My-Spot West Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the My-Spot West Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	4 6% 2 6% 4 4 5% 4 7% 8% 48%	136849
	Night	3 5% 2 6% 4 4 4 4 6% 8% 53%	131651
Transparency	Day	30% 6% 13% 25% 19% 6%	131616
	Night	27% 6% 14% 28% 19% 6%	127890
Seeing	Day	9% 13% 36% 30% 10% 2	17329
	Night	7% 7% 37% 37% 11% 1	27131

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	1 7% 4 9% 5% 5% 6% 5% 7% 9% 43%	3638
	11th to 20th	Day	4 3 10% 9% 5% 5% 4 7% 10% 43%	3852
	21st to end	Day	1 3 2 6% 4 4 3 3 5% 8% 60%	4510
February	1st to 10th	Day	1 6% 3 10% 5% 4 6% 5% 8% 9% 42%	1800
	11th to 20th	Day	1 6% 3 10% 6% 6% 4 4 6% 8% 44%	1755
	21st to end	Day	1 6% 4 6% 6% 5% 5% 5% 8% 10% 45%	1511
March	1st to 10th	Day	1 7% 3 6% 4 5% 4 4 6% 10% 50%	4944
	11th to 20th	Day	2 9% 3 6% 5% 4 4 5% 7% 9% 47%	5000
	21st to end	Day	4 10% 3 9% 7% 5% 7% 5% 8% 11% 31%	5486
April	1st to 10th	Day	4 11% 3 8% 4 4 5% 5% 7% 11% 38%	2383
	11th to 20th	Day	6% 11% 4 8% 6% 5% 5% 5% 6% 7% 37%	2423
	21st to end	Day	10% 11% 4 7% 6% 5% 5% 5% 6% 8% 33%	2600
May	1st to 10th	Day	9% 11% 3 8% 5% 5% 5% 4 5% 7% 37%	5967
	11th to 20th	Day	9% 7% 2 7% 4 5% 5% 5% 7% 8% 40%	6298
	21st to end	Day	7% 7% 3 7% 5% 5% 6% 5% 7% 8% 40%	6925
June	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 9% 40%	2800
	11th to 20th	Day	6% 5% 2 5% 4 4 5% 5% 8% 10% 46%	2830
	21st to end	Day	3 3 1 4 4 4 5% 5% 7% 9% 58%	3000
July	1st to 10th	Day	3 2 1 4 4 5% 6% 5% 8% 9% 52%	6542
	11th to 20th	Day	2 3 1 4 4 4 5% 5% 8% 9% 56%	6340
	21st to end	Day	2 3 1 5% 4 5% 6% 6% 9% 9% 48%	6596
August	1st to 10th	Day	5% 4 2 5% 4 4 5% 5% 7% 9% 50%	5779
	11th to 20th	Day	3 2 1 5% 5% 5% 6% 5% 8% 8% 53%	5798
	21st to end	Day	4 4 2 5% 4 3 4 4 5% 7% 59%	6258
September	1st to 10th	Day	5% 3 1 4 4 3 3 4 5% 7% 61%	2400
	11th to 20th	Day	7% 4 2 4 3 3 4 4 5% 7% 58%	2307
	21st to end	Day	7% 4 2 5% 3 3 4 3 4 7% 58%	2160
October	1st to 10th	Day	6% 5% 2 5% 2 3 3 2 3 4 65%	3270
	11th to 20th	Day	5% 6% 2 3 3 3 3 3 4 6% 62%	3250
	21st to end	Day	3 8% 2 6% 4 4 4 3 6% 8% 52%	3548
November	1st to 10th	Day	3 6% 2 7% 5% 5% 6% 5% 7% 7% 49%	1733
	11th to 20th	Day	1 6% 3 6% 4 4 6% 4 7% 9% 50%	1680
	21st to end	Day	2 7% 2 8% 4 4 4 3 6% 9% 52%	1680
December	1st to 10th	Day	3 2 7% 3 3 4 4 5% 7% 61%	2700
	11th to 20th	Day	1 4 2 6% 4 3 5% 4 8% 12% 50%	3082
	21st to end	Day	2 9% 3 8% 5% 5% 5% 4 7% 8% 45%	4004
January	1st to 10th	Night	4 3 8% 5% 5% 5% 4 7% 10% 48%	6327
	11th to 20th	Night	3 3 9% 8% 6% 5% 5% 6% 8% 47%	6158
	21st to end	Night	1 5% 3 7% 4 4 4 3 5% 7% 59%	6468
February	1st to 10th	Night	1 5% 3 11% 5% 5% 6% 5% 7% 8% 45%	2600
	11th to 20th	Night	1 6% 4 8% 6% 5% 5% 5% 7% 8% 45%	2532
	21st to end	Night	5% 3 9% 4 4 4 5% 7% 8% 50%	2117
March	1st to 10th	Night	3 10% 3 6% 3 4 4 4 5% 8% 51%	4946
