Millbrook
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Millbrook clear sky chart from 2005-11-06 to 2025-04-11.

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	14% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	113625
	Night	14% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	114639
Transparency	Day	48% 7% 13% 20% 11% 1	117255
	Night	47% 7% 13% 20% 12% 1	119172
Seeing	Day	20% 14% 31% 21% 10% 4	14894
	Night	19% 12% 31% 27% 11% 1	24369

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	13% 12% 4 10% 6% 5% 5% 4 5% 7% 31%	3597
	11th to 20th	Day	8% 17% 7% 18% 10% 6% 5% 3 4 4 18%	3727
	21st to end	Day	15% 16% 7% 13% 7% 5% 5% 3 5% 5% 20%	4466
February	1st to 10th	Day	13% 17% 5% 13% 6% 5% 5% 5% 4 5% 24%	1800
	11th to 20th	Day	10% 15% 6% 14% 7% 4 5% 3 4 5% 25%	1753
	21st to end	Day	17% 15% 5% 10% 6% 5% 4 3 4 5% 26%	1545
March	1st to 10th	Day	17% 12% 5% 10% 6% 4 3 3 4 6% 31%	4882
	11th to 20th	Day	10% 12% 5% 10% 6% 5% 5% 4 6% 8% 30%	4980
	21st to end	Day	17% 11% 3 7% 4 4 4 3 4 5% 37%	5499
April	1st to 10th	Day	17% 13% 4 8% 5% 5% 4 3 5% 5% 31%	2400
	11th to 20th	Day	20% 9% 2 7% 5% 4 3 3 4 6% 36%	2276
	21st to end	Day	19% 9% 3 8% 4 5% 4 3 5% 5% 33%	2470
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 6% 33%	4204
	11th to 20th	Day	16% 10% 4 8% 5% 4 4 4 6% 8% 32%	4398
	21st to end	Day	12% 6% 2 6% 4 4 4 4 6% 8% 45%	4837
June	1st to 10th	Day	17% 7% 2 7% 5% 4 4 5% 6% 8% 35%	2660
	11th to 20th	Day	12% 6% 3 7% 4 5% 4 4 6% 10% 39%	2660
	21st to end	Day	15% 7% 2 7% 4 5% 5% 4 6% 8% 37%	2660
July	1st to 10th	Day	10% 4 3 6% 5% 5% 6% 5% 8% 10% 38%	4440
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 6% 8% 10% 40%	4440
	21st to end	Day	6% 5% 2 7% 5% 5% 6% 5% 7% 10% 41%	4664
August	1st to 10th	Day	15% 9% 3 7% 4 4 5% 4 5% 7% 37%	4104
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 4 7% 10% 36%	4044
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 7% 10% 40%	4290
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 3 5% 8% 42%	2280
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 6% 8% 47%	2236
	21st to end	Day	14% 6% 2 8% 4 5% 5% 4 5% 7% 39%	2009
October	1st to 10th	Day	19% 7% 3 7% 5% 5% 4 5% 5% 6% 35%	1900
	11th to 20th	Day	19% 8% 4 9% 5% 5% 5% 3 5% 7% 31%	1900
	21st to end	Day	23% 12% 4 8% 5% 4 4 3 6% 6% 26%	2090
November	1st to 10th	Day	14% 10% 3 7% 5% 4 5% 4 6% 6% 35%	1668
	11th to 20th	Day	14% 11% 4 9% 6% 5% 6% 4 7% 7% 28%	1600
	21st to end	Day	17% 16% 3 10% 6% 4 4 3 6% 7% 24%	1600
December	1st to 10th	Day	16% 17% 6% 13% 8% 5% 5% 4 5% 5% 18%	2600
	11th to 20th	Day	16% 17% 5% 10% 6% 4 4 2 5% 5% 26%	2986
	21st to end	Day	21% 17% 5% 13% 6% 4 4 3 4 5% 18%	3960
January	1st to 10th	Night	14% 11% 5% 12% 6% 5% 5% 3 6% 6% 26%	6349
	11th to 20th	Night	10% 14% 6% 13% 9% 6% 6% 4 5% 7% 20%	6155
	21st to end	Night	13% 12% 6% 12% 5% 5% 5% 3 5% 6% 27%	6490
