StarHome Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StarHome Observatory clear sky chart from 2005-11-06 to 2024-02-10.

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	13% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	78356
	Night	13% 9% 4 7% 6% 3 5% 4 5% 6% 38%	80637
Transparency	Day	46% 7% 14% 23% 9% 1	78056
	Night	46% 6% 13% 23% 11% 1	80457
Seeing	Day	16% 9% 29% 28% 14% 4	14943
	Night	16% 8% 31% 33% 11% 1	25024

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	12% 16% 7% 13% 7% 4 5% 4 3 6% 23%	2527
	11th to 20th	Day	7% 10% 6% 12% 12% 6% 9% 9% 5% 4 21%	2530
	21st to end	Day	8% 19% 7% 12% 6% 4 5% 4 5% 6% 24%	3103
February	1st to 10th	Day	8% 11% 5% 9% 7% 4 5% 5% 6% 7% 33%	1701
	11th to 20th	Day	8% 12% 6% 8% 8% 3 5% 5% 4 7% 35%	1574
	21st to end	Day	11% 11% 7% 9% 6% 4 4 4 5% 7% 31%	1390
March	1st to 10th	Day	13% 11% 4 8% 7% 4 4 4 5% 8% 32%	1916
	11th to 20th	Day	14% 9% 4 5% 6% 3 5% 4 4 8% 37%	1980
	21st to end	Day	18% 11% 4 8% 6% 3 4 5% 5% 7% 30%	2212
April	1st to 10th	Day	17% 9% 4 7% 5% 3 4 5% 5% 7% 34%	2160
	11th to 20th	Day	20% 11% 4 7% 5% 3 5% 4 4 5% 32%	2138
	21st to end	Day	22% 10% 4 6% 5% 3 5% 4 4 6% 32%	2340
May	1st to 10th	Day	17% 7% 3 6% 7% 4 5% 4 5% 6% 34%	2463
	11th to 20th	Day	17% 7% 3 5% 5% 3 5% 4 5% 7% 37%	2479
	21st to end	Day	14% 6% 3 6% 6% 3 5% 5% 6% 8% 38%	2725
June	1st to 10th	Day	12% 5% 2 4 5% 3 4 5% 6% 8% 45%	2520
	11th to 20th	Day	12% 7% 2 6% 6% 3 5% 5% 7% 8% 39%	2520
	21st to end	Day	10% 4 2 5% 6% 4 5% 5% 7% 10% 42%	2647
July	1st to 10th	Day	7% 3 2 4 4 3 5% 5% 7% 10% 51%	2520
	11th to 20th	Day	8% 5% 2 4 5% 3 4 4 6% 8% 51%	2520
	21st to end	Day	7% 4 2 4 5% 3 5% 5% 6% 9% 50%	2743
August	1st to 10th	Day	9% 5% 2 5% 6% 4 5% 6% 7% 10% 42%	2336
	11th to 20th	Day	12% 5% 2 5% 5% 3 4 5% 6% 8% 44%	2301
	21st to end	Day	10% 5% 2 5% 4 2 4 4 5% 8% 52%	2461
September	1st to 10th	Day	11% 5% 2 5% 5% 3 4 4 5% 8% 45%	2160
	11th to 20th	Day	12% 5% 2 4 5% 3 6% 5% 6% 7% 45%	2152
	21st to end	Day	17% 5% 2 4 5% 3 4 4 6% 7% 42%	1882
October	1st to 10th	Day	17% 7% 2 5% 5% 2 4 5% 6% 7% 41%	1800
	11th to 20th	Day	16% 11% 3 7% 5% 3 4 4 5% 6% 36%	1800
	21st to end	Day	18% 9% 4 7% 6% 3 4 3 5% 5% 36%	1936
November	1st to 10th	Day	12% 12% 4 8% 6% 3 4 5% 5% 6% 35%	1559
	11th to 20th	Day	13% 14% 5% 10% 6% 3 4 4 5% 6% 28%	1520
	21st to end	Day	15% 10% 5% 7% 5% 3 5% 5% 4 7% 33%	1520
December	1st to 10th	Day	12% 13% 5% 11% 9% 4 5% 6% 5% 6% 25%	1520
	11th to 20th	Day	12% 13% 8% 9% 7% 4 5% 5% 5% 6% 27%	1918
	21st to end	Day	22% 16% 6% 13% 6% 3 3 3 3 4 18%	2783
January	1st to 10th	Night	11% 13% 8% 12% 7% 3 3 4 5% 5% 29%	4428
	11th to 20th	Night	8% 10% 6% 12% 9% 6% 8% 8% 6% 5% 22%	4440
	21st to end	Night	11% 20% 8% 11% 7% 3 4 4 4 4 26%	4560
February	1st to 10th	Night	6% 10% 5% 8% 8% 4 5% 4 6% 7% 36%	2464
	11th to 20th	Night	7% 10% 5% 7% 6% 4 5% 4 6% 8% 39%	2273
