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 2024-11-10.

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	16% 10% 3 7% 4 4 4 3 5% 7% 38%	105364
	Night	18% 12% 4 8% 5% 4 4 3 4 6% 33%	102505
Transparency	Day	45% 7% 12% 19% 13% 5%	105119
	Night	51% 7% 11% 16% 10% 5%	102339
Seeing	Day	22% 13% 30% 24% 9% 2	14990
	Night	26% 8% 17% 27% 19% 3	23942

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	23% 19% 5% 11% 6% 5% 4 3 4 5% 16%	2539
	11th to 20th	Day	35% 13% 3 7% 4 4 4 2 3 4 20%	2800
	21st to end	Day	29% 11% 3 8% 5% 5% 5% 3 5% 6% 18%	3146
February	1st to 10th	Day	23% 13% 4 9% 7% 4 5% 4 4 6% 21%	1710
	11th to 20th	Day	22% 14% 5% 9% 6% 4 4 3 4 5% 24%	1665
	21st to end	Day	25% 16% 4 9% 6% 4 4 2 4 4 21%	1472
March	1st to 10th	Day	21% 15% 3 8% 5% 5% 5% 3 5% 6% 23%	3442
	11th to 20th	Day	18% 9% 3 8% 4 4 4 3 5% 5% 35%	3490
	21st to end	Day	24% 13% 4 10% 6% 7% 6% 5% 6% 5% 14%	3825
April	1st to 10th	Day	26% 14% 4 10% 7% 5% 4 3 5% 5% 19%	2245
	11th to 20th	Day	22% 13% 4 9% 6% 5% 4 3 5% 6% 23%	2280
	21st to end	Day	21% 9% 3 8% 5% 5% 6% 4 6% 7% 26%	2470
May	1st to 10th	Day	18% 8% 3 7% 5% 4 5% 3 5% 6% 36%	4110
	11th to 20th	Day	11% 8% 2 5% 3 3 3 2 4 7% 51%	4311
	21st to end	Day	14% 8% 3 6% 4 4 5% 4 6% 10% 36%	4813
June	1st to 10th	Day	20% 10% 3 7% 5% 5% 4 3 5% 7% 31%	2660
	11th to 20th	Day	15% 10% 4 7% 5% 4 4 3 5% 7% 36%	2660
	21st to end	Day	10% 8% 2 6% 3 3 3 3 5% 6% 50%	2660
July	1st to 10th	Day	4 6% 1 3 2 2 2 1 3 6% 70%	4420
	11th to 20th	Day	4 6% 2 4 3 3 3 2 3 6% 61%	4396
	21st to end	Day	5% 10% 2 5% 3 3 2 2 4 6% 57%	4455
August	1st to 10th	Day	4 9% 3 6% 3 3 4 3 6% 9% 50%	4091
	11th to 20th	Day	6% 8% 2 5% 3 3 3 3 5% 8% 54%	4026
	21st to end	Day	8% 6% 2 4 3 3 3 2 4 6% 59%	4349
September	1st to 10th	Day	4 4 2 5% 3 3 3 3 4 7% 63%	2280
	11th to 20th	Day	9% 7% 3 6% 4 4 4 4 6% 8% 46%	2166
	21st to end	Day	8% 5% 2 6% 4 3 4 3 5% 8% 51%	2066
October	1st to 10th	Day	11% 6% 2 7% 4 3 3 3 4 6% 52%	3180
	11th to 20th	Day	17% 8% 2 7% 4 4 5% 4 6% 9% 34%	3161
	21st to end	Day	18% 8% 3 8% 6% 5% 6% 4 6% 8% 29%	3448
November	1st to 10th	Day	24% 10% 3 9% 6% 4 5% 5% 6% 6% 23%	1746
	11th to 20th	Day	22% 11% 5% 9% 5% 6% 5% 4 5% 6% 22%	1520
	21st to end	Day	21% 11% 3 7% 6% 5% 4 4 5% 6% 30%	1520
December	1st to 10th	Day	19% 10% 3 8% 6% 4 5% 5% 5% 7% 30%	1520
	11th to 20th	Day	23% 12% 3 10% 5% 5% 5% 5% 7% 6% 19%	1928
	21st to end	Day	30% 14% 4 9% 5% 5% 4 3 4 6% 15%	2794
January	1st to 10th	Night	32% 16% 4 8% 6% 5% 5% 3 4 3 14%	4406
	11th to 20th	Night	31% 17% 4 9% 4 3 3 2 3 4 20%	4164
	21st to end	Night	35% 13% 4 8% 5% 4 3 3 4 5% 17%	4521
February	1st to 10th	Night	22% 14% 4 8% 6% 5% 5% 4 5% 6% 20%	2470
	11th to 20th	Night	23% 13% 3 9% 5% 5% 4 4 4 4 26%	2402
	21st to end	Night	24% 15% 4 10% 5% 4 3 4 5% 5% 22%	1979
March	1st to 10th	Night	21% 16% 3 9% 6% 5% 4 3 4 5% 23%	3432
