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 2026-03-03.

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	2 5% 2 6% 5% 5% 5% 4 6% 8% 51%	139805
	Night	2 5% 2 6% 4 4 4 4 6% 7% 56%	138881
Transparency	Day	27% 7% 11% 24% 23% 7%	134768
	Night	24% 7% 12% 26% 24% 7%	134925
Seeing	Day	13% 20% 30% 23% 12% 3	17144
	Night	7% 12% 31% 32% 16% 2	27347

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	4 4 9% 5% 4 4 3 5% 6% 54%	4720
	11th to 20th	Day	2 2 6% 4 4 4 3 5% 6% 63%	5294
	21st to end	Day	4 3 7% 5% 5% 4 3 6% 6% 57%	5844
February	1st to 10th	Day	7% 3 7% 4 2 4 3 6% 8% 55%	1890
	11th to 20th	Day	1 5% 3 7% 5% 4 4 4 6% 8% 54%	2015
	21st to end	Day	1 5% 4 8% 4 5% 4 3 5% 7% 55%	1730
March	1st to 10th	Day	5% 3 7% 4 4 3 3 5% 7% 56%	4629
	11th to 20th	Day	1 9% 4 9% 6% 5% 6% 5% 6% 7% 42%	5000
	21st to end	Day	1 6% 3 8% 5% 5% 5% 4 6% 8% 48%	5735
April	1st to 10th	Day	2 8% 3 7% 4 5% 4 4 5% 7% 51%	2400
	11th to 20th	Day	3 12% 3 7% 4 4 5% 3 6% 7% 46%	2371
	21st to end	Day	4 10% 3 6% 5% 5% 5% 4 6% 8% 43%	2400
May	1st to 10th	Day	5% 11% 4 9% 6% 5% 5% 4 6% 6% 39%	5473
	11th to 20th	Day	3 7% 2 6% 5% 5% 5% 5% 6% 8% 48%	6254
	21st to end	Day	2 4 2 6% 4 5% 6% 5% 8% 8% 50%	6925
June	1st to 10th	Day	3 5% 3 5% 5% 5% 6% 6% 9% 9% 45%	2800
	11th to 20th	Day	4 4 1 4 4 4 5% 5% 7% 7% 57%	2800
	21st to end	Day	3 4 2 6% 5% 5% 6% 6% 8% 8% 47%	2800
July	1st to 10th	Day	3 3 2 5% 3 4 5% 5% 6% 7% 57%	6339
	11th to 20th	Day	2 3 2 6% 6% 8% 7% 6% 9% 10% 40%	6340
	21st to end	Day	4 5% 2 7% 6% 6% 7% 6% 8% 8% 41%	6406
August	1st to 10th	Day	4 6% 2 8% 7% 7% 8% 7% 8% 9% 33%	5779
	11th to 20th	Day	3 3 1 5% 5% 6% 7% 6% 9% 9% 46%	5367
	21st to end	Day	6% 6% 3 8% 6% 6% 7% 5% 7% 8% 35%	5984
September	1st to 10th	Day	5% 5% 2 5% 6% 4 5% 4 7% 8% 50%	2400
	11th to 20th	Day	5% 5% 2 5% 4 4 5% 4 6% 6% 56%	2400
	21st to end	Day	5% 4 2 4 3 3 4 3 4 5% 63%	2175
October	1st to 10th	Day	3 3 1 4 2 2 3 2 3 5% 71%	3270
	11th to 20th	Day	3 4 2 6% 4 4 5% 4 5% 8% 53%	3250
	21st to end	Day	2 4 1 5% 3 3 3 3 4 5% 67%	3548
November	1st to 10th	Day	1 4 1 4 3 3 3 3 4 6% 68%	1873
	11th to 20th	Day	1 3 1 4 2 2 2 4 4 6% 70%	1890
	21st to end	Day	2 5% 2 6% 3 3 3 2 4 7% 62%	1806
December	1st to 10th	Day	1 3 2 3 3 2 3 2 5% 7% 68%	2700
	11th to 20th	Day	4 2 4 2 3 3 3 6% 10% 62%	3106
	21st to end	Day	3 2 7% 4 5% 5% 4 6% 7% 56%	4092
January	1st to 10th	Night	3 3 10% 5% 5% 5% 4 6% 7% 51%	8201
	11th to 20th	Night	1 2 6% 3 4 3 3 5% 7% 66%	7752
	21st to end	Night	4 3 7% 4 4 4 3 5% 6% 61%	8370
February	1st to 10th	Night	1 6% 3 7% 4 3 3 2 5% 6% 61%	2730
	11th to 20th	Night	2 7% 3 6% 4 3 4 3 5% 6% 57%	2489
	21st to end	Night	1 6% 2 6% 4 3 3 3 6% 8% 58%	2076
March	1st to 10th	Night	1 7% 4 7% 4 3 3 3 5% 6% 57%	5279
	11th to 20th	Night	1 8% 3 7% 5% 4 4 4 6% 7% 50%	5000
