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 2026-06-01.

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 10% 6% 5% 4 3 5% 6% 30%	132443
	Night	16% 13% 5% 11% 6% 5% 4 3 5% 6% 26%	130675
Transparency	Day	53% 6% 12% 19% 10% 1	136032
	Night	57% 6% 11% 16% 9% 1	135172
Seeing	Day	24% 16% 30% 19% 8% 2	16456
	Night	25% 16% 28% 21% 9% 1	25883

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	15% 20% 10% 18% 8% 4 4 3 3 4 12%	4661
	11th to 20th	Day	10% 26% 10% 21% 9% 5% 4 3 3 3 7%	4965
	21st to end	Day	14% 17% 8% 17% 12% 10% 6% 3 3 3 8%	5780
February	1st to 10th	Day	16% 18% 7% 14% 8% 6% 6% 3 4 4 14%	1890
	11th to 20th	Day	13% 18% 8% 17% 9% 6% 4 4 3 4 15%	1843
	21st to end	Day	18% 19% 4 13% 9% 6% 5% 3 4 4 16%	1638
March	1st to 10th	Day	16% 14% 6% 12% 6% 5% 3 2 4 4 28%	5009
	11th to 20th	Day	11% 16% 5% 10% 5% 5% 4 4 4 6% 30%	5090
	21st to end	Day	17% 14% 4 10% 5% 4 4 3 4 5% 28%	5603
April	1st to 10th	Day	18% 14% 4 8% 5% 4 4 3 4 5% 30%	2520
	11th to 20th	Day	19% 12% 3 7% 4 4 4 3 4 6% 34%	2494
	21st to end	Day	20% 10% 3 8% 5% 4 4 3 5% 5% 33%	2723
May	1st to 10th	Day	22% 10% 3 8% 5% 5% 4 3 5% 6% 31%	6101
	11th to 20th	Day	16% 10% 3 8% 5% 4 4 4 5% 6% 34%	6458
	21st to end	Day	18% 8% 2 6% 4 4 4 4 6% 7% 37%	7103
June	1st to 10th	Day	15% 7% 3 6% 5% 4 5% 4 5% 8% 38%	2800
	11th to 20th	Day	11% 5% 2 7% 6% 5% 5% 5% 6% 9% 39%	2800
	21st to end	Day	14% 6% 2 6% 4 4 4 4 7% 9% 40%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	6360
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 6% 8% 46%	6610
August	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 4 6% 7% 38%	4234
	11th to 20th	Day	13% 6% 3 8% 5% 6% 5% 4 6% 10% 34%	4174
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 43%	4393
September	1st to 10th	Day	14% 6% 2 7% 4 4 4 3 6% 8% 43%	2400
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 7% 45%	2354
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 3 7% 9% 34%	2114
October	1st to 10th	Day	20% 7% 3 6% 4 5% 5% 3 5% 6% 35%	2000
	11th to 20th	Day	20% 10% 4 10% 4 4 4 4 5% 6% 28%	2000
	21st to end	Day	26% 13% 4 7% 6% 4 4 3 5% 5% 23%	2200
November	1st to 10th	Day	14% 15% 4 9% 5% 4 5% 4 5% 7% 29%	1812
	11th to 20th	Day	14% 17% 6% 10% 5% 4 5% 4 5% 6% 25%	1680
	21st to end	Day	19% 20% 5% 10% 6% 5% 5% 4 6% 5% 17%	1680
December	1st to 10th	Day	18% 22% 6% 13% 7% 6% 5% 4 5% 4 10%	2680
	11th to 20th	Day	21% 19% 6% 13% 5% 4 4 2 4 4 17%	3066
	21st to end	Day	22% 20% 8% 14% 8% 4 4 3 3 4 10%	4048
January	1st to 10th	Night	15% 19% 9% 22% 7% 5% 4 2 3 4 10%	8234
	11th to 20th	Night	10% 21% 10% 20% 9% 6% 4 3 4 3 9%	7901
	21st to end	Night	15% 16% 6% 16% 11% 9% 6% 3 4 4 10%	8401
February	1st to 10th	Night	15% 18% 6% 14% 8% 6% 5% 4 3 5% 16%	2730
	11th to 20th	Night	14% 15% 7% 16% 8% 6% 5% 3 4 5% 18%	2663
	21st to end	Night	15% 17% 5% 14% 6% 5% 4 4 5% 6% 18%	2167
