My-Spot Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the My-Spot Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	14% 10% 3 9% 5% 5% 5% 4 6% 7% 33%	124953
	Night	14% 11% 4 8% 5% 4 4 3 5% 6% 34%	123193
Transparency	Day	48% 6% 12% 21% 12% 1	128523
	Night	49% 5% 11% 21% 12% 1	127693
Seeing	Day	19% 12% 29% 22% 12% 5%	16433
	Night	20% 10% 29% 28% 11% 1	26213

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% 16% 6% 17% 6% 5% 3 3 4 5% 24%	3617
	11th to 20th	Day	15% 14% 6% 18% 8% 10% 7% 3 5% 5% 9%	3620
	21st to end	Day	21% 18% 5% 14% 6% 5% 5% 3 4 5% 14%	4394
February	1st to 10th	Day	14% 15% 5% 13% 9% 5% 5% 4 5% 5% 21%	1800
	11th to 20th	Day	14% 14% 4 12% 10% 6% 3 3 4 6% 23%	1889
	21st to end	Day	14% 16% 4 11% 7% 4 4 4 5% 6% 24%	1650
March	1st to 10th	Day	15% 12% 5% 11% 5% 4 4 3 4 6% 30%	4589
	11th to 20th	Day	12% 11% 3 9% 5% 5% 4 3 5% 8% 33%	4980
	21st to end	Day	19% 14% 4 9% 5% 4 4 3 5% 6% 28%	5465
April	1st to 10th	Day	16% 13% 4 8% 5% 5% 4 4 5% 6% 30%	2327
	11th to 20th	Day	17% 10% 3 7% 4 5% 4 3 5% 7% 34%	2537
	21st to end	Day	17% 9% 3 8% 4 5% 4 3 6% 6% 35%	2600
May	1st to 10th	Day	18% 10% 3 8% 5% 4 5% 3 5% 5% 34%	5862
	11th to 20th	Day	16% 8% 2 8% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	14% 7% 3 7% 4 4 4 3 5% 8% 41%	6433
June	1st to 10th	Day	10% 6% 2 6% 6% 4 4 4 7% 9% 40%	2848
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 4 7% 9% 40%	3000
	21st to end	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 10% 37%	2815
July	1st to 10th	Day	8% 5% 3 7% 5% 5% 6% 5% 8% 11% 38%	6360
	11th to 20th	Day	6% 5% 2 5% 5% 5% 5% 5% 7% 10% 45%	6360
	21st to end	Day	8% 6% 2 6% 4 5% 5% 5% 6% 9% 45%	6542
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 41%	4156
	11th to 20th	Day	10% 6% 2 6% 6% 6% 6% 4 8% 8% 36%	4044
	21st to end	Day	7% 6% 2 7% 6% 5% 7% 5% 7% 10% 38%	4482
September	1st to 10th	Day	10% 7% 2 6% 4 5% 6% 5% 6% 7% 40%	2148
	11th to 20th	Day	9% 6% 2 5% 3 4 5% 4 7% 10% 45%	2090
	21st to end	Day	14% 7% 2 8% 5% 5% 4 3 7% 8% 35%	2090
October	1st to 10th	Day	14% 7% 2 6% 5% 4 5% 4 6% 6% 41%	2038
	11th to 20th	Day	18% 10% 3 7% 5% 4 4 3 6% 7% 32%	1900
	21st to end	Day	20% 12% 3 7% 4 4 4 3 6% 6% 30%	1933
November	1st to 10th	Day	12% 10% 3 8% 4 5% 5% 4 6% 6% 37%	1762
	11th to 20th	Day	16% 12% 4 9% 6% 4 5% 3 5% 6% 30%	1800
	21st to end	Day	18% 14% 5% 9% 5% 4 4 3 4 6% 27%	1695
December	1st to 10th	Day	13% 14% 7% 13% 7% 4 5% 5% 5% 6% 21%	2600
	11th to 20th	Day	20% 21% 5% 9% 5% 5% 4 3 4 4 19%	2753
	21st to end	Day	26% 21% 7% 12% 5% 4 3 2 3 4 13%	3913
January	1st to 10th	Night	13% 15% 5% 12% 6% 5% 5% 3 4 5% 25%	6348
	11th to 20th	Night	12% 16% 7% 15% 9% 6% 7% 4 5% 6% 15%	6360
	21st to end	Night	17% 16% 8% 11% 5% 5% 5% 3 4 5% 22%	6563
February	1st to 10th	Night	15% 16% 4 12% 7% 5% 5% 3 5% 4 24%	2600
	11th to 20th	Night	14% 13% 5% 11% 7% 5% 4 3 4 6% 26%	2392
