Orient Land Trust
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Orient Land Trust clear sky chart from 2005-11-06 to 2024-12-02.

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	3 6% 2 7% 5% 5% 5% 4 6% 8% 50%	105351
	Night	3 5% 2 6% 4 4 4 4 6% 7% 54%	103012
Transparency	Day	28% 8% 12% 23% 23% 7%	105103
	Night	25% 7% 13% 26% 23% 6%	102850
Seeing	Day	11% 19% 30% 25% 12% 3	15508
	Night	6% 11% 31% 33% 16% 2	25603

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	3 2 7% 5% 5% 4 4 6% 8% 55%	2538
	11th to 20th	Day	1 4 3 10% 6% 4 5% 3 6% 7% 51%	2828
	21st to end	Day	1 6% 4 9% 5% 5% 4 3 6% 7% 50%	3146
February	1st to 10th	Day	7% 3 8% 4 3 4 4 6% 7% 54%	1710
	11th to 20th	Day	1 4 3 7% 5% 3 4 4 6% 7% 56%	1817
	21st to end	Day	1 6% 4 8% 5% 5% 4 3 5% 7% 52%	1570
March	1st to 10th	Day	1 6% 3 7% 5% 4 4 3 6% 9% 53%	3211
	11th to 20th	Day	2 11% 4 7% 5% 5% 6% 5% 6% 8% 42%	3490
	21st to end	Day	1 8% 3 8% 5% 6% 4 4 6% 8% 46%	3996
April	1st to 10th	Day	2 8% 3 7% 3 5% 4 4 5% 7% 51%	2280
	11th to 20th	Day	3 12% 3 7% 4 4 5% 3 6% 7% 47%	2251
	21st to end	Day	4 10% 3 6% 5% 5% 5% 4 6% 8% 42%	2280
May	1st to 10th	Day	6% 12% 4 8% 4 4 4 4 6% 6% 43%	3819
	11th to 20th	Day	4 8% 2 7% 6% 5% 6% 5% 7% 8% 40%	4357
	21st to end	Day	3 5% 3 6% 4 5% 6% 5% 7% 7% 50%	4825
June	1st to 10th	Day	3 5% 3 4 5% 5% 6% 6% 8% 9% 46%	2660
	11th to 20th	Day	4 4 1 4 4 4 5% 4 7% 7% 56%	2660
	21st to end	Day	3 4 2 6% 5% 5% 6% 6% 8% 8% 46%	2660
July	1st to 10th	Day	4 5% 2 5% 4 5% 6% 5% 6% 7% 53%	4430
	11th to 20th	Day	3 4 2 7% 7% 8% 7% 6% 8% 9% 39%	4430
	21st to end	Day	4 5% 2 7% 6% 6% 6% 6% 8% 8% 42%	4459
August	1st to 10th	Day	6% 7% 3 9% 7% 6% 7% 6% 8% 10% 31%	4092
	11th to 20th	Day	4 4 2 6% 5% 6% 7% 6% 9% 8% 43%	3727
	21st to end	Day	5% 6% 3 8% 6% 6% 7% 6% 8% 9% 35%	4180
September	1st to 10th	Day	5% 5% 2 5% 6% 4 5% 3 6% 8% 50%	2280
	11th to 20th	Day	5% 5% 2 4 4 4 4 3 6% 6% 57%	2280
	21st to end	Day	5% 4 2 5% 3 2 4 3 4 5% 64%	2067
October	1st to 10th	Day	3 3 1 4 2 2 3 2 3 5% 71%	3170
	11th to 20th	Day	3 4 2 6% 4 4 5% 4 5% 8% 53%	3150
	21st to end	Day	2 4 1 5% 3 3 3 3 4 4 67%	3438
November	1st to 10th	Day	1 4 1 4 3 3 3 3 4 6% 67%	1782
	11th to 20th	Day	1 3 1 3 3 2 2 3 4 6% 71%	1800
	21st to end	Day	2 5% 2 7% 4 3 3 2 4 7% 62%	1720
December	1st to 10th	Day	1 5% 2 5% 3 3 4 3 5% 8% 61%	1528
	11th to 20th	Day	1 6% 2 5% 3 3 4 3 7% 12% 55%	1926
	21st to end	Day	1 4 3 9% 4 5% 5% 4 6% 7% 54%	2794
January	1st to 10th	Night	4 2 9% 5% 6% 5% 4 5% 7% 51%	4417
	11th to 20th	Night	1 3 3 10% 4 5% 4 3 5% 6% 56%	4170
	21st to end	Night	1 5% 3 8% 4 4 5% 3 5% 8% 54%	4521
February	1st to 10th	Night	1 6% 4 8% 4 3 3 3 5% 6% 59%	2470
	11th to 20th	Night	2 7% 3 6% 3 3 3 3 5% 7% 59%	2247
	21st to end	Night	1 7% 2 6% 4 4 3 3 6% 8% 57%	1884
March	1st to 10th	Night	1 8% 3 6% 4 3 3 3 5% 6% 57%	3654
