Petroglyph
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Petroglyph 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	12% 7% 3 6% 4 4 4 4 6% 7% 43%	101156
	Night	13% 8% 3 6% 4 4 4 3 5% 7% 44%	97927
Transparency	Day	39% 4 11% 26% 18% 3	104790
	Night	39% 3 11% 25% 18% 3	102434
Seeing	Day	17% 12% 28% 25% 14% 4	15588
	Night	18% 12% 30% 27% 12% 1	25387

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	10% 12% 5% 11% 4 4 4 5% 6% 6% 34%	2527
	11th to 20th	Day	12% 10% 3 8% 4 4 4 4 4 5% 41%	2800
	21st to end	Day	20% 12% 5% 7% 4 3 3 3 4 5% 34%	3135
February	1st to 10th	Day	15% 12% 3 7% 5% 5% 4 3 5% 6% 36%	1710
	11th to 20th	Day	13% 10% 3 8% 5% 4 4 4 5% 6% 38%	1829
	21st to end	Day	13% 11% 4 6% 4 4 4 4 5% 6% 39%	1570
March	1st to 10th	Day	18% 12% 3 6% 3 3 3 2 3 4 41%	3208
	11th to 20th	Day	13% 7% 2 5% 3 3 3 3 5% 6% 50%	3480
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 6% 7% 37%	3996
April	1st to 10th	Day	10% 6% 2 7% 4 4 4 4 6% 8% 45%	2280
	11th to 20th	Day	17% 8% 2 5% 5% 4 4 3 4 7% 42%	2254
	21st to end	Day	19% 8% 2 6% 5% 4 4 3 5% 6% 37%	2280
May	1st to 10th	Day	12% 7% 3 7% 5% 6% 6% 5% 8% 9% 33%	3789
	11th to 20th	Day	18% 8% 3 6% 4 4 4 3 5% 8% 38%	4349
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 5% 7% 42%	4824
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 4 6% 8% 44%	2660
	11th to 20th	Day	6% 4 2 5% 4 5% 4 5% 7% 9% 49%	2660
	21st to end	Day	4 3 2 5% 5% 5% 5% 5% 8% 10% 48%	2660
July	1st to 10th	Day	13% 5% 2 5% 4 5% 6% 4 6% 9% 40%	4440
	11th to 20th	Day	4 3 2 6% 5% 4 5% 4 7% 11% 48%	4440
	21st to end	Day	7% 6% 3 7% 5% 6% 6% 5% 8% 9% 38%	4477
August	1st to 10th	Day	6% 4 1 4 3 4 5% 4 8% 9% 52%	4082
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 45%	3729
	21st to end	Day	7% 4 2 5% 4 4 4 3 6% 9% 53%	4180
September	1st to 10th	Day	13% 5% 1 4 4 4 4 4 5% 7% 49%	2280
	11th to 20th	Day	11% 5% 2 6% 3 4 5% 4 5% 8% 48%	2266
	21st to end	Day	9% 5% 2 6% 4 4 4 4 5% 7% 51%	2068
October	1st to 10th	Day	9% 5% 1 6% 4 3 3 3 4 6% 57%	1900
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 6% 53%	1900
	21st to end	Day	17% 7% 3 5% 3 3 3 2 4 5% 47%	2090
November	1st to 10th	Day	10% 6% 2 5% 3 3 4 3 3 5% 56%	1713
	11th to 20th	Day	12% 8% 3 5% 4 3 4 3 5% 6% 48%	1710
	21st to end	Day	14% 10% 3 6% 4 3 4 2 4 5% 44%	1649
December	1st to 10th	Day	14% 11% 3 5% 4 4 4 3 5% 7% 40%	1520
	11th to 20th	Day	12% 11% 5% 8% 5% 4 4 3 5% 6% 38%	1918
	21st to end	Day	16% 14% 6% 7% 5% 4 4 3 5% 5% 29%	2783
January	1st to 10th	Night	14% 9% 6% 10% 4 4 4 4 5% 5% 37%	4428
