Hiddenwell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hiddenwell Observatory clear sky chart from 2005-11-06 to 2024-10-20.

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% 4 9% 5% 4 4 4 5% 6% 31%	101364
	Night	15% 14% 5% 11% 6% 5% 4 3 4 5% 27%	97331
Transparency	Day	53% 6% 11% 19% 10% 1	101123
	Night	58% 6% 10% 16% 9% 1	97192
Seeing	Day	19% 12% 29% 24% 12% 4	15120
	Night	21% 11% 28% 28% 11% 1	24338

By Month

Percentage Cloud Cover NE (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% 23% 11% 20% 7% 5% 4 2 3 3 7%	2286
	11th to 20th	Day	9% 21% 11% 25% 15% 6% 3 2 2 1 5%	2624
	21st to end	Day	17% 23% 11% 18% 8% 5% 4 3 3 3 7%	3124
February	1st to 10th	Day	13% 21% 8% 17% 11% 6% 4 2 4 3 13%	1710
	11th to 20th	Day	12% 15% 10% 20% 10% 5% 5% 3 4 3 13%	1663
	21st to end	Day	15% 18% 6% 14% 8% 5% 4 2 4 5% 19%	1551
March	1st to 10th	Day	15% 17% 6% 11% 6% 3 3 3 3 5% 27%	3327
	11th to 20th	Day	11% 16% 9% 13% 5% 4 4 4 5% 5% 25%	3480
	21st to end	Day	15% 13% 6% 11% 5% 4 4 3 5% 6% 28%	3815
April	1st to 10th	Day	17% 15% 5% 9% 5% 4 3 4 5% 5% 28%	2180
	11th to 20th	Day	18% 12% 2 7% 4 4 3 4 4 6% 36%	2442
	21st to end	Day	19% 9% 4 7% 5% 4 4 3 5% 7% 34%	2470
May	1st to 10th	Day	17% 8% 3 6% 5% 4 4 4 6% 7% 37%	4108
	11th to 20th	Day	16% 9% 3 7% 4 4 3 3 5% 8% 38%	4194
	21st to end	Day	14% 5% 2 5% 4 4 4 3 5% 7% 47%	4837
June	1st to 10th	Day	13% 8% 2 7% 5% 4 4 4 6% 7% 39%	2779
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 4 7% 8% 43%	2850
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 5% 8% 42%	2850
July	1st to 10th	Day	12% 6% 2 6% 4 4 6% 5% 8% 9% 38%	4760
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 47%	4551
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 9% 46%	4466
August	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 8% 39%	4104
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 5% 6% 9% 37%	4044
	21st to end	Day	10% 6% 2 5% 4 4 4 4 7% 9% 44%	4508
September	1st to 10th	Day	12% 7% 2 8% 5% 4 5% 4 6% 8% 39%	2235
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 3 6% 7% 45%	2090
	21st to end	Day	16% 9% 3 7% 6% 4 5% 4 5% 8% 34%	2090
October	1st to 10th	Day	19% 8% 4 8% 4 5% 3 4 5% 7% 33%	1938
	11th to 20th	Day	24% 14% 4 9% 6% 4 4 4 5% 6% 20%	1890
	21st to end	Day	28% 18% 5% 9% 6% 4 4 2 4 5% 16%	1908
November	1st to 10th	Day	15% 20% 4 10% 6% 5% 4 4 4 5% 23%	1664
	11th to 20th	Day	15% 20% 5% 14% 6% 5% 5% 4 5% 5% 16%	1520
	21st to end	Day	18% 23% 8% 13% 6% 5% 5% 3 4 5% 10%	1520
December	1st to 10th	Day	15% 31% 11% 17% 6% 4 3 3 2 3 5%	1520
	11th to 20th	Day	18% 22% 8% 19% 6% 4 4 3 4 5% 7%	1835
	21st to end	Day	25% 26% 9% 13% 5% 4 3 2 3 4 7%	2431
January	1st to 10th	Night	15% 23% 11% 22% 8% 4 4 2 2 3 6%	4652
	11th to 20th	Night	13% 18% 9% 22% 13% 8% 5% 3 3 3 5%	4340
	21st to end	Night	17% 19% 10% 20% 8% 5% 4 3 4 3 7%	4532
