Two Mile Lane Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Two Mile Lane Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	15% 9% 3 7% 4 4 4 3 5% 6% 39%	135949
	Night	18% 12% 4 8% 4 4 4 3 4 5% 33%	131816
Transparency	Day	44% 7% 12% 19% 13% 5%	130733
	Night	51% 7% 11% 16% 10% 5%	128042
Seeing	Day	22% 13% 30% 24% 9% 2	16286
	Night	26% 8% 17% 27% 18% 3	25234

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	29% 17% 5% 10% 5% 4 4 3 4 5% 14%	3639
	11th to 20th	Day	24% 9% 2 6% 4 3 4 2 3 5% 38%	4026
	21st to end	Day	22% 8% 3 6% 4 4 4 3 5% 6% 34%	4510
February	1st to 10th	Day	23% 14% 4 9% 7% 4 4 4 4 6% 21%	1800
	11th to 20th	Day	23% 14% 5% 9% 5% 5% 4 3 4 5% 23%	1755
	21st to end	Day	24% 16% 4 9% 6% 4 4 2 4 4 22%	1548
March	1st to 10th	Day	19% 15% 4 8% 6% 6% 6% 4 6% 6% 20%	4952
	11th to 20th	Day	25% 12% 3 9% 5% 5% 4 3 4 4 26%	5000
	21st to end	Day	26% 13% 4 9% 6% 6% 6% 5% 6% 6% 13%	5486
April	1st to 10th	Day	26% 14% 4 9% 7% 5% 4 3 5% 5% 18%	2363
	11th to 20th	Day	21% 13% 4 8% 5% 5% 4 3 5% 6% 26%	2402
	21st to end	Day	21% 9% 3 8% 5% 5% 6% 4 6% 7% 27%	2600
May	1st to 10th	Day	15% 7% 3 6% 5% 4 4 3 5% 6% 43%	5880
	11th to 20th	Day	16% 10% 3 7% 4 4 4 3 5% 7% 38%	6185
	21st to end	Day	12% 8% 2 6% 4 4 4 4 6% 9% 40%	6902
June	1st to 10th	Day	19% 10% 3 7% 5% 4 4 3 5% 7% 32%	2800
	11th to 20th	Day	15% 10% 4 7% 5% 4 5% 3 5% 7% 36%	2800
	21st to end	Day	10% 8% 3 6% 3 3 4 3 5% 6% 49%	2800
July	1st to 10th	Day	3 8% 1 4 2 2 2 2 3 6% 68%	6319
	11th to 20th	Day	3 5% 2 4 3 3 2 2 3 6% 67%	6283
	21st to end	Day	5% 9% 2 5% 3 3 2 2 4 7% 59%	6402
August	1st to 10th	Day	5% 8% 2 6% 3 3 4 3 5% 9% 51%	5777
	11th to 20th	Day	7% 7% 2 5% 3 3 3 3 4 8% 55%	5796
	21st to end	Day	6% 6% 2 5% 4 3 3 3 4 7% 57%	6218
September	1st to 10th	Day	4 5% 2 5% 3 3 3 3 4 7% 61%	2400
	11th to 20th	Day	9% 7% 3 6% 4 4 3 4 6% 8% 47%	2280
	21st to end	Day	9% 5% 3 6% 4 3 3 3 4 8% 51%	2175
October	1st to 10th	Day	11% 7% 2 7% 4 3 3 3 4 6% 51%	3280
	11th to 20th	Day	16% 8% 2 7% 4 4 5% 4 6% 9% 35%	3261
	21st to end	Day	18% 8% 3 8% 5% 5% 6% 4 6% 8% 29%	3558
November	1st to 10th	Day	24% 10% 3 8% 6% 4 5% 4 6% 6% 23%	1844
	11th to 20th	Day	23% 11% 5% 9% 5% 6% 5% 4 5% 6% 21%	1656
	21st to end	Day	20% 11% 3 7% 6% 5% 4 4 5% 6% 31%	1600
December	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 4 6% 7% 39%	2620
	11th to 20th	Day	28% 13% 3 11% 6% 6% 5% 4 5% 5% 15%	3028
	21st to end	Day	35% 15% 4 10% 5% 5% 4 3 4 5% 11%	4004
January	1st to 10th	Night	34% 14% 4 9% 6% 5% 4 3 4 3 14%	6306
	11th to 20th	Night	22% 14% 3 7% 3 3 3 2 3 4 36%	5947
	21st to end	Night	26% 10% 3 7% 4 3 3 2 3 4 34%	6468
February	1st to 10th	Night	22% 14% 4 8% 5% 5% 5% 4 5% 6% 20%	2600
	11th to 20th	Night	24% 13% 3 9% 6% 5% 4 3 4 5% 25%	2532
	21st to end	Night	24% 14% 3 10% 5% 4 3 4 5% 5% 23%	2079
