Land-o-Clouds Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Land-o-Clouds Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	12% 9% 3 8% 5% 4 4 4 6% 7% 37%	101663
	Night	13% 10% 4 8% 5% 4 4 4 5% 6% 37%	98029
Transparency	Day	44% 6% 12% 24% 13% 1	105195
	Night	45% 6% 11% 23% 14% 2	102638
Seeing	Day	16% 11% 30% 26% 13% 4	16020
	Night	17% 10% 32% 29% 11% 1	25903

By Month

Percentage Cloud Cover SE (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	11% 14% 7% 13% 6% 5% 4 4 5% 5% 26%	2527
	11th to 20th	Day	11% 13% 6% 14% 6% 5% 8% 8% 6% 6% 18%	2530
	21st to end	Day	17% 17% 7% 14% 5% 4 3 3 3 4 21%	2783
February	1st to 10th	Day	10% 14% 6% 9% 5% 5% 5% 4 5% 6% 30%	1704
	11th to 20th	Day	12% 12% 4 7% 4 5% 5% 3 5% 6% 37%	1819
	21st to end	Day	12% 15% 5% 9% 5% 4 5% 4 5% 6% 31%	1570
March	1st to 10th	Day	12% 9% 3 6% 4 4 4 3 5% 6% 45%	3120
	11th to 20th	Day	11% 8% 3 9% 5% 4 4 3 5% 6% 43%	3273
	21st to end	Day	20% 13% 4 10% 5% 4 4 2 4 5% 29%	4165
April	1st to 10th	Day	15% 13% 4 7% 5% 4 4 4 6% 6% 31%	2280
	11th to 20th	Day	18% 11% 3 6% 5% 4 5% 4 5% 5% 35%	2254
	21st to end	Day	18% 9% 3 7% 6% 4 4 4 5% 6% 33%	2416
May	1st to 10th	Day	13% 8% 3 7% 4 5% 5% 4 6% 7% 40%	4164
	11th to 20th	Day	12% 5% 2 7% 4 4 4 4 5% 8% 46%	4399
	21st to end	Day	12% 7% 2 7% 5% 4 4 4 6% 8% 40%	4824
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 9% 44%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 8% 42%	2660
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 10% 43%	2660
July	1st to 10th	Day	12% 7% 3 7% 4 5% 5% 5% 6% 10% 37%	4440
	11th to 20th	Day	5% 3 2 4 4 5% 5% 5% 8% 12% 49%	4440
	21st to end	Day	7% 6% 2 6% 5% 5% 5% 4 6% 8% 46%	4813
August	1st to 10th	Day	9% 5% 3 7% 4 5% 5% 5% 7% 9% 41%	4423
	11th to 20th	Day	12% 6% 2 5% 3 4 3 4 8% 9% 44%	3864
	21st to end	Day	10% 7% 3 7% 4 5% 5% 4 5% 8% 42%	4180
September	1st to 10th	Day	12% 6% 3 6% 5% 4 4 3 6% 7% 45%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 3 3 5% 8% 49%	2176
	21st to end	Day	13% 8% 3 6% 4 4 4 3 6% 8% 41%	2090
October	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 5% 7% 44%	2001
	11th to 20th	Day	20% 11% 3 8% 4 4 5% 4 5% 6% 30%	1867
	21st to end	Day	16% 12% 4 8% 4 4 5% 2 4 6% 34%	1881
November	1st to 10th	Day	14% 9% 3 8% 4 4 3 4 5% 7% 40%	1762
	11th to 20th	Day	11% 14% 5% 12% 7% 6% 5% 4 5% 6% 26%	1800
	21st to end	Day	15% 13% 4 9% 6% 4 3 3 4 6% 32%	1617
December	1st to 10th	Day	13% 12% 6% 12% 8% 7% 4 4 6% 6% 23%	1520
	11th to 20th	Day	12% 14% 5% 12% 7% 6% 6% 4 5% 6% 24%	1918
	21st to end	Day	17% 19% 7% 14% 7% 5% 4 3 4 4 15%	2783
January	1st to 10th	Night	12% 15% 7% 12% 6% 4 4 2 3 4 32%	4428
	11th to 20th	Night	12% 14% 6% 15% 7% 5% 6% 6% 5% 4 19%	4440
	21st to end	Night	14% 18% 7% 13% 5% 5% 3 3 4 4 24%	4884
February	1st to 10th	Night	10% 14% 5% 10% 6% 5% 4 3 4 5% 34%	2475
	11th to 20th	Night	9% 10% 4 10% 5% 5% 5% 4 4 5% 39%	2246
