Shepherds Lair Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shepherds Lair Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	21% 9% 2 6% 4 4 4 3 4 5% 39%	140398
	Night	25% 10% 2 6% 4 3 3 3 4 5% 35%	138416
Transparency	Day	49% 6% 10% 17% 13% 5%	135168
	Night	54% 5% 9% 15% 12% 5%	134656
Seeing	Day	25% 12% 23% 26% 12% 2	16231
	Night	29% 9% 17% 28% 15% 2	25342

By Month

Percentage Cloud Cover SW (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	39% 17% 3 7% 5% 4 3 3 4 4 10%	4722
	11th to 20th	Day	22% 11% 2 4 3 2 3 2 3 6% 41%	4740
	21st to end	Day	30% 8% 2 6% 4 4 4 3 5% 4 32%	5621
February	1st to 10th	Day	32% 14% 3 6% 4 4 4 3 6% 5% 19%	1890
	11th to 20th	Day	30% 16% 4 7% 5% 4 3 3 4 5% 19%	1845
	21st to end	Day	30% 18% 2 8% 3 4 3 2 3 5% 20%	1663
March	1st to 10th	Day	21% 13% 4 9% 5% 5% 6% 4 5% 7% 21%	4787
	11th to 20th	Day	29% 15% 3 6% 4 4 3 2 3 3 26%	5078
	21st to end	Day	30% 15% 3 8% 6% 5% 5% 4 5% 6% 13%	5650
April	1st to 10th	Day	30% 16% 3 9% 5% 4 3 3 4 5% 18%	2400
	11th to 20th	Day	26% 15% 3 7% 5% 4 4 3 4 5% 25%	2369
	21st to end	Day	28% 11% 3 8% 5% 4 4 3 5% 5% 24%	2563
May	1st to 10th	Day	20% 11% 2 5% 4 4 3 3 4 5% 40%	6271
	11th to 20th	Day	23% 13% 3 7% 4 4 4 3 4 6% 29%	6277
	21st to end	Day	22% 8% 2 6% 4 4 4 4 5% 8% 32%	6902
June	1st to 10th	Day	26% 9% 2 6% 4 3 4 3 4 6% 33%	2800
	11th to 20th	Day	26% 10% 3 7% 5% 4 5% 4 5% 6% 26%	2800
	21st to end	Day	12% 6% 2 5% 4 4 4 4 4 6% 50%	2800
July	1st to 10th	Day	6% 3 1 3 2 2 2 2 3 4 71%	6319
	11th to 20th	Day	4 2 1 2 2 2 2 2 2 3 79%	6320
	21st to end	Day	5% 3 1 2 2 2 2 2 3 5% 72%	6880
August	1st to 10th	Day	6% 4 1 4 3 3 3 3 4 6% 63%	5876
	11th to 20th	Day	8% 4 2 4 4 3 4 4 5% 6% 55%	5796
	21st to end	Day	10% 5% 1 5% 3 4 4 3 6% 7% 52%	6103
September	1st to 10th	Day	9% 4 2 4 3 2 4 3 4 7% 58%	2400
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 4 7% 44%	2400
	21st to end	Day	18% 6% 2 5% 4 3 4 2 4 7% 46%	2113
October	1st to 10th	Day	19% 6% 2 3 3 2 3 3 4 6% 50%	3280
	11th to 20th	Day	23% 7% 2 6% 4 3 4 4 5% 7% 34%	3261
	21st to end	Day	31% 10% 3 6% 4 4 4 3 5% 6% 23%	3442
November	1st to 10th	Day	34% 10% 4 7% 4 4 4 2 5% 7% 18%	1770
	11th to 20th	Day	36% 10% 3 7% 4 4 4 4 5% 5% 19%	1680
	21st to end	Day	29% 14% 3 6% 4 3 4 3 4 5% 24%	1680
December	1st to 10th	Day	26% 9% 2 5% 3 3 3 3 3 5% 38%	2700
	11th to 20th	Day	36% 14% 4 8% 5% 4 4 3 5% 4 14%	3108
	21st to end	Day	38% 15% 4 8% 4 3 3 2 3 3 15%	4092
January	1st to 10th	Night	37% 17% 3 7% 4 4 4 3 3 4 13%	8172
	11th to 20th	Night	24% 11% 2 4 2 2 2 2 3 6% 40%	8220
	21st to end	Night	27% 10% 3 7% 4 3 3 2 3 5% 33%	8566
February	1st to 10th	Night	30% 16% 4 7% 5% 4 3 3 4 5% 20%	2730
	11th to 20th	Night	33% 15% 3 5% 3 3 3 2 4 4 23%	2662
	21st to end	Night	32% 18% 2 5% 3 4 3 3 4 6% 20%	2142