	11th to 20th	Night	3 7% 2 5% 3 4 3 3 6% 11% 51%	5000
	21st to end	Night	4 9% 3 8% 5% 5% 5% 5% 7% 11% 40%	5489
April	1st to 10th	Night	4 10% 3 6% 4 5% 4 4 7% 7% 45%	2016
	11th to 20th	Night	6% 11% 3 8% 6% 4 4 4 6% 7% 40%	1920
	21st to end	Night	7% 12% 3 7% 5% 5% 4 4 6% 8% 40%	1800
May	1st to 10th	Night	10% 10% 3 6% 3 3 3 3 5% 6% 48%	4013
	11th to 20th	Night	10% 6% 2 5% 4 4 5% 4 6% 8% 46%	3617
	21st to end	Night	9% 6% 2 6% 4 5% 6% 4 8% 10% 41%	3972
June	1st to 10th	Night	7% 5% 2 5% 4 4 6% 5% 7% 9% 46%	1600
	11th to 20th	Night	6% 4 1 4 3 5% 5% 4 5% 9% 54%	1559
	21st to end	Night	3 3 2 4 3 3 4 3 6% 6% 64%	1400
July	1st to 10th	Night	2 2 1 3 2 3 4 4 7% 9% 63%	3409
	11th to 20th	Night	3 2 1 2 3 4 5% 4 9% 11% 58%	3640
	21st to end	Night	2 2 1 3 2 3 3 4 7% 8% 65%	4260
August	1st to 10th	Night	4 3 1 4 3 4 4 4 7% 8% 57%	4033
	11th to 20th	Night	2 2 1 3 3 4 4 5% 7% 9% 61%	4051
	21st to end	Night	3 4 2 5% 3 3 3 3 7% 7% 60%	4719
September	1st to 10th	Night	5% 3 1 3 3 3 3 3 5% 8% 65%	2000
	11th to 20th	Night	6% 4 2 4 3 2 3 4 5% 8% 61%	2090
	21st to end	Night	8% 5% 2 4 3 3 3 2 4 6% 61%	2238
October	1st to 10th	Night	6% 4 1 4 2 2 2 2 4 6% 67%	3920
	11th to 20th	Night	6% 6% 2 5% 4 4 4 3 4 6% 57%	3898
	21st to end	Night	4 7% 3 5% 4 3 4 3 5% 7% 55%	4252
November	1st to 10th	Night	3 5% 2 6% 3 4 4 4 7% 9% 52%	2763
	11th to 20th	Night	2 6% 2 6% 4 4 4 4 7% 9% 51%	2940
	21st to end	Night	2 7% 3 5% 4 4 4 4 6% 9% 51%	2940
December	1st to 10th	Night	4 2 5% 4 3 3 3 6% 8% 62%	4710
	11th to 20th	Night	1 4 2 6% 4 4 5% 4 7% 10% 53%	5280
	21st to end	Night	1 6% 2 7% 6% 4 5% 4 7% 9% 50%	6974

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	34% 6% 16% 25% 17% 1	3639
	11th to 20th	Day	31% 9% 17% 22% 19% 2	3852
	21st to end	Day	22% 6% 16% 30% 22% 4	4510
February	1st to 10th	Day	33% 9% 20% 25% 11% 2	1800
	11th to 20th	Day	37% 8% 19% 21% 13% 3	1764
	21st to end	Day	31% 9% 17% 23% 16% 5%	1511
March	1st to 10th	Day	28% 7% 13% 30% 16% 7%	4944
	11th to 20th	Day	30% 7% 15% 27% 21% 1	5000
	21st to end	Day	40% 10% 17% 26% 6% 1	5486
April	1st to 10th	Day	38% 9% 16% 25% 11% 1	2383
	11th to 20th	Day	42% 8% 18% 24% 8% 1	2426
	21st to end	Day	47% 8% 15% 22% 8%	2600
May	1st to 10th	Day	43% 6% 12% 24% 13% 2	5967
	11th to 20th	Day	37% 8% 14% 25% 13% 3	6302
	21st to end	Day	36% 8% 15% 30% 10% 1	6926
June	1st to 10th	Day	35% 11% 19% 22% 12% 2	2800
	11th to 20th	Day	30% 10% 15% 20% 20% 5%	2830
	21st to end	Day	21% 8% 12% 25% 21% 12%	3000
July	1st to 10th	Day	26% 4 7% 25% 26% 13%	6474
	11th to 20th	Day	23% 3 6% 23% 28% 18%	6265
	21st to end	Day	29% 3 8% 19% 29% 12%	6407
August	1st to 10th	Day	32% 4 9% 22% 18% 14%	4208
	11th to 20th	Day	26% 4 8% 27% 24% 11%	4197
	21st to end	Day	20% 4 6% 23% 32% 15%	4513
September	1st to 10th	Day	23% 5% 8% 21% 31% 13%	2400
