Astro Hut
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro Hut clear sky chart from 2005-11-06 to 2024-03-06.

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% 11% 5% 7% 6% 3 4 4 5% 7% 33%	78456
	Night	16% 13% 6% 9% 6% 3 4 4 4 6% 29%	81115
Transparency	Day	52% 6% 11% 20% 11% 1	78157
	Night	58% 5% 10% 17% 10% 1	80935
Seeing	Day	20% 17% 30% 21% 9% 3	14790
	Night	22% 13% 27% 25% 11% 1	24840

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	15% 25% 10% 14% 9% 4 4 4 3 4 10%	2527
	11th to 20th	Day	13% 19% 10% 17% 16% 9% 5% 2 2 2 6%	2530
	21st to end	Day	22% 23% 10% 13% 9% 3 4 3 2 2 8%	2783
February	1st to 10th	Day	14% 15% 8% 13% 15% 5% 4 3 4 3 16%	1767
	11th to 20th	Day	14% 13% 7% 16% 10% 3 5% 3 3 4 21%	1848
	21st to end	Day	16% 13% 6% 12% 8% 3 5% 5% 4 6% 22%	1570
March	1st to 10th	Day	13% 12% 6% 9% 7% 4 5% 4 6% 7% 27%	1850
	11th to 20th	Day	12% 10% 4 7% 7% 3 4 4 6% 7% 35%	1903
	21st to end	Day	17% 11% 5% 8% 7% 3 4 4 5% 6% 30%	2286
April	1st to 10th	Day	19% 11% 4 5% 6% 3 5% 5% 5% 7% 29%	2160
	11th to 20th	Day	17% 11% 3 5% 4 2 5% 5% 4 7% 38%	2156
	21st to end	Day	18% 9% 3 6% 5% 2 5% 5% 5% 7% 34%	2340
May	1st to 10th	Day	16% 7% 3 6% 7% 3 5% 5% 6% 8% 35%	2340
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 4 5% 8% 39%	2314
	21st to end	Day	11% 5% 2 5% 5% 3 5% 5% 6% 8% 44%	2725
June	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	2520
	11th to 20th	Day	10% 5% 2 4 5% 3 4 4 6% 8% 49%	2520
	21st to end	Day	11% 6% 2 5% 5% 3 4 5% 5% 8% 46%	2520
July	1st to 10th	Day	7% 4 1 3 3 3 4 4 6% 10% 53%	2520
	11th to 20th	Day	9% 5% 2 4 4 3 4 4 6% 9% 50%	2520
	21st to end	Day	7% 4 1 4 5% 3 5% 5% 6% 9% 51%	2743
August	1st to 10th	Day	9% 4 2 5% 5% 3 4 5% 6% 10% 47%	2368
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 5% 6% 9% 47%	2290
	21st to end	Day	9% 5% 2 5% 4 2 4 4 6% 8% 50%	2376
September	1st to 10th	Day	11% 7% 3 4 6% 3 4 5% 5% 8% 44%	2150
	11th to 20th	Day	12% 5% 3 4 4 2 4 4 6% 9% 47%	1980
	21st to end	Day	12% 6% 3 7% 5% 4 5% 4 6% 8% 41%	1980
October	1st to 10th	Day	17% 7% 3 6% 5% 4 4 3 3 5% 43%	1980
	11th to 20th	Day	24% 11% 3 7% 7% 3 5% 4 6% 7% 24%	1757
	21st to end	Day	27% 11% 4 7% 7% 4 4 5% 5% 5% 21%	1782
November	1st to 10th	Day	14% 15% 3 6% 5% 3 4 4 4 6% 36%	1672
	11th to 20th	Day	17% 23% 6% 8% 6% 2 5% 4 3 5% 20%	1710
	21st to end	Day	23% 19% 7% 9% 5% 3 3 3 4 5% 20%	1710
December	1st to 10th	Day	16% 26% 13% 10% 5% 3 3 3 3 3 14%	1558
	11th to 20th	Day	14% 24% 13% 13% 6% 3 3 4 4 4 11%	1918
	21st to end	Day	21% 25% 9% 11% 8% 2 3 3 3 4 10%	2783
January	1st to 10th	Night	16% 25% 11% 14% 9% 3 3 3 3 3 9%	4428
	11th to 20th	Night	15% 21% 12% 16% 12% 7% 3 3 2 3 8%	4440
	21st to end	Night	24% 21% 11% 15% 7% 3 4 2 2 3 8%	4884
February	1st to 10th	Night	14% 17% 7% 15% 11% 3 4 3 4 4 18%	2408
	11th to 20th	Night	14% 15% 9% 13% 9% 4 3 3 5% 4 21%	2218
