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 2024-11-04.

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 8% 5% 5% 5% 4 6% 7% 33%	100918
	Night	14% 11% 4 8% 5% 4 4 3 5% 7% 35%	98065
Transparency	Day	48% 6% 12% 21% 12% 1	104532
	Night	47% 6% 11% 22% 12% 1	102591
Seeing	Day	19% 12% 29% 22% 13% 5%	15436
	Night	19% 10% 29% 29% 12% 1	25218

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	16% 18% 6% 11% 6% 5% 4 3 4 5% 22%	2527
	11th to 20th	Day	16% 17% 6% 12% 8% 9% 7% 3 5% 5% 12%	2530
	21st to end	Day	26% 21% 6% 11% 5% 4 4 2 4 4 12%	3065
February	1st to 10th	Day	14% 15% 5% 13% 9% 5% 5% 4 5% 5% 21%	1710
	11th to 20th	Day	14% 14% 4 12% 10% 6% 3 4 4 6% 24%	1792
	21st to end	Day	14% 17% 4 11% 7% 4 4 4 5% 6% 24%	1570
March	1st to 10th	Day	17% 16% 5% 10% 5% 4 4 3 4 6% 27%	3201
	11th to 20th	Day	13% 12% 3 10% 6% 5% 5% 4 5% 7% 31%	3480
	21st to end	Day	20% 12% 4 9% 6% 4 5% 3 5% 5% 28%	3815
April	1st to 10th	Day	16% 13% 4 8% 5% 5% 4 4 5% 7% 31%	2210
	11th to 20th	Day	17% 10% 3 7% 4 5% 4 3 5% 6% 34%	2409
	21st to end	Day	18% 9% 3 8% 4 5% 4 3 6% 6% 34%	2470
May	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 6% 35%	4082
	11th to 20th	Day	15% 8% 2 8% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	10% 6% 3 6% 4 4 4 3 4 9% 45%	4476
June	1st to 10th	Day	10% 6% 2 7% 6% 5% 4 4 7% 9% 40%	2705
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 4 6% 9% 41%	2850
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 36%	2675
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 9% 37%	4440
	11th to 20th	Day	7% 5% 2 5% 5% 4 5% 5% 8% 10% 45%	4440
	21st to end	Day	7% 6% 2 6% 5% 6% 6% 5% 7% 9% 41%	4542
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 41%	4104
	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% 9% 3 8% 5% 5% 5% 4 6% 6% 37%	1699
	11th to 20th	Day	15% 13% 4 9% 6% 4 5% 3 5% 5% 31%	1710
	21st to end	Day	19% 13% 4 8% 5% 4 4 3 4 6% 28%	1611
December	1st to 10th	Day	15% 15% 7% 12% 7% 4 5% 4 4 5% 22%	1520
	11th to 20th	Day	17% 16% 5% 10% 5% 5% 5% 4 5% 6% 22%	1751
	21st to end	Day	27% 21% 7% 11% 5% 4 3 3 4 5% 11%	2728
January	1st to 10th	Night	16% 16% 5% 12% 6% 5% 4 3 4 5% 24%	4428
	11th to 20th	Night	14% 18% 7% 14% 7% 5% 7% 4 5% 5% 14%	4440
	21st to end	Night	23% 18% 8% 11% 5% 4 4 2 3 4 18%	4591
February	1st to 10th	Night	16% 16% 4 11% 7% 5% 5% 3 5% 4 24%	2470
	11th to 20th	Night	14% 13% 5% 11% 7% 5% 4 3 4 6% 27%	2270
	21st to end	Night	18% 12% 4 10% 6% 5% 4 3 5% 6% 29%	1884
March	1st to 10th	Night	15% 13% 4 9% 5% 4 3 3 4 6% 35%	3651
	11th to 20th	Night	10% 8% 3 6% 5% 5% 5% 5% 6% 8% 41%	3480
	21st to end	Night	16% 9% 3 9% 5% 5% 4 4 4 5% 36%	3817
April	1st to 10th	Night	15% 10% 3 7% 5% 4 5% 4 6% 6% 36%	1986
	11th to 20th	Night	19% 9% 2 6% 3 3 4 4 5% 7% 37%	1720
	21st to end	Night	16% 9% 2 6% 4 3 4 5% 5% 6% 39%	1710
May	1st to 10th	Night	18% 9% 2 7% 4 4 4 3 5% 7% 37%	2813
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 5% 8% 42%	2812
	21st to end	Night	10% 6% 2 5% 4 4 4 4 4 8% 49%	3079
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 3 5% 9% 47%	1460
	11th to 20th	Night	9% 6% 2 6% 5% 3 5% 4 6% 9% 45%	1330
	21st to end	Night	9% 6% 2 7% 4 3 3 3 5% 9% 49%	1492
July	1st to 10th	Night	8% 3 2 5% 4 5% 5% 4 7% 10% 47%	2530
	11th to 20th	Night	8% 5% 2 4 3 4 3 4 6% 9% 53%	2530
	21st to end	Night	5% 4 2 7% 5% 5% 6% 4 7% 10% 46%	3013
August	1st to 10th	Night	10% 6% 2 6% 3 4 4 3 6% 8% 46%	2825
	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% 7% 43%	2452
	11th to 20th	Night	16% 11% 4 8% 4 5% 4 5% 6% 6% 31%	2470
	21st to end	Night	19% 14% 4 7% 4 3 4 4 5% 6% 29%	2565
December	1st to 10th	Night	17% 15% 6% 10% 6% 5% 5% 4 5% 5% 22%	2660
	11th to 20th	Night	15% 15% 5% 12% 6% 4 5% 3 5% 7% 23%	3441
	21st to end	Night	27% 24% 7% 9% 5% 3 3 2 3 4 12%	4916