	11th to 20th	Night	13% 9% 4 12% 6% 7% 6% 4 4 4 31%	4152
	21st to end	Night	23% 14% 5% 7% 4 3 3 2 4 4 31%	4532
February	1st to 10th	Night	18% 11% 4 8% 5% 4 3 3 4 5% 36%	2470
	11th to 20th	Night	13% 8% 4 8% 4 4 4 3 4 5% 43%	2237
	21st to end	Night	16% 10% 3 7% 4 3 3 3 5% 4 40%	1884
March	1st to 10th	Night	20% 12% 3 5% 3 3 3 2 3 4 42%	3674
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	3480
	21st to end	Night	15% 10% 3 8% 5% 5% 5% 4 5% 7% 34%	3633
April	1st to 10th	Night	10% 7% 2 5% 4 3 4 3 6% 6% 49%	1900
	11th to 20th	Night	13% 8% 3 5% 4 3 4 3 5% 6% 45%	1875
	21st to end	Night	19% 8% 2 6% 4 4 3 4 4 6% 40%	1900
May	1st to 10th	Night	17% 7% 2 5% 4 4 4 4 5% 8% 41%	3103
	11th to 20th	Night	14% 7% 3 5% 3 3 3 3 4 7% 46%	2551
	21st to end	Night	9% 7% 2 7% 5% 5% 5% 5% 7% 9% 40%	2743
June	1st to 10th	Night	8% 5% 2 6% 3 5% 4 4 7% 8% 49%	1520
	11th to 20th	Night	5% 4 2 3 3 3 4 4 7% 10% 56%	1520
	21st to end	Night	3 3 2 3 4 3 5% 5% 7% 9% 55%	1520
July	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 5% 9% 48%	2530
	11th to 20th	Night	5% 4 1 5% 4 3 4 4 5% 8% 58%	2530
	21st to end	Night	6% 6% 2 8% 4 6% 5% 5% 7% 8% 43%	3083
August	1st to 10th	Night	5% 3 2 4 4 3 4 3 6% 9% 59%	2857
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	3100
	21st to end	Night	6% 3 2 4 3 3 4 3 5% 9% 57%	3476
September	1st to 10th	Night	12% 5% 3 5% 3 3 4 3 4 6% 53%	1900
	11th to 20th	Night	9% 5% 2 5% 3 3 4 3 6% 7% 53%	1900
	21st to end	Night	10% 5% 1 5% 4 3 4 4 5% 6% 54%	2103
October	1st to 10th	Night	7% 3 1 4 3 3 2 2 5% 8% 62%	2280
	11th to 20th	Night	13% 5% 2 4 3 3 3 2 4 6% 54%	2280
	21st to end	Night	17% 8% 2 4 3 3 3 3 4 6% 48%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 4 5% 58%	2438
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 4 6% 49%	2470
	21st to end	Night	17% 10% 2 5% 3 3 3 2 3 5% 46%	2527
December	1st to 10th	Night	16% 11% 3 5% 4 4 4 4 4 8% 38%	2660
	11th to 20th	Night	15% 10% 4 7% 4 4 4 3 4 5% 40%	3279
	21st to end	Night	14% 17% 5% 9% 6% 4 5% 4 6% 5% 25%	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	49% 3 10% 26% 10% 2	2528
	11th to 20th	Day	45% 2 10% 25% 17% 1	2800
	21st to end	Day	54% 2 9% 18% 14% 4	3135
February	1st to 10th	Day	49% 3 10% 23% 12% 1	1710
	11th to 20th	Day	47% 4 11% 19% 15% 4	1839
	21st to end	Day	46% 3 8% 23% 14% 5%	1570
March	1st to 10th	Day	48% 3 6% 20% 15% 7%	3208
	11th to 20th	Day	36% 3 10% 28% 17% 6%	3480
	21st to end	Day	46% 4 17% 21% 12% 1	3996
April	1st to 10th	Day	37% 4 12% 26% 20% 1	2280
	11th to 20th	Day	44% 3 10% 22% 17% 4	2254