March	1st to 10th	Night	17% 12% 6% 11% 8% 4 4 3 4 5% 25%	5047
	11th to 20th	Night	13% 13% 4 9% 5% 4 4 4 5% 7% 32%	5089
	21st to end	Night	19% 10% 4 11% 5% 4 4 3 4 6% 30%	5557
April	1st to 10th	Night	20% 13% 5% 8% 5% 4 4 3 5% 5% 28%	2100
	11th to 20th	Night	17% 10% 3 6% 5% 5% 5% 3 6% 6% 34%	2075
	21st to end	Night	18% 9% 3 7% 4 4 5% 5% 5% 8% 32%	1915
May	1st to 10th	Night	26% 10% 3 7% 4 4 4 3 4 6% 28%	4057
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 8% 36%	3655
	21st to end	Night	18% 7% 3 7% 4 4 4 4 5% 7% 36%	4014
June	1st to 10th	Night	13% 7% 2 7% 5% 6% 6% 5% 7% 8% 35%	1600
	11th to 20th	Night	11% 5% 2 6% 6% 5% 5% 5% 6% 8% 42%	1600
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	1600
July	1st to 10th	Night	10% 7% 2 7% 5% 4 4 4 6% 9% 42%	3600
	11th to 20th	Night	11% 6% 3 6% 4 4 5% 3 7% 8% 44%	3600
	21st to end	Night	10% 6% 2 5% 4 3 4 4 5% 8% 51%	4238
August	1st to 10th	Night	10% 7% 2 7% 4 4 5% 3 7% 9% 42%	2915
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 5% 8% 40%	2875
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 6% 9% 44%	3472
September	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 6% 8% 45%	2000
	11th to 20th	Night	13% 4 2 5% 3 4 4 3 5% 7% 49%	2043
	21st to end	Night	18% 7% 3 5% 4 4 4 4 6% 8% 35%	2285
October	1st to 10th	Night	17% 6% 3 8% 6% 4 4 4 6% 6% 37%	2400
	11th to 20th	Night	21% 10% 4 7% 5% 5% 4 4 5% 7% 30%	2400
	21st to end	Night	27% 12% 3 6% 5% 4 5% 3 5% 5% 25%	2640
November	1st to 10th	Night	15% 12% 4 8% 4 4 4 4 6% 6% 34%	2704
	11th to 20th	Night	15% 14% 4 9% 6% 5% 5% 4 5% 7% 24%	2940
	21st to end	Night	18% 18% 5% 10% 6% 5% 5% 3 5% 6% 18%	2940
December	1st to 10th	Night	16% 20% 9% 15% 8% 6% 5% 3 4 4 9%	4720
	11th to 20th	Night	18% 22% 6% 12% 5% 4 4 4 4 4 16%	5348
	21st to end	Night	23% 19% 7% 13% 7% 5% 4 3 4 4 11%	7150

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	77% 6% 8% 6% 3	4662
	11th to 20th	Day	83% 4 5% 4 3 1	4998
	21st to end	Day	80% 5% 9% 4 2	5780
February	1st to 10th	Day	74% 6% 9% 10% 1	1890
	11th to 20th	Day	74% 6% 7% 10% 3	1852
	21st to end	Day	71% 6% 10% 9% 4	1638
March	1st to 10th	Day	58% 4 13% 16% 9%	5012
	11th to 20th	Day	52% 6% 19% 14% 8% 1	5090
	21st to end	Day	54% 6% 10% 21% 8% 1	5603
April	1st to 10th	Day	55% 5% 11% 18% 10% 1	2520
	11th to 20th	Day	51% 5% 10% 20% 13% 2	2494
	21st to end	Day	51% 5% 10% 19% 13% 2	2723
May	1st to 10th	Day	53% 6% 11% 16% 11% 4	6110
	11th to 20th	Day	49% 8% 11% 19% 13% 2	6462
	21st to end	Day	45% 5% 12% 22% 14% 2	7103
June	1st to 10th	Day	42% 6% 13% 22% 16% 1	2800
	11th to 20th	Day	39% 7% 14% 25% 14% 1	2800
	21st to end	Day	38% 7% 16% 24% 15% 1	2800
July	1st to 10th	Day	34% 5% 13% 29% 17% 1	6290
	11th to 20th	Day	36% 6% 14% 29% 13% 1	6285
	21st to end	Day	33% 6% 14% 31% 16%	6421
August	1st to 10th	Day	42% 6% 15% 22% 13% 1	4221