February	1st to 10th	Night	12% 21% 7% 16% 12% 6% 4 3 4 3 12%	2470
	11th to 20th	Night	12% 17% 8% 18% 8% 5% 4 3 4 5% 15%	2403
	21st to end	Night	15% 15% 6% 14% 7% 5% 5% 4 5% 6% 21%	1901
March	1st to 10th	Night	12% 15% 5% 10% 5% 4 4 3 4 6% 33%	3513
	11th to 20th	Night	9% 15% 6% 12% 5% 4 4 3 4 6% 31%	3479
	21st to end	Night	15% 10% 6% 12% 6% 5% 5% 3 5% 5% 30%	3817
April	1st to 10th	Night	18% 15% 5% 7% 5% 4 4 4 5% 5% 30%	2013
	11th to 20th	Night	19% 10% 3 6% 4 4 5% 3 4 6% 37%	1689
	21st to end	Night	18% 11% 3 8% 5% 5% 4 4 5% 5% 31%	1710
May	1st to 10th	Night	15% 8% 3 8% 4 4 4 4 5% 7% 39%	2833
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 3 5% 7% 40%	2697
	21st to end	Night	11% 6% 2 5% 4 5% 4 3 5% 7% 48%	2738
June	1st to 10th	Night	13% 8% 2 7% 5% 5% 3 3 5% 9% 40%	1397
	11th to 20th	Night	12% 5% 2 7% 4 4 5% 4 7% 9% 41%	1330
	21st to end	Night	14% 7% 2 5% 5% 5% 4 4 6% 7% 41%	1330
July	1st to 10th	Night	8% 7% 3 8% 4 4 3 3 5% 8% 47%	2210
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 5% 8% 50%	2400
	21st to end	Night	7% 7% 2 6% 3 4 5% 5% 6% 8% 48%	3087
August	1st to 10th	Night	11% 6% 3 7% 5% 4 4 4 5% 7% 45%	2825
	11th to 20th	Night	12% 8% 3 6% 5% 4 4 4 5% 8% 41%	2785
	21st to end	Night	11% 6% 2 6% 4 4 4 3 5% 7% 48%	3124
September	1st to 10th	Night	11% 5% 3 6% 4 4 5% 3 6% 7% 46%	1933
	11th to 20th	Night	11% 5% 2 6% 4 4 3 3 6% 7% 50%	2090
	21st to end	Night	16% 7% 3 8% 5% 4 5% 4 5% 6% 37%	2090
October	1st to 10th	Night	18% 8% 3 7% 5% 4 4 4 5% 5% 37%	2242
	11th to 20th	Night	23% 12% 3 9% 5% 5% 4 3 5% 7% 24%	2275
	21st to end	Night	25% 16% 4 11% 5% 5% 4 3 4 5% 19%	2443
November	1st to 10th	Night	17% 16% 4 9% 5% 6% 4 3 5% 6% 26%	2411
	11th to 20th	Night	17% 20% 5% 11% 6% 5% 5% 4 5% 4 17%	2660
	21st to end	Night	20% 22% 8% 12% 6% 4 3 2 4 5% 13%	2660
December	1st to 10th	Night	19% 29% 9% 14% 6% 4 4 2 4 3 7%	2660
	11th to 20th	Night	13% 22% 9% 18% 7% 6% 4 3 4 4 10%	3356
	21st to end	Night	23% 24% 10% 16% 6% 4 3 2 3 3 6%	5236

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	83% 4 7% 4 1	2287
	11th to 20th	Day	90% 3 3 2 2	2624
	21st to end	Day	83% 6% 6% 3 2	3124
February	1st to 10th	Day	77% 5% 7% 10% 1	1710
	11th to 20th	Day	76% 5% 7% 8% 4	1672
	21st to end	Day	68% 5% 8% 13% 5%	1551
March	1st to 10th	Day	60% 5% 11% 16% 8%	3330
	11th to 20th	Day	60% 6% 11% 16% 6% 1	3480
	21st to end	Day	57% 6% 11% 15% 10% 1	3815
April	1st to 10th	Day	57% 6% 11% 17% 9%	2180
	11th to 20th	Day	49% 5% 9% 22% 14% 1	2442
	21st to end	Day	50% 6% 11% 19% 11% 2	2470
May	1st to 10th	Day	44% 6% 11% 20% 17% 2	4120
	11th to 20th	Day	43% 7% 14% 26% 9% 1	4198
	21st to end	Day	37% 5% 11% 22% 20% 5%	4837
June	1st to 10th	Day	41% 7% 11% 24% 15% 1	2779
	11th to 20th	Day	36% 7% 12% 26% 18% 1	2850
	21st to end	Day	41% 6% 12% 26% 14% 1	2850
July	1st to 10th	Day	40% 7% 16% 20% 16% 2	4686
	11th to 20th	Day	32% 6% 13% 35% 13% 1	4471