	11th to 20th	Day	26% 5% 10% 26% 27% 7%	2307
	21st to end	Day	27% 4 10% 27% 26% 6%	2160
October	1st to 10th	Day	25% 4 7% 17% 30% 16%	3270
	11th to 20th	Day	25% 4 10% 33% 22% 7%	3250
	21st to end	Day	30% 3 10% 34% 20% 3	3548
November	1st to 10th	Day	31% 8% 15% 24% 19% 3	1733
	11th to 20th	Day	28% 8% 18% 27% 16% 3	1680
	21st to end	Day	29% 6% 17% 27% 18% 2	1680
December	1st to 10th	Day	21% 5% 16% 33% 21% 3	2700
	11th to 20th	Day	24% 6% 27% 30% 11% 1	3080
	21st to end	Day	36% 4 9% 26% 18% 7%	4004
January	1st to 10th	Night	28% 7% 18% 29% 17% 1	6327
	11th to 20th	Night	31% 6% 17% 27% 15% 3	6158
	21st to end	Night	24% 6% 16% 29% 22% 4	6468
February	1st to 10th	Night	32% 9% 20% 27% 10% 3	2600
	11th to 20th	Night	34% 9% 19% 23% 13% 3	2533
	21st to end	Night	29% 7% 18% 25% 18% 3	2117
March	1st to 10th	Night	30% 5% 11% 25% 21% 8%	4946
	11th to 20th	Night	25% 7% 16% 32% 20% 1	5000
	21st to end	Night	35% 8% 20% 27% 9% 1	5489
April	1st to 10th	Night	35% 8% 19% 24% 11% 2	2016
	11th to 20th	Night	40% 7% 18% 24% 10% 1	1920
	21st to end	Night	40% 7% 15% 29% 8%	1800
May	1st to 10th	Night	35% 5% 13% 26% 17% 3	4010
	11th to 20th	Night	33% 6% 16% 31% 12% 2	3618
	21st to end	Night	31% 8% 20% 30% 10%	3972
June	1st to 10th	Night	32% 10% 20% 22% 15% 1	1600
	11th to 20th	Night	25% 8% 19% 25% 17% 7%	1559
	21st to end	Night	19% 6% 12% 30% 22% 11%	1400
July	1st to 10th	Night	18% 3 8% 34% 27% 11%	3370
	11th to 20th	Night	18% 2 6% 29% 33% 12%	3592
	21st to end	Night	17% 2 8% 23% 32% 17%	4138
August	1st to 10th	Night	27% 4 12% 23% 24% 11%	2928
	11th to 20th	Night	22% 3 8% 32% 21% 13%	2928
	21st to end	Night	17% 3 7% 25% 32% 18%	3395
September	1st to 10th	Night	18% 4 10% 24% 31% 13%	2000
	11th to 20th	Night	22% 5% 11% 27% 26% 9%	2090
	21st to end	Night	24% 4 10% 27% 28% 6%	2238
October	1st to 10th	Night	21% 4 8% 23% 31% 14%	3920
	11th to 20th	Night	30% 3 9% 31% 23% 4	3898
	21st to end	Night	30% 5% 9% 29% 20% 8%	4252
November	1st to 10th	Night	26% 9% 16% 28% 17% 4	2763
	11th to 20th	Night	27% 7% 20% 29% 15% 2	2940
	21st to end	Night	28% 6% 18% 28% 17% 3	2940
December	1st to 10th	Night	20% 5% 19% 32% 21% 2	4710
	11th to 20th	Night	24% 6% 22% 34% 13% 1	5281
	21st to end	Night	29% 5% 11% 30% 16% 8%	6974

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	8% 12% 37% 29% 12% 2	300
	11th to 20th	Day	3 12% 48% 23% 10% 4	300
	21st to end	Day	6% 15% 42% 27% 7% 2	330
February	1st to 10th	Day	6% 21% 52% 20% 1	300
	11th to 20th	Day	5% 22% 42% 24% 7%	300
	21st to end	Day	10% 21% 41% 21% 8%	249
March	1st to 10th	Day	7% 22% 45% 23% 3	490
	11th to 20th	Day	8% 20% 42% 24% 5% 1	500
	21st to end	Day	14% 19% 37% 24% 5%	550
April	1st to 10th	Day	14% 23% 38% 22% 3	500
	11th to 20th	Day	14% 22% 39% 22% 3	492
	21st to end	Day	18% 17% 31% 25% 8% 1	500