	21st to end	Night	13% 10% 5% 7% 6% 5% 5% 4 4 6% 34%	1864
March	1st to 10th	Night	12% 10% 4 7% 7% 3 5% 5% 5% 6% 36%	2062
	11th to 20th	Night	14% 10% 4 7% 5% 3 4 4 4 6% 41%	1980
	21st to end	Night	19% 8% 4 6% 6% 3 4 3 4 6% 37%	2143
April	1st to 10th	Night	17% 9% 3 7% 6% 2 5% 4 4 7% 36%	1800
	11th to 20th	Night	22% 9% 4 6% 5% 2 4 4 4 6% 35%	1767
	21st to end	Night	20% 10% 3 5% 5% 3 4 4 5% 6% 35%	1620
May	1st to 10th	Night	16% 7% 3 6% 6% 3 5% 3 5% 7% 38%	1495
	11th to 20th	Night	14% 6% 2 5% 5% 3 4 5% 5% 9% 42%	1416
	21st to end	Night	12% 6% 2 5% 6% 4 5% 4 6% 8% 42%	1551
June	1st to 10th	Night	11% 4 2 4 5% 2 5% 5% 6% 9% 47%	1440
	11th to 20th	Night	9% 5% 1 6% 5% 4 5% 4 6% 8% 47%	1440
	21st to end	Night	7% 3 1 4 5% 3 6% 5% 7% 8% 51%	1296
July	1st to 10th	Night	5% 3 1 4 4 3 3 4 8% 9% 57%	1426
	11th to 20th	Night	6% 2 2 4 4 3 4 5% 6% 9% 54%	1440
	21st to end	Night	4 2 1 4 4 2 4 4 6% 8% 60%	1575
August	1st to 10th	Night	8% 4 2 5% 5% 2 5% 4 6% 7% 53%	1600
	11th to 20th	Night	12% 4 2 4 3 3 4 4 5% 7% 53%	1600
	21st to end	Night	9% 5% 2 4 4 3 4 4 4 6% 55%	1894
September	1st to 10th	Night	9% 4 2 4 5% 3 5% 4 5% 8% 53%	1800
	11th to 20th	Night	10% 4 2 4 5% 3 2 4 5% 8% 54%	1806
	21st to end	Night	15% 4 2 4 4 3 4 4 4 7% 49%	2059
October	1st to 10th	Night	15% 5% 3 5% 4 3 4 4 4 7% 47%	2160
	11th to 20th	Night	17% 8% 3 5% 5% 3 3 4 4 6% 40%	2160
	21st to end	Night	18% 10% 4 6% 6% 2 3 3 4 6% 36%	2420
November	1st to 10th	Night	15% 8% 4 6% 4 2 5% 5% 5% 6% 40%	2515
	11th to 20th	Night	14% 11% 5% 10% 7% 3 5% 4 4 5% 33%	2660
	21st to end	Night	13% 10% 4 7% 6% 3 4 4 5% 8% 36%	2660
December	1st to 10th	Night	10% 13% 5% 11% 8% 4 5% 4 4 6% 31%	2660
	11th to 20th	Night	12% 15% 5% 10% 7% 5% 5% 4 5% 6% 26%	3279
	21st to end	Night	22% 16% 5% 9% 6% 3 4 4 3 4 23%	4884

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% 6% 10% 17% 3	2528
	11th to 20th	Day	62% 7% 11% 14% 5%	2530
	21st to end	Day	59% 7% 14% 15% 5%	3103
February	1st to 10th	Day	49% 6% 14% 22% 8% 1	1701
	11th to 20th	Day	50% 6% 13% 24% 8% 1	1583
	21st to end	Day	53% 5% 11% 19% 10% 1	1390
March	1st to 10th	Day	52% 6% 16% 19% 7% 1	1916
	11th to 20th	Day	47% 6% 16% 24% 7% 1	1980
	21st to end	Day	54% 6% 11% 21% 8% 1	2212
April	1st to 10th	Day	49% 6% 12% 20% 12% 1	2160
	11th to 20th	Day	55% 6% 9% 18% 11% 2	2138
	21st to end	Day	54% 6% 12% 18% 9%	2340
May	1st to 10th	Day	49% 7% 13% 20% 9% 2	2463
	11th to 20th	Day	45% 7% 14% 22% 11% 1	2483
	21st to end	Day	44% 8% 12% 23% 13% 1	2725
June	1st to 10th	Day	35% 8% 15% 30% 11% 1	2520
	11th to 20th	Day	40% 8% 15% 23% 13% 1	2520
	21st to end	Day	35% 9% 18% 29% 8% 2	2647
July	1st to 10th	Day	26% 9% 17% 32% 15%	2451
	11th to 20th	Day	30% 8% 16% 26% 17% 3	2445
	21st to end	Day	28% 10% 18% 28% 16% 1	2591
August	1st to 10th	Day	36% 8% 17% 28% 9% 3	2321