	21st to end	Night	18% 14% 5% 12% 7% 3 4 3 4 5% 24%	1884
March	1st to 10th	Night	16% 11% 5% 8% 7% 3 4 3 5% 5% 32%	2227
	11th to 20th	Night	13% 10% 3 7% 6% 3 4 4 4 6% 40%	2057
	21st to end	Night	20% 10% 4 6% 5% 3 4 5% 4 6% 33%	2084
April	1st to 10th	Night	15% 10% 4 7% 5% 4 5% 6% 6% 6% 34%	1800
	11th to 20th	Night	19% 10% 2 5% 5% 2 4 5% 4 6% 37%	1752
	21st to end	Night	17% 9% 3 6% 5% 2 5% 4 5% 8% 37%	1620
May	1st to 10th	Night	16% 8% 4 8% 6% 3 4 5% 5% 8% 33%	1620
	11th to 20th	Night	19% 6% 3 5% 5% 2 5% 4 5% 8% 39%	1576
	21st to end	Night	11% 6% 3 7% 5% 3 4 5% 7% 8% 41%	1551
June	1st to 10th	Night	10% 5% 1 4 5% 2 5% 5% 6% 9% 46%	1440
	11th to 20th	Night	9% 6% 3 4 6% 4 4 4 6% 8% 46%	1440
	21st to end	Night	10% 5% 2 5% 5% 3 5% 5% 6% 9% 44%	1440
July	1st to 10th	Night	7% 4 2 6% 5% 3 3 4 7% 9% 51%	1440
	11th to 20th	Night	8% 3 2 4 4 3 4 5% 6% 9% 51%	1440
	21st to end	Night	7% 3 2 5% 5% 3 6% 6% 6% 9% 49%	1575
August	1st to 10th	Night	7% 3 2 5% 5% 3 5% 6% 7% 9% 48%	1570
	11th to 20th	Night	8% 4 3 4 5% 3 5% 5% 7% 7% 49%	1608
	21st to end	Night	8% 5% 2 6% 5% 3 4 4 5% 7% 51%	1980
September	1st to 10th	Night	11% 5% 2 5% 6% 3 4 4 5% 7% 48%	1800
	11th to 20th	Night	12% 4 1 4 4 3 4 5% 5% 7% 51%	1971
	21st to end	Night	13% 6% 3 7% 6% 3 3 4 5% 7% 42%	1980
October	1st to 10th	Night	18% 6% 3 5% 5% 3 4 4 5% 6% 43%	1980
	11th to 20th	Night	23% 11% 3 7% 5% 3 4 3 4 5% 33%	2200
	21st to end	Night	27% 12% 4 6% 5% 3 3 3 3 5% 29%	2574
November	1st to 10th	Night	17% 12% 5% 7% 5% 3 4 4 4 5% 35%	2406
	11th to 20th	Night	18% 19% 7% 9% 5% 3 4 4 4 5% 23%	2470
	21st to end	Night	19% 19% 7% 9% 6% 3 3 3 4 5% 22%	2470
December	1st to 10th	Night	19% 26% 10% 10% 5% 2 3 3 4 5% 13%	2619
	11th to 20th	Night	17% 23% 10% 12% 6% 3 4 4 4 5% 13%	3279
	21st to end	Night	25% 23% 8% 9% 6% 3 4 4 4 4 11%	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	79% 6% 6% 7% 1	2528
	11th to 20th	Day	86% 3 6% 3 1	2530
	21st to end	Day	84% 4 5% 6% 2	2783
February	1st to 10th	Day	74% 5% 7% 10% 4	1767
	11th to 20th	Day	67% 4 9% 13% 6% 1	1858
	21st to end	Day	65% 5% 11% 13% 5% 1	1570
March	1st to 10th	Day	57% 5% 13% 17% 8%	1850
	11th to 20th	Day	49% 5% 13% 21% 11% 2	1903
	21st to end	Day	56% 5% 10% 22% 6% 1	2286
April	1st to 10th	Day	54% 5% 10% 19% 11%	2160
	11th to 20th	Day	47% 5% 9% 22% 13% 3	2156
	21st to end	Day	48% 5% 13% 18% 13% 2	2340
May	1st to 10th	Day	46% 7% 13% 22% 10% 1	2340
	11th to 20th	Day	45% 6% 10% 22% 16% 2	2318
	21st to end	Day	38% 6% 13% 27% 14% 2	2725
June	1st to 10th	Day	35% 5% 11% 27% 19% 2	2520
	11th to 20th	Day	33% 6% 12% 27% 20% 2	2520
	21st to end	Day	36% 7% 15% 23% 18% 2	2520
July	1st to 10th	Day	27% 6% 14% 30% 19% 5%	2451
	11th to 20th	Day	30% 6% 12% 30% 19% 3	2445
	21st to end	Day	30% 7% 13% 28% 19% 3	2591
August	1st to 10th	Day	32% 8% 13% 31% 14% 2	2353