March	1st to 10th	Night	21% 15% 4 8% 5% 5% 5% 4 5% 6% 22%	5322
	11th to 20th	Night	28% 14% 3 6% 4 3 3 2 3 4 30%	5077
	21st to end	Night	34% 15% 3 7% 5% 5% 4 2 5% 6% 14%	5324
April	1st to 10th	Night	31% 17% 3 7% 5% 4 4 2 3 4 20%	2000
	11th to 20th	Night	26% 14% 3 6% 4 3 3 2 4 6% 29%	1979
	21st to end	Night	30% 13% 2 5% 3 4 3 2 5% 5% 27%	1835
May	1st to 10th	Night	26% 10% 2 5% 2 2 2 2 3 6% 40%	3691
	11th to 20th	Night	30% 12% 3 6% 4 2 3 2 3 4 32%	3616
	21st to end	Night	26% 9% 2 6% 4 3 4 3 5% 8% 31%	3972
June	1st to 10th	Night	24% 8% 3 6% 3 4 3 3 4 5% 37%	1600
	11th to 20th	Night	25% 12% 3 7% 5% 3 3 3 4 5% 32%	1600
	21st to end	Night	14% 6% 2 5% 3 3 3 3 4 5% 51%	1600
July	1st to 10th	Night	10% 3 1 2 1 2 2 3 3 4 70%	3640
	11th to 20th	Night	3 2 1 2 2 1 2 2 2 3 80%	3640
	21st to end	Night	6% 3 1 2 2 1 2 2 3 6% 72%	3970
August	1st to 10th	Night	8% 3 1 4 2 3 3 3 4 7% 61%	3916
	11th to 20th	Night	17% 6% 1 4 3 3 3 2 3 5% 52%	4052
	21st to end	Night	11% 5% 2 4 3 3 4 3 4 5% 56%	4877
September	1st to 10th	Night	10% 3 1 4 3 2 3 3 5% 5% 62%	2000
	11th to 20th	Night	17% 6% 1 5% 4 4 4 2 4 5% 48%	2000
	21st to end	Night	16% 6% 2 4 3 3 3 3 4 7% 48%	2284
October	1st to 10th	Night	18% 6% 2 4 3 2 2 2 3 6% 51%	3920
	11th to 20th	Night	19% 8% 3 6% 4 4 4 4 6% 7% 36%	3897
	21st to end	Night	31% 10% 3 7% 4 4 3 3 4 5% 25%	4364
November	1st to 10th	Night	35% 10% 3 7% 4 4 4 2 4 5% 21%	2746
	11th to 20th	Night	39% 11% 3 6% 4 3 3 3 3 4 21%	2940
	21st to end	Night	30% 11% 2 6% 4 3 4 2 4 4 27%	2940
December	1st to 10th	Night	23% 9% 2 5% 3 3 3 3 4 6% 39%	4700
	11th to 20th	Night	36% 14% 4 8% 4 4 3 3 4 5% 14%	5316
	21st to end	Night	37% 14% 4 7% 4 4 4 3 3 4 16%	7106

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	79% 5% 5% 7% 4	4723
	11th to 20th	Day	46% 4 4 17% 15% 13%	4740
	21st to end	Day	57% 4 4 12% 18% 5%	5621
February	1st to 10th	Day	68% 7% 8% 9% 6% 1	1890
	11th to 20th	Day	71% 5% 6% 11% 5% 3	1854
	21st to end	Day	70% 5% 8% 7% 9% 2	1663
March	1st to 10th	Day	63% 7% 10% 14% 6%	4787
	11th to 20th	Day	66% 4 4 17% 8%	5078
	21st to end	Day	72% 9% 9% 7% 3	5650
April	1st to 10th	Day	72% 6% 9% 10% 3	2400
	11th to 20th	Day	65% 5% 8% 12% 8% 2	2372
	21st to end	Day	64% 7% 9% 12% 6% 2	2563
May	1st to 10th	Day	48% 6% 9% 23% 13% 1	6271
	11th to 20th	Day	56% 8% 14% 15% 7%	6280
	21st to end	Day	51% 11% 19% 16% 3	6904
June	1st to 10th	Day	54% 7% 14% 17% 7%	2800
	11th to 20th	Day	60% 7% 14% 13% 7%	2800
	21st to end	Day	37% 6% 16% 26% 11% 4	2800
July	1st to 10th	Day	18% 4 9% 28% 19% 22%	6251
	11th to 20th	Day	12% 3 7% 21% 37% 21%	6245
	21st to end	Day	16% 4 16% 33% 25% 5%	6681
August	1st to 10th	Day	22% 5% 10% 26% 25% 13%	4280
	11th to 20th	Day	27% 7% 16% 32% 13% 5%	4195
	21st to end	Day	32% 6% 10% 22% 18% 11%	4394
September	1st to 10th	Day	27% 5% 10% 26% 19% 13%	2400
