The Top Of The Sky Mini Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Top Of The Sky Mini Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	15% 12% 5% 10% 5% 5% 4 4 5% 6% 29%	131731
	Night	15% 13% 5% 11% 5% 4 4 3 5% 6% 28%	131064
Transparency	Day	55% 4 10% 19% 10% 1	135363
	Night	57% 4 9% 18% 10% 1	135502
Seeing	Day	24% 17% 29% 18% 9% 3	16499
	Night	23% 15% 27% 23% 10% 1	26356

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	14% 20% 8% 27% 6% 3 3 2 2 4 10%	5258
	11th to 20th	Day	9% 20% 8% 22% 11% 7% 5% 2 3 3 9%	5280
	21st to end	Day	17% 17% 8% 14% 10% 8% 5% 2 3 3 12%	5780
February	1st to 10th	Day	15% 20% 7% 12% 8% 5% 3 3 4 5% 19%	1890
	11th to 20th	Day	14% 16% 7% 16% 7% 5% 4 3 4 5% 18%	1875
	21st to end	Day	16% 17% 5% 14% 6% 4 5% 3 5% 4 22%	1730
March	1st to 10th	Day	17% 11% 8% 13% 5% 3 3 3 4 6% 27%	4955
	11th to 20th	Day	12% 12% 5% 11% 5% 4 4 3 4 5% 34%	5090
	21st to end	Day	18% 14% 6% 10% 5% 4 3 3 4 5% 27%	5586
April	1st to 10th	Day	16% 15% 4 9% 4 4 4 3 5% 6% 30%	2369
	11th to 20th	Day	17% 13% 3 7% 6% 4 4 3 5% 5% 32%	2647
	21st to end	Day	19% 12% 3 7% 4 4 3 3 4 6% 34%	2730
May	1st to 10th	Day	20% 11% 3 9% 5% 5% 4 4 5% 6% 28%	5992
	11th to 20th	Day	19% 10% 3 8% 5% 4 4 4 5% 7% 31%	5939
	21st to end	Day	18% 9% 3 7% 5% 4 5% 4 6% 7% 34%	6656
June	1st to 10th	Day	15% 6% 3 7% 5% 5% 4 4 6% 8% 37%	2800
	11th to 20th	Day	14% 8% 2 8% 4 4 4 4 6% 9% 36%	2800
	21st to end	Day	12% 6% 3 5% 4 4 5% 5% 6% 10% 40%	2800
July	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 6% 10% 42%	6006
	11th to 20th	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 9% 37%	5900
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 41%	6424
August	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 4 7% 9% 33%	4234
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 6% 7% 8% 34%	4174
	21st to end	Day	10% 5% 2 6% 5% 5% 6% 4 7% 9% 40%	4578
September	1st to 10th	Day	14% 7% 2 5% 4 5% 4 4 6% 8% 41%	2322
	11th to 20th	Day	10% 5% 2 6% 5% 4 4 4 7% 8% 44%	2200
	21st to end	Day	20% 9% 2 7% 4 4 3 3 5% 7% 36%	2200
October	1st to 10th	Day	15% 7% 3 7% 4 4 5% 4 5% 7% 40%	2050
	11th to 20th	Day	21% 10% 3 8% 6% 4 4 3 4 5% 33%	2000
	21st to end	Day	22% 12% 4 8% 5% 4 4 3 5% 6% 26%	2200
November	1st to 10th	Day	13% 11% 5% 8% 6% 4 4 3 5% 7% 35%	1862
	11th to 20th	Day	18% 18% 5% 9% 5% 4 4 3 4 6% 24%	1814
	21st to end	Day	18% 20% 8% 11% 5% 4 4 3 5% 5% 16%	1680
December	1st to 10th	Day	16% 19% 11% 16% 6% 4 4 3 4 3 14%	2680
	11th to 20th	Day	22% 21% 8% 12% 6% 3 3 3 4 4 13%	3070
	21st to end	Day	23% 25% 8% 14% 6% 4 4 2 3 4 8%	4160
January	1st to 10th	Night	14% 20% 9% 27% 6% 4 3 3 3 4 9%	7635
	11th to 20th	Night	10% 18% 11% 21% 10% 6% 5% 3 3 3 10%	7680
	21st to end	Night	16% 17% 7% 14% 7% 6% 5% 3 4 5% 16%	8401
February	1st to 10th	Night	18% 17% 5% 15% 7% 4 3 3 4 4 19%	2730