May	1st to 10th	Day	14% 15% 32% 24% 12% 2	538
	11th to 20th	Day	17% 10% 27% 32% 12% 2	725
	21st to end	Day	15% 10% 28% 29% 15% 3	789
June	1st to 10th	Day	8% 12% 30% 32% 15% 3	730
	11th to 20th	Day	10% 15% 35% 31% 9% 1	730
	21st to end	Day	5% 9% 34% 36% 15% 2	730
July	1st to 10th	Day	4 6% 29% 35% 22% 3	721
	11th to 20th	Day	4 5% 33% 42% 15% 1	718
	21st to end	Day	4 5% 36% 35% 17% 2	666
August	1st to 10th	Day	8% 6% 38% 35% 12% 2	527
	11th to 20th	Day	5% 6% 33% 39% 14% 3	517
	21st to end	Day	4 9% 35% 35% 14% 3	583
September	1st to 10th	Day	6% 8% 37% 35% 11% 3	530
	11th to 20th	Day	9% 11% 33% 31% 12% 4	530
	21st to end	Day	10% 13% 33% 30% 12% 3	490
October	1st to 10th	Day	10% 13% 28% 33% 13% 3	330
	11th to 20th	Day	14% 13% 30% 28% 11% 4	330
	21st to end	Day	10% 17% 39% 27% 6% 2	363
November	1st to 10th	Day	10% 18% 39% 23% 7% 3	330
	11th to 20th	Day	12% 14% 40% 26% 7% 1	330
	21st to end	Day	9% 14% 41% 32% 4 1	330
December	1st to 10th	Day	7% 19% 42% 23% 8% 1	330
	11th to 20th	Day	10% 14% 48% 25% 3	321
	21st to end	Day	6% 13% 47% 24% 9% 1	330
January	1st to 10th	Night	3 11% 42% 35% 8% 1	1000
	11th to 20th	Night	3 10% 52% 23% 9% 2	922
	21st to end	Night	4 9% 49% 30% 6% 1	880
February	1st to 10th	Night	5% 8% 52% 29% 6%	800
	11th to 20th	Night	5% 8% 46% 31% 9% 1	784
	21st to end	Night	2 15% 46% 29% 8% 1	660
March	1st to 10th	Night	5% 11% 43% 33% 9%	800
	11th to 20th	Night	6% 8% 44% 35% 8%	800
	21st to end	Night	7% 9% 37% 40% 6% 1	880
April	1st to 10th	Night	9% 8% 44% 31% 8%	616
	11th to 20th	Night	12% 9% 41% 32% 5% 1	593
	21st to end	Night	13% 8% 34% 34% 12% 1	600
May	1st to 10th	Night	16% 8% 24% 38% 15%	600
	11th to 20th	Night	14% 5% 22% 41% 16% 1	591
	21st to end	Night	17% 6% 22% 39% 15% 1	650
June	1st to 10th	Night	8% 6% 25% 40% 19% 2	600
	11th to 20th	Night	7% 8% 33% 41% 10%	559
	21st to end	Night	4 2 21% 52% 21% 1	400
July	1st to 10th	Night	3 1 15% 53% 26% 1	485
	11th to 20th	Night	2 3 24% 55% 17%	582
	21st to end	Night	2 2 21% 54% 21% 1	657
August	1st to 10th	Night	5% 2 17% 57% 20%	597
	11th to 20th	Night	4 3 15% 56% 21% 1	594
	21st to end	Night	6% 2 25% 44% 22% 2	660
September	1st to 10th	Night	6% 2 24% 49% 18% 2	600
	11th to 20th	Night	7% 4 23% 47% 16% 4	690
	21st to end	Night	10% 3 27% 39% 17% 2	800
October	1st to 10th	Night	14% 4 28% 39% 14% 2	800
	11th to 20th	Night	9% 7% 38% 32% 14% 2	800
	21st to end	Night	6% 6% 46% 32% 8% 1	880
November	1st to 10th	Night	6% 8% 45% 32% 7% 1	965
	11th to 20th	Night	6% 11% 50% 29% 4	1050
	21st to end	Night	6% 9% 43% 36% 6%	1050
December	1st to 10th	Night	5% 12% 46% 30% 6% 1	1050
	11th to 20th	Night	5% 9% 50% 29% 6% 1	1036
	21st to end	Night	4 8% 51% 29% 7% 1	1100

Page generated at 2025-12-14 23:13:50. Version ac07

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