	21st to end	Night	17% 12% 3 10% 6% 5% 4 4 5% 6% 29%	1980
March	1st to 10th	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 35%	5263
	11th to 20th	Night	10% 8% 3 6% 5% 4 5% 4 6% 8% 41%	4980
	21st to end	Night	16% 9% 3 8% 5% 4 4 3 4 5% 37%	5467
April	1st to 10th	Night	16% 10% 3 7% 5% 4 4 4 6% 6% 35%	2090
	11th to 20th	Night	18% 9% 2 6% 4 3 4 4 6% 7% 37%	1812
	21st to end	Night	16% 9% 2 6% 4 4 4 5% 5% 6% 39%	1800
May	1st to 10th	Night	20% 10% 3 8% 4 4 4 3 5% 6% 33%	4033
	11th to 20th	Night	18% 9% 3 7% 4 3 4 3 4 7% 39%	4032
	21st to end	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	4421
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 3 5% 9% 47%	1537
	11th to 20th	Night	9% 6% 2 6% 5% 3 5% 4 6% 9% 44%	1400
	21st to end	Night	8% 6% 2 6% 4 3 3 3 5% 9% 49%	1571
July	1st to 10th	Night	7% 4 2 5% 4 5% 5% 4 7% 11% 46%	3618
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 9% 48%	3617
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 6% 9% 47%	4314
August	1st to 10th	Night	10% 6% 2 6% 3 4 4 3 6% 8% 47%	2861
	11th to 20th	Night	8% 7% 2 6% 4 4 4 4 5% 7% 49%	2785
	21st to end	Night	8% 6% 2 7% 5% 4 5% 4 5% 7% 48%	3171
September	1st to 10th	Night	10% 6% 2 6% 4 4 5% 3 5% 7% 48%	2011
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 5% 7% 55%	2090
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 44%	2090
October	1st to 10th	Night	14% 7% 3 6% 3 3 3 2 5% 6% 47%	2142
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 3 4 5% 36%	2280
	21st to end	Night	21% 9% 3 7% 5% 4 5% 4 5% 6% 32%	2661
November	1st to 10th	Night	13% 9% 2 6% 5% 4 4 4 5% 6% 43%	2536
	11th to 20th	Night	15% 11% 4 9% 5% 5% 5% 5% 6% 6% 31%	2600
	21st to end	Night	19% 15% 5% 8% 4 3 4 4 5% 6% 28%	2701
December	1st to 10th	Night	12% 13% 5% 14% 6% 6% 5% 4 5% 7% 23%	4580
	11th to 20th	Night	16% 19% 7% 12% 6% 4 4 3 4 6% 19%	5445
	21st to end	Night	25% 22% 7% 9% 5% 3 3 2 3 4 15%	7042

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	63% 5% 10% 14% 6% 1	3618
	11th to 20th	Day	75% 6% 8% 6% 4 1	3620
	21st to end	Day	73% 4 10% 8% 5%	4394
February	1st to 10th	Day	63% 6% 9% 14% 8%	1800
	11th to 20th	Day	63% 5% 8% 15% 8% 1	1899
	21st to end	Day	60% 5% 10% 16% 7% 2	1650
March	1st to 10th	Day	55% 5% 13% 17% 8% 2	4589
	11th to 20th	Day	49% 5% 15% 22% 8% 2	4980
	21st to end	Day	58% 4 11% 15% 11% 2	5465
April	1st to 10th	Day	52% 7% 11% 18% 11% 1	2327
	11th to 20th	Day	49% 5% 11% 21% 12% 2	2537
	21st to end	Day	48% 6% 13% 18% 13% 1	2600
May	1st to 10th	Day	50% 5% 10% 19% 13% 3	5897
	11th to 20th	Day	47% 7% 12% 21% 12% 1	5865
	21st to end	Day	42% 5% 9% 21% 19% 4	6447
June	1st to 10th	Day	38% 6% 13% 27% 14% 2	2848
	11th to 20th	Day	39% 7% 12% 24% 15% 3	3000
	21st to end	Day	40% 8% 12% 23% 16% 1	2815
July	1st to 10th	Day	39% 5% 13% 25% 16% 2	6291
	11th to 20th	Day	33% 5% 14% 29% 18% 1	6285
	21st to end	Day	34% 5% 14% 23% 22% 1	6356