February	1st to 10th	Night	11% 15% 4 11% 7% 6% 6% 4 5% 6% 25%	2600
	11th to 20th	Night	13% 12% 5% 11% 7% 6% 4 4 6% 6% 28%	2533
	21st to end	Night	12% 13% 4 9% 6% 5% 5% 4 4 7% 31%	2084
March	1st to 10th	Night	14% 10% 4 10% 6% 6% 4 3 5% 6% 32%	4956
	11th to 20th	Night	10% 8% 3 7% 5% 6% 6% 5% 7% 8% 36%	4979
	21st to end	Night	18% 8% 3 7% 4 4 4 3 4 6% 39%	5432
April	1st to 10th	Night	19% 9% 3 8% 5% 4 4 3 6% 6% 33%	2000
	11th to 20th	Night	17% 8% 2 7% 4 5% 5% 3 5% 6% 37%	1877
	21st to end	Night	16% 9% 3 7% 4 4 5% 4 7% 6% 35%	1710
May	1st to 10th	Night	17% 9% 2 6% 4 5% 4 4 5% 6% 37%	2721
	11th to 20th	Night	14% 8% 3 7% 4 5% 5% 4 6% 7% 37%	2495
	21st to end	Night	10% 5% 2 5% 4 3 4 3 6% 8% 49%	2738
June	1st to 10th	Night	13% 6% 3 6% 4 5% 6% 5% 7% 8% 38%	1520
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 5% 6% 9% 44%	1520
	21st to end	Night	12% 7% 2 7% 5% 4 4 3 6% 8% 41%	1520
July	1st to 10th	Night	6% 5% 2 7% 4 4 5% 5% 6% 9% 47%	2520
	11th to 20th	Night	10% 5% 2 5% 4 3 4 3 6% 9% 49%	2520
	21st to end	Night	8% 4 1 5% 4 3 4 4 6% 9% 52%	2886
August	1st to 10th	Night	9% 5% 2 6% 4 4 5% 3 5% 8% 48%	2825
	11th to 20th	Night	12% 5% 2 6% 3 3 3 3 5% 8% 49%	2785
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 49%	3334
September	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 47%	1900
	11th to 20th	Night	11% 4 1 4 4 3 4 4 5% 7% 53%	1942
	21st to end	Night	14% 7% 3 7% 4 5% 4 3 4 6% 44%	2171
October	1st to 10th	Night	16% 6% 2 7% 5% 4 4 4 6% 6% 40%	2280
	11th to 20th	Night	18% 8% 3 7% 6% 4 5% 4 4 7% 33%	2280
	21st to end	Night	20% 9% 4 8% 4 4 5% 4 6% 6% 31%	2508
November	1st to 10th	Night	16% 9% 3 6% 5% 4 5% 3 6% 7% 37%	2625
	11th to 20th	Night	16% 11% 3 8% 5% 5% 5% 5% 5% 7% 30%	2800
	21st to end	Night	18% 12% 3 8% 6% 5% 6% 5% 6% 6% 26%	2800
December	1st to 10th	Night	14% 15% 5% 11% 6% 6% 6% 4 7% 7% 20%	4580
	11th to 20th	Night	14% 16% 5% 11% 6% 5% 5% 4 5% 6% 23%	5208
	21st to end	Night	22% 15% 5% 13% 6% 5% 5% 3 4 5% 16%	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	50% 7% 13% 18% 10% 1	3598
	11th to 20th	Day	69% 5% 11% 9% 5% 1	3758
	21st to end	Day	65% 5% 10% 10% 8% 1	4466
February	1st to 10th	Day	63% 6% 10% 16% 5% 1	1800
	11th to 20th	Day	60% 5% 9% 15% 9% 1	1762
	21st to end	Day	60% 6% 10% 16% 8% 1	1545
March	1st to 10th	Day	53% 5% 12% 13% 15% 1	4885
	11th to 20th	Day	51% 5% 18% 17% 8% 2	4980
	21st to end	Day	48% 5% 7% 20% 18% 2	5499
April	1st to 10th	Day	54% 5% 10% 17% 12% 2	2400
	11th to 20th	Day	49% 5% 9% 20% 16% 1	2276
	21st to end	Day	51% 6% 10% 18% 13% 2	2470
May	1st to 10th	Day	47% 8% 11% 19% 12% 2	4216
	11th to 20th	Day	47% 10% 12% 20% 10% 1	4402
	21st to end	Day	35% 8% 14% 24% 15% 3	4837
June	1st to 10th	Day	44% 8% 13% 21% 14% 1	2660
	11th to 20th	Day	39% 9% 13% 24% 14% 1	2660
	21st to end	Day	42% 8% 14% 22% 13% 1	2660
July	1st to 10th	Day	35% 9% 16% 24% 15% 1	4370
	11th to 20th	Day	33% 8% 16% 33% 9% 1	4365