	11th to 20th	Night	2 9% 3 6% 4 4 5% 5% 7% 8% 46%	3490
	21st to end	Night	1 8% 3 8% 6% 4 4 4 6% 7% 49%	3649
April	1st to 10th	Night	3 7% 2 5% 4 3 4 3 6% 7% 56%	1900
	11th to 20th	Night	4 10% 2 6% 5% 3 4 3 5% 6% 51%	1877
	21st to end	Night	6% 9% 2 5% 4 4 4 4 5% 7% 51%	1900
May	1st to 10th	Night	5% 9% 2 5% 4 3 4 3 5% 7% 53%	3148
	11th to 20th	Night	6% 8% 2 6% 4 4 5% 5% 6% 8% 46%	2531
	21st to end	Night	5% 4 2 5% 3 3 4 3 6% 7% 59%	2762
June	1st to 10th	Night	2 3 1 3 4 5% 5% 6% 7% 8% 56%	1520
	11th to 20th	Night	3 3 1 3 4 3 3 4 5% 7% 63%	1520
	21st to end	Night	3 4 1 4 4 4 7% 6% 7% 8% 51%	1520
July	1st to 10th	Night	5% 5% 2 5% 5% 4 5% 7% 6% 6% 51%	2540
	11th to 20th	Night	3 3 2 6% 5% 5% 6% 6% 9% 12% 43%	2540
	21st to end	Night	3 4 1 5% 4 6% 6% 6% 9% 11% 46%	3099
August	1st to 10th	Night	7% 6% 3 9% 6% 6% 8% 7% 9% 9% 30%	2847
	11th to 20th	Night	2 4 2 7% 6% 6% 7% 5% 9% 13% 40%	3109
	21st to end	Night	4 6% 3 8% 7% 7% 7% 6% 9% 8% 36%	3487
September	1st to 10th	Night	5% 5% 3 6% 4 4 4 5% 7% 7% 50%	1900
	11th to 20th	Night	5% 3 2 4 4 4 4 4 6% 6% 59%	1900
	21st to end	Night	5% 5% 1 4 4 3 3 3 4 7% 61%	2109
October	1st to 10th	Night	3 3 1 3 3 2 2 3 4 5% 70%	3800
	11th to 20th	Night	3 6% 2 7% 3 3 4 3 5% 7% 57%	3778
	21st to end	Night	4 5% 1 4 2 2 2 2 3 5% 71%	4120
November	1st to 10th	Night	1 3 1 3 3 3 3 3 4 7% 69%	2496
	11th to 20th	Night	1 3 2 3 2 2 2 2 4 7% 72%	2600
	21st to end	Night	3 6% 2 5% 3 3 3 3 5% 7% 62%	2676
December	1st to 10th	Night	1 5% 2 5% 4 3 3 3 5% 8% 61%	2687
	11th to 20th	Night	1 5% 2 6% 4 3 3 4 7% 10% 56%	3271
	21st to end	Night	1 5% 2 7% 5% 4 4 4 6% 8% 54%	4873

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	20% 9% 17% 33% 20% 2	2539
	11th to 20th	Day	19% 11% 25% 28% 16% 2	2828
	21st to end	Day	30% 7% 11% 31% 18% 3	3146
February	1st to 10th	Day	27% 6% 15% 27% 19% 5%	1710
	11th to 20th	Day	25% 6% 12% 28% 24% 4	1827
	21st to end	Day	29% 5% 8% 24% 26% 7%	1570
March	1st to 10th	Day	21% 8% 9% 27% 30% 6%	3211
	11th to 20th	Day	32% 10% 11% 32% 13% 2	3490
	21st to end	Day	27% 11% 13% 30% 16% 2	3996
April	1st to 10th	Day	30% 6% 10% 24% 21% 9%	2280
	11th to 20th	Day	35% 5% 8% 25% 22% 5%	2254
	21st to end	Day	34% 8% 10% 24% 22% 3	2280
May	1st to 10th	Day	32% 8% 11% 17% 21% 10%	3819
	11th to 20th	Day	33% 11% 12% 25% 15% 4	4361
	21st to end	Day	21% 10% 12% 18% 32% 7%	4825
June	1st to 10th	Day	28% 8% 14% 21% 21% 9%	2660
	11th to 20th	Day	23% 6% 9% 17% 26% 19%	2660
	21st to end	Day	29% 8% 10% 22% 20% 11%	2660
July	1st to 10th	Day	27% 7% 9% 15% 22% 20%	4361
	11th to 20th	Day	35% 9% 14% 25% 13% 4	4355
	21st to end	Day	33% 10% 15% 17% 16% 8%	4318
August	1st to 10th	Day	42% 11% 18% 15% 10% 3	4078
	11th to 20th	Day	33% 9% 13% 17% 20% 6%	3763
	21st to end	Day	41% 8% 11% 17% 19% 4	4179
