Marshall Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marshall Hill Observatory clear sky chart from 2005-11-06 to 2025-02-06.

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	15% 12% 4 9% 5% 5% 4 4 5% 6% 30%	108650
	Night	16% 13% 5% 11% 6% 5% 4 3 5% 6% 27%	109594
Transparency	Day	52% 6% 13% 19% 9% 1	112284
	Night	56% 7% 11% 16% 9% 1	114123
Seeing	Day	23% 16% 30% 20% 9% 3	14739
	Night	24% 16% 28% 21% 9% 1	24229

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% 21% 10% 19% 8% 4 4 2 3 3 12%	3597
	11th to 20th	Day	12% 25% 10% 20% 10% 5% 4 3 4 3 6%	3815
	21st to end	Day	17% 18% 9% 17% 8% 7% 5% 3 3 3 9%	4466
February	1st to 10th	Day	16% 19% 6% 14% 8% 6% 5% 3 4 4 14%	1755
	11th to 20th	Day	12% 18% 9% 16% 9% 6% 4 3 3 4 16%	1663
	21st to end	Day	18% 17% 4 13% 9% 6% 5% 3 4 4 16%	1486
March	1st to 10th	Day	18% 14% 5% 11% 5% 5% 3 3 4 4 28%	3414
	11th to 20th	Day	11% 20% 5% 9% 5% 6% 4 4 4 5% 28%	3480
	21st to end	Day	15% 12% 4 10% 5% 4 4 4 5% 6% 30%	3832
April	1st to 10th	Day	18% 14% 5% 7% 5% 4 4 3 4 6% 31%	2280
	11th to 20th	Day	19% 10% 3 7% 4 4 3 3 4 6% 36%	2254
	21st to end	Day	20% 10% 3 8% 5% 4 4 3 5% 5% 33%	2463
May	1st to 10th	Day	19% 9% 3 7% 4 5% 4 4 5% 7% 34%	4162
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 5% 5% 7% 32%	4398
	21st to end	Day	10% 7% 2 6% 4 4 4 4 6% 8% 45%	4837
June	1st to 10th	Day	16% 7% 3 6% 5% 4 5% 4 5% 8% 38%	2660
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 6% 9% 40%	2660
	21st to end	Day	14% 6% 2 6% 4 4 4 4 7% 9% 40%	2660
July	1st to 10th	Day	10% 5% 2 5% 4 5% 5% 4 7% 9% 44%	4440
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 42%	4440
	21st to end	Day	8% 5% 2 6% 5% 6% 6% 5% 8% 9% 40%	4596
August	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 4 6% 7% 37%	4104
	11th to 20th	Day	12% 6% 3 8% 5% 6% 5% 4 6% 10% 34%	4044
	21st to end	Day	12% 6% 2 6% 4 4 5% 3 6% 9% 43%	4258
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 3 6% 8% 42%	2280
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 4 6% 7% 44%	2237
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 3 7% 9% 34%	2009
October	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 3 5% 6% 33%	1900
	11th to 20th	Day	20% 10% 4 10% 4 4 4 4 5% 6% 28%	1900
	21st to end	Day	26% 13% 4 7% 6% 4 4 3 4 5% 22%	2090
November	1st to 10th	Day	14% 14% 4 9% 5% 4 5% 4 5% 7% 29%	1724
	11th to 20th	Day	14% 16% 5% 10% 5% 4 5% 4 5% 7% 25%	1600
	21st to end	Day	19% 20% 5% 10% 6% 5% 5% 4 6% 5% 17%	1600
December	1st to 10th	Day	18% 22% 6% 13% 7% 6% 5% 4 5% 4 10%	2600
	11th to 20th	Day	21% 19% 6% 13% 5% 4 4 2 4 3 18%	2986
	21st to end	Day	22% 20% 7% 14% 8% 4 4 3 3 4 10%	3960
January	1st to 10th	Night	15% 19% 8% 23% 7% 5% 4 2 3 4 11%	6349
	11th to 20th	Night	10% 20% 10% 20% 9% 7% 5% 3 4 3 9%	6065
	21st to end	Night	18% 18% 7% 16% 8% 5% 4 3 4 4 12%	6490