	11th to 20th	Day	33% 8% 15% 28% 15% 2	2290
	21st to end	Day	31% 6% 15% 25% 19% 4	2376
September	1st to 10th	Day	38% 6% 12% 25% 15% 4	2150
	11th to 20th	Day	35% 4 13% 33% 14% 2	1980
	21st to end	Day	40% 6% 11% 26% 16% 1	1980
October	1st to 10th	Day	47% 4 8% 27% 14% 1	1980
	11th to 20th	Day	59% 4 11% 17% 8%	1757
	21st to end	Day	64% 5% 10% 14% 6% 1	1782
November	1st to 10th	Day	50% 6% 12% 19% 13% 1	1672
	11th to 20th	Day	67% 4 8% 15% 5% 1	1710
	21st to end	Day	69% 5% 9% 13% 4	1710
December	1st to 10th	Day	76% 4 7% 9% 3	1558
	11th to 20th	Day	75% 6% 9% 8% 1	1915
	21st to end	Day	80% 4 6% 9% 1	2783
January	1st to 10th	Night	80% 4 7% 6% 3	4428
	11th to 20th	Night	85% 3 6% 4 2	4440
	21st to end	Night	84% 3 6% 5% 1	4884
February	1st to 10th	Night	72% 4 8% 10% 6%	2408
	11th to 20th	Night	68% 3 8% 15% 6%	2219
	21st to end	Night	64% 4 9% 15% 7% 1	1884
March	1st to 10th	Night	54% 5% 11% 20% 8% 1	2227
	11th to 20th	Night	46% 5% 12% 22% 13% 3	2057
	21st to end	Night	54% 5% 9% 21% 10% 1	2084
April	1st to 10th	Night	49% 7% 11% 19% 13% 1	1800
	11th to 20th	Night	48% 5% 7% 18% 18% 3	1752
	21st to end	Night	46% 6% 12% 18% 15% 3	1620
May	1st to 10th	Night	50% 5% 11% 21% 11% 1	1620
	11th to 20th	Night	45% 4 10% 21% 16% 3	1577
	21st to end	Night	39% 7% 13% 24% 15% 2	1552
June	1st to 10th	Night	34% 6% 11% 29% 17% 3	1440
	11th to 20th	Night	36% 6% 12% 24% 19% 3	1440
	21st to end	Night	36% 7% 16% 23% 16% 3	1440
July	1st to 10th	Night	30% 5% 14% 28% 19% 5%	1400
	11th to 20th	Night	30% 6% 12% 28% 21% 3	1392
	21st to end	Night	30% 8% 13% 24% 21% 3	1487
August	1st to 10th	Night	29% 8% 15% 26% 19% 2	1561
	11th to 20th	Night	33% 7% 12% 29% 17% 2	1608
	21st to end	Night	33% 6% 10% 26% 21% 3	1980
September	1st to 10th	Night	36% 5% 12% 26% 17% 4	1800
	11th to 20th	Night	33% 5% 9% 32% 19% 2	1971
	21st to end	Night	42% 6% 11% 25% 16% 1	1980
October	1st to 10th	Night	43% 5% 7% 25% 17% 2	1980
	11th to 20th	Night	55% 5% 10% 20% 11% 1	2200
	21st to end	Night	60% 4 11% 17% 8% 1	2574
November	1st to 10th	Night	53% 5% 9% 20% 13% 1	2406
	11th to 20th	Night	65% 4 10% 15% 6% 1	2470
	21st to end	Night	65% 4 13% 15% 3	2470
December	1st to 10th	Night	76% 5% 6% 9% 4	2619
	11th to 20th	Night	75% 6% 9% 8% 2	3281
	21st to end	Night	78% 5% 10% 6% 2	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	24% 25% 30% 16% 4 1	270
	11th to 20th	Day	26% 21% 39% 11% 3	270
	21st to end	Day	31% 21% 34% 13% 1	297
February	1st to 10th	Day	28% 27% 26% 14% 3 1	270
	11th to 20th	Day	26% 34% 29% 10% 1	270
	21st to end	Day	29% 37% 25% 8% 1	225
March	1st to 10th	Day	23% 34% 32% 10% 2	255
	11th to 20th	Day	23% 24% 30% 17% 5% 1	343
	21st to end	Day	31% 19% 29% 15% 5% 1	462
April	1st to 10th	Day	27% 21% 32% 14% 5%	420
	11th to 20th	Day	27% 25% 26% 14% 5% 2	412
	21st to end	Day	28% 20% 29% 14% 8% 1	420