	21st to end	Night	16% 13% 3 7% 5% 4 4 3 5% 5% 36%	1884
March	1st to 10th	Night	15% 9% 2 6% 4 4 4 3 5% 6% 42%	3733
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 4 6% 7% 41%	3716
	21st to end	Night	19% 11% 4 10% 4 4 4 3 4 5% 30%	3466
April	1st to 10th	Night	18% 12% 3 6% 4 3 4 4 4 7% 34%	1900
	11th to 20th	Night	19% 10% 3 7% 4 4 3 3 5% 5% 37%	1875
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 37%	1762
May	1st to 10th	Night	14% 8% 2 7% 5% 4 4 4 5% 7% 39%	2732
	11th to 20th	Night	11% 6% 2 7% 5% 6% 5% 4 7% 7% 40%	2506
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 6% 40%	2743
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	1520
	11th to 20th	Night	10% 6% 3 6% 4 4 4 4 6% 9% 45%	1520
	21st to end	Night	8% 5% 2 7% 4 4 4 4 6% 8% 49%	1520
July	1st to 10th	Night	13% 7% 3 6% 4 4 4 3 4 8% 43%	2530
	11th to 20th	Night	4 3 2 4 4 4 4 5% 6% 10% 53%	2530
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 9% 50%	2750
August	1st to 10th	Night	10% 5% 2 5% 3 4 4 4 7% 9% 47%	2517
	11th to 20th	Night	11% 7% 3 6% 4 3 5% 3 5% 8% 46%	2961
	21st to end	Night	7% 6% 2 5% 4 4 5% 4 6% 8% 48%	3476
September	1st to 10th	Night	9% 6% 2 5% 4 3 4 3 5% 7% 52%	1900
	11th to 20th	Night	12% 4 2 5% 4 3 3 4 5% 6% 52%	1984
	21st to end	Night	12% 7% 3 7% 4 3 3 3 5% 6% 48%	2090
October	1st to 10th	Night	11% 7% 3 7% 4 4 3 3 6% 6% 47%	2175
	11th to 20th	Night	16% 10% 3 6% 4 4 4 3 5% 7% 38%	2313
	21st to end	Night	17% 12% 3 7% 3 4 4 4 6% 7% 33%	2717
November	1st to 10th	Night	13% 9% 3 6% 5% 3 4 3 5% 6% 44%	2536
	11th to 20th	Night	13% 14% 4 11% 7% 5% 4 3 5% 5% 30%	2600
	21st to end	Night	15% 12% 4 8% 6% 4 4 3 5% 6% 32%	2777
December	1st to 10th	Night	13% 13% 4 11% 6% 5% 6% 4 5% 6% 26%	2660
	11th to 20th	Night	12% 15% 6% 11% 6% 5% 5% 4 4 4 27%	3279
	21st to end	Night	17% 18% 6% 12% 6% 5% 4 3 4 5% 20%	4884

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	59% 6% 14% 15% 5%	2528
	11th to 20th	Day	64% 6% 9% 16% 5%	2530
	21st to end	Day	68% 4 8% 16% 4	2783
February	1st to 10th	Day	54% 5% 11% 20% 10% 1	1704
	11th to 20th	Day	47% 5% 11% 24% 11% 2	1829
	21st to end	Day	53% 6% 10% 19% 10% 3	1570
March	1st to 10th	Day	40% 5% 10% 27% 16% 3	3120
	11th to 20th	Day	41% 4 12% 25% 16% 2	3273
	21st to end	Day	58% 4 9% 20% 7% 1	4165
April	1st to 10th	Day	53% 6% 12% 19% 10%	2280
	11th to 20th	Day	50% 6% 9% 20% 13% 2	2254
	21st to end	Day	50% 7% 11% 20% 11% 1	2416
May	1st to 10th	Day	42% 6% 11% 23% 17% 1	4202
	11th to 20th	Day	36% 6% 11% 32% 14% 1	4403
	21st to end	Day	40% 7% 11% 22% 17% 2	4837
June	1st to 10th	Day	36% 7% 12% 28% 16% 1	2660
	11th to 20th	Day	39% 7% 13% 25% 16% 1	2660
	21st to end	Day	35% 8% 13% 27% 15% 1	2660
July	1st to 10th	Day	41% 8% 13% 26% 10% 2	4371
	11th to 20th	Day	25% 8% 16% 34% 16% 1	4365
	21st to end	Day	33% 9% 13% 21% 21% 2	4662
August	1st to 10th	Day	36% 8% 15% 28% 12% 1	4408
