StarHome Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StarHome Observatory clear sky chart from 2005-11-05 to 2024-10-14.

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% 3 8% 5% 4 4 4 5% 7% 38%	101537
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 39%	97096
Transparency	Day	45% 7% 14% 24% 10% 1	101328
	Night	45% 6% 13% 24% 11% 1	96981
Seeing	Day	16% 9% 29% 28% 14% 4	15985
	Night	16% 8% 31% 33% 11% 1	25823

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% 17% 6% 15% 6% 5% 4 3 4 6% 23%	2527
	11th to 20th	Day	6% 11% 5% 14% 10% 9% 9% 6% 5% 4 21%	2530
	21st to end	Day	8% 20% 7% 13% 5% 5% 5% 3 5% 5% 24%	3103
February	1st to 10th	Day	9% 12% 5% 10% 6% 5% 5% 4 5% 7% 33%	1710
	11th to 20th	Day	8% 12% 5% 9% 6% 4 4 4 5% 7% 36%	1664
	21st to end	Day	11% 11% 6% 9% 6% 4 4 3 5% 6% 33%	1475
March	1st to 10th	Day	12% 11% 3 8% 5% 4 4 3 5% 7% 39%	3334
	11th to 20th	Day	10% 9% 3 8% 6% 5% 5% 3 5% 8% 40%	3480
	21st to end	Day	21% 11% 4 11% 6% 4 5% 4 4 6% 24%	3875
April	1st to 10th	Day	18% 10% 3 8% 4 4 5% 3 5% 7% 34%	2280
	11th to 20th	Day	20% 12% 4 8% 5% 4 4 3 4 4 32%	2259
	21st to end	Day	21% 11% 3 6% 5% 5% 4 4 4 6% 31%	2470
May	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 5% 6% 41%	4302
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 44%	4399
	21st to end	Day	17% 7% 2 8% 5% 4 5% 3 6% 8% 35%	4824
June	1st to 10th	Day	12% 5% 2 4 4 5% 5% 4 6% 8% 46%	2660
	11th to 20th	Day	12% 7% 2 7% 5% 4 4 4 7% 8% 38%	2660
	21st to end	Day	11% 4 2 5% 6% 5% 5% 4 7% 9% 42%	2795
July	1st to 10th	Day	11% 5% 2 6% 4 6% 6% 5% 8% 10% 38%	4440
	11th to 20th	Day	5% 3 1 4 4 3 4 3 6% 8% 59%	4440
	21st to end	Day	8% 5% 2 5% 4 4 4 4 6% 9% 50%	4813
August	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4112
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 44%	4041
	21st to end	Day	12% 5% 2 7% 4 4 4 3 5% 7% 46%	4321
September	1st to 10th	Day	11% 5% 2 6% 5% 4 4 3 5% 8% 47%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 7% 45%	2271
	21st to end	Day	16% 5% 2 5% 4 4 4 3 6% 7% 44%	1986
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 4 5% 7% 42%	1900
	11th to 20th	Day	15% 11% 2 7% 4 4 5% 3 5% 6% 36%	1830
	21st to end	Day	18% 10% 4 9% 5% 4 4 2 5% 5% 35%	1936
November	1st to 10th	Day	11% 13% 3 9% 6% 3 4 4 5% 6% 35%	1559
	11th to 20th	Day	13% 14% 5% 11% 6% 5% 4 3 5% 6% 27%	1520
	21st to end	Day	14% 11% 5% 8% 4 5% 5% 4 5% 6% 33%	1520
December	1st to 10th	Day	12% 13% 5% 13% 7% 5% 5% 4 5% 6% 25%	1520
	11th to 20th	Day	12% 14% 7% 11% 5% 6% 4 4 5% 6% 27%	1918
	21st to end	Day	22% 17% 6% 15% 5% 4 3 2 4 4 18%	2783
January	1st to 10th	Night	11% 14% 7% 13% 5% 4 3 3 5% 5% 29%	4428
	11th to 20th	Night	7% 10% 5% 13% 8% 7% 8% 7% 7% 5% 22%	4440
	21st to end	Night	10% 21% 7% 13% 5% 4 4 3 4 4 26%	4560
February	1st to 10th	Night	6% 11% 5% 10% 7% 6% 4 3 6% 7% 36%	2470
	11th to 20th	Night	7% 10% 4 8% 5% 6% 5% 3 6% 8% 39%	2403
	21st to end	Night	12% 11% 4 8% 5% 6% 5% 3 5% 6% 36%	1977
March	1st to 10th	Night	9% 8% 2 6% 5% 4 4 3 5% 7% 46%	3551
	11th to 20th	Night	9% 8% 3 8% 5% 5% 5% 4 6% 7% 40%	3480
	21st to end	Night	26% 11% 3 11% 4 3 3 2 4 4 30%	3755
April	1st to 10th	Night	17% 10% 3 8% 5% 3 4 3 5% 7% 36%	1900