	11th to 20th	Day	41% 6% 11% 18% 16% 9%	2400
	21st to end	Day	41% 6% 10% 20% 15% 8%	2113
October	1st to 10th	Day	38% 5% 7% 22% 22% 6%	3280
	11th to 20th	Day	50% 6% 13% 17% 12% 2	3261
	21st to end	Day	60% 7% 7% 16% 10% 1	3442
November	1st to 10th	Day	68% 5% 9% 12% 5% 1	1770
	11th to 20th	Day	68% 6% 7% 10% 6% 2	1680
	21st to end	Day	64% 6% 7% 7% 15% 2	1680
December	1st to 10th	Day	51% 4 8% 11% 22% 4	2700
	11th to 20th	Day	74% 5% 8% 8% 5%	3108
	21st to end	Day	76% 5% 6% 11% 2 1	4092
January	1st to 10th	Night	77% 4 6% 8% 4 1	8172
	11th to 20th	Night	48% 4 5% 16% 15% 12%	8220
	21st to end	Night	57% 3 4 10% 18% 7%	8566
February	1st to 10th	Night	69% 6% 8% 10% 7% 2	2730
	11th to 20th	Night	67% 5% 8% 10% 6% 3	2664
	21st to end	Night	69% 4 7% 10% 9% 2	2142
March	1st to 10th	Night	63% 7% 10% 13% 5% 2	5322
	11th to 20th	Night	62% 5% 8% 16% 9% 1	5077
	21st to end	Night	74% 6% 9% 10% 1	5324
April	1st to 10th	Night	70% 6% 10% 10% 4	2000
	11th to 20th	Night	59% 7% 11% 13% 7% 3	1979
	21st to end	Night	63% 6% 11% 12% 6% 2	1835
May	1st to 10th	Night	49% 6% 8% 24% 13% 1	3688
	11th to 20th	Night	57% 10% 12% 16% 4 1	3616
	21st to end	Night	51% 12% 18% 15% 4	3972
June	1st to 10th	Night	53% 6% 15% 19% 6% 1	1600
	11th to 20th	Night	58% 5% 14% 17% 5% 1	1600
	21st to end	Night	38% 6% 16% 25% 10% 5%	1600
July	1st to 10th	Night	20% 4 9% 26% 23% 17%	3600
	11th to 20th	Night	14% 3 7% 20% 39% 17%	3592
	21st to end	Night	14% 4 20% 31% 25% 5%	3857
August	1st to 10th	Night	18% 4 9% 31% 25% 12%	2849
	11th to 20th	Night	32% 7% 19% 24% 12% 6%	2929
	21st to end	Night	32% 5% 9% 25% 15% 13%	3509
September	1st to 10th	Night	26% 7% 13% 25% 18% 12%	2000
	11th to 20th	Night	42% 6% 10% 19% 12% 11%	2000
	21st to end	Night	37% 7% 13% 19% 15% 10%	2284
October	1st to 10th	Night	38% 4 6% 22% 26% 4	3920
	11th to 20th	Night	47% 7% 12% 18% 14% 3	3897
	21st to end	Night	62% 6% 7% 14% 10% 3	4364
November	1st to 10th	Night	68% 5% 6% 10% 9% 1	2746
	11th to 20th	Night	69% 5% 7% 9% 7% 3	2940
	21st to end	Night	61% 5% 7% 10% 13% 4	2940
December	1st to 10th	Night	48% 5% 8% 13% 22% 4	4700
	11th to 20th	Night	74% 6% 8% 7% 5%	5316
	21st to end	Night	73% 7% 8% 8% 3	7106

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	47% 15% 15% 17% 6%	220
	11th to 20th	Day	35% 10% 28% 15% 10% 2	220
	21st to end	Day	36% 11% 20% 21% 10% 2	394
February	1st to 10th	Day	40% 15% 18% 20% 7%	430
	11th to 20th	Day	37% 16% 26% 16% 3 1	425
	21st to end	Day	40% 14% 23% 19% 4	411
March	1st to 10th	Day	35% 17% 26% 16% 5%	540
	11th to 20th	Day	35% 16% 23% 19% 6% 1	529
	21st to end	Day	37% 20% 22% 17% 4	550
April	1st to 10th	Day	37% 17% 20% 19% 7% 1	500
	11th to 20th	Day	30% 18% 28% 15% 7% 2	492
	21st to end	Day	28% 15% 27% 21% 8% 1	500
May	1st to 10th	Day	23% 16% 26% 23% 9% 3	503