	11th to 20th	Day	35% 7% 12% 35% 10% 1	3903
	21st to end	Day	39% 8% 12% 24% 17% 1	4179
September	1st to 10th	Day	37% 7% 12% 27% 16% 1	2280
	11th to 20th	Day	35% 5% 14% 32% 14% 1	2176
	21st to end	Day	41% 5% 12% 30% 12% 1	2090
October	1st to 10th	Day	24% 4 7% 38% 26% 2	3326
	11th to 20th	Day	42% 6% 10% 24% 18% 1	3083
	21st to end	Day	44% 5% 12% 26% 12% 1	3081
November	1st to 10th	Day	44% 5% 13% 24% 13% 1	1762
	11th to 20th	Day	56% 7% 13% 17% 7% 1	1800
	21st to end	Day	54% 6% 14% 20% 6%	1617
December	1st to 10th	Day	59% 10% 11% 14% 6% 1	1520
	11th to 20th	Day	59% 6% 14% 14% 5% 2	1915
	21st to end	Day	73% 5% 7% 10% 5%	2783
January	1st to 10th	Night	58% 4 9% 20% 9% 1	4428
	11th to 20th	Night	66% 5% 8% 13% 7%	4440
	21st to end	Night	65% 5% 9% 16% 5%	4884
February	1st to 10th	Night	53% 4 9% 20% 12% 2	2475
	11th to 20th	Night	47% 5% 10% 24% 13% 2	2247
	21st to end	Night	51% 4 12% 19% 11% 2	1884
March	1st to 10th	Night	43% 5% 12% 26% 11% 3	3733
	11th to 20th	Night	39% 5% 13% 24% 17% 2	3716
	21st to end	Night	56% 5% 8% 19% 10% 2	3466
April	1st to 10th	Night	50% 5% 10% 17% 17% 1	1900
	11th to 20th	Night	49% 4 9% 20% 16% 3	1875
	21st to end	Night	47% 5% 11% 22% 14% 1	1762
May	1st to 10th	Night	44% 6% 13% 19% 17% 2	2759
	11th to 20th	Night	39% 8% 10% 28% 14% 1	2507
	21st to end	Night	44% 6% 10% 18% 16% 6%	2751
June	1st to 10th	Night	33% 8% 12% 29% 17% 1	1520
	11th to 20th	Night	34% 7% 11% 28% 18% 2	1520
	21st to end	Night	33% 8% 16% 25% 16% 3	1520
July	1st to 10th	Night	40% 6% 14% 22% 13% 5%	2490
	11th to 20th	Night	23% 8% 17% 31% 18% 2	2482
	21st to end	Night	29% 8% 15% 23% 22% 3	2662
August	1st to 10th	Night	32% 8% 14% 27% 18% 2	2508
	11th to 20th	Night	35% 7% 11% 31% 15%	2988
	21st to end	Night	31% 8% 14% 24% 21% 3	3476
September	1st to 10th	Night	32% 6% 11% 25% 24% 2	1900
	11th to 20th	Night	32% 5% 10% 33% 17% 2	1984
	21st to end	Night	37% 6% 10% 30% 17% 1	2090
October	1st to 10th	Night	27% 4 8% 37% 22% 2	3630
	11th to 20th	Night	34% 5% 11% 30% 17% 3	3836
	21st to end	Night	44% 5% 10% 24% 16%	4467
November	1st to 10th	Night	41% 5% 11% 25% 16% 2	2536
	11th to 20th	Night	55% 5% 11% 17% 10% 1	2600
	21st to end	Night	52% 6% 15% 21% 6% 1	2777
December	1st to 10th	Night	56% 9% 11% 17% 7% 1	2660
	11th to 20th	Night	59% 5% 11% 16% 8% 1	3281
	21st to end	Night	66% 6% 9% 13% 5%	4884

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	16% 15% 34% 27% 7% 1	270
	11th to 20th	Day	18% 13% 37% 24% 7%	270
	21st to end	Day	21% 12% 36% 25% 6% 1	297
February	1st to 10th	Day	13% 19% 41% 19% 9% 1	270
	11th to 20th	Day	18% 22% 36% 19% 6%	270
	21st to end	Day	19% 20% 44% 15% 1	225
March	1st to 10th	Day	18% 18% 43% 17% 4	270
	11th to 20th	Day	20% 19% 31% 22% 4 4	284
	21st to end	Day	26% 13% 29% 24% 7% 1	506
April	1st to 10th	Day	24% 12% 37% 21% 5%	460
	11th to 20th	Day	25% 17% 27% 19% 9% 3	452
