Spring Gulch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spring Gulch clear sky chart from 2005-11-06 to 2024-11-04.

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	3 6% 2 6% 5% 5% 5% 5% 6% 8% 50%	105192
	Night	2 6% 2 6% 4 4 5% 4 6% 7% 53%	102554
Transparency	Day	31% 9% 13% 20% 18% 8%	104947
	Night	29% 9% 14% 22% 19% 7%	102389
Seeing	Day	9% 14% 29% 29% 15% 2	15456
	Night	6% 6% 25% 40% 21% 1	25536

By Month

Percentage Cloud Cover SW (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	6% 3 15% 8% 6% 7% 5% 7% 8% 36%	2584
	11th to 20th	Day	1 9% 7% 13% 7% 5% 4 4 7% 7% 37%	2860
	21st to end	Day	1 9% 4 8% 5% 5% 4 4 5% 6% 49%	3146
February	1st to 10th	Day	2 10% 4 12% 6% 6% 5% 4 6% 6% 39%	1710
	11th to 20th	Day	1 11% 4 9% 6% 5% 5% 5% 5% 7% 41%	1707
	21st to end	Day	1 9% 5% 10% 6% 6% 6% 5% 7% 6% 40%	1570
March	1st to 10th	Day	1 12% 6% 9% 5% 5% 5% 4 5% 6% 41%	3368
	11th to 20th	Day	1 9% 3 8% 5% 5% 6% 5% 7% 7% 45%	3490
	21st to end	Day	1 10% 4 9% 6% 6% 9% 8% 8% 8% 31%	3881
April	1st to 10th	Day	3 8% 3 7% 5% 6% 4 5% 6% 7% 45%	2280
	11th to 20th	Day	4 9% 2 8% 6% 5% 6% 6% 6% 7% 39%	2251
	21st to end	Day	3 7% 3 8% 6% 6% 5% 5% 7% 7% 42%	2280
May	1st to 10th	Day	4 9% 3 9% 7% 6% 6% 5% 7% 8% 36%	4046
	11th to 20th	Day	4 5% 2 6% 5% 5% 6% 5% 8% 9% 46%	4291
	21st to end	Day	4 5% 1 6% 5% 5% 6% 4 7% 8% 49%	4825
June	1st to 10th	Day	3 3 1 3 3 4 4 4 7% 9% 57%	2660
	11th to 20th	Day	3 3 1 3 3 4 5% 4 6% 7% 62%	2660
	21st to end	Day	2 2 1 3 3 4 5% 4 7% 8% 62%	2660
July	1st to 10th	Day	3 2 1 3 3 3 4 4 5% 7% 66%	4430
	11th to 20th	Day	2 2 1 3 4 4 6% 6% 10% 14% 49%	4314
	21st to end	Day	3 3 2 5% 4 5% 5% 4 7% 9% 53%	4454
August	1st to 10th	Day	4 3 1 5% 5% 5% 8% 6% 9% 11% 42%	4092
	11th to 20th	Day	3 2 1 4 5% 5% 7% 6% 8% 9% 48%	3912
	21st to end	Day	2 2 1 5% 4 4 5% 5% 8% 11% 53%	4180
September	1st to 10th	Day	5% 2 1 5% 3 4 5% 4 5% 8% 58%	2280
	11th to 20th	Day	4 2 1 4 4 4 5% 3 6% 8% 58%	2280
	21st to end	Day	5% 3 1 5% 2 3 4 3 5% 6% 63%	2010
October	1st to 10th	Day	4 2 1 3 2 2 2 3 4 5% 72%	3170
	11th to 20th	Day	3 3 1 3 4 4 5% 5% 8% 11% 53%	3150
	21st to end	Day	3 5% 2 4 2 3 3 3 4 6% 64%	3438
November	1st to 10th	Day	2 5% 2 4 3 3 3 4 5% 6% 63%	1785
	11th to 20th	Day	2 8% 2 6% 3 3 3 3 5% 6% 58%	1668
	21st to end	Day	1 7% 3 9% 4 4 3 4 4 5% 56%	1520
December	1st to 10th	Day	2 9% 3 11% 7% 5% 5% 3 4 5% 45%	1520
	11th to 20th	Day	1 9% 4 9% 5% 4 4 4 5% 7% 47%	1926
	21st to end	Day	1 8% 4 10% 6% 6% 5% 4 5% 6% 44%	2794
January	1st to 10th	Night	9% 5% 13% 7% 6% 6% 4 5% 6% 40%	4385
	11th to 20th	Night	1 11% 4 11% 9% 6% 5% 4 5% 6% 39%	4110
	21st to end	Night	1 11% 5% 9% 5% 5% 4 3 5% 6% 45%	4521
February	1st to 10th	Night	1 7% 4 9% 6% 5% 5% 5% 6% 7% 46%	2470
	11th to 20th	Night	1 7% 5% 8% 5% 5% 6% 5% 6% 8% 44%	2360
	21st to end	Night	2 10% 5% 10% 4 5% 5% 4 5% 7% 45%	1884
