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 2024-11-02.

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% 11% 4 8% 5% 4 4 3 5% 7% 35%	101214
	Night	15% 13% 5% 10% 5% 4 4 3 5% 6% 31%	97804
Transparency	Day	49% 6% 11% 21% 12% 1	101005
	Night	55% 5% 10% 18% 11% 1	97685
Seeing	Day	20% 17% 31% 21% 9% 3	15801
	Night	22% 13% 27% 25% 11% 1	25633

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% 26% 9% 15% 8% 5% 3 3 3 4 9%	2527
	11th to 20th	Day	13% 20% 9% 19% 13% 10% 3 2 2 2 6%	2530
	21st to end	Day	22% 25% 9% 15% 7% 4 3 2 3 2 8%	2783
February	1st to 10th	Day	13% 18% 6% 15% 13% 6% 4 3 4 3 16%	1767
	11th to 20th	Day	14% 15% 6% 18% 9% 5% 3 3 4 3 21%	1848
	21st to end	Day	16% 15% 6% 14% 6% 5% 5% 4 4 6% 21%	1570
March	1st to 10th	Day	15% 13% 6% 8% 5% 5% 4 3 5% 7% 28%	3120
	11th to 20th	Day	11% 10% 7% 11% 5% 4 4 3 6% 7% 32%	3347
	21st to end	Day	19% 13% 7% 10% 6% 4 4 3 4 5% 26%	4005
April	1st to 10th	Day	18% 13% 3 6% 5% 5% 5% 4 5% 7% 30%	2280
	11th to 20th	Day	16% 12% 3 6% 4 3 4 4 4 6% 38%	2278
	21st to end	Day	17% 10% 3 7% 4 4 4 4 6% 7% 35%	2470
May	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 8% 38%	4082
	11th to 20th	Day	13% 7% 2 6% 4 4 4 4 6% 9% 42%	4108
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 9% 46%	4824
June	1st to 10th	Day	10% 6% 2 6% 3 5% 4 4 6% 8% 47%	2660
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 7% 8% 48%	2660
	21st to end	Day	10% 7% 2 6% 4 4 3 4 6% 7% 45%	2660
July	1st to 10th	Day	12% 7% 3 6% 4 4 5% 3 6% 10% 40%	4440
	11th to 20th	Day	7% 4 1 4 4 4 4 4 6% 9% 53%	4440
	21st to end	Day	6% 5% 2 6% 4 5% 5% 4 6% 9% 48%	4813
August	1st to 10th	Day	11% 6% 2 5% 4 3 4 3 6% 9% 46%	4172
	11th to 20th	Day	10% 5% 2 5% 3 4 4 3 6% 7% 50%	4017
	21st to end	Day	7% 6% 3 6% 4 4 4 4 6% 8% 49%	4180
September	1st to 10th	Day	10% 7% 2 5% 4 4 5% 4 5% 8% 45%	2269
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 6% 8% 49%	2090
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 3 6% 8% 40%	2090
October	1st to 10th	Day	16% 7% 2 6% 5% 4 4 2 3 5% 44%	2090
	11th to 20th	Day	23% 11% 3 8% 5% 4 5% 3 6% 7% 25%	1853
	21st to end	Day	26% 12% 3 8% 5% 5% 5% 4 6% 5% 22%	1881
November	1st to 10th	Day	14% 16% 3 6% 5% 4 4 4 5% 5% 36%	1681
	11th to 20th	Day	16% 25% 4 9% 5% 4 5% 3 3 5% 20%	1710
	21st to end	Day	22% 21% 6% 9% 4 3 3 3 3 5% 20%	1710
December	1st to 10th	Day	16% 29% 9% 11% 5% 4 3 2 4 3 14%	1558
	11th to 20th	Day	13% 26% 12% 14% 5% 4 3 4 4 4 11%	1918
	21st to end	Day	21% 26% 9% 13% 6% 3 3 2 3 4 10%	2783
January	1st to 10th	Night	16% 26% 11% 16% 6% 4 3 3 3 3 9%	4428
	11th to 20th	Night	15% 22% 11% 18% 10% 8% 3 2 2 2 8%	4440
	21st to end	Night	24% 22% 10% 17% 6% 4 3 2 2 3 8%	4884
February	1st to 10th	Night	13% 19% 6% 17% 9% 4 3 2 4 4 18%	2408
	11th to 20th	Night	14% 17% 8% 15% 7% 5% 3 2 5% 4 21%	2218
	21st to end	Night	18% 15% 5% 13% 7% 4 3 2 4 5% 24%	1884
March	1st to 10th	Night	17% 11% 5% 9% 5% 4 4 3 4 5% 34%	3733
	11th to 20th	Night	9% 9% 3 9% 6% 4 4 3 5% 6% 42%	3613
	21st to end	Night	18% 13% 7% 11% 5% 4 4 3 4 5% 26%	3640
April	1st to 10th	Night	15% 11% 3 7% 4 5% 4 5% 5% 6% 34%	1900
	11th to 20th	Night	20% 11% 2 6% 4 3 4 4 4 5% 37%	1850
