% 4 3 5% 7% 40%	3936
April	1st to 10th	Day	12% 7% 3 7% 5% 4 5% 4 6% 8% 40%	2280
	11th to 20th	Day	15% 8% 3 7% 5% 4 3 4 5% 5% 42%	2254
	21st to end	Day	16% 7% 2 7% 5% 4 4 4 5% 7% 38%	2399
May	1st to 10th	Day	19% 8% 2 7% 4 5% 5% 4 6% 8% 31%	4082
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 5% 6% 38%	4081
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 8% 39%	4476
June	1st to 10th	Day	10% 6% 2 6% 6% 5% 5% 4 7% 8% 42%	2513
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 5% 7% 10% 45%	2660
	21st to end	Day	7% 5% 2 5% 5% 5% 6% 6% 7% 11% 42%	2660
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 9% 38%	4440
	11th to 20th	Day	7% 5% 2 6% 5% 4 5% 5% 8% 10% 43%	4381
	21st to end	Day	13% 12% 4 10% 7% 6% 6% 5% 6% 8% 25%	4466
August	1st to 10th	Day	5% 4 2 5% 4 4 5% 4 7% 11% 49%	4104
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 39%	3742
	21st to end	Day	5% 4 1 4 4 4 4 4 6% 9% 55%	4180
September	1st to 10th	Day	13% 5% 2 7% 5% 5% 5% 4 5% 7% 42%	2225
	11th to 20th	Day	11% 7% 2 5% 4 4 4 4 6% 8% 45%	2090
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 8% 47%	2090
October	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 4 6% 57%	2090
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 5% 6% 51%	2025
	21st to end	Day	19% 6% 2 6% 4 3 4 3 4 5% 44%	1881
November	1st to 10th	Day	12% 6% 2 5% 4 3 4 2 4 6% 54%	1735
	11th to 20th	Day	15% 8% 2 6% 4 4 3 3 4 5% 46%	1710
	21st to end	Day	17% 8% 3 5% 4 3 4 3 5% 6% 42%	1710
December	1st to 10th	Day	22% 11% 3 7% 4 3 3 3 5% 5% 34%	1710
	11th to 20th	Day	16% 11% 4 9% 5% 5% 4 3 4 5% 34%	2153
	21st to end	Day	18% 15% 5% 8% 5% 4 5% 3 5% 6% 26%	2898
January	1st to 10th	Night	16% 15% 5% 8% 5% 4 4 4 4 6% 30%	4428
	11th to 20th	Night	17% 13% 5% 10% 6% 5% 4 2 4 4 29%	4344
	21st to end	Night	24% 15% 3 7% 4 3 3 2 4 5% 30%	4404
February	1st to 10th	Night	21% 11% 3 8% 5% 4 4 4 5% 6% 30%	2280
	11th to 20th	Night	19% 9% 4 8% 4 3 4 3 4 4 36%	2218
	21st to end	Night	21% 11% 4 8% 4 4 4 3 4 6% 31%	1884
March	1st to 10th	Night	22% 14% 3 7% 4 4 4 3 4 5% 31%	3733
	11th to 20th	Night	13% 8% 2 6% 4 3 3 3 5% 6% 48%	3578
	21st to end	Night	12% 8% 2 8% 5% 4 4 4 5% 6% 42%	3722
April	1st to 10th	Night	11% 6% 2 6% 4 4 5% 4 6% 7% 44%	1900
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 5% 6% 43%	1875
	21st to end	Night	15% 8% 2 8% 6% 3 4 3 6% 6% 39%	1779
May	1st to 10th	Night	18% 9% 3 8% 5% 5% 4 3 6% 6% 32%	2813
	11th to 20th	Night	15% 6% 3 6% 4 4 5% 4 6% 7% 41%	2812
	21st to end	Night	6% 6% 2 7% 5% 5% 5% 5% 7% 9% 42%	3079
June	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 7% 9% 47%	1662
	11th to 20th	Night	5% 4 2 5% 4 4 4 4 7% 10% 52%	1520
	21st to end	Night	3 4 1 5% 4 5% 4 5% 9% 13% 47%	1520
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 4 4 7% 11% 46%	2530
	11th to 20th	Night	7% 4 2 4 3 3 4 4 6% 10% 54%	2582
	21st to end	Night	9% 8% 3 12% 7% 7% 6% 5% 7% 8% 29%	3087
August	1st to 10th	Night	5% 4 2 5% 3 3 4 3 6% 8% 57%	2825
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 4 8% 10% 48%	3084
	21st to end	Night	4 3 1 3 2 3 3 4 5% 9% 63%	3476
September	1st to 10th	Night	11% 5% 2 4 4 4 4 4 6% 7% 50%	1934
	11th to 20th	Night	10% 6% 2 5% 4 4 3 3 5% 8% 50%	2090