	21st to end	Night	1 7% 3 8% 5% 5% 4 4 5% 7% 51%	5226
April	1st to 10th	Night	3 7% 2 5% 4 3 4 3 6% 7% 56%	2000
	11th to 20th	Night	3 11% 2 6% 5% 3 3 4 5% 6% 52%	1977
	21st to end	Night	5% 9% 2 5% 4 4 3 4 5% 7% 52%	2000
May	1st to 10th	Night	4 7% 2 6% 6% 5% 6% 4 6% 8% 45%	4504
	11th to 20th	Night	5% 7% 2 5% 4 4 4 5% 6% 7% 51%	3644
	21st to end	Night	4 4 2 5% 3 4 4 4 7% 7% 57%	3972
June	1st to 10th	Night	3 3 1 3 4 5% 5% 6% 6% 8% 56%	1600
	11th to 20th	Night	2 3 1 4 4 3 4 4 5% 7% 64%	1600
	21st to end	Night	3 5% 1 4 4 4 6% 6% 7% 8% 52%	1600
July	1st to 10th	Night	3 4 2 5% 5% 4 5% 6% 7% 7% 53%	3640
	11th to 20th	Night	2 3 2 7% 5% 6% 7% 6% 10% 12% 41%	3640
	21st to end	Night	3 4 2 7% 5% 6% 7% 6% 9% 10% 42%	4462
August	1st to 10th	Night	5% 4 2 6% 5% 5% 6% 6% 9% 9% 42%	4033
	11th to 20th	Night	2 3 1 6% 5% 6% 6% 5% 9% 14% 44%	4479
	21st to end	Night	6% 7% 3 9% 7% 7% 7% 6% 8% 8% 31%	4993
September	1st to 10th	Night	5% 5% 3 6% 4 4 4 5% 7% 7% 51%	2000
	11th to 20th	Night	5% 3 2 5% 4 4 4 4 6% 6% 58%	2000
	21st to end	Night	5% 5% 1 4 4 3 3 3 4 7% 60%	2221
October	1st to 10th	Night	3 3 1 3 3 2 2 3 4 5% 70%	3920
	11th to 20th	Night	3 6% 2 7% 3 4 4 3 5% 7% 57%	3898
	21st to end	Night	4 5% 1 4 2 2 2 2 3 5% 71%	4252
November	1st to 10th	Night	1 3 1 3 2 3 3 3 3 7% 70%	2624
	11th to 20th	Night	1 3 2 3 2 2 3 2 4 7% 71%	2730
	21st to end	Night	2 5% 2 5% 3 3 3 3 5% 7% 61%	2810
December	1st to 10th	Night	3 1 4 3 2 2 2 4 6% 71%	4710
	11th to 20th	Night	1 3 1 4 3 2 3 3 5% 8% 68%	5321
	21st to end	Night	4 2 7% 5% 5% 5% 4 6% 8% 54%	7128

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	26% 7% 11% 36% 15% 5%	4723
	11th to 20th	Day	16% 6% 16% 24% 27% 11%	5294
	21st to end	Day	27% 5% 8% 27% 24% 10%	5844
February	1st to 10th	Day	26% 6% 15% 26% 21% 6%	1890
	11th to 20th	Day	26% 6% 11% 28% 24% 4	2025
	21st to end	Day	27% 5% 8% 24% 27% 9%	1730
March	1st to 10th	Day	21% 6% 8% 21% 32% 12%	4629
	11th to 20th	Day	35% 8% 9% 30% 16% 3	5000
	21st to end	Day	27% 10% 15% 33% 13% 1	5735
April	1st to 10th	Day	30% 6% 10% 24% 21% 9%	2400
	11th to 20th	Day	35% 5% 8% 25% 22% 5%	2374
	21st to end	Day	33% 7% 9% 24% 22% 3	2400
May	1st to 10th	Day	37% 9% 10% 19% 18% 7%	5473
	11th to 20th	Day	28% 9% 10% 28% 21% 4	6258
	21st to end	Day	21% 10% 11% 22% 30% 6%	6926
June	1st to 10th	Day	29% 8% 13% 21% 20% 8%	2800
	11th to 20th	Day	23% 6% 10% 17% 26% 19%	2800
	21st to end	Day	28% 8% 9% 22% 21% 12%	2800
July	1st to 10th	Day	23% 6% 8% 14% 26% 22%	6271
	11th to 20th	Day	32% 11% 17% 25% 12% 3	6265
	21st to end	Day	35% 10% 16% 16% 17% 6%	6221
August	1st to 10th	Day	42% 11% 17% 16% 10% 3	4208
	11th to 20th	Day	33% 9% 13% 18% 20% 6%	3883
	21st to end	Day	41% 8% 11% 17% 18% 4	4311
September	1st to 10th	Day	29% 6% 12% 21% 22% 9%	2400
	11th to 20th	Day	25% 6% 11% 20% 28% 10%	2400