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	64% 6% 11% 12% 6% 1	2528
	11th to 20th	Day	71% 9% 10% 6% 3	2530
	21st to end	Day	76% 6% 8% 6% 3 1	3065
February	1st to 10th	Day	63% 6% 8% 14% 8%	1710
	11th to 20th	Day	62% 5% 8% 16% 9% 1	1802
	21st to end	Day	60% 5% 11% 16% 7% 2	1570
March	1st to 10th	Day	59% 5% 12% 18% 6% 1	3201
	11th to 20th	Day	52% 6% 12% 20% 8% 2	3480
	21st to end	Day	57% 5% 12% 14% 9% 2	3815
April	1st to 10th	Day	52% 7% 11% 18% 12% 1	2210
	11th to 20th	Day	49% 5% 11% 21% 12% 2	2409
	21st to end	Day	49% 6% 13% 17% 13% 1	2470
May	1st to 10th	Day	47% 7% 11% 23% 11% 2	4120
	11th to 20th	Day	45% 8% 14% 23% 9% 1	4085
	21st to end	Day	37% 6% 11% 19% 23% 4	4489
June	1st to 10th	Day	39% 6% 13% 26% 14% 2	2705
	11th to 20th	Day	38% 7% 12% 24% 16% 3	2850
	21st to end	Day	40% 8% 12% 23% 15% 2	2675
July	1st to 10th	Day	42% 5% 14% 20% 16% 2	4371
	11th to 20th	Day	31% 6% 13% 33% 16% 1	4365
	21st to end	Day	37% 7% 14% 18% 23% 2	4398
August	1st to 10th	Day	38% 8% 12% 25% 15% 1	4091
	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% 7% 12% 22% 12% 1	1699
	11th to 20th	Day	53% 6% 12% 19% 9% 1	1710
	21st to end	Day	57% 5% 12% 18% 7% 1	1611
December	1st to 10th	Day	64% 6% 11% 15% 4	1520
	11th to 20th	Day	62% 6% 12% 14% 6%	1747
	21st to end	Day	78% 3 9% 6% 3	2728
January	1st to 10th	Night	62% 6% 10% 15% 6% 1	4428
	11th to 20th	Night	70% 6% 10% 9% 4	4440
	21st to end	Night	71% 5% 8% 13% 3	4591
February	1st to 10th	Night	62% 6% 10% 13% 8% 1	2470
	11th to 20th	Night	57% 6% 9% 18% 10% 1	2271
	21st to end	Night	56% 4 12% 17% 9% 1	1884
March	1st to 10th	Night	51% 4 12% 25% 7% 1	3651
	11th to 20th	Night	38% 7% 15% 20% 19% 1	3480
	21st to end	Night	49% 5% 11% 22% 11% 1	3817
April	1st to 10th	Night	48% 5% 10% 22% 13% 2	1986
	11th to 20th	Night	45% 6% 11% 18% 18% 2	1720
	21st to end	Night	45% 6% 10% 19% 16% 3	1710
May	1st to 10th	Night	46% 6% 10% 21% 13% 3	2840
	11th to 20th	Night	41% 6% 11% 27% 13% 2	2813
	21st to end	Night	33% 8% 11% 22% 21% 6%	3087
June	1st to 10th	Night	34% 5% 11% 29% 18% 3	1460
	11th to 20th	Night	35% 8% 12% 25% 17% 2	1330
	21st to end	Night	33% 6% 12% 27% 20% 1	1492
July	1st to 10th	Night	31% 5% 13% 31% 15% 6%	2490
	11th to 20th	Night	28% 5% 13% 33% 19% 2	2482
	21st to end	Night	32% 7% 18% 20% 21% 3	2917
August	1st to 10th	Night	35% 5% 13% 29% 17% 1	2814
	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	2452
	11th to 20th	Night	52% 6% 13% 20% 9% 1	2470
	21st to end	Night	56% 5% 12% 19% 7%	2565
December	1st to 10th	Night	64% 6% 10% 15% 5%	2660
	11th to 20th	Night	60% 7% 12% 14% 6% 1	3444
	21st to end	Night	77% 5% 10% 6% 2	4916