September	1st to 10th	Day	30% 6% 11% 20% 23% 9%	2280
	11th to 20th	Day	24% 6% 11% 20% 28% 11%	2280
	21st to end	Day	22% 4 6% 26% 35% 7%	2067
October	1st to 10th	Day	17% 4 5% 20% 46% 9%	3170
	11th to 20th	Day	26% 8% 8% 28% 24% 6%	3150
	21st to end	Day	19% 5% 7% 26% 39% 5%	3438
November	1st to 10th	Day	18% 5% 9% 26% 36% 7%	1782
	11th to 20th	Day	14% 4 10% 36% 30% 6%	1800
	21st to end	Day	22% 4 8% 30% 27% 9%	1720
December	1st to 10th	Day	20% 6% 17% 31% 20% 6%	1528
	11th to 20th	Day	20% 8% 14% 31% 23% 3	1924
	21st to end	Day	23% 11% 12% 22% 23% 9%	2794
January	1st to 10th	Night	24% 11% 13% 28% 21% 2	4417
	11th to 20th	Night	16% 12% 19% 31% 17% 4	4170
	21st to end	Night	27% 7% 14% 31% 18% 3	4521
February	1st to 10th	Night	27% 3 16% 28% 20% 6%	2470
	11th to 20th	Night	26% 4 12% 29% 25% 4	2247
	21st to end	Night	25% 4 9% 25% 31% 6%	1884
March	1st to 10th	Night	20% 8% 9% 25% 33% 4	3654
	11th to 20th	Night	28% 9% 13% 32% 16% 2	3490
	21st to end	Night	27% 11% 13% 30% 17% 2	3649
April	1st to 10th	Night	26% 6% 9% 29% 24% 6%	1900
	11th to 20th	Night	32% 5% 9% 26% 22% 5%	1877
	21st to end	Night	30% 6% 13% 27% 22% 3	1900
May	1st to 10th	Night	26% 8% 11% 21% 29% 6%	3148
	11th to 20th	Night	31% 8% 14% 27% 15% 4	2532
	21st to end	Night	18% 9% 11% 24% 29% 8%	2762
June	1st to 10th	Night	22% 7% 15% 23% 20% 13%	1520
	11th to 20th	Night	18% 6% 9% 22% 25% 21%	1520
	21st to end	Night	25% 7% 14% 20% 23% 12%	1520
July	1st to 10th	Night	28% 7% 12% 14% 17% 22%	2500
	11th to 20th	Night	28% 9% 20% 29% 9% 4	2492
	21st to end	Night	27% 11% 20% 24% 15% 3	3002
August	1st to 10th	Night	43% 11% 21% 12% 8% 5%	2838
	11th to 20th	Night	32% 11% 20% 17% 17% 4	3139
	21st to end	Night	42% 9% 14% 19% 14% 3	3487
September	1st to 10th	Night	29% 7% 14% 21% 21% 8%	1900
	11th to 20th	Night	23% 6% 12% 22% 24% 12%	1900
	21st to end	Night	23% 5% 7% 25% 32% 8%	2109
October	1st to 10th	Night	17% 4 6% 25% 38% 9%	3800
	11th to 20th	Night	24% 7% 9% 27% 26% 7%	3778
	21st to end	Night	16% 4 4 30% 40% 6%	4120
November	1st to 10th	Night	16% 4 10% 30% 33% 7%	2496
	11th to 20th	Night	15% 3 9% 35% 31% 7%	2600
	21st to end	Night	22% 4 8% 28% 30% 8%	2676
December	1st to 10th	Night	20% 4 17% 31% 22% 5%	2687
	11th to 20th	Night	22% 7% 15% 28% 27% 3	3272
	21st to end	Night	24% 9% 13% 22% 24% 8%	4873

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	9% 23% 38% 21% 7% 2	270
	11th to 20th	Day	13% 22% 40% 22% 3	270
	21st to end	Day	4 16% 43% 29% 7% 1	297
February	1st to 10th	Day	5% 32% 44% 17% 1 1	270
	11th to 20th	Day	7% 38% 39% 11% 4 1	270
	21st to end	Day	18% 33% 32% 15% 2	225
March	1st to 10th	Day	13% 31% 31% 19% 5%	312
	11th to 20th	Day	13% 27% 36% 17% 7% 1	460
	21st to end	Day	16% 36% 35% 10% 2	506
April	1st to 10th	Day	17% 32% 34% 13% 4 1	460
	11th to 20th	Day	22% 34% 29% 12% 3	452