	21st to end	Night	16% 9% 2 7% 4 3 5% 3 6% 8% 37%	1710
May	1st to 10th	Night	14% 9% 3 9% 5% 4 4 3 6% 8% 35%	2813
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 4 7% 40%	2783
	21st to end	Night	15% 7% 3 7% 4 4 4 4 5% 7% 42%	2743
June	1st to 10th	Night	10% 6% 1 4 5% 3 5% 4 7% 9% 46%	1520
	11th to 20th	Night	9% 6% 2 5% 6% 5% 4 4 6% 8% 44%	1520
	21st to end	Night	10% 5% 2 6% 5% 5% 4 4 6% 9% 44%	1520
July	1st to 10th	Night	9% 6% 2 8% 5% 5% 5% 4 7% 9% 41%	2530
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 6% 8% 54%	2530
	21st to end	Night	6% 5% 2 7% 4 5% 5% 5% 6% 10% 45%	2750
August	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 6% 9% 46%	2759
	11th to 20th	Night	10% 6% 2 5% 3 4 4 3 5% 7% 50%	2809
	21st to end	Night	8% 6% 3 8% 4 5% 5% 4 5% 7% 46%	3476
September	1st to 10th	Night	10% 5% 2 5% 5% 4 4 3 4 7% 50%	1900
	11th to 20th	Night	11% 4 1 4 3 4 4 4 5% 7% 52%	2081
	21st to end	Night	13% 7% 3 9% 5% 4 3 3 5% 7% 41%	2090
October	1st to 10th	Night	17% 6% 2 6% 4 4 3 3 5% 6% 45%	2090
	11th to 20th	Night	22% 12% 3 7% 4 4 4 2 4 5% 34%	2323
	21st to end	Night	26% 12% 4 6% 4 3 4 2 4 5% 30%	2717
November	1st to 10th	Night	17% 13% 5% 7% 4 4 3 3 4 5% 35%	2420
	11th to 20th	Night	18% 20% 6% 10% 4 3 4 3 4 5% 22%	2470
	21st to end	Night	19% 21% 6% 9% 5% 3 3 2 4 6% 22%	2470
December	1st to 10th	Night	19% 30% 7% 10% 5% 3 3 2 4 4 13%	2619
	11th to 20th	Night	16% 25% 9% 13% 5% 4 4 3 4 4 13%	3279
	21st to end	Night	25% 24% 7% 11% 5% 4 3 3 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	55% 5% 13% 19% 8%	3120
	11th to 20th	Day	51% 5% 12% 23% 7% 1	3347
	21st to end	Day	61% 5% 9% 19% 5% 1	4005
April	1st to 10th	Day	54% 5% 10% 19% 12%	2280
	11th to 20th	Day	46% 5% 9% 23% 14% 3	2278
	21st to end	Day	48% 5% 13% 19% 14% 2	2470
May	1st to 10th	Day	43% 7% 11% 25% 13% 1	4120
	11th to 20th	Day	39% 7% 12% 26% 15% 1	4112
	21st to end	Day	34% 6% 12% 24% 23% 1	4837
June	1st to 10th	Day	35% 5% 11% 27% 19% 2	2660
	11th to 20th	Day	32% 7% 12% 27% 20% 2	2660
	21st to end	Day	36% 7% 14% 23% 18% 3	2660
July	1st to 10th	Day	40% 6% 14% 22% 15% 3	4371
	11th to 20th	Day	27% 6% 14% 33% 18% 2	4365
	21st to end	Day	32% 8% 16% 21% 21% 2	4662
August	1st to 10th	Day	34% 7% 12% 31% 15% 1	4157
	11th to 20th	Day	32% 7% 13% 32% 15% 1	4056
	21st to end	Day	32% 6% 15% 28% 18% 2	4179
September	1st to 10th	Day	36% 6% 13% 26% 15% 4	2269
	11th to 20th	Day	33% 4 12% 32% 17% 2	2090
	21st to end	Day	41% 6% 11% 25% 16% 1	2090
October	1st to 10th	Day	45% 4 8% 29% 14% 1	2090
	11th to 20th	Day	57% 4 11% 18% 9%	1853
	21st to end	Day	62% 6% 11% 15% 6% 1	1881
November	1st to 10th	Day	50% 6% 12% 19% 12% 1	1681
	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	52% 5% 12% 23% 7%	3733
	11th to 20th	Night	42% 5% 13% 24% 13% 2	3613
	21st to end	Night	60% 5% 10% 17% 7% 1	3640
April	1st to 10th	Night	49% 7% 11% 19% 13% 1	1900
	11th to 20th	Night	49% 5% 7% 18% 18% 3	1850
	21st to end	Night	45% 7% 12% 18% 15% 3	1710
May	1st to 10th	Night	47% 6% 11% 22% 13% 1	2840
	11th to 20th	Night	44% 5% 9% 23% 17% 3	2784
	21st to end	Night	41% 6% 9% 19% 23% 2	2751
June	1st to 10th	Night	34% 6% 12% 29% 17% 3	1520
	11th to 20th	Night	36% 6% 12% 24% 19% 3	1520