	21st to end	Night	11% 5% 2 5% 3 3 4 4 4 7% 52%	2090
October	1st to 10th	Night	9% 4 1 4 2 3 3 3 3 6% 60%	2090
	11th to 20th	Night	14% 5% 1 5% 4 3 3 3 4 6% 51%	2151
	21st to end	Night	16% 7% 2 6% 3 3 4 3 4 6% 47%	2717
November	1st to 10th	Night	11% 7% 2 5% 4 4 4 3 4 4 52%	2504
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 46%	2470
	21st to end	Night	17% 8% 4 6% 4 3 4 3 5% 5% 42%	2470
December	1st to 10th	Night	18% 12% 5% 7% 5% 4 4 3 4 5% 33%	2470
	11th to 20th	Night	19% 9% 4 9% 5% 3 3 4 4 6% 35%	3042
	21st to end	Night	17% 16% 4 10% 6% 6% 5% 3 4 5% 24%	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	56% 5% 8% 20% 10% 1	2528
	11th to 20th	Day	59% 4 8% 19% 9% 1	2605
	21st to end	Day	60% 4 7% 22% 6% 1	3234
February	1st to 10th	Day	53% 4 8% 21% 13% 1	1900
	11th to 20th	Day	51% 4 8% 21% 12% 3	1858
	21st to end	Day	53% 5% 8% 16% 13% 5%	1570
March	1st to 10th	Day	56% 4 6% 17% 13% 4	3120
	11th to 20th	Day	38% 5% 10% 26% 18% 3	3381
	21st to end	Day	43% 6% 11% 25% 14% 1	3936
April	1st to 10th	Day	41% 7% 11% 23% 16% 2	2280
	11th to 20th	Day	43% 5% 9% 22% 18% 3	2254
	21st to end	Day	45% 5% 10% 25% 12% 2	2399
May	1st to 10th	Day	49% 9% 13% 20% 9%	4120
	11th to 20th	Day	46% 7% 9% 23% 14%	4085
	21st to end	Day	40% 8% 12% 17% 20% 2	4489
June	1st to 10th	Day	39% 7% 10% 23% 18% 2	2513
	11th to 20th	Day	30% 10% 15% 25% 19% 2	2660
	21st to end	Day	34% 8% 16% 20% 18% 4	2660
July	1st to 10th	Day	41% 7% 14% 24% 14% 2	4371
	11th to 20th	Day	34% 7% 12% 33% 11% 2	4306
	21st to end	Day	56% 6% 12% 17% 7% 1	4324
August	1st to 10th	Day	28% 6% 15% 34% 17% 1	4091
	11th to 20th	Day	37% 6% 13% 25% 17% 1	3778
	21st to end	Day	27% 5% 11% 38% 16% 3	4179
September	1st to 10th	Day	40% 8% 11% 20% 18% 4	2225
	11th to 20th	Day	37% 6% 10% 23% 22% 3	2090
	21st to end	Day	36% 5% 9% 26% 22% 3	2090
October	1st to 10th	Day	20% 3 6% 23% 44% 4	3480
	11th to 20th	Day	24% 3 10% 26% 33% 4	3344
	21st to end	Day	31% 3 6% 29% 22% 8%	3081
November	1st to 10th	Day	33% 3 9% 27% 22% 5%	1735
	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	54% 5% 9% 20% 11% 1	4428
	11th to 20th	Night	59% 3 7% 18% 11% 2	4344
	21st to end	Night	57% 2 8% 19% 11% 1	4404
February	1st to 10th	Night	55% 5% 9% 18% 12% 1	2280
	11th to 20th	Night	51% 4 7% 22% 14% 2	2219
	21st to end	Night	55% 4 9% 15% 13% 3	1884
March	1st to 10th	Night	57% 4 7% 17% 14% 3	3733
	11th to 20th	Night	37% 5% 9% 27% 13% 7%	3578
	21st to end	Night	43% 6% 11% 23% 17% 1	3722
April	1st to 10th	Night	38% 6% 10% 24% 19% 3	1900
	11th to 20th	Night	42% 4 8% 18% 22% 5%	1875
	21st to end	Night	44% 6% 8% 24% 15% 2	1779
May	1st to 10th	Night	50% 8% 10% 23% 7% 2	2840
	11th to 20th	Night	42% 8% 12% 25% 13% 1	2813
	21st to end	Night	34% 9% 15% 22% 18% 2	3087
June	1st to 10th	Night	32% 7% 13% 26% 18% 4	1662
	11th to 20th	Night	27% 7% 15% 25% 23% 4	1520
	21st to end	Night	27% 10% 18% 21% 19% 6%	1520
July	1st to 10th	Night	33% 6% 13% 30% 16% 2	2490
	11th to 20th	Night	26% 6% 14% 37% 15% 3	2534
	21st to end	Night	51% 7% 12% 19% 9% 1	2988
August	1st to 10th	Night	25% 5% 13% 38% 18% 1	2814
	11th to 20th	Night	29% 6% 16% 29% 18% 1	3114
	21st to end	Night	19% 5% 13% 37% 23% 4	3476
September	1st to 10th	Night	32% 7% 10% 22% 24% 5%	1934
	11th to 20th	Night	33% 5% 9% 26% 23% 4	2090