February	1st to 10th	Night	16% 19% 6% 14% 8% 5% 5% 4 3 5% 16%	2536
	11th to 20th	Night	13% 15% 7% 15% 8% 6% 5% 3 4 5% 18%	2403
	21st to end	Night	16% 16% 5% 14% 6% 4 4 4 5% 6% 19%	1967
March	1st to 10th	Night	20% 13% 4 10% 6% 4 3 3 4 5% 29%	3422
	11th to 20th	Night	17% 16% 5% 10% 5% 4 4 4 4 6% 26%	3479
	21st to end	Night	15% 9% 4 10% 4 4 4 3 4 6% 36%	3800
April	1st to 10th	Night	20% 13% 4 7% 5% 4 4 3 5% 5% 29%	1900
	11th to 20th	Night	17% 10% 3 6% 6% 5% 5% 3 6% 6% 35%	1875
	21st to end	Night	18% 9% 3 7% 4 4 5% 5% 5% 8% 32%	1733
May	1st to 10th	Night	22% 10% 3 7% 4 4 4 3 4 7% 32%	2777
	11th to 20th	Night	14% 8% 3 8% 6% 5% 5% 4 6% 8% 34%	2495
	21st to end	Night	11% 5% 2 6% 4 4 4 4 6% 9% 44%	2738
June	1st to 10th	Night	13% 7% 2 7% 5% 6% 6% 5% 7% 8% 34%	1520
	11th to 20th	Night	12% 4 2 6% 6% 5% 5% 4 6% 8% 42%	1520
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	1520
July	1st to 10th	Night	9% 7% 3 7% 5% 4 4 4 6% 9% 42%	2520
	11th to 20th	Night	11% 6% 3 6% 4 4 5% 3 7% 9% 44%	2520
	21st to end	Night	10% 6% 2 4 4 3 4 4 6% 8% 49%	2953
August	1st to 10th	Night	10% 7% 2 7% 4 4 5% 3 6% 9% 42%	2825
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 6% 8% 40%	2785
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 6% 9% 44%	3366
September	1st to 10th	Night	13% 7% 2 7% 4 3 4 3 6% 8% 45%	1900
	11th to 20th	Night	14% 5% 2 5% 4 4 4 3 5% 7% 48%	1941
	21st to end	Night	18% 7% 3 5% 4 4 4 4 7% 8% 35%	2171
October	1st to 10th	Night	17% 7% 3 8% 6% 4 4 4 6% 6% 35%	2280
	11th to 20th	Night	22% 10% 4 7% 5% 5% 4 3 5% 6% 29%	2280
	21st to end	Night	28% 12% 2 6% 5% 4 5% 3 5% 5% 25%	2508
November	1st to 10th	Night	15% 12% 4 8% 4 4 4 4 5% 6% 34%	2572
	11th to 20th	Night	15% 14% 4 9% 6% 5% 5% 4 6% 7% 25%	2800
	21st to end	Night	18% 18% 5% 10% 6% 5% 5% 4 5% 6% 18%	2800
December	1st to 10th	Night	17% 20% 9% 15% 8% 6% 5% 3 4 4 9%	4580
	11th to 20th	Night	19% 22% 6% 12% 5% 4 4 4 4 4 16%	5208
	21st to end	Night	23% 19% 7% 12% 8% 5% 4 3 4 4 11%	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	76% 7% 7% 7% 3	3598
	11th to 20th	Day	84% 5% 6% 3 2 1	3848
	21st to end	Day	78% 5% 10% 4 2	4466
February	1st to 10th	Day	73% 6% 10% 10% 1	1755
	11th to 20th	Day	73% 6% 7% 11% 3	1672
	21st to end	Day	70% 6% 10% 9% 4	1486
March	1st to 10th	Day	57% 5% 12% 20% 6%	3417
	11th to 20th	Day	54% 8% 16% 14% 8% 1	3480
	21st to end	Day	48% 8% 12% 23% 8% 1	3832
April	1st to 10th	Day	54% 5% 11% 18% 10% 1	2280
	11th to 20th	Day	49% 5% 10% 20% 14% 2	2254
	21st to end	Day	51% 5% 10% 18% 13% 2	2463
May	1st to 10th	Day	48% 7% 13% 19% 12% 2	4174
	11th to 20th	Day	48% 9% 13% 20% 8% 1	4402
	21st to end	Day	35% 7% 14% 25% 17% 2	4837
June	1st to 10th	Day	43% 6% 13% 22% 16% 1	2660
	11th to 20th	Day	38% 7% 14% 25% 15% 1	2660
	21st to end	Day	37% 7% 16% 24% 15% 1	2660
July	1st to 10th	Day	34% 5% 13% 30% 16% 1	4370
	11th to 20th	Day	34% 7% 15% 31% 12% 1	4365