	11th to 20th	Day	42% 7% 16% 26% 9%	4213
	21st to end	Day	36% 5% 14% 34% 10% 1	4393
September	1st to 10th	Day	39% 6% 15% 28% 12% 1	2400
	11th to 20th	Day	36% 7% 14% 29% 14% 1	2354
	21st to end	Day	45% 7% 15% 22% 10% 1	2114
October	1st to 10th	Day	42% 6% 16% 24% 13%	3260
	11th to 20th	Day	47% 6% 16% 19% 12%	3240
	21st to end	Day	51% 6% 16% 21% 7%	3538
November	1st to 10th	Day	54% 7% 11% 18% 9% 1	1812
	11th to 20th	Day	58% 7% 15% 13% 6% 1	1680
	21st to end	Day	68% 8% 11% 10% 3	1680
December	1st to 10th	Day	75% 8% 11% 5% 1	2680
	11th to 20th	Day	72% 6% 8% 7% 6% 1	3066
	21st to end	Day	78% 6% 9% 5% 2	4048
January	1st to 10th	Night	79% 7% 7% 6% 1	8234
	11th to 20th	Night	78% 6% 7% 5% 4	7943
	21st to end	Night	75% 5% 7% 6% 6%	8401
February	1st to 10th	Night	71% 8% 9% 11% 2	2730
	11th to 20th	Night	69% 6% 10% 12% 3	2665
	21st to end	Night	64% 8% 11% 12% 5%	2167
March	1st to 10th	Night	61% 5% 11% 16% 7%	5057
	11th to 20th	Night	50% 6% 17% 18% 9% 1	5090
	21st to end	Night	52% 6% 9% 20% 12% 1	5557
April	1st to 10th	Night	57% 5% 8% 18% 10% 1	2100
	11th to 20th	Night	49% 6% 11% 20% 12% 2	2075
	21st to end	Night	48% 7% 11% 19% 13% 2	1915
May	1st to 10th	Night	57% 5% 9% 14% 12% 4	4064
	11th to 20th	Night	43% 8% 15% 20% 12% 2	3656
	21st to end	Night	45% 7% 12% 20% 15% 1	4015
June	1st to 10th	Night	42% 9% 11% 21% 15% 2	1600
	11th to 20th	Night	39% 7% 12% 23% 17% 2	1600
	21st to end	Night	40% 7% 13% 21% 17% 1	1600
July	1st to 10th	Night	38% 5% 13% 24% 19% 1	3560
	11th to 20th	Night	36% 6% 12% 28% 16% 2	3552
	21st to end	Night	32% 6% 13% 26% 22% 1	4112
August	1st to 10th	Night	37% 6% 15% 27% 14% 1	2904
	11th to 20th	Night	40% 6% 16% 27% 11%	2902
	21st to end	Night	34% 7% 13% 32% 13% 1	3472
September	1st to 10th	Night	36% 8% 14% 28% 14% 1	2000
	11th to 20th	Night	34% 6% 12% 30% 17% 1	2043
	21st to end	Night	45% 7% 12% 23% 12% 1	2285
October	1st to 10th	Night	42% 6% 11% 23% 17% 1	3920
	11th to 20th	Night	49% 5% 14% 20% 11%	3899
	21st to end	Night	48% 5% 13% 24% 8% 1	4252
November	1st to 10th	Night	50% 7% 11% 20% 10% 2	2704
	11th to 20th	Night	58% 8% 12% 14% 8% 1	2940
	21st to end	Night	65% 8% 13% 12% 2	2940
December	1st to 10th	Night	77% 7% 9% 5% 1	4720
	11th to 20th	Night	71% 7% 7% 9% 6%	5348
	21st to end	Night	75% 8% 8% 7% 2	7150

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	35% 27% 26% 10% 1 1	220
	11th to 20th	Day	36% 31% 24% 7% 1 1	278
	21st to end	Day	33% 25% 30% 10% 3	363
February	1st to 10th	Day	35% 27% 28% 8% 2	330
	11th to 20th	Day	34% 31% 27% 7% 1	330
	21st to end	Day	40% 25% 26% 9% 1	354
March	1st to 10th	Day	28% 25% 29% 15% 3	540
	11th to 20th	Day	33% 22% 30% 12% 2 1	540
	21st to end	Day	31% 21% 32% 12% 4	594
April	1st to 10th	Day	31% 23% 29% 13% 4	540
	11th to 20th	Day	33% 20% 26% 14% 6% 1	532
	21st to end	Day	27% 17% 37% 13% 5% 1	540