	21st to end	Day	33% 7% 13% 27% 18% 2	4324
August	1st to 10th	Day	39% 8% 13% 24% 14% 1	4091
	11th to 20th	Day	42% 7% 16% 25% 10%	4083
	21st to end	Day	35% 6% 16% 31% 12% 1	4508
September	1st to 10th	Day	42% 7% 13% 25% 11% 2	2235
	11th to 20th	Day	37% 7% 14% 28% 14%	2090
	21st to end	Day	47% 7% 13% 23% 8% 1	2090
October	1st to 10th	Day	51% 6% 12% 20% 10% 2	1938
	11th to 20th	Day	64% 6% 11% 13% 5% 1	1890
	21st to end	Day	72% 5% 8% 11% 4	1908
November	1st to 10th	Day	63% 5% 9% 15% 7%	1664
	11th to 20th	Day	67% 8% 11% 8% 5% 1	1520
	21st to end	Day	77% 6% 8% 8% 1	1520
December	1st to 10th	Day	86% 4 5% 3 1	1520
	11th to 20th	Day	81% 6% 7% 5% 1	1835
	21st to end	Day	83% 5% 8% 3	2431
January	1st to 10th	Night	84% 5% 6% 4 1	4652
	11th to 20th	Night	85% 6% 5% 3 2	4340
	21st to end	Night	82% 6% 7% 3 2	4532
February	1st to 10th	Night	78% 5% 7% 9% 2	2470
	11th to 20th	Night	72% 5% 11% 10% 3	2405
	21st to end	Night	64% 6% 11% 12% 7%	1901
March	1st to 10th	Night	52% 6% 15% 20% 7%	3523
	11th to 20th	Night	53% 5% 14% 17% 9% 1	3480
	21st to end	Night	56% 5% 10% 18% 9% 1	3817
April	1st to 10th	Night	55% 6% 9% 18% 11% 1	2013
	11th to 20th	Night	48% 5% 8% 20% 17% 2	1689
	21st to end	Night	53% 5% 9% 16% 15% 2	1710
May	1st to 10th	Night	43% 7% 11% 20% 16% 4	2840
	11th to 20th	Night	43% 7% 10% 23% 15% 2	2698
	21st to end	Night	35% 6% 11% 23% 21% 3	2739
June	1st to 10th	Night	42% 6% 10% 23% 17% 2	1397
	11th to 20th	Night	38% 6% 13% 24% 17% 2	1330
	21st to end	Night	41% 6% 10% 24% 17% 2	1330
July	1st to 10th	Night	35% 5% 12% 25% 17% 6%	2175
	11th to 20th	Night	31% 7% 12% 34% 16% 1	2357
	21st to end	Night	32% 7% 14% 25% 20% 2	2988
August	1st to 10th	Night	39% 6% 10% 29% 15% 1	2814
	11th to 20th	Night	40% 7% 15% 22% 14% 1	2812
	21st to end	Night	36% 6% 12% 32% 14% 1	3124
September	1st to 10th	Night	35% 7% 12% 29% 15% 2	1933
	11th to 20th	Night	34% 6% 14% 29% 17% 1	2090
	21st to end	Night	47% 7% 12% 24% 10%	2090
October	1st to 10th	Night	47% 7% 10% 20% 14% 1	2242
	11th to 20th	Night	60% 6% 12% 16% 6%	2275
	21st to end	Night	69% 6% 9% 10% 6%	2443
November	1st to 10th	Night	58% 7% 10% 18% 7%	2411
	11th to 20th	Night	68% 7% 9% 11% 4 1	2660
	21st to end	Night	76% 5% 11% 8% 1	2660
December	1st to 10th	Night	83% 5% 6% 4 1	2660
	11th to 20th	Night	78% 6% 10% 5% 1	3356
	21st to end	Night	84% 5% 7% 3	5236

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	19% 20% 37% 17% 7% 1	204
	11th to 20th	Day	21% 19% 38% 14% 7% 1	270
	21st to end	Day	25% 15% 30% 25% 5%	297
February	1st to 10th	Day	21% 25% 35% 15% 4	270
	11th to 20th	Day	23% 20% 41% 13% 3	270
	21st to end	Day	23% 33% 30% 11% 3	225
March	1st to 10th	Day	21% 18% 38% 17% 6% 1	389
	11th to 20th	Day	22% 16% 32% 22% 6% 1	460
	21st to end	Day	26% 12% 33% 22% 6% 1	506
April	1st to 10th	Day	23% 17% 33% 20% 8%	460
	11th to 20th	Day	27% 16% 28% 16% 8% 5%	452