	21st to end	Night	32% 6% 8% 24% 25% 6%	2090
October	1st to 10th	Night	20% 3 6% 18% 48% 5%	3480
	11th to 20th	Night	23% 3 6% 25% 37% 7%	3571
	21st to end	Night	30% 2 7% 21% 31% 9%	4467
November	1st to 10th	Night	35% 4 7% 25% 24% 5%	2504
	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	23% 17% 32% 18% 9%	270
	11th to 20th	Day	25% 20% 35% 14% 5%	270
	21st to end	Day	23% 18% 39% 15% 5%	297
February	1st to 10th	Day	23% 19% 35% 19% 3 1	270
	11th to 20th	Day	26% 20% 35% 16% 3 1	270
	21st to end	Day	29% 18% 37% 13% 3	225
March	1st to 10th	Day	23% 20% 34% 19% 4	270
	11th to 20th	Day	21% 16% 37% 17% 7% 1	400
	21st to end	Day	21% 21% 32% 20% 5% 1	506
April	1st to 10th	Day	17% 18% 27% 26% 10% 2	460
	11th to 20th	Day	18% 15% 28% 23% 14% 3	452
	21st to end	Day	19% 11% 30% 27% 9% 4	460
May	1st to 10th	Day	19% 12% 28% 24% 13% 5%	463
	11th to 20th	Day	16% 7% 24% 29% 16% 7%	487
	21st to end	Day	13% 4 19% 29% 21% 15%	529
June	1st to 10th	Day	12% 4 19% 27% 23% 15%	533
	11th to 20th	Day	6% 2 16% 35% 28% 12%	680
	21st to end	Day	8% 4 21% 30% 24% 14%	680
July	1st to 10th	Day	10% 4 14% 23% 29% 21%	671
	11th to 20th	Day	8% 3 14% 30% 28% 17%	635
	21st to end	Day	11% 3 11% 26% 31% 18%	532
August	1st to 10th	Day	10% 2 12% 23% 33% 20%	486
	11th to 20th	Day	13% 2 12% 29% 33% 11%	479
	21st to end	Day	10% 2 6% 23% 35% 23%	539
September	1st to 10th	Day	12% 3 11% 22% 37% 14%	490
	11th to 20th	Day	11% 2 11% 24% 34% 17%	490
	21st to end	Day	17% 4 18% 25% 24% 12%	490
October	1st to 10th	Day	10% 3 18% 33% 28% 9%	490
	11th to 20th	Day	12% 11% 30% 25% 14% 7%	477
	21st to end	Day	21% 14% 26% 26% 13% 1	330
November	1st to 10th	Day	17% 8% 26% 33% 13% 3	291
	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% 13% 34% 22% 4	893
	21st to end	Night	22% 13% 38% 26% 1	836
February	1st to 10th	Night	26% 10% 35% 26% 3	760
	11th to 20th	Night	23% 11% 41% 23% 2	743
	21st to end	Night	26% 11% 40% 21% 2	628
March	1st to 10th	Night	21% 11% 36% 27% 5%	760
	11th to 20th	Night	23% 10% 34% 27% 6% 1	760
	21st to end	Night	19% 12% 30% 31% 7%	779
April	1st to 10th	Night	13% 8% 37% 32% 9%	570
	11th to 20th	Night	20% 8% 31% 28% 12% 1	563
	21st to end	Night	18% 7% 33% 31% 10% 1	570
May	1st to 10th	Night	21% 4 29% 31% 14% 1	570
	11th to 20th	Night	14% 3 20% 40% 20% 3	561
	21st to end	Night	6% 2 16% 40% 30% 6%	617
June	1st to 10th	Night	11% 2 16% 38% 30% 4	570
	11th to 20th	Night	6% 3 12% 42% 34% 3	570
	21st to end	Night	4 1 15% 42% 32% 5%	570
July	1st to 10th	Night	10% 2 8% 27% 43% 10%	555
	11th to 20th	Night	9% 2 6% 33% 43% 6%	552
	21st to end	Night	9% 3 7% 37% 38% 7%	624
August	1st to 10th	Night	8% 1 6% 33% 46% 7%	567
	11th to 20th	Night	7% 1 9% 37% 41% 6%	564
	21st to end	Night	7% 1 5% 32% 47% 8%	627
September	1st to 10th	Night	13% 2 9% 31% 40% 5%	604
	11th to 20th	Night	11% 3 9% 33% 38% 6%	760
	21st to end	Night	12% 4 18% 30% 31% 5%	760
October	1st to 10th	Night	11% 3 16% 35% 29% 6%	760
	11th to 20th	Night	16% 6% 23% 35% 19% 2	760
	21st to end	Night	18% 6% 31% 30% 14% 1	836
November	1st to 10th	Night	13% 7% 28% 34% 16% 2	771
	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 2024-11-05 08:15:51. Version ac07

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