	11th to 20th	Night	15% 11% 4 10% 6% 4 4 3 4 5% 36%	3490
	21st to end	Night	25% 16% 5% 10% 6% 5% 4 3 4 5% 18%	3828
April	1st to 10th	Night	24% 13% 5% 8% 5% 4 4 4 5% 6% 23%	1933
	11th to 20th	Night	20% 14% 3 7% 5% 4 4 4 5% 5% 29%	1843
	21st to end	Night	21% 11% 4 7% 5% 5% 5% 4 6% 7% 26%	1710
May	1st to 10th	Night	18% 12% 3 8% 4 4 3 3 4 5% 37%	2860
	11th to 20th	Night	11% 12% 3 10% 4 2 3 3 4 5% 43%	2581
	21st to end	Night	19% 13% 3 5% 3 3 2 3 4 6% 39%	2762
June	1st to 10th	Night	18% 12% 2 6% 5% 4 5% 3 5% 7% 34%	1520
	11th to 20th	Night	12% 10% 3 6% 4 4 4 3 6% 8% 41%	1520
	21st to end	Night	8% 11% 2 6% 3 3 3 3 4 7% 50%	1520
July	1st to 10th	Night	4 8% 2 3 2 2 2 1 3 5% 68%	2540
	11th to 20th	Night	4 11% 4 8% 4 3 3 2 3 7% 52%	2562
	21st to end	Night	8% 19% 2 6% 3 2 2 2 4 5% 45%	3109
August	1st to 10th	Night	4 23% 6% 11% 4 3 3 2 4 6% 33%	2848
	11th to 20th	Night	7% 17% 3 7% 4 3 3 2 4 6% 43%	2812
	21st to end	Night	7% 7% 2 4 3 3 2 2 4 5% 62%	3324
September	1st to 10th	Night	5% 5% 2 4 3 3 3 3 5% 6% 61%	1900
	11th to 20th	Night	8% 7% 2 5% 4 3 3 3 5% 7% 53%	2009
	21st to end	Night	10% 7% 2 6% 3 3 4 3 5% 7% 48%	2114
October	1st to 10th	Night	8% 9% 2 6% 3 3 3 2 4 5% 54%	3800
	11th to 20th	Night	12% 9% 4 9% 5% 4 4 4 5% 8% 35%	3777
	21st to end	Night	23% 10% 3 9% 5% 5% 4 4 5% 6% 27%	4121
November	1st to 10th	Night	21% 10% 4 8% 5% 5% 5% 3 5% 7% 27%	2530
	11th to 20th	Night	23% 10% 3 9% 7% 6% 4 4 5% 6% 23%	2660
	21st to end	Night	19% 9% 3 9% 5% 6% 4 4 5% 6% 31%	2660
December	1st to 10th	Night	18% 10% 4 8% 6% 5% 5% 4 5% 6% 29%	2660
	11th to 20th	Night	26% 12% 4 9% 5% 4 4 4 5% 6% 21%	3276
	21st to end	Night	25% 12% 5% 10% 6% 5% 4 3 4 5% 20%	4862

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	72% 7% 9% 8% 3 1	2539
	11th to 20th	Day	69% 5% 5% 9% 6% 5%	2800
	21st to end	Day	67% 8% 7% 8% 7% 3	3146
February	1st to 10th	Day	64% 6% 10% 12% 5% 2	1710
	11th to 20th	Day	65% 6% 9% 13% 6% 2	1674
	21st to end	Day	66% 8% 7% 12% 6% 2	1472
March	1st to 10th	Day	62% 8% 9% 17% 4	3442
	11th to 20th	Day	49% 7% 11% 17% 14% 2	3490
	21st to end	Day	70% 10% 8% 10% 3	3825
April	1st to 10th	Day	67% 8% 9% 10% 5%	2245
	11th to 20th	Day	62% 7% 10% 12% 7% 2	2283
	21st to end	Day	56% 8% 13% 15% 7% 1	2470
May	1st to 10th	Day	46% 11% 13% 18% 10% 2	4110
	11th to 20th	Day	30% 7% 16% 30% 15% 2	4314
	21st to end	Day	40% 10% 28% 20% 2	4814
June	1st to 10th	Day	52% 8% 13% 21% 5%	2660
	11th to 20th	Day	45% 8% 18% 22% 7%	2660
	21st to end	Day	32% 5% 16% 30% 14% 3	2660
July	1st to 10th	Day	16% 3 8% 17% 21% 35%	4351
	11th to 20th	Day	20% 4 8% 20% 30% 19%	4321
	21st to end	Day	23% 4 12% 26% 29% 6%	4315
August	1st to 10th	Day	23% 6% 15% 27% 23% 6%	4076
	11th to 20th	Day	24% 5% 19% 32% 16% 2	4065
	21st to end	Day	25% 6% 15% 26% 26% 3	4348
September	1st to 10th	Day	20% 5% 9% 31% 23% 12%	2280