	21st to end	Day	17% 33% 33% 13% 3	460
May	1st to 10th	Day	20% 29% 32% 15% 3	469
	11th to 20th	Day	11% 17% 31% 27% 13% 2	675
	21st to end	Day	9% 21% 31% 25% 12% 3	734
June	1st to 10th	Day	5% 14% 27% 33% 18% 4	680
	11th to 20th	Day	7% 15% 35% 25% 15% 3	680
	21st to end	Day	6% 12% 31% 26% 20% 4	680
July	1st to 10th	Day	8% 6% 24% 33% 23% 5%	671
	11th to 20th	Day	7% 3 16% 36% 29% 8%	668
	21st to end	Day	10% 7% 18% 37% 23% 6%	536
August	1st to 10th	Day	10% 7% 23% 38% 18% 4	487
	11th to 20th	Day	9% 8% 28% 34% 18% 4	477
	21st to end	Day	12% 10% 23% 34% 17% 4	539
September	1st to 10th	Day	9% 8% 26% 30% 21% 6%	490
	11th to 20th	Day	9% 16% 34% 25% 12% 3	490
	21st to end	Day	8% 15% 27% 28% 17% 4	310
October	1st to 10th	Day	8% 21% 24% 24% 17% 6%	300
	11th to 20th	Day	9% 25% 29% 23% 11% 4	300
	21st to end	Day	16% 21% 27% 20% 14% 2	330
November	1st to 10th	Day	12% 27% 30% 20% 10% 2	300
	11th to 20th	Day	10% 22% 37% 21% 8% 1	300
	21st to end	Day	11% 22% 35% 25% 7% 1	300
December	1st to 10th	Day	12% 21% 40% 18% 7% 2	273
	11th to 20th	Day	7% 24% 39% 23% 7% 1	270
	21st to end	Day	12% 24% 39% 21% 4 1	297
January	1st to 10th	Night	5% 14% 39% 32% 9% 2	950
	11th to 20th	Night	4 14% 41% 30% 9% 2	794
	21st to end	Night	2 10% 39% 37% 11% 1	836
February	1st to 10th	Night	4 12% 47% 32% 5% 1	760
	11th to 20th	Night	4 14% 51% 24% 7%	744
	21st to end	Night	5% 17% 47% 27% 3	628
March	1st to 10th	Night	6% 16% 39% 32% 6% 1	760
	11th to 20th	Night	4 10% 37% 36% 12%	760
	21st to end	Night	5% 17% 39% 32% 6%	734
April	1st to 10th	Night	9% 17% 39% 29% 6%	570
	11th to 20th	Night	11% 17% 33% 31% 7% 1	563
	21st to end	Night	9% 17% 33% 33% 7% 1	570
May	1st to 10th	Night	11% 11% 32% 35% 11% 1	570
	11th to 20th	Night	12% 9% 27% 34% 17% 2	561
	21st to end	Night	10% 15% 27% 33% 14% 1	617
June	1st to 10th	Night	4 8% 22% 41% 22% 3	570
	11th to 20th	Night	5% 11% 22% 44% 17% 2	570
	21st to end	Night	4 5% 19% 38% 32% 3	570
July	1st to 10th	Night	10% 4 12% 34% 37% 4	555
	11th to 20th	Night	6% 3 8% 31% 43% 8%	552
	21st to end	Night	6% 2 8% 30% 47% 8%	624
August	1st to 10th	Night	8% 4 7% 32% 41% 6%	567
	11th to 20th	Night	4 4 12% 40% 34% 6%	564
	21st to end	Night	7% 4 11% 35% 38% 5%	627
September	1st to 10th	Night	6% 4 17% 34% 34% 6%	570
	11th to 20th	Night	6% 5% 19% 38% 27% 5%	570
	21st to end	Night	7% 8% 19% 38% 24% 4	600
October	1st to 10th	Night	7% 11% 24% 38% 17% 4	760
	11th to 20th	Night	5% 6% 34% 36% 17% 3	760
	21st to end	Night	8% 9% 29% 37% 16% 2	836
November	1st to 10th	Night	6% 12% 32% 34% 14% 1	936
	11th to 20th	Night	4 13% 38% 32% 11% 2	1000
	21st to end	Night	7% 14% 31% 35% 12% 1	1000
December	1st to 10th	Night	5% 13% 44% 29% 9% 1	960
	11th to 20th	Night	4 15% 43% 28% 9% 1	950
	21st to end	Night	4 16% 45% 27% 7% 1	1045

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