	21st to end	Day	23% 4 6% 26% 33% 7%	2175
October	1st to 10th	Day	17% 4 5% 20% 46% 9%	3270
	11th to 20th	Day	26% 8% 8% 28% 24% 6%	3250
	21st to end	Day	19% 5% 6% 26% 40% 5%	3548
November	1st to 10th	Day	17% 5% 9% 27% 36% 7%	1873
	11th to 20th	Day	15% 4 10% 37% 29% 6%	1890
	21st to end	Day	22% 4 9% 30% 26% 9%	1806
December	1st to 10th	Day	16% 4 11% 36% 24% 8%	2700
	11th to 20th	Day	17% 6% 14% 33% 28% 2	3104
	21st to end	Day	22% 9% 12% 23% 28% 6%	4092
January	1st to 10th	Night	28% 8% 9% 32% 19% 4	8202
	11th to 20th	Night	13% 7% 15% 28% 22% 15%	7752
	21st to end	Night	24% 5% 9% 24% 26% 13%	8370
February	1st to 10th	Night	25% 3 15% 28% 22% 7%	2730
	11th to 20th	Night	26% 6% 12% 29% 24% 4	2489
	21st to end	Night	23% 4 9% 26% 31% 8%	2076
March	1st to 10th	Night	22% 7% 7% 18% 37% 8%	5279
	11th to 20th	Night	28% 8% 10% 25% 26% 3	5000
	21st to end	Night	26% 11% 14% 34% 14% 1	5226
April	1st to 10th	Night	26% 5% 9% 29% 24% 6%	2000
	11th to 20th	Night	31% 5% 9% 26% 23% 5%	1977
	21st to end	Night	29% 6% 13% 27% 22% 3	2000
May	1st to 10th	Night	30% 9% 12% 24% 21% 4	4501
	11th to 20th	Night	27% 7% 12% 31% 19% 3	3645
	21st to end	Night	20% 9% 13% 22% 29% 6%	3972
June	1st to 10th	Night	23% 7% 15% 23% 19% 12%	1600
	11th to 20th	Night	18% 6% 9% 21% 25% 21%	1600
	21st to end	Night	25% 7% 13% 19% 23% 13%	1600
July	1st to 10th	Night	26% 6% 11% 16% 21% 21%	3600
	11th to 20th	Night	30% 10% 23% 24% 8% 4	3592
	21st to end	Night	33% 11% 18% 21% 15% 2	4335
August	1st to 10th	Night	42% 11% 21% 13% 8% 5%	2928
	11th to 20th	Night	31% 11% 21% 17% 17% 3	3239
	21st to end	Night	42% 9% 13% 19% 13% 3	3597
September	1st to 10th	Night	29% 7% 14% 23% 21% 8%	2000
	11th to 20th	Night	24% 6% 12% 22% 24% 12%	2000
	21st to end	Night	24% 5% 7% 25% 31% 8%	2221
October	1st to 10th	Night	17% 4 6% 26% 38% 9%	3920
	11th to 20th	Night	25% 7% 9% 26% 26% 7%	3898
	21st to end	Night	16% 4 4 30% 40% 6%	4252
November	1st to 10th	Night	15% 4 9% 32% 33% 7%	2624
	11th to 20th	Night	15% 3 8% 37% 29% 7%	2730
	21st to end	Night	22% 4 9% 28% 29% 8%	2810
December	1st to 10th	Night	15% 3 11% 38% 27% 7%	4710
	11th to 20th	Night	16% 4 12% 35% 31% 3	5322
	21st to end	Night	25% 8% 14% 22% 26% 6%	7128

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	15% 25% 35% 18% 6% 2	330
	11th to 20th	Day	15% 25% 37% 19% 3	330
	21st to end	Day	12% 18% 38% 26% 6% 1	363
February	1st to 10th	Day	13% 31% 39% 15% 1 1	330
	11th to 20th	Day	19% 34% 33% 10% 4 1	330
	21st to end	Day	22% 31% 30% 15% 1	273
March	1st to 10th	Day	17% 30% 30% 17% 5%	350
	11th to 20th	Day	17% 26% 33% 16% 6% 1	500
	21st to end	Day	18% 35% 34% 10% 3	550
April	1st to 10th	Day	18% 33% 34% 12% 4 1	500
	11th to 20th	Day	25% 33% 28% 11% 2	492
	21st to end	Day	17% 34% 32% 13% 3	500
May	1st to 10th	Day	20% 29% 33% 15% 3	510