May	1st to 10th	Day	25% 15% 34% 17% 7% 2	543
	11th to 20th	Day	20% 14% 34% 23% 8% 2	567
	21st to end	Day	19% 13% 31% 24% 11% 2	617
June	1st to 10th	Day	18% 15% 33% 25% 8% 1	530
	11th to 20th	Day	14% 15% 31% 27% 11% 2	530
	21st to end	Day	17% 11% 35% 25% 9% 2	530
July	1st to 10th	Day	10% 8% 29% 31% 18% 3	525
	11th to 20th	Day	15% 7% 28% 33% 14% 4	524
	21st to end	Day	11% 8% 36% 31% 12% 3	576
August	1st to 10th	Day	11% 9% 28% 31% 16% 6%	526
	11th to 20th	Day	13% 7% 27% 32% 15% 6%	519
	21st to end	Day	13% 8% 32% 28% 14% 5%	583
September	1st to 10th	Day	14% 13% 24% 27% 15% 6%	530
	11th to 20th	Day	12% 7% 25% 26% 18% 12%	530
	21st to end	Day	16% 13% 31% 22% 13% 5%	530
October	1st to 10th	Day	24% 13% 26% 20% 13% 5%	530
	11th to 20th	Day	27% 19% 31% 16% 6% 1	530
	21st to end	Day	35% 18% 29% 13% 5%	583
November	1st to 10th	Day	31% 20% 30% 12% 6% 1	370
	11th to 20th	Day	32% 24% 26% 11% 4 2	220
	21st to end	Day	39% 22% 27% 10% 1	220
December	1st to 10th	Day	40% 20% 28% 9% 2	220
	11th to 20th	Day	39% 23% 30% 7% 1	220
	21st to end	Day	40% 25% 24% 8% 2 1	242
January	1st to 10th	Night	32% 28% 26% 11% 3	840
	11th to 20th	Night	35% 24% 27% 12% 2	840
	21st to end	Night	27% 27% 29% 14% 3	924
February	1st to 10th	Night	31% 27% 28% 13% 1	840
	11th to 20th	Night	29% 26% 30% 13% 2	823
	21st to end	Night	30% 24% 32% 13% 2	692
March	1st to 10th	Night	27% 24% 30% 17% 3	840
	11th to 20th	Night	30% 18% 30% 17% 4 1	840
	21st to end	Night	25% 17% 32% 19% 7% 1	924
April	1st to 10th	Night	29% 17% 30% 21% 3	840
	11th to 20th	Night	24% 17% 30% 21% 6% 1	831
	21st to end	Night	24% 12% 32% 25% 8%	655
May	1st to 10th	Night	21% 12% 26% 26% 14% 2	596
	11th to 20th	Night	15% 8% 34% 32% 10%	414
	21st to end	Night	13% 10% 30% 31% 15% 2	455
June	1st to 10th	Night	16% 7% 33% 31% 13% 1	400
	11th to 20th	Night	13% 10% 28% 35% 14% 1	400
	21st to end	Night	14% 6% 34% 35% 12%	400
July	1st to 10th	Night	9% 5% 26% 38% 20% 2	390
	11th to 20th	Night	12% 4 29% 36% 18% 2	388
	21st to end	Night	12% 4 26% 40% 16% 2	571
August	1st to 10th	Night	10% 4 24% 37% 21% 3	597
	11th to 20th	Night	12% 4 23% 35% 22% 4	594
	21st to end	Night	12% 6% 27% 31% 22% 2	805
September	1st to 10th	Night	14% 9% 23% 31% 20% 3	800
	11th to 20th	Night	15% 5% 19% 29% 26% 6%	800
	21st to end	Night	22% 11% 21% 27% 16% 4	800
October	1st to 10th	Night	21% 12% 27% 23% 16% 2	800
	11th to 20th	Night	27% 17% 29% 19% 7% 1	800
	21st to end	Night	36% 14% 27% 19% 4	880
November	1st to 10th	Night	30% 15% 29% 19% 6% 1	820
	11th to 20th	Night	27% 22% 29% 15% 6% 1	840
	21st to end	Night	34% 23% 24% 14% 5%	840
December	1st to 10th	Night	36% 25% 24% 11% 3	840
	11th to 20th	Night	33% 26% 28% 11% 2	840
	21st to end	Night	35% 24% 26% 13% 2	924

Page generated at 2026-05-29 01:53:27. Version ac07

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