	21st to end	Day	36% 8% 16% 25% 14%	4449
August	1st to 10th	Day	43% 6% 15% 22% 13% 1	4091
	11th to 20th	Day	42% 7% 17% 25% 8%	4083
	21st to end	Day	36% 5% 14% 34% 10% 1	4258
September	1st to 10th	Day	39% 6% 15% 27% 11% 1	2280
	11th to 20th	Day	37% 7% 14% 28% 14% 1	2237
	21st to end	Day	45% 8% 15% 22% 10% 1	2009
October	1st to 10th	Day	43% 6% 16% 23% 12%	3160
	11th to 20th	Day	47% 6% 16% 19% 12%	3140
	21st to end	Day	50% 6% 16% 21% 6%	3428
November	1st to 10th	Day	54% 7% 12% 18% 9% 1	1724
	11th to 20th	Day	57% 8% 15% 13% 6% 1	1600
	21st to end	Day	67% 8% 11% 11% 3	1600
December	1st to 10th	Day	75% 8% 11% 5% 1	2600
	11th to 20th	Day	72% 5% 8% 7% 6% 1	2986
	21st to end	Day	77% 6% 9% 6% 2	3960
January	1st to 10th	Night	78% 8% 7% 6% 1	6349
	11th to 20th	Night	78% 7% 7% 5% 2	6107
	21st to end	Night	75% 5% 8% 5% 7%	6490
February	1st to 10th	Night	70% 8% 9% 11% 2	2536
	11th to 20th	Night	68% 6% 10% 12% 4	2405
	21st to end	Night	63% 9% 11% 13% 4	1967
March	1st to 10th	Night	58% 6% 10% 21% 5%	3432
	11th to 20th	Night	56% 7% 14% 14% 7% 1	3480
	21st to end	Night	44% 8% 11% 24% 13% 1	3800
April	1st to 10th	Night	56% 6% 8% 19% 10% 1	1900
	11th to 20th	Night	48% 6% 11% 20% 12% 2	1875
	21st to end	Night	48% 7% 11% 18% 13% 2	1733
May	1st to 10th	Night	50% 6% 11% 18% 12% 2	2784
	11th to 20th	Night	45% 9% 16% 20% 8% 2	2496
	21st to end	Night	34% 8% 13% 23% 20% 2	2739
June	1st to 10th	Night	42% 9% 11% 21% 16% 2	1520
	11th to 20th	Night	38% 7% 12% 23% 18% 2	1520
	21st to end	Night	41% 8% 13% 21% 17% 1	1520
July	1st to 10th	Night	38% 6% 12% 23% 19% 2	2480
	11th to 20th	Night	35% 6% 14% 30% 14% 1	2472
	21st to end	Night	33% 7% 16% 26% 17% 1	2859
August	1st to 10th	Night	37% 6% 14% 27% 14% 1	2814
	11th to 20th	Night	40% 6% 16% 27% 10%	2812
	21st to end	Night	34% 7% 13% 32% 13% 1	3366
September	1st to 10th	Night	37% 8% 14% 26% 14% 1	1900
	11th to 20th	Night	35% 6% 12% 29% 17% 1	1941
	21st to end	Night	45% 7% 13% 23% 12%	2171
October	1st to 10th	Night	43% 6% 11% 22% 17% 1	3800
	11th to 20th	Night	49% 5% 14% 20% 11%	3779
	21st to end	Night	48% 5% 14% 25% 8% 1	4120
November	1st to 10th	Night	49% 8% 11% 20% 11% 2	2572
	11th to 20th	Night	57% 8% 12% 14% 8% 1	2800
	21st to end	Night	64% 8% 13% 12% 2	2800
December	1st to 10th	Night	78% 7% 10% 5% 1	4580
	11th to 20th	Night	71% 7% 7% 8% 6%	5208
	21st to end	Night	75% 8% 8% 7% 2	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	33% 28% 26% 11% 2 1	200
	11th to 20th	Day	34% 31% 25% 8% 1 1	253
	21st to end	Day	31% 25% 32% 9% 4	330
February	1st to 10th	Day	33% 29% 28% 8% 2	285
	11th to 20th	Day	29% 34% 29% 7% 1	270
	21st to end	Day	39% 24% 27% 9% 1	298
March	1st to 10th	Day	27% 24% 30% 15% 3	460
	11th to 20th	Day	29% 22% 32% 13% 3 1	460
	21st to end	Day	29% 21% 32% 13% 5%	506
April	1st to 10th	Day	29% 21% 30% 15% 4	460
	11th to 20th	Day	31% 21% 25% 15% 7% 1	452