	21st to end	Day	48% 3 11% 23% 13% 1	2280
May	1st to 10th	Day	45% 6% 13% 23% 12% 1	3824
	11th to 20th	Day	46% 4 11% 26% 13% 1	4353
	21st to end	Day	40% 6% 10% 21% 23% 1	4837
June	1st to 10th	Day	37% 5% 12% 27% 17% 2	2660
	11th to 20th	Day	30% 5% 12% 27% 22% 4	2660
	21st to end	Day	28% 6% 18% 26% 16% 5%	2660
July	1st to 10th	Day	40% 5% 11% 30% 13% 2	4371
	11th to 20th	Day	28% 4 13% 35% 17% 2	4365
	21st to end	Day	40% 5% 17% 28% 10% 1	4336
August	1st to 10th	Day	27% 4 17% 34% 18%	4068
	11th to 20th	Day	38% 5% 12% 26% 17% 2	3765
	21st to end	Day	30% 4 13% 36% 14% 3	4179
September	1st to 10th	Day	33% 4 11% 27% 19% 5%	2280
	11th to 20th	Day	35% 5% 7% 27% 24% 2	2266
	21st to end	Day	32% 5% 7% 28% 24% 3	2068
October	1st to 10th	Day	18% 2 6% 20% 45% 9%	3160
	11th to 20th	Day	23% 1 7% 21% 37% 10%	3140
	21st to end	Day	32% 2 6% 29% 27% 4	3428
November	1st to 10th	Day	33% 3 11% 27% 21% 6%	1713
	11th to 20th	Day	38% 3 8% 23% 22% 6%	1710
	21st to end	Day	44% 3 9% 25% 18% 1	1649
December	1st to 10th	Day	45% 3 11% 27% 13% 2	1520
	11th to 20th	Day	49% 3 9% 21% 18% 1	1915
	21st to end	Day	58% 2 11% 16% 10% 2	2783
January	1st to 10th	Night	50% 2 11% 23% 12% 1	4428
	11th to 20th	Night	55% 2 9% 22% 11% 2	4152
	21st to end	Night	59% 2 8% 22% 9% 1	4532
February	1st to 10th	Night	52% 3 9% 21% 13% 1	2470
	11th to 20th	Night	44% 3 10% 22% 17% 3	2238
	21st to end	Night	47% 3 10% 21% 15% 4	1884
March	1st to 10th	Night	49% 2 7% 22% 16% 3	3674
	11th to 20th	Night	36% 3 12% 25% 19% 5%	3480
	21st to end	Night	49% 3 14% 26% 7% 1	3633
April	1st to 10th	Night	35% 4 11% 27% 20% 3	1900
	11th to 20th	Night	40% 4 10% 24% 18% 5%	1875
	21st to end	Night	45% 4 9% 25% 16% 2	1900
May	1st to 10th	Night	43% 5% 16% 21% 14% 2	3133
	11th to 20th	Night	39% 4 16% 25% 15% 1	2552
	21st to end	Night	39% 7% 13% 25% 16% 1	2751
June	1st to 10th	Night	32% 4 16% 26% 19% 3	1520
	11th to 20th	Night	23% 7% 11% 31% 24% 4	1520
	21st to end	Night	23% 4 17% 30% 19% 7%	1520
July	1st to 10th	Night	35% 4 13% 29% 15% 3	2490
	11th to 20th	Night	25% 5% 11% 38% 20% 3	2482
	21st to end	Night	37% 4 18% 24% 14% 2	2985
August	1st to 10th	Night	23% 5% 18% 33% 20% 1	2847
	11th to 20th	Night	35% 5% 15% 25% 17% 3	3130
	21st to end	Night	25% 4 14% 36% 18% 3	3476
September	1st to 10th	Night	32% 4 9% 28% 21% 6%	1900
	11th to 20th	Night	30% 5% 10% 27% 26% 3	1900
	21st to end	Night	31% 5% 9% 24% 28% 4	2103
October	1st to 10th	Night	14% 2 7% 17% 50% 11%	3800
	11th to 20th	Night	23% 2 6% 21% 35% 13%	3779
	21st to end	Night	31% 2 7% 27% 27% 6%	4120