	21st to end	Night	37% 7% 15% 22% 16% 3	1520
July	1st to 10th	Night	39% 6% 13% 23% 12% 7%	2490
	11th to 20th	Night	27% 7% 12% 25% 26% 3	2482
	21st to end	Night	33% 8% 13% 17% 26% 2	2662
August	1st to 10th	Night	33% 8% 13% 28% 18% 1	2750
	11th to 20th	Night	33% 6% 9% 31% 19% 1	2836
	21st to end	Night	36% 6% 9% 27% 20% 2	3476
September	1st to 10th	Night	34% 5% 12% 26% 18% 4	1900
	11th to 20th	Night	32% 5% 8% 31% 21% 2	2081
	21st to end	Night	42% 6% 10% 24% 16% 1	2090
October	1st to 10th	Night	42% 5% 7% 27% 17% 2	2090
	11th to 20th	Night	53% 5% 10% 20% 12% 1	2323
	21st to end	Night	58% 4 11% 18% 9% 1	2717
November	1st to 10th	Night	53% 5% 9% 20% 13% 1	2420
	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	22% 34% 32% 10% 2	270
	11th to 20th	Day	25% 24% 30% 16% 5% 1	379
	21st to end	Day	31% 19% 29% 14% 5% 1	506
April	1st to 10th	Day	28% 22% 31% 13% 5%	460
	11th to 20th	Day	29% 24% 27% 13% 5% 2	452
	21st to end	Day	28% 20% 29% 14% 8% 1	460
May	1st to 10th	Day	23% 21% 33% 16% 5% 2	463
	11th to 20th	Day	21% 15% 32% 21% 8% 3	501
	21st to end	Day	13% 14% 35% 23% 11% 4	734
June	1st to 10th	Day	13% 13% 40% 23% 9% 3	680
	11th to 20th	Day	14% 12% 35% 28% 9% 2	680
	21st to end	Day	15% 12% 36% 24% 11% 2	680
July	1st to 10th	Day	9% 9% 30% 31% 15% 7%	671
	11th to 20th	Day	10% 9% 33% 31% 11% 6%	668
	21st to end	Day	8% 6% 33% 33% 14% 6%	739
August	1st to 10th	Day	11% 6% 26% 26% 21% 10%	529
	11th to 20th	Day	11% 7% 26% 30% 18% 7%	479
	21st to end	Day	13% 5% 27% 30% 18% 6%	539
September	1st to 10th	Day	15% 9% 26% 27% 17% 6%	490
	11th to 20th	Day	9% 7% 24% 28% 25% 7%	490
	21st to end	Day	18% 15% 23% 23% 15% 5%	490
October	1st to 10th	Day	19% 16% 25% 25% 13% 2	490
	11th to 20th	Day	29% 21% 28% 18% 4	369
	21st to end	Day	36% 20% 30% 12% 2 1	330
November	1st to 10th	Day	26% 20% 33% 16% 4 1	273
	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	23% 19% 28% 23% 7% 1	760
	11th to 20th	Night	20% 18% 34% 23% 6% 1	760
	21st to end	Night	24% 15% 29% 25% 6% 1	736
April	1st to 10th	Night	20% 17% 31% 24% 6% 1	570
	11th to 20th	Night	27% 12% 30% 23% 6% 1	563
	21st to end	Night	21% 13% 31% 26% 8% 1	570
May	1st to 10th	Night	21% 11% 33% 26% 8% 1	570
	11th to 20th	Night	20% 9% 28% 31% 12%	561
	21st to end	Night	15% 5% 28% 33% 18% 1	617
June	1st to 10th	Night	12% 6% 27% 37% 17% 1	570
	11th to 20th	Night	12% 5% 32% 34% 16% 1	570
	21st to end	Night	11% 7% 24% 35% 21% 2	570
July	1st to 10th	Night	9% 3 19% 38% 29% 3	555
	11th to 20th	Night	10% 4 21% 35% 26% 3	552
	21st to end	Night	8% 3 18% 42% 26% 2	624
August	1st to 10th	Night	8% 3 17% 42% 27% 4	567
	11th to 20th	Night	12% 4 15% 39% 26% 5%	564
	21st to end	Night	10% 4 20% 36% 26% 4	627
September	1st to 10th	Night	11% 6% 21% 36% 22% 4	570
	11th to 20th	Night	13% 5% 19% 34% 26% 3	751
	21st to end	Night	17% 10% 24% 26% 18% 5%	760
October	1st to 10th	Night	20% 9% 23% 29% 18% 2	760
	11th to 20th	Night	29% 15% 26% 23% 6% 1	760
	21st to end	Night	35% 14% 27% 19% 5%	836
November	1st to 10th	Night	26% 16% 29% 20% 9% 1	745
	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 2024-10-30 01:22:57. Version ac07

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