Sturk Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sturk Road Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	18% 14% 4 10% 6% 5% 4 3 4 6% 27%	132560
	Night	18% 15% 5% 12% 7% 4 4 3 4 5% 24%	130345
Transparency	Day	60% 3 9% 18% 9% 1	136130
	Night	62% 3 9% 17% 8% 1	134863
Seeing	Day	27% 20% 31% 17% 5% 1	19040
	Night	27% 18% 30% 20% 5%	26664

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% 24% 8% 18% 11% 6% 3 1 2 3 11%	4167
	11th to 20th	Day	16% 22% 11% 26% 5% 4 3 2 3 3 7%	4980
	21st to end	Day	12% 14% 7% 17% 15% 9% 6% 2 3 4 11%	5781
February	1st to 10th	Day	16% 17% 6% 15% 11% 6% 4 3 3 4 13%	1890
	11th to 20th	Day	15% 18% 6% 15% 10% 6% 4 3 3 4 18%	1843
	21st to end	Day	11% 17% 6% 11% 7% 5% 3 3 4 5% 28%	1594
March	1st to 10th	Day	20% 18% 6% 14% 5% 7% 3 2 3 3 18%	5029
	11th to 20th	Day	14% 17% 5% 11% 5% 5% 4 3 4 5% 27%	5090
	21st to end	Day	21% 20% 6% 12% 5% 4 4 3 3 4 19%	5587
April	1st to 10th	Day	22% 17% 5% 8% 4 3 3 3 4 4 27%	2476
	11th to 20th	Day	20% 15% 3 6% 4 4 4 2 4 6% 33%	2624
	21st to end	Day	20% 13% 3 6% 4 5% 4 3 4 6% 31%	2730
May	1st to 10th	Day	24% 17% 3 8% 4 5% 4 3 5% 5% 20%	6058
	11th to 20th	Day	20% 12% 3 8% 5% 5% 5% 3 4 6% 30%	6402
	21st to end	Day	16% 12% 3 8% 4 4 4 3 6% 8% 32%	6948
June	1st to 10th	Day	24% 9% 3 6% 5% 4 5% 4 6% 6% 28%	2840
	11th to 20th	Day	20% 8% 2 7% 5% 4 4 4 5% 7% 34%	3000
	21st to end	Day	24% 11% 3 7% 5% 4 4 4 5% 5% 28%	3000
July	1st to 10th	Day	21% 9% 3 6% 4 4 5% 4 6% 8% 30%	6820
	11th to 20th	Day	18% 9% 3 6% 4 4 4 3 6% 7% 38%	6480
	21st to end	Day	12% 7% 2 7% 4 4 5% 4 7% 9% 38%	6608
August	1st to 10th	Day	20% 10% 3 7% 5% 4 5% 3 5% 6% 31%	4235
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	4175
	21st to end	Day	11% 6% 2 5% 4 4 4 4 5% 8% 47%	4551
September	1st to 10th	Day	18% 6% 2 5% 4 4 3 3 6% 7% 44%	2400
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 3 6% 45%	2271
	21st to end	Day	18% 8% 3 5% 4 3 3 3 5% 6% 43%	2169
October	1st to 10th	Day	17% 9% 3 6% 4 3 4 3 6% 7% 39%	2000
	11th to 20th	Day	19% 12% 3 6% 4 4 4 3 5% 7% 33%	2000
	21st to end	Day	19% 15% 3 8% 5% 3 3 3 4 5% 30%	2195
November	1st to 10th	Day	20% 17% 3 8% 5% 4 4 2 4 6% 27%	1705
	11th to 20th	Day	18% 23% 9% 9% 5% 4 4 2 4 4 20%	1680
	21st to end	Day	19% 22% 7% 12% 6% 4 4 3 3 5% 17%	1680
December	1st to 10th	Day	18% 19% 9% 21% 6% 4 3 2 3 4 11%	2690
	11th to 20th	Day	21% 18% 7% 18% 8% 4 3 3 2 2 14%	3076
	21st to end	Day	18% 18% 10% 16% 8% 4 3 3 4 4 14%	3786
January	1st to 10th	Night	12% 20% 9% 22% 11% 5% 2 2 3 3 10%	8736
	11th to 20th	Night	17% 20% 9% 23% 6% 4 3 2 2 3 10%	7905
	21st to end	Night	11% 13% 6% 19% 18% 8% 4 2 4 4 12%	8400
February	1st to 10th	Night	17% 16% 7% 18% 9% 6% 3 3 4 3 14%	2730
	11th to 20th	Night	16% 15% 6% 15% 10% 5% 3 4 4 4 18%	2663