	21st to end	Day	27% 16% 37% 13% 5% 1	460
May	1st to 10th	Day	24% 14% 33% 19% 8% 3	463
	11th to 20th	Day	18% 12% 34% 25% 8% 3	487
	21st to end	Day	16% 11% 31% 27% 13% 3	529
June	1st to 10th	Day	19% 13% 33% 26% 9% 1	490
	11th to 20th	Day	14% 14% 30% 28% 12% 2	490
	21st to end	Day	18% 11% 34% 25% 9% 2	490
July	1st to 10th	Day	10% 8% 28% 33% 18% 4	485
	11th to 20th	Day	15% 7% 27% 33% 15% 4	484
	21st to end	Day	11% 8% 35% 32% 12% 3	532
August	1st to 10th	Day	11% 9% 28% 31% 16% 6%	486
	11th to 20th	Day	13% 7% 27% 32% 15% 6%	479
	21st to end	Day	13% 7% 32% 28% 14% 6%	539
September	1st to 10th	Day	14% 13% 24% 27% 16% 6%	490
	11th to 20th	Day	13% 8% 24% 26% 18% 12%	490
	21st to end	Day	17% 13% 31% 21% 13% 5%	490
October	1st to 10th	Day	25% 14% 25% 18% 12% 5%	490
	11th to 20th	Day	27% 18% 30% 17% 7% 1	490
	21st to end	Day	35% 18% 29% 13% 6%	539
November	1st to 10th	Day	30% 18% 31% 13% 7% 1	342
	11th to 20th	Day	32% 23% 28% 13% 4 2	200
	21st to end	Day	38% 22% 28% 12% 2	200
December	1st to 10th	Day	41% 19% 29% 10% 3 1	200
	11th to 20th	Day	40% 22% 30% 8% 1	200
	21st to end	Day	38% 26% 25% 9% 2 1	220
January	1st to 10th	Night	31% 28% 27% 11% 3	800
	11th to 20th	Night	33% 24% 28% 13% 2	800
	21st to end	Night	26% 27% 30% 14% 3	880
February	1st to 10th	Night	32% 27% 27% 13% 1	781
	11th to 20th	Night	28% 26% 31% 12% 2	743
	21st to end	Night	29% 23% 32% 14% 2	628
March	1st to 10th	Night	26% 24% 31% 17% 3	760
	11th to 20th	Night	29% 19% 30% 18% 4 1	760
	21st to end	Night	23% 16% 33% 19% 8% 1	836
April	1st to 10th	Night	28% 17% 29% 22% 3	760
	11th to 20th	Night	24% 17% 30% 22% 7% 1	751
	21st to end	Night	25% 11% 31% 25% 8%	593
May	1st to 10th	Night	20% 11% 25% 26% 15% 2	540
	11th to 20th	Night	14% 8% 33% 34% 10% 1	374
	21st to end	Night	11% 9% 29% 32% 16% 2	411
June	1st to 10th	Night	16% 7% 32% 31% 13% 1	380
	11th to 20th	Night	13% 9% 28% 35% 14% 1	380
	21st to end	Night	14% 6% 33% 35% 12%	380
July	1st to 10th	Night	9% 6% 26% 38% 21% 2	370
	11th to 20th	Night	12% 4 29% 34% 18% 2	368
	21st to end	Night	12% 4 27% 39% 17% 2	542
August	1st to 10th	Night	11% 4 24% 35% 22% 3	567
	11th to 20th	Night	12% 4 24% 35% 21% 4	564
	21st to end	Night	12% 5% 27% 32% 22% 2	765
September	1st to 10th	Night	15% 9% 23% 31% 20% 3	760
	11th to 20th	Night	16% 6% 20% 28% 25% 6%	760
	21st to end	Night	22% 12% 21% 26% 16% 4	760
October	1st to 10th	Night	22% 13% 27% 21% 16% 2	760
	11th to 20th	Night	28% 17% 28% 18% 8% 1	760
	21st to end	Night	37% 14% 26% 19% 4	836
November	1st to 10th	Night	29% 15% 29% 20% 6% 1	780
	11th to 20th	Night	26% 22% 30% 15% 6% 2	800
	21st to end	Night	33% 23% 25% 14% 5%	800
December	1st to 10th	Night	37% 25% 24% 11% 4	800
	11th to 20th	Night	33% 25% 28% 12% 2	800
	21st to end	Night	35% 23% 27% 13% 2	880

Page generated at 2025-02-03 03:05:55. Version ac07

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