	11th to 20th	Night	15% 15% 7% 15% 8% 5% 4 3 5% 5% 19%	2626
	21st to end	Night	19% 14% 5% 13% 7% 4 4 3 4 4 23%	2076
March	1st to 10th	Night	20% 14% 5% 9% 5% 3 3 3 4 4 30%	5118
	11th to 20th	Night	12% 10% 5% 9% 4 4 4 3 5% 5% 36%	5090
	21st to end	Night	18% 12% 5% 10% 5% 4 4 3 4 5% 31%	5588
April	1st to 10th	Night	18% 14% 5% 8% 5% 4 4 3 4 6% 29%	2250
	11th to 20th	Night	15% 10% 3 8% 5% 5% 5% 4 6% 6% 32%	1940
	21st to end	Night	14% 10% 4 8% 4 4 5% 4 5% 7% 35%	1890
May	1st to 10th	Night	24% 11% 2 7% 5% 4 4 3 5% 6% 27%	4123
	11th to 20th	Night	16% 11% 3 6% 4 3 4 3 5% 8% 36%	4086
	21st to end	Night	17% 8% 2 5% 4 4 4 3 6% 9% 38%	4196
June	1st to 10th	Night	11% 7% 2 6% 5% 5% 6% 4 6% 7% 41%	1600
	11th to 20th	Night	10% 5% 3 8% 5% 6% 5% 4 6% 8% 41%	1600
	21st to end	Night	11% 5% 2 6% 4 4 5% 5% 7% 10% 41%	1600
July	1st to 10th	Night	9% 7% 2 6% 4 4 4 4 6% 10% 44%	3919
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 4 6% 9% 44%	4057
	21st to end	Night	8% 6% 2 6% 4 4 4 4 5% 8% 48%	4427
August	1st to 10th	Night	10% 8% 3 7% 5% 5% 5% 4 6% 8% 40%	2915
	11th to 20th	Night	14% 9% 2 6% 3 4 3 3 4 7% 43%	2875
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 7% 8% 43%	3285
September	1st to 10th	Night	12% 6% 2 5% 4 4 5% 3 5% 7% 47%	2075
	11th to 20th	Night	9% 4 2 5% 4 3 4 3 5% 8% 52%	2200
	21st to end	Night	17% 7% 3 8% 5% 4 4 3 5% 5% 39%	2200
October	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 6% 7% 45%	2350
	11th to 20th	Night	17% 10% 4 8% 4 4 5% 4 5% 6% 34%	2400
	21st to end	Night	24% 10% 2 6% 4 4 3 4 5% 7% 30%	2640
November	1st to 10th	Night	14% 10% 3 8% 5% 3 4 3 5% 7% 37%	2651
	11th to 20th	Night	16% 18% 5% 10% 4 4 3 3 5% 6% 25%	2803
	21st to end	Night	19% 19% 6% 10% 5% 4 5% 3 5% 5% 20%	2940
December	1st to 10th	Night	14% 19% 9% 16% 6% 5% 4 3 4 4 16%	4720
	11th to 20th	Night	19% 22% 10% 12% 5% 4 3 2 3 4 16%	5345
	21st to end	Night	22% 23% 8% 12% 5% 4 4 3 4 4 12%	7033

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	82% 2 7% 6% 2	5259
	11th to 20th	Day	83% 3 6% 6% 3	5280
	21st to end	Day	80% 2 4 10% 3	5780
February	1st to 10th	Day	70% 4 8% 13% 6%	1890
	11th to 20th	Day	70% 5% 7% 13% 5%	1884
	21st to end	Day	65% 5% 10% 13% 6% 1	1730
March	1st to 10th	Day	59% 4 11% 19% 6%	4955
	11th to 20th	Day	53% 4 14% 21% 6% 2	5090
	21st to end	Day	60% 5% 6% 17% 11% 1	5586
April	1st to 10th	Day	54% 5% 11% 16% 12% 1	2369
	11th to 20th	Day	53% 6% 10% 17% 12% 2	2647
	21st to end	Day	52% 4 10% 21% 10% 2	2730
May	1st to 10th	Day	56% 5% 8% 19% 9% 3	6027
	11th to 20th	Day	51% 5% 10% 22% 11% 1	5943
	21st to end	Day	48% 6% 10% 19% 12% 4	6671
June	1st to 10th	Day	45% 6% 10% 25% 13% 1	2800
	11th to 20th	Day	43% 8% 14% 21% 14% 1	2800
	21st to end	Day	38% 7% 13% 24% 16% 2	2800
July	1st to 10th	Day	39% 3 11% 28% 16% 2	5941
	11th to 20th	Day	42% 3 13% 26% 15% 1	5831
	21st to end	Day	40% 4 11% 23% 20% 1	6240