	21st to end	Night	11% 16% 6% 12% 7% 6% 4 4 4 5% 25%	2208
March	1st to 10th	Night	19% 18% 5% 14% 9% 6% 3 2 2 3 18%	5031
	11th to 20th	Night	16% 18% 7% 9% 6% 5% 4 3 4 5% 25%	5089
	21st to end	Night	24% 15% 6% 10% 5% 4 3 2 3 3 24%	5587
April	1st to 10th	Night	23% 18% 5% 6% 5% 4 3 3 4 4 26%	2143
	11th to 20th	Night	22% 12% 3 7% 4 3 4 3 4 7% 30%	1942
	21st to end	Night	21% 14% 3 7% 6% 4 3 3 4 5% 31%	1890
May	1st to 10th	Night	23% 19% 4 7% 5% 4 4 3 4 5% 22%	4103
	11th to 20th	Night	22% 10% 3 7% 5% 4 4 4 6% 6% 30%	3645
	21st to end	Night	16% 13% 3 8% 4 4 4 3 5% 6% 32%	3948
June	1st to 10th	Night	24% 8% 3 6% 5% 5% 4 3 5% 8% 28%	1574
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 3 4 6% 35%	1400
	21st to end	Night	22% 9% 3 6% 6% 5% 5% 4 5% 7% 29%	1400
July	1st to 10th	Night	20% 11% 4 9% 5% 4 5% 4 6% 7% 26%	3160
	11th to 20th	Night	16% 8% 2 6% 4 4 5% 4 6% 9% 35%	3436
	21st to end	Night	14% 7% 2 5% 4 4 4 4 7% 9% 40%	4249
August	1st to 10th	Night	21% 11% 2 6% 3 4 4 3 5% 7% 33%	2915
	11th to 20th	Night	15% 7% 2 6% 5% 4 6% 4 7% 9% 34%	2872
	21st to end	Night	12% 6% 2 5% 4 3 4 3 6% 7% 47%	3346
September	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 6% 6% 45%	2000
	11th to 20th	Night	18% 6% 3 6% 3 3 3 3 4 6% 45%	2125
	21st to end	Night	17% 8% 2 7% 4 3 4 4 5% 7% 39%	2230
October	1st to 10th	Night	18% 6% 2 7% 4 4 4 3 4 7% 41%	2400
	11th to 20th	Night	18% 11% 3 8% 4 4 4 3 5% 7% 34%	2400
	21st to end	Night	25% 13% 3 8% 4 3 4 2 4 5% 28%	2643
November	1st to 10th	Night	21% 15% 3 8% 4 4 3 3 4 5% 29%	2811
	11th to 20th	Night	18% 23% 7% 10% 5% 4 3 3 3 4 20%	2940
	21st to end	Night	19% 21% 6% 11% 5% 3 3 3 4 5% 20%	2940
December	1st to 10th	Night	14% 20% 9% 21% 6% 4 3 3 4 4 12%	4720
	11th to 20th	Night	16% 18% 7% 18% 7% 4 3 2 3 3 18%	5353
	21st to end	Night	21% 19% 6% 14% 7% 3 3 2 3 3 18%	7411

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	82% 1 9% 7% 1	4168
	11th to 20th	Day	85% 2 4 7% 2	5012
	21st to end	Day	75% 3 7% 8% 7% 1	5781
February	1st to 10th	Day	76% 2 7% 9% 6% 1	1890
	11th to 20th	Day	73% 2 7% 12% 5%	1851
	21st to end	Day	60% 3 9% 18% 9% 1	1594
March	1st to 10th	Day	73% 1 5% 12% 8% 1	5033
	11th to 20th	Day	61% 3 11% 18% 7% 1	5090
	21st to end	Day	72% 2 3 13% 9% 1	5587
April	1st to 10th	Day	64% 2 6% 14% 13% 2	2476
	11th to 20th	Day	55% 2 6% 23% 13% 1	2624
	21st to end	Day	56% 2 8% 19% 12% 3	2730
May	1st to 10th	Day	67% 2 8% 15% 6% 1	6069
	11th to 20th	Day	58% 3 10% 21% 7% 1	6406
	21st to end	Day	53% 2 13% 19% 12%	6948
June	1st to 10th	Day	58% 3 8% 22% 8% 1	2840
	11th to 20th	Day	51% 3 12% 22% 11% 1	3000
	21st to end	Day	60% 2 9% 18% 10% 1	3000
July	1st to 10th	Day	54% 4 14% 18% 10% 1	6745
	11th to 20th	Day	45% 4 13% 27% 10%	6401
	21st to end	Day	43% 6% 15% 26% 10%	6421
August	1st to 10th	Day	56% 4 9% 20% 11%	4222