	11th to 20th	Day	34% 7% 15% 24% 16% 4	2166
	21st to end	Day	31% 7% 13% 25% 18% 6%	2066
October	1st to 10th	Day	34% 4 8% 17% 19% 18%	3180
	11th to 20th	Day	44% 8% 17% 19% 10% 2	3161
	21st to end	Day	50% 7% 12% 16% 11% 3	3448
November	1st to 10th	Day	61% 9% 8% 11% 10% 1	1746
	11th to 20th	Day	62% 7% 9% 8% 8% 5%	1520
	21st to end	Day	56% 5% 10% 9% 11% 9%	1520
December	1st to 10th	Day	54% 7% 11% 12% 12% 3	1520
	11th to 20th	Day	65% 8% 11% 10% 5% 1	1928
	21st to end	Day	71% 7% 7% 9% 4 2	2794
January	1st to 10th	Night	75% 7% 7% 6% 3 2	4406
	11th to 20th	Night	70% 4 7% 9% 5% 5%	4164
	21st to end	Night	72% 5% 6% 7% 8% 3	4521
February	1st to 10th	Night	63% 7% 10% 11% 6% 2	2470
	11th to 20th	Night	62% 6% 8% 16% 6% 2	2404
	21st to end	Night	65% 8% 8% 10% 7% 2	1979
March	1st to 10th	Night	65% 6% 10% 16% 4	3432
	11th to 20th	Night	49% 8% 13% 14% 12% 4	3490
	21st to end	Night	69% 9% 11% 9% 2	3828
April	1st to 10th	Night	63% 8% 11% 13% 5% 1	1933
	11th to 20th	Night	57% 9% 12% 14% 5% 3	1843
	21st to end	Night	57% 8% 11% 17% 6% 1	1710
May	1st to 10th	Night	49% 10% 11% 19% 9% 3	2860
	11th to 20th	Night	35% 8% 15% 28% 10% 5%	2581
	21st to end	Night	47% 10% 27% 15% 1	2762
June	1st to 10th	Night	49% 6% 17% 23% 3	1520
	11th to 20th	Night	40% 7% 20% 25% 7% 1	1520
	21st to end	Night	34% 4 17% 32% 10% 4	1520
July	1st to 10th	Night	19% 4 8% 17% 20% 32%	2500
	11th to 20th	Night	23% 7% 12% 18% 24% 16%	2514
	21st to end	Night	36% 3 10% 22% 24% 5%	3011
August	1st to 10th	Night	37% 14% 13% 19% 13% 3	2839
	11th to 20th	Night	37% 6% 15% 25% 14% 3	2839
	21st to end	Night	23% 5% 12% 31% 25% 4	3324
September	1st to 10th	Night	21% 4 10% 28% 23% 13%	1900
	11th to 20th	Night	29% 7% 17% 26% 18% 4	2009
	21st to end	Night	34% 7% 13% 21% 16% 10%	2114
October	1st to 10th	Night	32% 5% 9% 16% 22% 16%	3800
	11th to 20th	Night	45% 6% 18% 20% 9% 2	3777
	21st to end	Night	57% 6% 9% 14% 9% 5%	4121
November	1st to 10th	Night	57% 7% 11% 12% 10% 2	2530
	11th to 20th	Night	64% 6% 9% 10% 7% 5%	2660
	21st to end	Night	53% 7% 9% 11% 11% 9%	2660
December	1st to 10th	Night	56% 7% 8% 14% 13% 3	2660
	11th to 20th	Night	65% 6% 9% 10% 8% 2	3276
	21st to end	Night	67% 7% 8% 13% 4 2	4862

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	41% 17% 19% 12% 7% 4	180
	11th to 20th	Day	41% 11% 16% 16% 11% 5%	336
	21st to end	Day	34% 13% 21% 19% 10% 4	407
February	1st to 10th	Day	33% 11% 25% 21% 9% 1	370
	11th to 20th	Day	39% 13% 27% 16% 3 2	365
	21st to end	Day	34% 17% 27% 19% 3 1	323
March	1st to 10th	Day	39% 14% 24% 17% 6%	460
	11th to 20th	Day	30% 12% 28% 20% 8% 2	460
	21st to end	Day	33% 19% 24% 18% 5% 1	506
April	1st to 10th	Day	33% 18% 25% 17% 5% 2	460
	11th to 20th	Day	29% 20% 28% 16% 4 3	452
	21st to end	Day	27% 18% 32% 16% 7% 1	460
May	1st to 10th	Day	22% 18% 33% 19% 6% 1	463