March	1st to 10th	Night	1 9% 4 8% 5% 6% 7% 5% 6% 7% 43%	3502
	11th to 20th	Night	1 8% 3 7% 5% 4 5% 3 5% 6% 53%	3490
	21st to end	Night	2 9% 3 7% 5% 6% 7% 6% 9% 10% 36%	3768
April	1st to 10th	Night	3 7% 3 6% 5% 5% 5% 4 7% 7% 48%	1900
	11th to 20th	Night	4 10% 3 7% 4 4 4 5% 5% 8% 47%	1877
	21st to end	Night	5% 7% 2 7% 4 4 4 3 6% 6% 50%	1900
May	1st to 10th	Night	4 7% 3 7% 5% 4 5% 4 6% 8% 46%	2909
	11th to 20th	Night	5% 4 2 5% 4 4 4 4 8% 9% 50%	2613
	21st to end	Night	3 3 1 4 4 4 5% 4 6% 7% 60%	2762
June	1st to 10th	Night	2 2 1 3 3 3 4 4 6% 8% 64%	1520
	11th to 20th	Night	4 2 1 4 3 3 4 4 6% 7% 63%	1520
	21st to end	Night	1 1 1 2 2 3 3 4 7% 8% 68%	1520
July	1st to 10th	Night	3 2 1 3 4 3 4 3 5% 7% 67%	2540
	11th to 20th	Night	1 1 3 3 4 5% 5% 10% 12% 56%	2652
	21st to end	Night	3 2 1 4 4 4 5% 4 8% 8% 57%	3109
August	1st to 10th	Night	3 2 1 4 4 4 6% 7% 10% 12% 47%	2847
	11th to 20th	Night	2 3 1 3 3 4 5% 4 7% 10% 58%	2925
	21st to end	Night	2 2 1 4 4 4 5% 4 7% 9% 57%	3487
September	1st to 10th	Night	3 3 2 4 4 4 3 4 5% 6% 63%	1900
	11th to 20th	Night	3 2 1 4 4 2 4 3 6% 9% 62%	1900
	21st to end	Night	3 2 1 3 2 3 3 2 4 6% 69%	2166
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 5% 76%	3800
	11th to 20th	Night	2 2 1 3 2 2 3 4 7% 10% 63%	3778
	21st to end	Night	4 5% 1 3 2 2 3 2 4 6% 67%	4120
November	1st to 10th	Night	2 5% 1 3 3 2 3 4 6% 6% 63%	2346
	11th to 20th	Night	2 8% 2 4 3 3 4 4 4 6% 59%	2509
	21st to end	Night	2 7% 3 6% 3 4 3 4 5% 6% 56%	2660
December	1st to 10th	Night	1 9% 3 8% 6% 5% 5% 4 5% 5% 50%	2660
	11th to 20th	Night	2 9% 3 7% 5% 4 5% 4 6% 9% 45%	3271
	21st to end	Night	1 8% 4 10% 6% 5% 5% 4 5% 6% 46%	4873

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	41% 13% 16% 20% 10% 1	2585
	11th to 20th	Day	46% 12% 19% 15% 8%	2860
	21st to end	Day	35% 10% 19% 25% 10% 1	3146
February	1st to 10th	Day	47% 8% 16% 17% 10% 2	1710
	11th to 20th	Day	48% 6% 10% 21% 13% 2	1716
	21st to end	Day	46% 8% 13% 18% 13% 3	1570
March	1st to 10th	Day	37% 13% 16% 19% 14% 1	3368
	11th to 20th	Day	31% 14% 19% 27% 7% 2	3490
	21st to end	Day	46% 14% 15% 17% 8% 1	3881
April	1st to 10th	Day	38% 8% 14% 18% 16% 5%	2280
	11th to 20th	Day	43% 9% 12% 18% 16% 2	2254
	21st to end	Day	38% 9% 14% 23% 15% 1	2280
May	1st to 10th	Day	39% 13% 13% 18% 15% 2	4046
	11th to 20th	Day	27% 14% 15% 26% 14% 5%	4295
	21st to end	Day	27% 9% 11% 24% 25% 3	4825
June	1st to 10th	Day	22% 7% 12% 22% 26% 10%	2660
	11th to 20th	Day	22% 7% 8% 15% 26% 22%	2660
	21st to end	Day	20% 6% 9% 19% 26% 20%	2660
July	1st to 10th	Day	19% 4 8% 10% 20% 38%	4361
	11th to 20th	Day	21% 9% 13% 20% 24% 12%	4239
	21st to end	Day	25% 9% 12% 15% 19% 19%	4314
August	1st to 10th	Day	32% 12% 18% 12% 16% 11%	4078
	11th to 20th	Day	30% 12% 14% 13% 21% 10%	3951
	21st to end	Day	26% 10% 14% 18% 22% 11%	4179