March	1st to 10th	Night	20% 18% 4 10% 6% 6% 5% 4 5% 5% 18%	4942
	11th to 20th	Night	22% 15% 4 10% 6% 4 4 3 3 4 25%	5000
	21st to end	Night	27% 15% 4 10% 6% 5% 4 3 4 6% 15%	5489
April	1st to 10th	Night	23% 13% 5% 8% 5% 4 4 4 5% 6% 23%	2035
	11th to 20th	Night	19% 14% 3 6% 5% 4 4 4 4 5% 31%	1941
	21st to end	Night	21% 11% 4 6% 5% 4 5% 4 6% 6% 27%	1800
May	1st to 10th	Night	15% 10% 2 7% 3 3 4 3 4 5% 44%	4100
	11th to 20th	Night	17% 15% 4 9% 4 3 3 3 4 5% 34%	3709
	21st to end	Night	15% 10% 3 4 3 3 3 3 5% 7% 45%	3972
June	1st to 10th	Night	17% 12% 2 6% 5% 4 5% 3 5% 7% 35%	1600
	11th to 20th	Night	12% 11% 3 6% 4 4 4 3 6% 8% 41%	1600
	21st to end	Night	8% 11% 2 7% 3 3 3 3 4 7% 49%	1600
July	1st to 10th	Night	4 14% 4 7% 2 2 2 1 3 4 56%	3640
	11th to 20th	Night	3 9% 5% 10% 4 3 3 2 2 6% 54%	3676
	21st to end	Night	8% 20% 6% 8% 3 2 2 2 3 6% 40%	4472
August	1st to 10th	Night	7% 23% 5% 12% 4 3 3 2 3 5% 34%	4035
	11th to 20th	Night	9% 16% 4 8% 3 3 3 2 4 6% 43%	4052
	21st to end	Night	6% 9% 4 7% 4 3 3 3 4 5% 53%	4757
September	1st to 10th	Night	5% 7% 3 4 3 3 3 3 5% 6% 59%	2000
	11th to 20th	Night	8% 7% 2 6% 4 3 3 3 5% 7% 53%	2115
	21st to end	Night	10% 7% 2 6% 3 3 4 3 4 7% 50%	2225
October	1st to 10th	Night	8% 9% 2 6% 3 3 2 2 4 4 54%	3920
	11th to 20th	Night	12% 9% 4 9% 5% 4 4 4 5% 8% 36%	3897
	21st to end	Night	22% 10% 3 9% 5% 5% 4 4 5% 6% 27%	4253
November	1st to 10th	Night	22% 10% 4 8% 5% 5% 4 3 5% 7% 27%	2668
	11th to 20th	Night	24% 11% 4 9% 7% 6% 4 4 5% 6% 22%	2898
	21st to end	Night	20% 8% 3 9% 5% 6% 4 4 5% 6% 31%	2800
December	1st to 10th	Night	14% 8% 3 6% 5% 4 4 3 5% 5% 42%	4560
	11th to 20th	Night	30% 12% 4 9% 6% 5% 5% 4 5% 5% 16%	5176
	21st to end	Night	31% 12% 4 9% 5% 5% 4 3 4 5% 18%	6952

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	73% 6% 7% 8% 5% 1	3639
	11th to 20th	Day	50% 5% 6% 16% 17% 6%	4026
	21st to end	Day	52% 6% 6% 10% 12% 15%	4510
February	1st to 10th	Day	64% 6% 10% 13% 5% 2	1800
	11th to 20th	Day	65% 6% 9% 12% 6% 2	1764
	21st to end	Day	65% 7% 7% 12% 6% 2	1548
March	1st to 10th	Day	63% 8% 10% 16% 3 1	4952
	11th to 20th	Day	60% 7% 9% 13% 10% 1	5000
	21st to end	Day	69% 9% 10% 10% 2	5486
April	1st to 10th	Day	67% 8% 10% 10% 5%	2363
	11th to 20th	Day	59% 7% 9% 14% 8% 2	2405
	21st to end	Day	56% 8% 13% 16% 7% 1	2600
May	1st to 10th	Day	41% 9% 11% 25% 13% 2	5880
	11th to 20th	Day	41% 8% 16% 22% 11% 1	6188
	21st to end	Day	37% 9% 28% 23% 2	6904
June	1st to 10th	Day	50% 8% 13% 22% 6%	2800
	11th to 20th	Day	44% 8% 17% 23% 7%	2800
	21st to end	Day	33% 5% 16% 30% 13% 3	2800
July	1st to 10th	Day	17% 4 10% 25% 19% 24%	6251
	11th to 20th	Day	15% 4 8% 16% 34% 23%	6208
	21st to end	Day	21% 5% 14% 35% 20% 4	6217
August	1st to 10th	Day	23% 6% 15% 27% 22% 6%	4206
	11th to 20th	Day	25% 5% 19% 32% 16% 2	4195
	21st to end	Day	24% 6% 15% 26% 26% 4	4485