	11th to 20th	Day	36% 8% 15% 30% 10%	2301
	21st to end	Day	31% 6% 15% 31% 16% 1	2461
September	1st to 10th	Day	36% 8% 13% 32% 11%	2160
	11th to 20th	Day	36% 7% 17% 32% 8%	2152
	21st to end	Day	40% 7% 13% 31% 8%	1882
October	1st to 10th	Day	41% 7% 14% 28% 10%	1800
	11th to 20th	Day	47% 7% 13% 22% 10% 1	1800
	21st to end	Day	50% 6% 13% 22% 9%	1936
November	1st to 10th	Day	49% 6% 13% 18% 12% 2	1559
	11th to 20th	Day	58% 6% 13% 16% 7%	1520
	21st to end	Day	50% 8% 16% 19% 6% 2	1520
December	1st to 10th	Day	59% 7% 16% 14% 5%	1520
	11th to 20th	Day	58% 6% 16% 15% 4	1915
	21st to end	Day	69% 7% 8% 10% 6%	2783
January	1st to 10th	Night	57% 6% 12% 19% 6% 1	4428
	11th to 20th	Night	59% 8% 13% 14% 5%	4440
	21st to end	Night	62% 5% 9% 18% 5% 1	4560
February	1st to 10th	Night	47% 6% 13% 23% 10% 1	2464
	11th to 20th	Night	43% 6% 15% 25% 9% 1	2274
	21st to end	Night	51% 5% 12% 20% 11% 1	1864
March	1st to 10th	Night	49% 7% 14% 22% 8%	2062
	11th to 20th	Night	45% 5% 13% 21% 15% 1	1980
	21st to end	Night	49% 5% 12% 24% 10% 1	2143
April	1st to 10th	Night	48% 6% 8% 21% 15% 1	1800
	11th to 20th	Night	51% 5% 8% 20% 14% 2	1767
	21st to end	Night	49% 6% 10% 20% 13% 2	1620
May	1st to 10th	Night	46% 7% 12% 18% 15% 1	1495
	11th to 20th	Night	40% 7% 14% 23% 14% 2	1417
	21st to end	Night	40% 8% 13% 22% 16% 2	1552
June	1st to 10th	Night	32% 9% 15% 29% 15% 1	1440
	11th to 20th	Night	34% 8% 14% 25% 17% 3	1440
	21st to end	Night	28% 9% 16% 30% 14% 3	1296
July	1st to 10th	Night	22% 7% 17% 35% 18% 1	1386
	11th to 20th	Night	25% 9% 15% 31% 17% 4	1392
	21st to end	Night	21% 7% 17% 32% 19% 5%	1487
August	1st to 10th	Night	30% 6% 15% 31% 14% 4	1591
	11th to 20th	Night	31% 7% 16% 30% 15% 2	1600
	21st to end	Night	31% 7% 13% 30% 17% 3	1894
September	1st to 10th	Night	30% 7% 14% 32% 16% 1	1800
	11th to 20th	Night	28% 7% 18% 32% 16%	1806
	21st to end	Night	36% 6% 13% 33% 12% 1	2059
October	1st to 10th	Night	38% 6% 11% 31% 14% 2	2160
	11th to 20th	Night	44% 7% 13% 22% 12% 1	2160
	21st to end	Night	50% 6% 12% 22% 10% 1	2420
November	1st to 10th	Night	43% 6% 13% 24% 12% 1	2515
	11th to 20th	Night	54% 5% 13% 17% 10% 1	2660
	21st to end	Night	48% 6% 17% 22% 6% 1	2660
December	1st to 10th	Night	55% 6% 15% 19% 5%	2660
	11th to 20th	Night	58% 7% 12% 16% 5% 1	3281
	21st to end	Night	65% 5% 11% 15% 4	4884

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	13% 9% 33% 32% 10% 3	270
	11th to 20th	Day	16% 9% 37% 30% 7% 1	270
	21st to end	Day	16% 5% 38% 27% 12% 1	297
February	1st to 10th	Day	13% 11% 45% 24% 6% 1	267
	11th to 20th	Day	16% 15% 41% 21% 5% 2	240
	21st to end	Day	19% 14% 37% 26% 4	198
March	1st to 10th	Day	18% 13% 38% 23% 8% 1	240
	11th to 20th	Day	19% 7% 35% 28% 9% 3	240
	21st to end	Day	24% 13% 28% 22% 9% 4	298
April	1st to 10th	Day	21% 15% 34% 24% 6% 1	420
	11th to 20th	Day	28% 12% 22% 25% 11% 1	416