	11th to 20th	Night	21% 10% 3 7% 5% 3 3 3 4 6% 36%	1866
	21st to end	Night	19% 10% 3 6% 4 4 4 4 5% 6% 35%	1710
May	1st to 10th	Night	13% 7% 3 8% 5% 5% 4 3 5% 6% 41%	2601
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 5% 8% 50%	2506
	21st to end	Night	14% 8% 2 6% 5% 4 4 4 6% 7% 40%	2743
June	1st to 10th	Night	12% 5% 1 5% 5% 3 4 4 6% 8% 47%	1520
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 4 6% 9% 46%	1520
	21st to end	Night	8% 4 1 6% 3 4 5% 4 7% 7% 50%	1367
July	1st to 10th	Night	12% 4 2 5% 4 4 4 4 8% 8% 48%	2516
	11th to 20th	Night	4 2 1 5% 3 3 4 4 6% 8% 59%	2530
	21st to end	Night	6% 3 1 4 3 3 3 3 6% 8% 59%	2750
August	1st to 10th	Night	13% 5% 2 6% 4 3 3 3 5% 7% 48%	2825
	11th to 20th	Night	13% 4 2 4 3 3 4 3 7% 7% 49%	2798
	21st to end	Night	11% 6% 2 6% 4 5% 4 3 5% 8% 46%	3338
September	1st to 10th	Night	8% 5% 2 5% 4 4 5% 3 5% 8% 53%	1900
	11th to 20th	Night	9% 4 2 5% 4 3 3 3 5% 8% 55%	1907
	21st to end	Night	15% 4 2 4 3 3 4 3 4 6% 51%	2174
October	1st to 10th	Night	14% 5% 3 5% 3 3 4 3 4 7% 49%	2280
	11th to 20th	Night	17% 9% 3 5% 5% 4 4 4 4 6% 39%	2203
	21st to end	Night	18% 11% 3 7% 5% 3 3 3 5% 6% 36%	2420
November	1st to 10th	Night	15% 9% 3 6% 4 4 4 4 5% 6% 40%	2515
	11th to 20th	Night	13% 12% 5% 11% 6% 4 5% 3 4 5% 33%	2660
	21st to end	Night	13% 10% 4 9% 5% 4 4 3 5% 8% 35%	2660
December	1st to 10th	Night	10% 14% 4 12% 7% 5% 5% 3 4 6% 30%	2660
	11th to 20th	Night	12% 15% 4 12% 6% 6% 5% 3 5% 5% 26%	3279
	21st to end	Night	22% 17% 5% 10% 5% 4 3 3 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	1710
	11th to 20th	Day	48% 6% 13% 25% 8%	1673
	21st to end	Day	51% 5% 12% 20% 10% 1	1475
March	1st to 10th	Day	44% 7% 15% 23% 10% 1	3334
	11th to 20th	Day	41% 7% 13% 31% 7% 1	3480
	21st to end	Day	61% 7% 9% 18% 5%	3875
April	1st to 10th	Day	49% 6% 11% 20% 12% 1	2280
	11th to 20th	Day	54% 6% 9% 18% 12% 2	2259
	21st to end	Day	54% 6% 12% 19% 9%	2470
May	1st to 10th	Day	41% 7% 12% 25% 13% 1	4341
	11th to 20th	Day	38% 8% 13% 26% 14% 2	4403
	21st to end	Day	45% 9% 13% 22% 10% 1	4837
June	1st to 10th	Day	34% 8% 15% 30% 12% 1	2660
	11th to 20th	Day	42% 7% 15% 22% 13% 1	2660
	21st to end	Day	36% 8% 17% 28% 9% 2	2795
July	1st to 10th	Day	40% 6% 19% 24% 12%	4371
	11th to 20th	Day	24% 5% 17% 37% 17% 1	4365
	21st to end	Day	30% 6% 16% 30% 17% 1	4662
August	1st to 10th	Day	37% 5% 14% 29% 13% 1	4097
	11th to 20th	Day	37% 5% 13% 35% 9%	4080
	21st to end	Day	38% 4 17% 27% 13% 1	4320
September	1st to 10th	Day	35% 7% 13% 33% 11%	2280
	11th to 20th	Day	36% 7% 17% 32% 8%	2271
	21st to end	Day	38% 7% 13% 32% 10% 1	1986
October	1st to 10th	Day	40% 7% 13% 29% 11%	1900
	11th to 20th	Day	47% 7% 13% 21% 10% 1	1830
	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	2470
	11th to 20th	Night	43% 7% 15% 25% 9% 1	2404
	21st to end	Night	49% 6% 13% 21% 11% 1	1977
March	1st to 10th	Night	37% 6% 13% 26% 17% 1	3551
	11th to 20th	Night	39% 8% 11% 27% 14% 1	3480
	21st to end	Night	59% 4 9% 19% 8% 1	3755
April	1st to 10th	Night	48% 6% 8% 21% 16% 1	1900
	11th to 20th	Night	49% 5% 8% 21% 14% 2	1866
	21st to end	Night	48% 7% 10% 20% 14% 2	1710
May	1st to 10th	Night	42% 8% 11% 20% 18% 1	2627
	11th to 20th	Night	33% 6% 12% 26% 20% 3	2507
	21st to end	Night	41% 8% 13% 23% 14% 1	2751