	11th to 20th	Day	30% 14% 21% 22% 10% 2	528
	21st to end	Day	26% 9% 24% 25% 13% 3	573
June	1st to 10th	Day	24% 10% 20% 28% 14% 4	530
	11th to 20th	Day	23% 15% 29% 24% 9% 1	530
	21st to end	Day	12% 7% 27% 35% 16% 4	530
July	1st to 10th	Day	7% 5% 24% 43% 17% 3	526
	11th to 20th	Day	4 7% 25% 40% 20% 4	524
	21st to end	Day	6% 3 20% 44% 21% 6%	575
August	1st to 10th	Day	6% 5% 16% 49% 18% 7%	527
	11th to 20th	Day	5% 5% 21% 38% 25% 6%	517
	21st to end	Day	8% 8% 26% 36% 17% 5%	583
September	1st to 10th	Day	10% 7% 26% 35% 19% 3	530
	11th to 20th	Day	16% 8% 21% 35% 17% 3	530
	21st to end	Day	18% 10% 26% 30% 14% 2	530
October	1st to 10th	Day	22% 11% 23% 27% 15% 2	530
	11th to 20th	Day	26% 11% 21% 27% 14% 1	530
	21st to end	Day	34% 13% 20% 22% 9% 2	516
November	1st to 10th	Day	37% 15% 17% 22% 8% 1	286
	11th to 20th	Day	36% 14% 25% 15% 8% 2	220
	21st to end	Day	32% 15% 19% 24% 10%	220
December	1st to 10th	Day	37% 15% 18% 16% 11% 3	220
	11th to 20th	Day	49% 13% 20% 12% 5%	220
	21st to end	Day	46% 14% 19% 13% 8%	242
January	1st to 10th	Night	44% 13% 20% 18% 5% 1	840
	11th to 20th	Night	37% 10% 22% 21% 9%	840
	21st to end	Night	34% 10% 23% 25% 7% 1	924
February	1st to 10th	Night	35% 11% 21% 27% 5% 1	840
	11th to 20th	Night	39% 12% 18% 24% 7%	824
	21st to end	Night	41% 13% 20% 22% 5%	692
March	1st to 10th	Night	34% 13% 18% 25% 9% 1	840
	11th to 20th	Night	34% 11% 19% 26% 9%	828
	21st to end	Night	38% 13% 17% 22% 9% 1	880
April	1st to 10th	Night	37% 11% 17% 23% 11% 1	800
	11th to 20th	Night	31% 10% 19% 25% 13% 2	791
	21st to end	Night	35% 10% 12% 29% 12% 1	635
May	1st to 10th	Night	27% 10% 14% 32% 15% 2	425
	11th to 20th	Night	26% 8% 13% 29% 23% 1	394
	21st to end	Night	28% 6% 16% 28% 20% 2	433
June	1st to 10th	Night	27% 5% 15% 30% 22% 2	400
	11th to 20th	Night	27% 9% 17% 32% 16% 1	400
	21st to end	Night	15% 5% 13% 39% 26% 1	400
July	1st to 10th	Night	9% 3 9% 41% 37% 1	390
	11th to 20th	Night	6% 2 13% 46% 32% 1	388
	21st to end	Night	4 2 7% 42% 42% 3	438
August	1st to 10th	Night	7% 2 7% 33% 45% 6%	542
	11th to 20th	Night	8% 1 9% 32% 43% 7%	594
	21st to end	Night	8% 4 13% 41% 30% 4	820
September	1st to 10th	Night	11% 3 15% 36% 33% 3	800
	11th to 20th	Night	16% 5% 12% 34% 29% 4	800
	21st to end	Night	18% 5% 15% 34% 26% 3	800
October	1st to 10th	Night	24% 5% 15% 34% 20% 2	800
	11th to 20th	Night	24% 8% 15% 33% 18% 2	800
	21st to end	Night	30% 10% 22% 25% 11% 2	880
November	1st to 10th	Night	39% 10% 18% 26% 6%	820
	11th to 20th	Night	42% 11% 21% 20% 5%	840
	21st to end	Night	32% 11% 22% 28% 7%	840
December	1st to 10th	Night	35% 11% 22% 24% 7% 1	840
	11th to 20th	Night	42% 12% 21% 20% 5%	840
	21st to end	Night	44% 13% 22% 16% 4 1	924

Page generated at 2026-03-14 23:23:56. Version ac07

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