August	1st to 10th	Day	46% 5% 14% 24% 11%	4221
	11th to 20th	Day	44% 6% 10% 30% 9% 1	4213
	21st to end	Day	39% 5% 14% 30% 11% 1	4578
September	1st to 10th	Day	40% 6% 13% 21% 16% 3	2322
	11th to 20th	Day	37% 6% 14% 26% 16% 1	2200
	21st to end	Day	48% 5% 10% 23% 14% 1	2200
October	1st to 10th	Day	35% 4 13% 28% 20% 1	3336
	11th to 20th	Day	50% 2 10% 20% 15% 3	3240
	21st to end	Day	46% 3 13% 23% 14% 1	3538
November	1st to 10th	Day	49% 6% 10% 21% 13% 1	1862
	11th to 20th	Day	62% 4 10% 14% 7% 2	1814
	21st to end	Day	69% 6% 8% 12% 5%	1680
December	1st to 10th	Day	76% 3 9% 10% 2	2680
	11th to 20th	Day	76% 4 6% 12% 2	3066
	21st to end	Day	83% 3 6% 7% 2	4160
January	1st to 10th	Night	82% 3 5% 8% 3	7636
	11th to 20th	Night	82% 3 5% 7% 3 1	7680
	21st to end	Night	73% 2 7% 12% 5%	8401
February	1st to 10th	Night	69% 4 8% 14% 5%	2730
	11th to 20th	Night	68% 4 8% 14% 5%	2628
	21st to end	Night	65% 5% 8% 14% 7% 1	2076
March	1st to 10th	Night	58% 3 12% 18% 9%	5118
	11th to 20th	Night	49% 4 13% 23% 9% 1	5090
	21st to end	Night	56% 5% 7% 18% 13% 1	5588
April	1st to 10th	Night	56% 6% 10% 16% 11% 2	2250
	11th to 20th	Night	51% 5% 10% 19% 13% 2	1940
	21st to end	Night	47% 6% 10% 21% 14% 3	1890
May	1st to 10th	Night	58% 4 8% 15% 9% 5%	4150
	11th to 20th	Night	46% 6% 9% 23% 13% 2	4087
	21st to end	Night	42% 6% 12% 23% 14% 2	4203
June	1st to 10th	Night	40% 7% 10% 24% 15% 3	1600
	11th to 20th	Night	40% 6% 13% 21% 18% 2	1600
	21st to end	Night	37% 9% 12% 24% 17% 1	1600
July	1st to 10th	Night	38% 3 12% 26% 18% 3	3876
	11th to 20th	Night	38% 4 12% 28% 15% 2	4006
	21st to end	Night	36% 4 11% 20% 26% 2	4297
August	1st to 10th	Night	42% 5% 11% 26% 14% 1	2904
	11th to 20th	Night	43% 5% 9% 31% 12% 1	2902
	21st to end	Night	39% 4 11% 30% 14% 2	3285
September	1st to 10th	Night	37% 6% 11% 28% 16% 3	2075
	11th to 20th	Night	30% 6% 13% 29% 20% 2	2200
	21st to end	Night	46% 6% 9% 21% 17% 1	2200
October	1st to 10th	Night	35% 3 8% 29% 23% 1	3844
	11th to 20th	Night	46% 3 10% 21% 17% 3	3899
	21st to end	Night	44% 3 12% 25% 15% 1	4252
November	1st to 10th	Night	46% 6% 11% 22% 13% 2	2651
	11th to 20th	Night	61% 4 9% 16% 7% 3	2803
	21st to end	Night	66% 5% 10% 15% 4	2940
December	1st to 10th	Night	73% 3 11% 10% 3	4720
	11th to 20th	Night	74% 3 8% 13% 2	5348
	21st to end	Night	77% 4 8% 10% 1	7033

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	35% 26% 29% 7% 2	330
	11th to 20th	Day	36% 32% 27% 4 1 1	330
	21st to end	Day	33% 31% 28% 6% 1	363
February	1st to 10th	Day	36% 24% 33% 6% 1	330
	11th to 20th	Day	35% 32% 28% 5%	330
	21st to end	Day	38% 30% 28% 4	273
March	1st to 10th	Day	30% 26% 30% 12% 2	498
	11th to 20th	Day	34% 24% 29% 10% 3 1	540
	21st to end	Day	35% 24% 25% 12% 2	594
April	1st to 10th	Day	29% 25% 28% 13% 5%	540
	11th to 20th	Day	33% 20% 26% 14% 7% 1	532
	21st to end	Day	30% 18% 31% 15% 4 1	540