	11th to 20th	Day	14% 18% 30% 26% 12% 2	725
	21st to end	Day	10% 20% 31% 24% 11% 2	789
June	1st to 10th	Day	6% 15% 26% 32% 18% 3	730
	11th to 20th	Day	7% 17% 35% 24% 15% 3	730
	21st to end	Day	8% 13% 31% 25% 20% 4	730
July	1st to 10th	Day	8% 8% 24% 33% 22% 5%	721
	11th to 20th	Day	7% 4 17% 37% 27% 8%	718
	21st to end	Day	11% 7% 19% 36% 22% 6%	580
August	1st to 10th	Day	11% 9% 24% 35% 17% 4	527
	11th to 20th	Day	8% 8% 28% 35% 17% 3	517
	21st to end	Day	12% 10% 25% 33% 16% 4	583
September	1st to 10th	Day	9% 9% 28% 29% 19% 5%	530
	11th to 20th	Day	10% 18% 34% 24% 12% 3	530
	21st to end	Day	10% 17% 27% 27% 16% 4	340
October	1st to 10th	Day	11% 22% 25% 22% 15% 5%	330
	11th to 20th	Day	15% 24% 27% 21% 10% 3	330
	21st to end	Day	17% 23% 26% 20% 13% 1	363
November	1st to 10th	Day	14% 28% 29% 19% 9% 2	330
	11th to 20th	Day	11% 24% 36% 21% 8% 1	330
	21st to end	Day	14% 21% 34% 23% 7% 1	330
December	1st to 10th	Day	14% 22% 38% 17% 7% 2	330
	11th to 20th	Day	12% 24% 37% 20% 6% 1	330
	21st to end	Day	17% 25% 35% 18% 4 1	363
January	1st to 10th	Night	6% 15% 38% 30% 8% 2	1050
	11th to 20th	Night	5% 15% 41% 29% 8% 2	877
	21st to end	Night	2 12% 40% 34% 10% 1	924
February	1st to 10th	Night	6% 13% 45% 31% 5%	840
	11th to 20th	Night	9% 15% 48% 22% 6%	824
	21st to end	Night	6% 17% 47% 27% 3	692
March	1st to 10th	Night	7% 17% 38% 31% 6% 1	808
	11th to 20th	Night	6% 11% 37% 35% 12%	800
	21st to end	Night	6% 17% 39% 31% 6%	772
April	1st to 10th	Night	10% 18% 39% 28% 6%	600
	11th to 20th	Night	12% 17% 33% 30% 7% 1	593
	21st to end	Night	10% 17% 34% 32% 7% 1	600
May	1st to 10th	Night	11% 11% 33% 34% 11% 1	600
	11th to 20th	Night	13% 9% 27% 33% 16% 2	591
	21st to end	Night	9% 14% 29% 32% 14% 1	650
June	1st to 10th	Night	4 8% 23% 41% 21% 3	600
	11th to 20th	Night	5% 11% 23% 44% 17% 2	600
	21st to end	Night	5% 6% 19% 37% 31% 3	600
July	1st to 10th	Night	9% 4 13% 35% 35% 4	585
	11th to 20th	Night	7% 3 10% 32% 41% 8%	582
	21st to end	Night	6% 2 8% 30% 46% 7%	657
August	1st to 10th	Night	8% 4 9% 34% 39% 6%	597
	11th to 20th	Night	4 4 12% 40% 33% 6%	594
	21st to end	Night	8% 5% 12% 35% 37% 5%	660
September	1st to 10th	Night	6% 5% 17% 34% 33% 5%	600
	11th to 20th	Night	7% 5% 20% 37% 26% 5%	600
	21st to end	Night	7% 8% 20% 39% 23% 3	632
October	1st to 10th	Night	7% 12% 25% 37% 16% 4	800
	11th to 20th	Night	8% 6% 33% 35% 16% 2	800
	21st to end	Night	8% 9% 30% 36% 15% 2	880
November	1st to 10th	Night	7% 13% 32% 33% 14% 1	984
	11th to 20th	Night	5% 13% 39% 31% 11% 2	1050
	21st to end	Night	7% 15% 32% 34% 12% 1	1050
December	1st to 10th	Night	5% 15% 42% 29% 8%	1050
	11th to 20th	Night	6% 16% 42% 27% 9% 1	1050
	21st to end	Night	7% 16% 44% 26% 6% 1	1155

Page generated at 2026-02-27 23:40:25. Version ac07

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