August	1st to 10th	Day	37% 8% 12% 25% 16% 1	4143
	11th to 20th	Day	40% 9% 10% 30% 11% 1	4083
	21st to end	Day	39% 7% 12% 27% 13% 1	4481
September	1st to 10th	Day	40% 8% 14% 24% 13% 2	2148
	11th to 20th	Day	32% 8% 13% 32% 14% 1	2090
	21st to end	Day	44% 7% 12% 23% 13%	2090
October	1st to 10th	Day	32% 5% 11% 30% 21% 1	3389
	11th to 20th	Day	40% 5% 11% 25% 19%	3140
	21st to end	Day	45% 4 13% 24% 12% 1	3157
November	1st to 10th	Day	45% 6% 12% 23% 12% 1	1762
	11th to 20th	Day	55% 6% 11% 18% 9% 1	1800
	21st to end	Day	59% 5% 11% 18% 7% 1	1695
December	1st to 10th	Day	64% 5% 16% 12% 4	2600
	11th to 20th	Day	69% 4 9% 12% 6%	2749
	21st to end	Day	77% 3 8% 8% 5%	3913
January	1st to 10th	Night	60% 5% 9% 18% 6% 2	6348
	11th to 20th	Night	69% 5% 10% 10% 5%	6360
	21st to end	Night	66% 4 9% 13% 7%	6563
February	1st to 10th	Night	63% 6% 10% 14% 8% 1	2600
	11th to 20th	Night	58% 5% 9% 18% 10% 1	2393
	21st to end	Night	56% 4 13% 17% 9% 1	1980
March	1st to 10th	Night	50% 4 13% 22% 10% 1	5263
	11th to 20th	Night	40% 6% 14% 24% 15% 1	4980
	21st to end	Night	49% 4 11% 20% 14% 2	5467
April	1st to 10th	Night	49% 5% 10% 21% 13% 2	2090
	11th to 20th	Night	45% 6% 11% 18% 19% 2	1812
	21st to end	Night	45% 6% 10% 20% 17% 3	1800
May	1st to 10th	Night	52% 5% 10% 16% 13% 4	4060
	11th to 20th	Night	45% 5% 10% 26% 11% 3	4033
	21st to end	Night	38% 6% 11% 23% 18% 4	4429
June	1st to 10th	Night	34% 5% 11% 28% 19% 2	1537
	11th to 20th	Night	35% 8% 12% 26% 16% 2	1400
	21st to end	Night	33% 6% 12% 27% 21% 1	1571
July	1st to 10th	Night	31% 5% 14% 30% 15% 5%	3580
	11th to 20th	Night	32% 5% 14% 30% 18% 2	3572
	21st to end	Night	34% 6% 14% 22% 23% 2	4187
August	1st to 10th	Night	35% 5% 13% 28% 18% 1	2850
	11th to 20th	Night	34% 6% 10% 37% 11% 2	2812
	21st to end	Night	35% 6% 12% 32% 14% 1	3171
September	1st to 10th	Night	34% 7% 13% 27% 17% 2	2011
	11th to 20th	Night	29% 6% 10% 32% 21% 2	2090
	21st to end	Night	40% 6% 11% 28% 14% 1	2090
October	1st to 10th	Night	27% 3 9% 33% 27% 1	3571
	11th to 20th	Night	43% 4 11% 24% 18%	3779
	21st to end	Night	41% 4 11% 30% 13% 1	4387
November	1st to 10th	Night	41% 6% 11% 26% 15% 1	2536
	11th to 20th	Night	52% 6% 12% 19% 9% 1	2600
	21st to end	Night	57% 5% 12% 19% 7%	2701
December	1st to 10th	Night	61% 5% 16% 15% 3	4580
	11th to 20th	Night	67% 5% 9% 13% 5% 1	5448
	21st to end	Night	74% 4 9% 7% 6%	7042

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	23% 22% 36% 12% 5% 1	300
	11th to 20th	Day	25% 16% 45% 12% 1 1	300
	21st to end	Day	29% 17% 33% 16% 4 1	330
February	1st to 10th	Day	24% 23% 36% 13% 2 1	300
	11th to 20th	Day	26% 21% 36% 15% 2	300
	21st to end	Day	28% 28% 33% 10% 2	249
March	1st to 10th	Day	27% 21% 35% 13% 3	344
	11th to 20th	Day	24% 14% 33% 23% 5%	500
	21st to end	Day	29% 14% 32% 19% 5% 1	550
April	1st to 10th	Day	24% 17% 33% 18% 7% 1	500
	11th to 20th	Day	27% 18% 29% 15% 6% 4	492