May	1st to 10th	Day	28% 22% 29% 15% 3 2	543
	11th to 20th	Day	25% 13% 32% 21% 7% 2	551
	21st to end	Day	19% 11% 31% 24% 12% 4	573
June	1st to 10th	Day	17% 14% 37% 20% 9% 2	530
	11th to 20th	Day	19% 11% 30% 27% 11% 2	530
	21st to end	Day	16% 13% 31% 26% 11% 3	530
July	1st to 10th	Day	13% 8% 24% 30% 18% 6%	525
	11th to 20th	Day	13% 10% 28% 26% 17% 6%	524
	21st to end	Day	11% 8% 30% 28% 16% 7%	576
August	1st to 10th	Day	11% 8% 24% 31% 20% 6%	526
	11th to 20th	Day	15% 7% 29% 25% 15% 8%	519
	21st to end	Day	12% 7% 28% 28% 18% 8%	583
September	1st to 10th	Day	16% 10% 24% 25% 19% 6%	530
	11th to 20th	Day	10% 5% 22% 29% 23% 10%	530
	21st to end	Day	25% 11% 25% 23% 12% 5%	530
October	1st to 10th	Day	15% 14% 28% 22% 18% 3	530
	11th to 20th	Day	25% 20% 33% 15% 5% 2	530
	21st to end	Day	34% 19% 30% 14% 3	583
November	1st to 10th	Day	28% 21% 31% 13% 6% 1	340
	11th to 20th	Day	37% 19% 27% 12% 4	289
	21st to end	Day	37% 21% 36% 6%	220
December	1st to 10th	Day	33% 26% 27% 11% 1 1	220
	11th to 20th	Day	38% 26% 26% 8% 1	220
	21st to end	Day	36% 29% 27% 8%	267
January	1st to 10th	Night	31% 25% 30% 11% 3	840
	11th to 20th	Night	31% 28% 27% 12% 2	840
	21st to end	Night	28% 26% 29% 15% 2	924
February	1st to 10th	Night	32% 22% 28% 17% 1	840
	11th to 20th	Night	30% 23% 31% 15% 1	823
	21st to end	Night	32% 25% 30% 13% 1	692
March	1st to 10th	Night	27% 22% 31% 17% 3	840
	11th to 20th	Night	27% 21% 30% 17% 5% 1	840
	21st to end	Night	27% 19% 30% 19% 5%	924
April	1st to 10th	Night	29% 16% 30% 20% 4	840
	11th to 20th	Night	23% 18% 33% 18% 7% 1	696
	21st to end	Night	20% 15% 33% 25% 6%	630
May	1st to 10th	Night	25% 10% 30% 26% 8% 1	630
	11th to 20th	Night	20% 10% 29% 31% 9% 2	609
	21st to end	Night	13% 5% 29% 34% 17% 1	554
June	1st to 10th	Night	12% 5% 35% 36% 12% 1	400
	11th to 20th	Night	13% 6% 31% 38% 13% 1	400
	21st to end	Night	12% 7% 29% 36% 16% 1	400
July	1st to 10th	Night	9% 4 19% 39% 26% 3	524
	11th to 20th	Night	10% 3 24% 35% 24% 3	582
	21st to end	Night	10% 4 18% 42% 24% 2	657
August	1st to 10th	Night	9% 5% 18% 41% 25% 2	597
	11th to 20th	Night	13% 5% 16% 36% 27% 3	594
	21st to end	Night	12% 4 19% 34% 29% 3	696
September	1st to 10th	Night	14% 7% 20% 32% 24% 3	800
	11th to 20th	Night	12% 4 19% 34% 26% 5%	800
	21st to end	Night	20% 9% 24% 28% 16% 3	800
October	1st to 10th	Night	18% 9% 27% 27% 17% 2	800
	11th to 20th	Night	25% 16% 28% 23% 8% 1	800
	21st to end	Night	32% 15% 30% 19% 4	880
November	1st to 10th	Night	22% 17% 29% 23% 8% 1	820
	11th to 20th	Night	29% 22% 28% 15% 5% 1	840
	21st to end	Night	34% 20% 30% 13% 4	840
December	1st to 10th	Night	32% 23% 29% 12% 4	840
	11th to 20th	Night	30% 27% 28% 13% 2	840
	21st to end	Night	34% 21% 31% 12% 1	924

Page generated at 2026-05-14 01:58:59. Version ac07

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