	21st to end	Day	27% 11% 28% 24% 9% 2	460
May	1st to 10th	Day	24% 9% 30% 23% 12% 3	463
	11th to 20th	Day	21% 8% 28% 24% 15% 5%	487
	21st to end	Day	17% 7% 27% 27% 15% 6%	529
June	1st to 10th	Day	15% 9% 31% 29% 15% 1	609
	11th to 20th	Day	13% 10% 29% 30% 14% 3	680
	21st to end	Day	17% 7% 35% 27% 11% 2	680
July	1st to 10th	Day	11% 6% 28% 30% 20% 5%	671
	11th to 20th	Day	13% 6% 32% 28% 15% 7%	649
	21st to end	Day	11% 5% 26% 32% 18% 8%	532
August	1st to 10th	Day	12% 4 25% 30% 18% 11%	486
	11th to 20th	Day	14% 4 20% 29% 21% 12%	479
	21st to end	Day	15% 5% 22% 31% 20% 7%	539
September	1st to 10th	Day	12% 9% 23% 30% 17% 8%	490
	11th to 20th	Day	11% 6% 20% 27% 24% 12%	490
	21st to end	Day	17% 9% 25% 27% 16% 7%	490
October	1st to 10th	Day	23% 10% 26% 22% 13% 6%	490
	11th to 20th	Day	26% 17% 30% 20% 5% 2	486
	21st to end	Day	35% 15% 30% 15% 5%	423
November	1st to 10th	Day	23% 19% 33% 17% 8% 1	266
	11th to 20th	Day	24% 23% 28% 18% 4 3	180
	21st to end	Day	25% 23% 35% 16% 1	180
December	1st to 10th	Day	29% 18% 36% 14% 2 1	180
	11th to 20th	Day	29% 23% 30% 13% 4 1	180
	21st to end	Day	35% 20% 32% 10% 3	198
January	1st to 10th	Night	24% 18% 32% 18% 6% 1	950
	11th to 20th	Night	25% 17% 32% 20% 5% 1	889
	21st to end	Night	20% 11% 36% 25% 7% 1	836
February	1st to 10th	Night	22% 17% 35% 22% 4	760
	11th to 20th	Night	22% 15% 35% 23% 5% 1	743
	21st to end	Night	22% 19% 33% 23% 3	628
March	1st to 10th	Night	18% 16% 36% 24% 6% 1	760
	11th to 20th	Night	16% 17% 36% 25% 5% 1	760
	21st to end	Night	22% 12% 29% 26% 10% 1	836
April	1st to 10th	Night	24% 12% 33% 25% 6%	760
	11th to 20th	Night	22% 14% 26% 27% 10% 1	565
	21st to end	Night	25% 10% 24% 29% 11% 1	570
May	1st to 10th	Night	19% 5% 29% 33% 12% 1	570
	11th to 20th	Night	19% 5% 25% 37% 13% 1	498
	21st to end	Night	13% 3 26% 40% 17% 1	411
June	1st to 10th	Night	17% 6% 24% 39% 13% 1	380
	11th to 20th	Night	14% 6% 28% 39% 12%	380
	21st to end	Night	17% 4 23% 41% 14% 1	380
July	1st to 10th	Night	8% 4 19% 49% 19% 1	370
	11th to 20th	Night	9% 5% 23% 38% 24% 2	462
	21st to end	Night	11% 2 19% 43% 24% 1	624
August	1st to 10th	Night	11% 4 16% 39% 28% 1	567
	11th to 20th	Night	10% 3 16% 39% 27% 4	564
	21st to end	Night	13% 4 20% 33% 28% 2	627
September	1st to 10th	Night	12% 4 22% 36% 19% 6%	750
	11th to 20th	Night	13% 4 19% 37% 23% 4	760
	21st to end	Night	17% 8% 23% 31% 18% 4	760
October	1st to 10th	Night	22% 7% 27% 28% 14% 1	760
	11th to 20th	Night	28% 12% 24% 27% 8% 1	759
	21st to end	Night	34% 12% 28% 22% 4 1	792
November	1st to 10th	Night	28% 13% 27% 24% 7% 1	740
	11th to 20th	Night	25% 21% 29% 18% 7%	760
	21st to end	Night	30% 16% 30% 21% 4	760
December	1st to 10th	Night	32% 16% 31% 17% 4	760
	11th to 20th	Night	23% 18% 38% 16% 3 1	802
	21st to end	Night	32% 14% 32% 18% 3	1045

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