	11th to 20th	Day	45% 5% 14% 25% 12%	4214
	21st to end	Day	36% 5% 12% 29% 18% 1	4551
September	1st to 10th	Day	42% 3 11% 27% 15% 2	2400
	11th to 20th	Day	44% 2 11% 25% 18% 2	2271
	21st to end	Day	43% 2 8% 27% 18% 1	2169
October	1st to 10th	Day	45% 5% 10% 26% 13%	3260
	11th to 20th	Day	49% 2 8% 26% 14%	3240
	21st to end	Day	49% 3 13% 22% 13% 1	3521
November	1st to 10th	Day	62% 2 9% 18% 8% 1	1705
	11th to 20th	Day	68% 3 10% 14% 4	1680
	21st to end	Day	72% 4 9% 12% 3	1680
December	1st to 10th	Day	77% 3 6% 12% 2	2690
	11th to 20th	Day	77% 2 5% 11% 5%	3075
	21st to end	Day	75% 2 9% 8% 5%	3786
January	1st to 10th	Night	81% 2 6% 9% 1	8736
	11th to 20th	Night	81% 3 5% 8% 3	7948
	21st to end	Night	76% 3 6% 10% 4 1	8400
February	1st to 10th	Night	74% 3 7% 9% 6% 1	2730
	11th to 20th	Night	70% 2 8% 14% 5% 1	2666
	21st to end	Night	62% 3 8% 18% 9% 1	2208
March	1st to 10th	Night	73% 2 5% 11% 9% 1	5040
	11th to 20th	Night	63% 2 10% 17% 7% 1	5090
	21st to end	Night	68% 2 5% 15% 9% 1	5587
April	1st to 10th	Night	64% 2 7% 12% 13% 2	2143
	11th to 20th	Night	56% 3 7% 19% 14% 1	1942
	21st to end	Night	58% 3 9% 17% 12% 2	1890
May	1st to 10th	Night	67% 2 9% 14% 7%	4109
	11th to 20th	Night	57% 2 10% 23% 6% 2	3646
	21st to end	Night	54% 3 14% 21% 7% 1	3949
June	1st to 10th	Night	55% 3 12% 23% 7%	1574
	11th to 20th	Night	50% 4 11% 24% 10% 1	1400
	21st to end	Night	56% 5% 11% 17% 10% 2	1400
July	1st to 10th	Night	56% 5% 13% 18% 8%	3125
	11th to 20th	Night	44% 5% 17% 21% 12% 1	3392
	21st to end	Night	39% 5% 17% 26% 12%	4123
August	1st to 10th	Night	51% 5% 13% 24% 7%	2903
	11th to 20th	Night	45% 5% 17% 25% 8%	2899
	21st to end	Night	34% 5% 15% 28% 16% 1	3345
September	1st to 10th	Night	40% 3 10% 30% 15% 2	2000
	11th to 20th	Night	42% 2 10% 30% 16% 1	2125
	21st to end	Night	46% 3 9% 27% 14% 1	2230
October	1st to 10th	Night	48% 3 10% 25% 12% 1	3920
	11th to 20th	Night	44% 3 12% 25% 15%	3899
	21st to end	Night	49% 2 11% 23% 14%	4267
November	1st to 10th	Night	58% 3 11% 18% 9%	2811
	11th to 20th	Night	69% 3 9% 14% 5%	2940
	21st to end	Night	67% 3 13% 13% 3	2940
December	1st to 10th	Night	73% 4 8% 13% 2	4720
	11th to 20th	Night	71% 3 7% 14% 4	5355
	21st to end	Night	72% 2 7% 13% 5%	7411

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	31% 29% 29% 11%	242
	11th to 20th	Day	33% 25% 27% 12% 3	430
	21st to end	Day	26% 25% 37% 10% 2	473
February	1st to 10th	Day	27% 27% 30% 14% 2	430
	11th to 20th	Day	31% 28% 29% 10% 2	426
	21st to end	Day	25% 27% 33% 12% 3	375
March	1st to 10th	Day	30% 26% 32% 10% 2	540
	11th to 20th	Day	28% 28% 31% 10% 2	540
	21st to end	Day	32% 29% 26% 10% 3	594
April	1st to 10th	Day	38% 26% 26% 7% 2	540
	11th to 20th	Day	31% 27% 28% 10% 3	532
	21st to end	Day	26% 23% 30% 16% 6%	540