September	1st to 10th	Day	22% 5% 10% 30% 22% 11%	2400
	11th to 20th	Day	33% 7% 15% 25% 16% 4	2280
	21st to end	Day	31% 6% 13% 25% 19% 6%	2175
October	1st to 10th	Day	34% 4 8% 17% 19% 18%	3280
	11th to 20th	Day	43% 7% 17% 20% 10% 2	3261
	21st to end	Day	50% 7% 12% 17% 11% 3	3558
November	1st to 10th	Day	61% 9% 8% 12% 9% 1	1844
	11th to 20th	Day	63% 8% 9% 8% 7% 5%	1656
	21st to end	Day	55% 5% 10% 10% 11% 8%	1600
December	1st to 10th	Day	43% 6% 11% 14% 16% 10%	2620
	11th to 20th	Day	71% 6% 9% 8% 4 1	3028
	21st to end	Day	77% 6% 5% 7% 3 2	4004
January	1st to 10th	Night	75% 6% 6% 7% 4 2	6306
	11th to 20th	Night	53% 4 7% 14% 16% 6%	5947
	21st to end	Night	56% 4 4 8% 13% 15%	6468
February	1st to 10th	Night	63% 7% 10% 12% 6% 2	2600
	11th to 20th	Night	63% 6% 8% 15% 6% 2	2534
	21st to end	Night	64% 7% 8% 10% 7% 3	2079
March	1st to 10th	Night	67% 6% 10% 13% 4	4942
	11th to 20th	Night	61% 7% 10% 10% 8% 3	5000
	21st to end	Night	69% 10% 11% 9% 1	5489
April	1st to 10th	Night	63% 8% 11% 13% 4 1	2035
	11th to 20th	Night	56% 8% 12% 16% 5% 3	1941
	21st to end	Night	57% 8% 11% 18% 6% 1	1800
May	1st to 10th	Night	43% 8% 10% 26% 11% 2	4097
	11th to 20th	Night	46% 8% 16% 21% 7% 3	3709
	21st to end	Night	39% 9% 31% 19% 2	3972
June	1st to 10th	Night	48% 7% 17% 24% 4	1600
	11th to 20th	Night	40% 7% 19% 26% 8% 1	1600
	21st to end	Night	34% 5% 17% 31% 10% 4	1600
July	1st to 10th	Night	29% 6% 8% 19% 16% 22%	3600
	11th to 20th	Night	20% 9% 11% 18% 27% 16%	3628
	21st to end	Night	39% 7% 14% 20% 17% 3	4345
August	1st to 10th	Night	37% 14% 14% 19% 12% 3	2929
	11th to 20th	Night	37% 6% 15% 25% 14% 3	2929
	21st to end	Night	23% 5% 12% 31% 24% 5%	3428
September	1st to 10th	Night	23% 5% 10% 28% 22% 12%	2000
	11th to 20th	Night	29% 7% 17% 26% 18% 4	2115
	21st to end	Night	33% 7% 12% 21% 17% 9%	2225
October	1st to 10th	Night	32% 5% 9% 16% 22% 15%	3920
	11th to 20th	Night	44% 6% 18% 20% 10% 2	3897
	21st to end	Night	57% 6% 9% 14% 9% 5%	4253
November	1st to 10th	Night	58% 7% 12% 12% 9% 2	2668
	11th to 20th	Night	65% 6% 9% 10% 7% 4	2898
	21st to end	Night	53% 7% 9% 11% 12% 8%	2800
December	1st to 10th	Night	43% 6% 7% 17% 19% 7%	4560
	11th to 20th	Night	70% 6% 10% 7% 5% 1	5176
	21st to end	Night	69% 7% 8% 12% 3 1	6952

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	43% 17% 18% 12% 7% 4	200
	11th to 20th	Day	38% 11% 19% 16% 12% 5%	365
	21st to end	Day	33% 12% 21% 20% 10% 4	440
February	1st to 10th	Day	34% 12% 25% 21% 8% 1	400
	11th to 20th	Day	40% 14% 25% 16% 3 2	395
	21st to end	Day	34% 15% 27% 19% 3 1	349
March	1st to 10th	Day	37% 16% 24% 17% 6%	500
	11th to 20th	Day	33% 12% 27% 18% 8% 2	500
	21st to end	Day	35% 18% 24% 17% 5% 1	550
April	1st to 10th	Day	34% 18% 25% 16% 5% 2	500
	11th to 20th	Day	28% 20% 28% 16% 4 3	492
	21st to end	Day	26% 18% 32% 16% 7% 1	500