	21st to end	Day	24% 15% 29% 18% 11% 2	500
May	1st to 10th	Day	24% 15% 26% 21% 11% 3	503
	11th to 20th	Day	21% 10% 27% 23% 13% 7%	527
	21st to end	Day	16% 10% 23% 26% 17% 8%	573
June	1st to 10th	Day	13% 9% 31% 30% 14% 4	723
	11th to 20th	Day	14% 9% 32% 27% 15% 3	730
	21st to end	Day	13% 10% 29% 30% 13% 4	730
July	1st to 10th	Day	9% 7% 24% 30% 20% 10%	721
	11th to 20th	Day	11% 7% 30% 28% 18% 7%	718
	21st to end	Day	9% 5% 28% 28% 21% 9%	629
August	1st to 10th	Day	9% 5% 23% 27% 25% 12%	502
	11th to 20th	Day	12% 6% 19% 30% 20% 13%	479
	21st to end	Day	12% 5% 22% 26% 23% 12%	539
September	1st to 10th	Day	14% 8% 22% 23% 22% 10%	490
	11th to 20th	Day	10% 4 20% 25% 28% 13%	490
	21st to end	Day	18% 10% 24% 22% 18% 8%	490
October	1st to 10th	Day	19% 9% 27% 21% 16% 8%	490
	11th to 20th	Day	23% 14% 30% 24% 8% 1	490
	21st to end	Day	30% 15% 35% 15% 5%	382
November	1st to 10th	Day	16% 23% 35% 17% 8% 1	300
	11th to 20th	Day	25% 23% 30% 15% 4 2	300
	21st to end	Day	30% 17% 36% 13% 4	247
December	1st to 10th	Day	25% 20% 35% 13% 8%	200
	11th to 20th	Day	31% 22% 35% 9% 4 1	200
	21st to end	Day	30% 20% 36% 13% 1	315
January	1st to 10th	Night	23% 16% 36% 20% 5% 1	1000
	11th to 20th	Night	26% 15% 33% 22% 4 1	1000
	21st to end	Night	22% 13% 34% 26% 5%	914
February	1st to 10th	Night	24% 14% 35% 24% 3	800
	11th to 20th	Night	24% 15% 31% 26% 4	783
	21st to end	Night	25% 15% 39% 19% 2	660
March	1st to 10th	Night	22% 13% 33% 26% 5% 1	800
	11th to 20th	Night	19% 16% 35% 25% 4 1	800
	21st to end	Night	23% 13% 31% 25% 8% 1	880
April	1st to 10th	Night	21% 15% 33% 26% 4	690
	11th to 20th	Night	22% 10% 34% 26% 7% 2	593
	21st to end	Night	20% 11% 29% 31% 8%	600
May	1st to 10th	Night	21% 8% 32% 30% 8% 1	600
	11th to 20th	Night	22% 6% 27% 31% 14% 1	591
	21st to end	Night	13% 5% 24% 38% 19% 2	650
June	1st to 10th	Night	12% 5% 24% 45% 13%	535
	11th to 20th	Night	13% 3 26% 46% 13%	400
	21st to end	Night	11% 5% 23% 40% 19% 1	571
July	1st to 10th	Night	8% 3 18% 41% 26% 4	585
	11th to 20th	Night	9% 4 20% 39% 22% 4	582
	21st to end	Night	7% 3 17% 48% 23% 2	657
August	1st to 10th	Night	8% 4 18% 39% 29% 3	579
	11th to 20th	Night	10% 2 19% 35% 29% 4	564
	21st to end	Night	10% 3 18% 34% 31% 4	627
September	1st to 10th	Night	12% 6% 20% 31% 25% 5%	681
	11th to 20th	Night	11% 5% 16% 33% 29% 5%	760
	21st to end	Night	17% 7% 21% 30% 20% 5%	760
October	1st to 10th	Night	18% 7% 26% 28% 18% 3	760
	11th to 20th	Night	24% 13% 27% 26% 9% 1	760
	21st to end	Night	25% 13% 33% 24% 5%	836
November	1st to 10th	Night	18% 12% 31% 26% 12% 1	780
	11th to 20th	Night	23% 15% 33% 22% 6% 1	800
	21st to end	Night	28% 13% 33% 20% 6% 1	800
December	1st to 10th	Night	24% 17% 33% 19% 6%	800
	11th to 20th	Night	27% 14% 34% 22% 3	915
	21st to end	Night	33% 13% 34% 18% 2	1100

Page generated at 2025-08-02 01:32:28. Version ac07

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