September	1st to 10th	Day	26% 7% 10% 17% 22% 18%	2280
	11th to 20th	Day	26% 7% 11% 18% 26% 12%	2280
	21st to end	Day	25% 5% 7% 23% 32% 8%	2010
October	1st to 10th	Day	18% 5% 8% 19% 33% 16%	3170
	11th to 20th	Day	24% 9% 10% 33% 19% 4	3150
	21st to end	Day	21% 8% 11% 34% 23% 2	3438
November	1st to 10th	Day	24% 7% 14% 26% 27% 3	1785
	11th to 20th	Day	28% 6% 16% 28% 19% 3	1668
	21st to end	Day	34% 5% 12% 30% 15% 4	1520
December	1st to 10th	Day	43% 8% 16% 20% 10% 3	1520
	11th to 20th	Day	39% 8% 12% 27% 13%	1924
	21st to end	Day	39% 11% 11% 18% 18% 3	2794
January	1st to 10th	Night	38% 17% 14% 20% 10% 1	4385
	11th to 20th	Night	47% 9% 17% 17% 9% 1	4110
	21st to end	Night	40% 10% 20% 19% 9% 2	4521
February	1st to 10th	Night	40% 10% 16% 22% 11% 1	2470
	11th to 20th	Night	39% 8% 14% 24% 12% 2	2361
	21st to end	Night	42% 7% 13% 22% 14% 3	1884
March	1st to 10th	Night	34% 13% 15% 21% 17%	3502
	11th to 20th	Night	29% 11% 17% 31% 10% 2	3490
	21st to end	Night	39% 15% 18% 20% 8% 1	3768
April	1st to 10th	Night	36% 8% 13% 21% 19% 3	1900
	11th to 20th	Night	36% 10% 13% 22% 17% 3	1877
	21st to end	Night	33% 9% 13% 24% 19% 2	1900
May	1st to 10th	Night	33% 11% 12% 25% 16% 3	2909
	11th to 20th	Night	23% 12% 17% 31% 13% 2	2614
	21st to end	Night	21% 9% 14% 24% 27% 5%	2762
June	1st to 10th	Night	17% 7% 12% 24% 28% 11%	1520
	11th to 20th	Night	21% 7% 8% 15% 23% 26%	1520
	21st to end	Night	14% 5% 12% 18% 27% 24%	1520
July	1st to 10th	Night	18% 7% 8% 10% 22% 35%	2500
	11th to 20th	Night	16% 10% 15% 29% 16% 15%	2604
	21st to end	Night	20% 8% 16% 19% 18% 18%	3011
August	1st to 10th	Night	26% 10% 24% 15% 15% 11%	2838
	11th to 20th	Night	22% 10% 20% 15% 20% 13%	2952
	21st to end	Night	24% 10% 11% 17% 22% 16%	3487
September	1st to 10th	Night	23% 6% 11% 16% 24% 20%	1900
	11th to 20th	Night	21% 7% 13% 18% 26% 14%	1900
	21st to end	Night	18% 5% 8% 23% 36% 10%	2166
October	1st to 10th	Night	14% 5% 7% 18% 41% 16%	3800
	11th to 20th	Night	16% 8% 13% 36% 22% 4	3778
	21st to end	Night	20% 6% 11% 33% 27% 3	4120
November	1st to 10th	Night	22% 7% 13% 27% 27% 4	2346
	11th to 20th	Night	28% 7% 13% 29% 20% 3	2509
	21st to end	Night	31% 6% 11% 29% 19% 4	2660
December	1st to 10th	Night	39% 6% 17% 21% 15% 2	2660
	11th to 20th	Night	37% 9% 15% 25% 15% 1	3272
	21st to end	Night	39% 9% 13% 17% 21% 1	4873

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	11% 13% 24% 35% 16% 1	270
	11th to 20th	Day	13% 12% 34% 30% 11%	270
	21st to end	Day	7% 11% 33% 34% 15% 1	297
February	1st to 10th	Day	14% 16% 37% 26% 5% 1	270
	11th to 20th	Day	15% 19% 31% 29% 6% 1	270
	21st to end	Day	20% 19% 35% 20% 6%	225
March	1st to 10th	Day	20% 23% 29% 22% 6%	406
	11th to 20th	Day	12% 22% 33% 26% 7%	460
	21st to end	Day	16% 30% 35% 15% 4	506
April	1st to 10th	Day	17% 27% 31% 16% 8% 1	460