May	1st to 10th	Day	22% 18% 34% 19% 6% 1	503
	11th to 20th	Day	30% 13% 27% 21% 7% 2	528
	21st to end	Day	20% 14% 37% 22% 6% 1	573
June	1st to 10th	Day	18% 14% 35% 24% 8% 1	530
	11th to 20th	Day	17% 23% 39% 17% 3 1	530
	21st to end	Day	9% 11% 42% 30% 5% 1	530
July	1st to 10th	Day	8% 10% 44% 29% 9%	526
	11th to 20th	Day	6% 8% 47% 32% 6% 1	524
	21st to end	Day	7% 6% 42% 35% 8% 2	575
August	1st to 10th	Day	6% 9% 32% 36% 13% 3	527
	11th to 20th	Day	7% 6% 34% 38% 12% 2	517
	21st to end	Day	8% 8% 37% 33% 10% 4	583
September	1st to 10th	Day	5% 8% 35% 33% 16% 2	530
	11th to 20th	Day	14% 12% 29% 31% 13% 2	530
	21st to end	Day	14% 13% 31% 27% 12% 3	530
October	1st to 10th	Day	18% 13% 25% 28% 12% 4	530
	11th to 20th	Day	20% 13% 27% 22% 13% 5%	530
	21st to end	Day	27% 15% 23% 23% 9% 3	583
November	1st to 10th	Day	31% 14% 25% 19% 9% 2	382
	11th to 20th	Day	32% 14% 18% 21% 10% 5%	214
	21st to end	Day	32% 12% 18% 18% 18% 4	200
December	1st to 10th	Day	32% 7% 17% 21% 15% 9%	200
	11th to 20th	Day	42% 13% 14% 21% 8% 3	200
	21st to end	Day	43% 12% 15% 19% 10% 1	220
January	1st to 10th	Night	40% 11% 18% 18% 11% 3	800
	11th to 20th	Night	34% 11% 18% 17% 16% 5%	800
	21st to end	Night	29% 9% 15% 25% 19% 4	880
February	1st to 10th	Night	32% 9% 16% 25% 17% 2	800
	11th to 20th	Night	32% 10% 20% 24% 13% 2	784
	21st to end	Night	35% 10% 19% 25% 11%	660
March	1st to 10th	Night	37% 10% 18% 23% 11% 1	800
	11th to 20th	Night	29% 10% 19% 27% 14% 2	800
	21st to end	Night	38% 8% 20% 21% 12% 1	880
April	1st to 10th	Night	35% 9% 17% 21% 16% 2	800
	11th to 20th	Night	30% 9% 21% 24% 14% 2	753
	21st to end	Night	29% 7% 16% 27% 19% 1	600
May	1st to 10th	Night	24% 8% 19% 29% 20% 1	600
	11th to 20th	Night	26% 6% 15% 26% 24% 3	435
	21st to end	Night	28% 8% 16% 28% 17% 3	433
June	1st to 10th	Night	23% 6% 21% 31% 18% 2	400
	11th to 20th	Night	18% 6% 27% 35% 14% 2	400
	21st to end	Night	15% 6% 17% 38% 22% 3	400
July	1st to 10th	Night	15% 3 14% 40% 26% 1	390
	11th to 20th	Night	14% 5% 22% 37% 20% 2	398
	21st to end	Night	17% 6% 14% 35% 25% 3	657
August	1st to 10th	Night	15% 10% 10% 27% 30% 7%	597
	11th to 20th	Night	11% 7% 8% 30% 39% 6%	594
	21st to end	Night	11% 5% 16% 36% 26% 5%	765
September	1st to 10th	Night	7% 5% 17% 38% 30% 4	800
	11th to 20th	Night	10% 7% 14% 31% 29% 9%	800
	21st to end	Night	15% 6% 19% 30% 24% 6%	800
October	1st to 10th	Night	14% 8% 22% 28% 23% 6%	800
	11th to 20th	Night	18% 8% 18% 28% 23% 5%	800
	21st to end	Night	24% 10% 21% 25% 14% 4	880
November	1st to 10th	Night	29% 11% 18% 27% 14% 2	820
	11th to 20th	Night	34% 10% 15% 25% 13% 2	828
	21st to end	Night	25% 10% 17% 28% 18% 3	800
December	1st to 10th	Night	27% 9% 16% 27% 17% 4	800
	11th to 20th	Night	36% 10% 17% 21% 14% 2	800
	21st to end	Night	41% 12% 15% 21% 10% 1	880

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