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% 35% 13% 5% 1	270
	11th to 20th	Day	25% 15% 44% 13% 1 1	270
	21st to end	Day	30% 15% 33% 17% 4 1	297
February	1st to 10th	Day	24% 21% 37% 14% 2 1	270
	11th to 20th	Day	24% 20% 37% 16% 2	270
	21st to end	Day	27% 29% 33% 9% 2	225
March	1st to 10th	Day	27% 21% 33% 15% 4	312
	11th to 20th	Day	23% 14% 33% 23% 6%	460
	21st to end	Day	28% 14% 33% 19% 5% 1	506
April	1st to 10th	Day	22% 17% 34% 19% 8% 1	460
	11th to 20th	Day	27% 18% 29% 15% 6% 5%	452
	21st to end	Day	25% 15% 29% 18% 11% 2	460
May	1st to 10th	Day	24% 14% 26% 22% 12% 3	463
	11th to 20th	Day	20% 10% 26% 23% 14% 7%	487
	21st to end	Day	15% 8% 23% 28% 18% 9%	529
June	1st to 10th	Day	12% 8% 30% 31% 15% 4	673
	11th to 20th	Day	14% 8% 30% 27% 16% 4	680
	21st to end	Day	14% 10% 29% 29% 14% 5%	680
July	1st to 10th	Day	9% 7% 23% 30% 21% 11%	671
	11th to 20th	Day	11% 7% 29% 28% 19% 7%	668
	21st to end	Day	9% 5% 27% 29% 21% 9%	582
August	1st to 10th	Day	9% 4 22% 27% 25% 12%	486
	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	15% 22% 36% 18% 8% 1	279
	11th to 20th	Day	23% 23% 31% 17% 4 1	270
	21st to end	Day	27% 17% 37% 13% 5%	223
December	1st to 10th	Day	26% 18% 35% 12% 9%	180
	11th to 20th	Day	27% 24% 35% 9% 4 1	180
	21st to end	Day	30% 20% 37% 13% 1	283
January	1st to 10th	Night	23% 15% 35% 21% 5% 1	950
	11th to 20th	Night	26% 13% 33% 23% 4 1	950
	21st to end	Night	21% 12% 34% 27% 5%	869
February	1st to 10th	Night	24% 13% 36% 24% 3	760
	11th to 20th	Night	23% 15% 31% 26% 4	743
	21st to end	Night	25% 15% 38% 20% 2	628
March	1st to 10th	Night	22% 12% 34% 26% 6% 1	760
	11th to 20th	Night	18% 15% 35% 26% 4 1	760
	21st to end	Night	22% 12% 31% 25% 8% 1	836
April	1st to 10th	Night	20% 15% 34% 27% 5%	656
	11th to 20th	Night	21% 10% 35% 25% 7% 2	563
	21st to end	Night	21% 10% 29% 31% 8%	570
May	1st to 10th	Night	20% 7% 32% 31% 8% 1	570
	11th to 20th	Night	22% 4 27% 32% 14% 1	561
	21st to end	Night	12% 4 23% 39% 20% 2	617
June	1st to 10th	Night	12% 4 24% 46% 13%	508
	11th to 20th	Night	12% 3 25% 46% 13%	380
	21st to end	Night	11% 5% 23% 41% 19% 1	542
July	1st to 10th	Night	8% 3 17% 41% 27% 4	555
	11th to 20th	Night	9% 4 20% 39% 22% 5%	552
	21st to end	Night	6% 3 17% 49% 23% 2	624
August	1st to 10th	Night	8% 4 18% 39% 29% 3	567
	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% 30% 26% 12% 1	755
	11th to 20th	Night	22% 15% 34% 22% 6% 1	760
	21st to end	Night	27% 13% 33% 20% 6% 1	760
December	1st to 10th	Night	25% 16% 33% 19% 7%	760
	11th to 20th	Night	26% 14% 35% 22% 3	869
	21st to end	Night	33% 12% 35% 18% 2	1045

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