	11th to 20th	Day	19% 29% 29% 17% 5% 1	452
	21st to end	Day	14% 28% 32% 20% 6% 1	460
May	1st to 10th	Day	16% 24% 36% 17% 6% 1	463
	11th to 20th	Day	11% 14% 28% 32% 14% 2	617
	21st to end	Day	8% 16% 28% 31% 16% 2	734
June	1st to 10th	Day	5% 13% 28% 30% 22% 2	680
	11th to 20th	Day	4 15% 32% 32% 16% 1	680
	21st to end	Day	4 11% 31% 32% 20% 2	680
July	1st to 10th	Day	4 5% 23% 35% 29% 4	671
	11th to 20th	Day	2 3 23% 37% 31% 4	604
	21st to end	Day	5% 5% 24% 38% 22% 6%	531
August	1st to 10th	Day	7% 6% 26% 35% 21% 5%	487
	11th to 20th	Day	3 5% 30% 38% 20% 4	477
	21st to end	Day	3 6% 24% 39% 23% 5%	539
September	1st to 10th	Day	6% 8% 25% 32% 23% 6%	490
	11th to 20th	Day	6% 11% 33% 31% 14% 4	490
	21st to end	Day	12% 11% 23% 29% 19% 6%	381
October	1st to 10th	Day	10% 12% 20% 31% 18% 9%	300
	11th to 20th	Day	9% 16% 25% 29% 17% 4	300
	21st to end	Day	12% 13% 34% 27% 13% 2	330
November	1st to 10th	Day	14% 17% 29% 25% 13% 1	279
	11th to 20th	Day	8% 14% 28% 34% 14% 2	270
	21st to end	Day	9% 13% 30% 30% 15% 2	270
December	1st to 10th	Day	10% 13% 34% 28% 13% 2	270
	11th to 20th	Day	10% 10% 30% 35% 14%	270
	21st to end	Day	12% 14% 36% 28% 9%	297
January	1st to 10th	Night	5% 9% 29% 39% 16% 2	931
	11th to 20th	Night	7% 7% 32% 38% 15% 1	760
	21st to end	Night	7% 5% 28% 44% 14% 2	836
February	1st to 10th	Night	6% 8% 36% 43% 8%	760
	11th to 20th	Night	6% 8% 35% 40% 11%	744
	21st to end	Night	9% 8% 40% 38% 5%	628
March	1st to 10th	Night	6% 10% 35% 38% 10% 1	760
	11th to 20th	Night	5% 8% 26% 45% 16%	760
	21st to end	Night	8% 9% 32% 40% 11%	801
April	1st to 10th	Night	8% 12% 35% 36% 8%	570
	11th to 20th	Night	9% 10% 33% 34% 12% 1	563
	21st to end	Night	10% 10% 28% 36% 15% 2	570
May	1st to 10th	Night	9% 7% 31% 39% 12% 1	570
	11th to 20th	Night	8% 4 22% 42% 23% 2	561
	21st to end	Night	8% 7% 22% 37% 23% 3	617
June	1st to 10th	Night	4 4 17% 44% 29% 1	570
	11th to 20th	Night	5% 6% 24% 44% 19% 1	570
	21st to end	Night	1 2 19% 44% 32% 1	570
July	1st to 10th	Night	4 1 13% 40% 40% 3	555
	11th to 20th	Night	2 1 7% 44% 43% 4	552
	21st to end	Night	3 1 5% 36% 52% 3	624
August	1st to 10th	Night	7% 1 8% 41% 42% 2	567
	11th to 20th	Night	4 1 12% 43% 38% 3	564
	21st to end	Night	2 1 11% 40% 42% 3	627
September	1st to 10th	Night	4 2 12% 46% 35% 1	570
	11th to 20th	Night	4 3 15% 45% 32% 2	570
	21st to end	Night	4 3 18% 39% 33% 4	727
October	1st to 10th	Night	6% 4 22% 42% 25% 2	760
	11th to 20th	Night	5% 4 26% 40% 22% 2	760
	21st to end	Night	9% 7% 24% 39% 19% 2	836
November	1st to 10th	Night	5% 7% 31% 37% 20% 1	838
	11th to 20th	Night	8% 9% 28% 38% 15% 2	950
	21st to end	Night	7% 9% 26% 39% 18% 1	950
December	1st to 10th	Night	7% 7% 33% 40% 12% 1	950
	11th to 20th	Night	9% 10% 31% 35% 14% 1	950
	21st to end	Night	9% 9% 33% 37% 10% 1	1045

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