June	1st to 10th	Night	32% 8% 14% 29% 15% 2	1520
	11th to 20th	Night	34% 7% 14% 25% 17% 2	1520
	21st to end	Night	29% 9% 15% 30% 14% 3	1367
July	1st to 10th	Night	33% 5% 22% 26% 13% 1	2476
	11th to 20th	Night	22% 5% 14% 37% 20% 2	2482
	21st to end	Night	23% 5% 16% 34% 19% 3	2662
August	1st to 10th	Night	37% 4 12% 30% 14% 3	2816
	11th to 20th	Night	34% 5% 14% 33% 13% 1	2825
	21st to end	Night	38% 5% 16% 28% 10% 2	3338
September	1st to 10th	Night	29% 6% 15% 33% 16% 1	1900
	11th to 20th	Night	27% 6% 18% 32% 16%	1907
	21st to end	Night	34% 6% 13% 33% 13% 1	2174
October	1st to 10th	Night	36% 5% 11% 31% 15% 2	2280
	11th to 20th	Night	44% 7% 13% 23% 12% 1	2208
	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% 46% 23% 6% 1	270
	11th to 20th	Day	14% 15% 42% 21% 6% 2	270
	21st to end	Day	17% 15% 37% 27% 4	225
March	1st to 10th	Day	18% 13% 39% 23% 7% 1	270
	11th to 20th	Day	17% 8% 34% 29% 9% 2	270
	21st to end	Day	24% 13% 28% 24% 8% 3	333
April	1st to 10th	Day	22% 15% 33% 24% 6% 1	460
	11th to 20th	Day	27% 14% 23% 25% 10% 1	457
	21st to end	Day	27% 9% 31% 21% 10% 3	650
May	1st to 10th	Day	21% 11% 31% 26% 9% 1	653
	11th to 20th	Day	18% 12% 28% 26% 13% 4	675
	21st to end	Day	17% 11% 25% 27% 16% 4	734
June	1st to 10th	Day	11% 7% 26% 33% 18% 5%	680
	11th to 20th	Day	14% 8% 33% 32% 10% 3	680
	21st to end	Day	14% 6% 29% 29% 17% 6%	680
July	1st to 10th	Day	9% 4 20% 31% 24% 12%	671
	11th to 20th	Day	8% 5% 25% 32% 20% 10%	668
	21st to end	Day	6% 4 23% 34% 24% 9%	739
August	1st to 10th	Day	10% 5% 20% 32% 26% 7%	676
	11th to 20th	Day	12% 5% 17% 31% 27% 7%	665
	21st to end	Day	11% 7% 22% 33% 21% 7%	628
September	1st to 10th	Day	14% 6% 26% 31% 19% 5%	490
	11th to 20th	Day	12% 4 27% 32% 21% 4	481
	21st to end	Day	20% 9% 28% 22% 15% 7%	300
October	1st to 10th	Day	18% 8% 30% 21% 16% 6%	300
	11th to 20th	Day	18% 16% 31% 24% 9% 1	279
	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% 11% 38% 31% 8% 1	950
	11th to 20th	Night	11% 10% 43% 28% 8% 1	929
	21st to end	Night	18% 9% 35% 29% 9%	721
March	1st to 10th	Night	15% 10% 36% 30% 9% 1	656
	11th to 20th	Night	18% 7% 40% 28% 7% 1	570
	21st to end	Night	23% 8% 31% 26% 10% 2	627
April	1st to 10th	Night	20% 11% 35% 28% 5%	570
	11th to 20th	Night	25% 9% 31% 28% 7% 1	563
	21st to end	Night	24% 8% 29% 29% 8% 1	570
May	1st to 10th	Night	19% 10% 34% 32% 6%	570
	11th to 20th	Night	15% 8% 30% 35% 11% 1	561
	21st to end	Night	12% 7% 26% 39% 15% 1	617
June	1st to 10th	Night	12% 4 27% 42% 14% 1	570
	11th to 20th	Night	10% 5% 31% 44% 10% 1	570
	21st to end	Night	10% 3 24% 44% 16% 2	570
July	1st to 10th	Night	6% 2 21% 43% 27% 2	555
	11th to 20th	Night	8% 3 22% 43% 23% 1	552
	21st to end	Night	5% 1 21% 47% 23% 3	624
August	1st to 10th	Night	10% 3 16% 50% 20% 1	567
	11th to 20th	Night	14% 3 18% 37% 26% 3	564
	21st to end	Night	10% 4 22% 38% 24% 2	627
September	1st to 10th	Night	9% 6% 24% 40% 18% 3	570
	11th to 20th	Night	10% 3 27% 38% 19% 2	570
	21st to end	Night	15% 7% 32% 30% 15% 1	654
October	1st to 10th	Night	18% 7% 28% 32% 13% 1	760
	11th to 20th	Night	21% 9% 33% 29% 7% 1	735
	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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