	21st to end	Day	34% 10% 15% 26% 14% 1	4515
August	1st to 10th	Day	44% 8% 13% 21% 12% 1	4091
	11th to 20th	Day	39% 7% 15% 28% 9%	4083
	21st to end	Day	35% 8% 15% 31% 10% 1	4290
September	1st to 10th	Day	39% 8% 13% 29% 10% 1	2280
	11th to 20th	Day	33% 9% 13% 30% 15% 1	2236
	21st to end	Day	41% 10% 14% 25% 10% 1	2009
October	1st to 10th	Day	38% 7% 14% 28% 13% 1	3160
	11th to 20th	Day	40% 7% 17% 22% 14% 1	3140
	21st to end	Day	49% 7% 17% 20% 8%	3428
November	1st to 10th	Day	47% 8% 14% 20% 10% 1	1668
	11th to 20th	Day	53% 8% 14% 15% 10% 1	1600
	21st to end	Day	59% 7% 13% 15% 6%	1600
December	1st to 10th	Day	67% 7% 14% 10% 3	2600
	11th to 20th	Day	59% 8% 10% 14% 9%	2986
	21st to end	Day	68% 6% 9% 10% 7%	3960
January	1st to 10th	Night	56% 7% 13% 14% 9%	6349
	11th to 20th	Night	59% 8% 12% 13% 6% 1	6199
	21st to end	Night	55% 8% 12% 13% 12% 1	6490
February	1st to 10th	Night	59% 7% 10% 17% 6% 1	2600
	11th to 20th	Night	55% 7% 10% 17% 10%	2535
	21st to end	Night	53% 6% 12% 20% 8% 1	2084
March	1st to 10th	Night	51% 5% 11% 17% 12% 4	4966
	11th to 20th	Night	42% 7% 19% 21% 10% 1	4980
	21st to end	Night	45% 5% 10% 20% 19% 2	5432
April	1st to 10th	Night	51% 6% 9% 20% 13% 2	2000
	11th to 20th	Night	46% 7% 10% 18% 18% 2	1877
	21st to end	Night	46% 8% 11% 17% 15% 2	1710
May	1st to 10th	Night	45% 7% 13% 16% 16% 2	2728
	11th to 20th	Night	43% 10% 12% 22% 10% 2	2496
	21st to end	Night	32% 8% 13% 25% 20% 2	2739
June	1st to 10th	Night	41% 9% 10% 22% 15% 3	1520
	11th to 20th	Night	34% 8% 13% 22% 20% 3	1520
	21st to end	Night	40% 7% 13% 24% 15% 2	1520
July	1st to 10th	Night	32% 7% 15% 25% 20% 2	2480
	11th to 20th	Night	32% 5% 13% 33% 15% 1	2472
	21st to end	Night	29% 7% 16% 33% 14% 1	2794
August	1st to 10th	Night	34% 6% 14% 28% 16% 2	2814
	11th to 20th	Night	33% 5% 17% 31% 12% 1	2812
	21st to end	Night	31% 7% 13% 33% 15% 1	3334
September	1st to 10th	Night	35% 8% 14% 28% 14% 1	1900
	11th to 20th	Night	29% 8% 13% 32% 18% 1	1942
	21st to end	Night	41% 7% 11% 28% 13% 1	2171
October	1st to 10th	Night	32% 6% 11% 28% 22% 1	3800
	11th to 20th	Night	41% 5% 15% 28% 10%	3779
	21st to end	Night	40% 8% 16% 26% 9% 1	4120
November	1st to 10th	Night	45% 7% 14% 21% 13% 1	2625
	11th to 20th	Night	50% 8% 13% 17% 10% 2	2800
	21st to end	Night	56% 9% 15% 15% 5%	2800
December	1st to 10th	Night	58% 7% 17% 13% 4	4580
	11th to 20th	Night	59% 9% 10% 13% 9% 1	5208
	21st to end	Night	67% 8% 10% 9% 6%	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	24% 25% 29% 16% 6% 2	200
	11th to 20th	Day	26% 26% 29% 13% 4 2	235
	21st to end	Day	24% 25% 32% 16% 3	330
February	1st to 10th	Day	29% 26% 31% 12% 2	300
	11th to 20th	Day	26% 28% 36% 9% 1	300
	21st to end	Day	34% 20% 37% 9%	309
March	1st to 10th	Day	24% 19% 37% 15% 4	500
	11th to 20th	Day	25% 21% 33% 18% 3 1	500
	21st to end	Day	33% 14% 33% 14% 5% 1	550
April	1st to 10th	Day	28% 19% 32% 15% 6% 1	500