May	1st to 10th	Day	23% 22% 31% 17% 5% 2	423
	11th to 20th	Day	22% 15% 32% 21% 7% 4	460
	21st to end	Day	13% 13% 35% 23% 12% 4	679
June	1st to 10th	Day	11% 12% 41% 23% 9% 3	630
	11th to 20th	Day	15% 12% 33% 29% 9% 3	630
	21st to end	Day	15% 11% 36% 23% 12% 3	630
July	1st to 10th	Day	8% 9% 29% 31% 16% 7%	621
	11th to 20th	Day	10% 10% 33% 30% 11% 6%	618
	21st to end	Day	8% 7% 34% 32% 13% 6%	684
August	1st to 10th	Day	11% 5% 26% 26% 22% 10%	487
	11th to 20th	Day	12% 7% 23% 30% 19% 8%	439
	21st to end	Day	14% 6% 28% 30% 16% 7%	495
September	1st to 10th	Day	16% 9% 25% 28% 16% 6%	450
	11th to 20th	Day	10% 8% 25% 28% 22% 7%	450
	21st to end	Day	18% 16% 22% 24% 15% 6%	450
October	1st to 10th	Day	20% 16% 24% 24% 14% 2	450
	11th to 20th	Day	31% 21% 27% 18% 3	336
	21st to end	Day	38% 20% 30% 10% 1 1	297
November	1st to 10th	Day	26% 20% 33% 16% 4 1	270
	11th to 20th	Day	31% 24% 29% 13% 3	270
	21st to end	Day	34% 21% 36% 7% 1	270
December	1st to 10th	Day	29% 32% 24% 11% 3	270
	11th to 20th	Day	24% 32% 30% 11% 1 1	270
	21st to end	Day	28% 28% 33% 8% 1 1	297
January	1st to 10th	Night	27% 22% 29% 18% 4	950
	11th to 20th	Night	30% 18% 32% 17% 3 1	950
	21st to end	Night	26% 17% 31% 23% 3	1045
February	1st to 10th	Night	24% 19% 33% 19% 4	805
	11th to 20th	Night	24% 21% 31% 20% 4	743
	21st to end	Night	28% 22% 31% 17% 2	628
March	1st to 10th	Night	22% 19% 28% 24% 7% 1	743
	11th to 20th	Night	20% 17% 33% 23% 6% 1	720
	21st to end	Night	24% 14% 29% 25% 7% 1	698
April	1st to 10th	Night	20% 17% 30% 24% 6% 1	540
	11th to 20th	Night	26% 12% 31% 24% 6% 1	533
	21st to end	Night	21% 13% 31% 26% 8% 1	540
May	1st to 10th	Night	21% 11% 33% 26% 8% 1	540
	11th to 20th	Night	21% 9% 27% 31% 12%	531
	21st to end	Night	15% 5% 28% 33% 18% 1	584
June	1st to 10th	Night	12% 6% 26% 39% 17% 1	540
	11th to 20th	Night	12% 4 32% 35% 17% 1	540
	21st to end	Night	11% 7% 24% 35% 21% 2	540
July	1st to 10th	Night	9% 3 18% 38% 30% 3	525
	11th to 20th	Night	9% 4 22% 36% 26% 3	522
	21st to end	Night	8% 3 19% 42% 25% 2	591
August	1st to 10th	Night	8% 3 16% 42% 27% 4	537
	11th to 20th	Night	11% 3 15% 38% 27% 5%	534
	21st to end	Night	11% 4 21% 35% 26% 4	594
September	1st to 10th	Night	11% 6% 21% 36% 22% 4	540
	11th to 20th	Night	13% 6% 20% 33% 25% 4	711
	21st to end	Night	17% 10% 24% 25% 19% 5%	720
October	1st to 10th	Night	21% 9% 23% 28% 18% 2	720
	11th to 20th	Night	29% 16% 26% 22% 6% 1	720
	21st to end	Night	36% 14% 26% 19% 5%	792
November	1st to 10th	Night	25% 16% 29% 20% 9% 1	740
	11th to 20th	Night	30% 21% 28% 16% 4 1	760
	21st to end	Night	33% 15% 31% 18% 3	760
December	1st to 10th	Night	32% 21% 29% 14% 3	909
	11th to 20th	Night	30% 20% 32% 15% 3	950
	21st to end	Night	37% 18% 29% 14% 1	1045

Page generated at 2024-03-03 06:28:58. Version ac07

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