	11th to 20th	Day	30% 12% 27% 22% 8% 2	488
	21st to end	Day	21% 14% 36% 23% 6%	529
June	1st to 10th	Day	19% 13% 35% 24% 8% 1	490
	11th to 20th	Day	16% 22% 40% 18% 3 1	490
	21st to end	Day	10% 11% 41% 31% 5% 1	490
July	1st to 10th	Day	9% 9% 44% 30% 8%	486
	11th to 20th	Day	6% 8% 46% 32% 6% 1	484
	21st to end	Day	8% 6% 43% 35% 7% 2	531
August	1st to 10th	Day	7% 9% 32% 36% 14% 3	487
	11th to 20th	Day	6% 7% 33% 40% 12% 2	477
	21st to end	Day	9% 8% 37% 33% 10% 4	539
September	1st to 10th	Day	5% 8% 34% 33% 17% 3	490
	11th to 20th	Day	15% 11% 28% 31% 13% 2	490
	21st to end	Day	13% 13% 32% 26% 12% 3	490
October	1st to 10th	Day	18% 12% 24% 28% 13% 4	490
	11th to 20th	Day	20% 12% 26% 22% 14% 5%	490
	21st to end	Day	27% 16% 23% 23% 9% 3	539
November	1st to 10th	Day	29% 14% 26% 19% 9% 2	350
	11th to 20th	Day	28% 12% 21% 22% 12% 5%	180
	21st to end	Day	34% 11% 16% 17% 18% 4	180
December	1st to 10th	Day	34% 7% 16% 21% 14% 7%	180
	11th to 20th	Day	40% 12% 14% 22% 9% 3	180
	21st to end	Day	40% 12% 16% 21% 11% 1	198
January	1st to 10th	Night	39% 11% 17% 19% 11% 3	760
	11th to 20th	Night	35% 11% 16% 18% 16% 5%	760
	21st to end	Night	30% 8% 15% 24% 18% 4	836
February	1st to 10th	Night	32% 8% 15% 25% 18% 2	760
	11th to 20th	Night	31% 10% 19% 25% 13% 2	744
	21st to end	Night	35% 10% 19% 24% 12%	628
March	1st to 10th	Night	38% 9% 18% 23% 12% 1	760
	11th to 20th	Night	27% 10% 19% 27% 15% 2	760
	21st to end	Night	37% 8% 20% 21% 12% 1	836
April	1st to 10th	Night	35% 9% 17% 21% 16% 2	760
	11th to 20th	Night	31% 9% 21% 24% 14% 2	715
	21st to end	Night	29% 7% 16% 28% 19% 1	570
May	1st to 10th	Night	25% 8% 19% 29% 19% 1	570
	11th to 20th	Night	25% 6% 15% 27% 25% 3	413
	21st to end	Night	29% 8% 16% 28% 17% 3	411
June	1st to 10th	Night	24% 5% 21% 31% 17% 2	380
	11th to 20th	Night	18% 5% 26% 36% 13% 2	380
	21st to end	Night	16% 6% 17% 37% 21% 3	380
July	1st to 10th	Night	15% 2 14% 41% 26% 1	370
	11th to 20th	Night	15% 5% 23% 36% 20% 2	377
	21st to end	Night	17% 5% 14% 36% 25% 3	624
August	1st to 10th	Night	15% 10% 10% 27% 31% 7%	567
	11th to 20th	Night	10% 7% 8% 30% 38% 7%	564
	21st to end	Night	12% 5% 17% 35% 26% 6%	727
September	1st to 10th	Night	7% 4 16% 39% 31% 4	760
	11th to 20th	Night	10% 7% 14% 30% 30% 9%	760
	21st to end	Night	15% 6% 19% 30% 23% 6%	760
October	1st to 10th	Night	14% 7% 22% 28% 24% 6%	760
	11th to 20th	Night	18% 8% 17% 28% 23% 6%	760
	21st to end	Night	25% 10% 20% 25% 14% 5%	836
November	1st to 10th	Night	29% 10% 18% 27% 15% 2	778
	11th to 20th	Night	33% 9% 15% 27% 14% 2	760
	21st to end	Night	24% 10% 17% 28% 18% 3	760
December	1st to 10th	Night	28% 9% 16% 27% 16% 4	760
	11th to 20th	Night	35% 10% 17% 21% 14% 2	760
	21st to end	Night	40% 12% 15% 22% 10% 1	836

Page generated at 2024-11-07 03:55:47. Version ac07

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