	11th to 20th	Day	31% 16% 27% 15% 8% 3	452
	21st to end	Day	27% 14% 36% 15% 7% 1	460
May	1st to 10th	Day	22% 11% 32% 22% 8% 5%	463
	11th to 20th	Day	18% 11% 31% 25% 11% 3	487
	21st to end	Day	13% 10% 32% 26% 14% 5%	529
June	1st to 10th	Day	17% 11% 33% 26% 10% 2	490
	11th to 20th	Day	14% 11% 28% 27% 16% 3	490
	21st to end	Day	19% 10% 29% 27% 11% 4	490
July	1st to 10th	Day	10% 7% 27% 31% 19% 6%	485
	11th to 20th	Day	14% 6% 27% 28% 18% 7%	484
	21st to end	Day	9% 5% 31% 35% 14% 5%	532
August	1st to 10th	Day	11% 6% 29% 29% 17% 8%	486
	11th to 20th	Day	10% 7% 25% 28% 21% 9%	479
	21st to end	Day	12% 6% 29% 27% 19% 7%	539
September	1st to 10th	Day	13% 9% 26% 25% 19% 8%	490
	11th to 20th	Day	11% 5% 27% 23% 20% 15%	490
	21st to end	Day	16% 11% 29% 24% 15% 6%	490
October	1st to 10th	Day	21% 11% 31% 17% 14% 5%	490
	11th to 20th	Day	20% 18% 32% 20% 7% 2	490
	21st to end	Day	28% 18% 28% 21% 5%	539
November	1st to 10th	Day	24% 17% 28% 22% 8%	295
	11th to 20th	Day	24% 22% 33% 15% 5% 2	200
	21st to end	Day	25% 20% 38% 14% 4	200
December	1st to 10th	Day	28% 19% 35% 15% 3 1	200
	11th to 20th	Day	27% 27% 32% 12% 3 1	200
	21st to end	Day	29% 21% 33% 14% 3	220
January	1st to 10th	Night	22% 20% 35% 16% 6%	800
	11th to 20th	Night	27% 16% 33% 19% 5% 1	800
	21st to end	Night	20% 15% 35% 25% 5%	880
February	1st to 10th	Night	21% 17% 39% 21% 2	800
	11th to 20th	Night	20% 19% 38% 19% 3	783
	21st to end	Night	19% 20% 38% 20% 3	660
March	1st to 10th	Night	21% 16% 37% 23% 4	800
	11th to 20th	Night	21% 17% 36% 22% 4 1	800
	21st to end	Night	21% 14% 35% 23% 8%	880
April	1st to 10th	Night	24% 13% 34% 24% 4	800
	11th to 20th	Night	20% 14% 34% 23% 9% 1	749
	21st to end	Night	21% 9% 32% 27% 11%	570
May	1st to 10th	Night	20% 6% 30% 29% 12% 2	510
	11th to 20th	Night	15% 9% 30% 35% 11%	374
	21st to end	Night	13% 6% 25% 39% 17% 1	411
June	1st to 10th	Night	14% 6% 31% 37% 13%	380
	11th to 20th	Night	9% 7% 28% 39% 17% 1	380
	21st to end	Night	14% 5% 26% 39% 15% 1	380
July	1st to 10th	Night	6% 4 22% 47% 19% 1	370
	11th to 20th	Night	9% 3 24% 38% 25% 1	368
	21st to end	Night	9% 2 25% 42% 19% 2	501
August	1st to 10th	Night	11% 2 17% 41% 26% 2	567
	11th to 20th	Night	9% 2 19% 38% 28% 4	564
	21st to end	Night	11% 4 25% 34% 24% 2	746
September	1st to 10th	Night	12% 5% 25% 36% 20% 3	760
	11th to 20th	Night	12% 4 21% 29% 27% 7%	760
	21st to end	Night	18% 8% 21% 30% 19% 4	760
October	1st to 10th	Night	18% 7% 29% 27% 16% 3	760
	11th to 20th	Night	21% 12% 32% 25% 9% 1	760
	21st to end	Night	24% 13% 32% 25% 6%	836
November	1st to 10th	Night	22% 13% 28% 26% 10% 1	780
	11th to 20th	Night	20% 16% 35% 20% 7% 1	800
	21st to end	Night	24% 15% 34% 22% 6%	800
December	1st to 10th	Night	25% 18% 35% 19% 4	800
	11th to 20th	Night	22% 17% 35% 20% 5% 1	800
	21st to end	Night	29% 16% 34% 19% 3	880

Page generated at 2025-04-08 09:17:29. Version ac07

Copyright © 2025 Attilla Danko.