	21st to end	Day	22% 13% 28% 24% 12% 2	460
May	1st to 10th	Day	20% 12% 31% 26% 9% 2	506
	11th to 20th	Day	16% 11% 32% 25% 12% 4	675
	21st to end	Day	15% 9% 29% 26% 16% 5%	734
June	1st to 10th	Day	11% 7% 31% 32% 15% 4	680
	11th to 20th	Day	13% 9% 31% 27% 15% 4	680
	21st to end	Day	13% 10% 27% 30% 16% 4	680
July	1st to 10th	Day	8% 5% 21% 31% 26% 10%	671
	11th to 20th	Day	9% 6% 24% 33% 20% 8%	668
	21st to end	Day	7% 4 27% 33% 21% 8%	739
August	1st to 10th	Day	8% 4 23% 34% 22% 9%	676
	11th to 20th	Day	14% 4 16% 28% 25% 12%	501
	21st to end	Day	12% 8% 20% 28% 22% 10%	539
September	1st to 10th	Day	14% 6% 22% 29% 21% 9%	490
	11th to 20th	Day	11% 4 21% 35% 20% 9%	490
	21st to end	Day	16% 11% 26% 25% 19% 4	490
October	1st to 10th	Day	17% 12% 24% 26% 15% 5%	401
	11th to 20th	Day	21% 18% 31% 22% 7% 1	300
	21st to end	Day	25% 12% 39% 17% 5% 2	330
November	1st to 10th	Day	18% 15% 36% 21% 9% 1	300
	11th to 20th	Day	16% 18% 40% 21% 5%	300
	21st to end	Day	23% 16% 36% 19% 4	299
December	1st to 10th	Day	20% 20% 40% 15% 5%	270
	11th to 20th	Day	22% 16% 39% 16% 6% 1	270
	21st to end	Day	23% 21% 36% 16% 4	297
January	1st to 10th	Night	17% 13% 38% 24% 7%	950
	11th to 20th	Night	19% 13% 38% 22% 6% 1	950
	21st to end	Night	19% 11% 38% 26% 6%	1045
February	1st to 10th	Night	19% 13% 37% 24% 6%	950
	11th to 20th	Night	15% 15% 39% 25% 6%	771
	21st to end	Night	22% 13% 37% 22% 5%	628
March	1st to 10th	Night	21% 11% 34% 24% 8% 1	760
	11th to 20th	Night	16% 12% 36% 28% 7% 1	729
	21st to end	Night	23% 7% 28% 32% 9% 1	627
April	1st to 10th	Night	22% 10% 37% 25% 5%	570
	11th to 20th	Night	23% 11% 31% 28% 6% 1	563
	21st to end	Night	20% 9% 32% 30% 8% 1	570
May	1st to 10th	Night	19% 9% 33% 31% 7% 1	570
	11th to 20th	Night	17% 8% 35% 30% 9% 1	561
	21st to end	Night	12% 6% 27% 38% 15% 1	617
June	1st to 10th	Night	12% 5% 28% 38% 17% 1	570
	11th to 20th	Night	13% 5% 33% 36% 12% 1	570
	21st to end	Night	10% 4 27% 40% 18% 1	570
July	1st to 10th	Night	9% 3 18% 41% 27% 2	555
	11th to 20th	Night	7% 3 24% 41% 23% 2	552
	21st to end	Night	7% 2 20% 44% 25% 1	624
August	1st to 10th	Night	9% 1 17% 46% 25% 3	567
	11th to 20th	Night	10% 3 16% 41% 27% 3	564
	21st to end	Night	10% 3 20% 43% 22% 3	627
September	1st to 10th	Night	11% 4 23% 35% 22% 5%	570
	11th to 20th	Night	13% 5% 20% 35% 22% 5%	654
	21st to end	Night	15% 7% 26% 30% 19% 3	760
October	1st to 10th	Night	14% 8% 29% 30% 17% 2	760
	11th to 20th	Night	21% 14% 32% 25% 8%	760
	21st to end	Night	23% 12% 29% 26% 9% 1	836
November	1st to 10th	Night	17% 13% 34% 26% 9% 1	780
	11th to 20th	Night	20% 17% 36% 22% 6%	800
	21st to end	Night	21% 15% 37% 21% 6%	978
December	1st to 10th	Night	19% 16% 38% 20% 6%	950
	11th to 20th	Night	21% 15% 37% 21% 6% 1	950
	21st to end	Night	24% 14% 37% 22% 3	1045

Page generated at 2024-11-27 16:30:52. Version ac07

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