May	1st to 10th	Day	31% 26% 27% 12% 3 1	559
	11th to 20th	Day	27% 18% 28% 21% 6% 1	754
	21st to end	Day	27% 18% 34% 17% 4 1	790
June	1st to 10th	Day	31% 14% 29% 20% 6% 1	730
	11th to 20th	Day	22% 17% 34% 19% 7% 1	730
	21st to end	Day	29% 11% 33% 20% 6% 1	730
July	1st to 10th	Day	22% 9% 37% 22% 9% 1	720
	11th to 20th	Day	20% 8% 34% 26% 10% 1	718
	21st to end	Day	22% 8% 36% 25% 9%	655
August	1st to 10th	Day	15% 8% 36% 29% 11% 1	526
	11th to 20th	Day	18% 11% 31% 26% 12% 2	519
	21st to end	Day	17% 13% 32% 25% 11% 3	583
September	1st to 10th	Day	20% 10% 34% 26% 8% 1	530
	11th to 20th	Day	21% 15% 30% 22% 10% 2	530
	21st to end	Day	19% 17% 31% 21% 11% 1	512
October	1st to 10th	Day	21% 17% 28% 23% 8% 3	420
	11th to 20th	Day	27% 21% 26% 18% 7% 1	420
	21st to end	Day	26% 24% 29% 17% 4 1	462
November	1st to 10th	Day	30% 24% 28% 14% 3 1	425
	11th to 20th	Day	36% 25% 24% 11% 4	430
	21st to end	Day	31% 29% 30% 8% 2	430
December	1st to 10th	Day	34% 24% 29% 11% 2	430
	11th to 20th	Day	39% 30% 21% 9%	430
	21st to end	Day	32% 27% 31% 7% 2 1	345
January	1st to 10th	Night	28% 27% 31% 13% 2	950
	11th to 20th	Night	30% 25% 28% 14% 4	891
	21st to end	Night	23% 28% 32% 13% 3	924
February	1st to 10th	Night	29% 26% 28% 15% 2	840
	11th to 20th	Night	31% 26% 28% 13% 2	823
	21st to end	Night	22% 23% 33% 19% 2	692
March	1st to 10th	Night	28% 25% 30% 14% 3	840
	11th to 20th	Night	25% 22% 31% 18% 4	840
	21st to end	Night	27% 22% 33% 14% 4	924
April	1st to 10th	Night	36% 19% 28% 14% 2	673
	11th to 20th	Night	31% 19% 31% 17% 3	624
	21st to end	Night	29% 14% 31% 21% 4	630
May	1st to 10th	Night	30% 15% 32% 18% 5%	630
	11th to 20th	Night	28% 12% 31% 25% 4	611
	21st to end	Night	22% 13% 38% 22% 6%	649
June	1st to 10th	Night	29% 10% 31% 24% 6%	574
	11th to 20th	Night	23% 7% 34% 29% 6% 1	400
	21st to end	Night	26% 7% 35% 26% 7% 1	400
July	1st to 10th	Night	19% 6% 35% 33% 7%	390
	11th to 20th	Night	16% 6% 37% 30% 11%	506
	21st to end	Night	20% 5% 30% 36% 9%	656
August	1st to 10th	Night	18% 7% 27% 40% 8%	598
	11th to 20th	Night	17% 7% 29% 33% 14% 1	592
	21st to end	Night	17% 7% 30% 32% 14% 1	660
September	1st to 10th	Night	17% 8% 32% 32% 11% 1	600
	11th to 20th	Night	18% 11% 30% 28% 12% 1	725
	21st to end	Night	22% 13% 30% 24% 10% 1	800
October	1st to 10th	Night	20% 12% 33% 26% 9% 1	800
	11th to 20th	Night	26% 15% 32% 19% 7% 1	800
	21st to end	Night	33% 20% 24% 19% 4	880
November	1st to 10th	Night	32% 18% 28% 16% 4 1	897
	11th to 20th	Night	32% 23% 26% 14% 4	950
	21st to end	Night	32% 23% 27% 15% 2	950
December	1st to 10th	Night	32% 23% 30% 13% 2	950
	11th to 20th	Night	34% 27% 27% 10% 2	950
	21st to end	Night	32% 24% 27% 14% 3	1045

Page generated at 2026-05-14 02:58:58. Version ac07

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