	21st to end	Day	27% 9% 31% 21% 10% 3	600
May	1st to 10th	Day	21% 11% 30% 27% 10% 1	603
	11th to 20th	Day	20% 12% 28% 25% 12% 4	625
	21st to end	Day	16% 10% 25% 27% 17% 4	679
June	1st to 10th	Day	11% 6% 26% 33% 18% 6%	630
	11th to 20th	Day	12% 8% 33% 33% 11% 3	630
	21st to end	Day	12% 6% 28% 30% 18% 7%	630
July	1st to 10th	Day	8% 4 21% 31% 25% 11%	621
	11th to 20th	Day	9% 5% 25% 32% 20% 9%	618
	21st to end	Day	6% 4 24% 34% 23% 8%	684
August	1st to 10th	Day	10% 4 19% 33% 27% 7%	626
	11th to 20th	Day	12% 5% 18% 31% 27% 7%	615
	21st to end	Day	11% 7% 21% 33% 20% 7%	580
September	1st to 10th	Day	15% 6% 25% 31% 18% 5%	450
	11th to 20th	Day	13% 4 27% 32% 20% 4	442
	21st to end	Day	22% 10% 27% 23% 11% 6%	270
October	1st to 10th	Day	19% 8% 31% 19% 17% 7%	270
	11th to 20th	Day	17% 16% 30% 25% 9% 1	270
	21st to end	Day	22% 14% 41% 14% 6% 2	297
November	1st to 10th	Day	20% 12% 32% 25% 10% 1	270
	11th to 20th	Day	17% 16% 43% 19% 5%	270
	21st to end	Day	21% 13% 38% 22% 5%	270
December	1st to 10th	Day	14% 10% 40% 25% 11%	270
	11th to 20th	Day	20% 14% 37% 24% 5%	270
	21st to end	Day	29% 11% 34% 23% 2	297
January	1st to 10th	Night	14% 10% 35% 33% 8% 1	950
	11th to 20th	Night	16% 8% 37% 30% 8% 1	950
	21st to end	Night	18% 9% 35% 29% 8% 1	1045
February	1st to 10th	Night	12% 10% 38% 31% 8% 1	948
	11th to 20th	Night	11% 10% 42% 28% 8% 1	879
	21st to end	Night	19% 9% 35% 28% 9%	680
March	1st to 10th	Night	15% 10% 35% 30% 9% 1	622
	11th to 20th	Night	19% 6% 39% 28% 7% 1	540
	21st to end	Night	23% 8% 30% 26% 11% 2	594
April	1st to 10th	Night	20% 11% 36% 28% 6%	540
	11th to 20th	Night	26% 8% 30% 28% 8% 1	533
	21st to end	Night	24% 8% 29% 29% 9% 1	540
May	1st to 10th	Night	19% 9% 34% 32% 6%	540
	11th to 20th	Night	15% 8% 30% 34% 11% 1	531
	21st to end	Night	13% 6% 26% 39% 15% 1	584
June	1st to 10th	Night	11% 4 27% 43% 14% 1	540
	11th to 20th	Night	9% 5% 31% 44% 10% 1	540
	21st to end	Night	9% 3 23% 45% 17% 2	540
July	1st to 10th	Night	6% 2 21% 42% 27% 2	525
	11th to 20th	Night	8% 3 23% 43% 22% 1	522
	21st to end	Night	4 2 22% 48% 22% 3	591
August	1st to 10th	Night	9% 3 15% 51% 21% 1	537
	11th to 20th	Night	14% 3 18% 35% 27% 3	534
	21st to end	Night	9% 4 22% 38% 25% 2	594
September	1st to 10th	Night	10% 6% 24% 39% 18% 3	540
	11th to 20th	Night	10% 4 28% 37% 19% 2	540
	21st to end	Night	16% 7% 33% 29% 14% 1	619
October	1st to 10th	Night	19% 6% 28% 32% 13% 1	720
	11th to 20th	Night	21% 9% 33% 29% 8% 1	720
	21st to end	Night	26% 8% 29% 29% 8%	792
November	1st to 10th	Night	19% 9% 31% 32% 8% 1	849
	11th to 20th	Night	19% 11% 35% 27% 8% 1	950
	21st to end	Night	17% 12% 37% 26% 7%	950
December	1st to 10th	Night	16% 9% 35% 31% 8% 1	950
	11th to 20th	Night	20% 9% 37% 26% 8% 1	950
	21st to end	Night	26% 11% 31% 27% 6%	1045

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