November	1st to 10th	Night	30% 3 12% 26% 24% 6%	2438
	11th to 20th	Night	37% 2 8% 24% 24% 5%	2470
	21st to end	Night	43% 2 10% 24% 20% 2	2527
December	1st to 10th	Night	46% 4 11% 24% 15% 2	2660
	11th to 20th	Night	47% 3 9% 23% 17% 1	3281
	21st to end	Night	60% 3 11% 16% 8% 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	27% 21% 36% 14% 2 1	270
	11th to 20th	Day	31% 25% 34% 9% 1	270
	21st to end	Day	19% 32% 35% 12% 2	297
February	1st to 10th	Day	27% 24% 35% 12% 1	270
	11th to 20th	Day	28% 27% 31% 10% 2 1	270
	21st to end	Day	32% 24% 34% 8% 1	225
March	1st to 10th	Day	26% 23% 42% 7% 3	270
	11th to 20th	Day	34% 21% 33% 9% 3	270
	21st to end	Day	32% 21% 33% 12% 2	402
April	1st to 10th	Day	20% 22% 35% 17% 6%	460
	11th to 20th	Day	27% 19% 30% 18% 6% 1	452
	21st to end	Day	23% 17% 36% 17% 6% 1	460
May	1st to 10th	Day	20% 19% 38% 16% 6% 1	463
	11th to 20th	Day	19% 12% 31% 25% 10% 3	638
	21st to end	Day	12% 10% 35% 25% 14% 4	734
June	1st to 10th	Day	12% 7% 27% 28% 22% 5%	680
	11th to 20th	Day	7% 5% 27% 39% 18% 4	680
	21st to end	Day	5% 7% 33% 36% 15% 4	680
July	1st to 10th	Day	12% 3 19% 28% 27% 10%	671
	11th to 20th	Day	6% 4 21% 40% 24% 6%	668
	21st to end	Day	10% 3 21% 35% 25% 6%	739
August	1st to 10th	Day	10% 3 18% 37% 24% 8%	667
	11th to 20th	Day	13% 3 22% 30% 26% 6%	479
	21st to end	Day	12% 2 15% 31% 29% 11%	539
September	1st to 10th	Day	14% 4 19% 30% 27% 6%	490
	11th to 20th	Day	10% 3 18% 33% 24% 12%	490
	21st to end	Day	13% 9% 22% 30% 18% 7%	468
October	1st to 10th	Day	13% 12% 27% 32% 12% 4	300
	11th to 20th	Day	20% 17% 33% 24% 5% 1	300
	21st to end	Day	30% 19% 30% 18% 2	330
November	1st to 10th	Day	16% 14% 39% 21% 8% 2	279
	11th to 20th	Day	24% 20% 30% 19% 7% 1	270
	21st to end	Day	25% 19% 34% 17% 4 1	270
December	1st to 10th	Day	19% 25% 32% 13% 8% 2	270
	11th to 20th	Day	27% 20% 33% 16% 3 1	270
	21st to end	Day	27% 26% 33% 12% 2	297
January	1st to 10th	Night	21% 22% 35% 18% 3	950
	11th to 20th	Night	24% 24% 32% 16% 3	950
	21st to end	Night	21% 22% 38% 18% 1	1045
February	1st to 10th	Night	29% 18% 34% 17% 2	950
	11th to 20th	Night	21% 22% 39% 17% 2	762
	21st to end	Night	26% 21% 33% 19% 1	628
March	1st to 10th	Night	25% 17% 37% 18% 2	727
	11th to 20th	Night	24% 14% 37% 22% 3	570
	21st to end	Night	27% 15% 34% 21% 4	627
April	1st to 10th	Night	16% 19% 35% 26% 4	570
	11th to 20th	Night	20% 15% 33% 23% 9%	563
	21st to end	Night	25% 10% 38% 21% 6%	570
May	1st to 10th	Night	17% 8% 37% 28% 9%	570
	11th to 20th	Night	19% 7% 29% 31% 14% 1	561
	21st to end	Night	12% 6% 28% 34% 18% 3	617
June	1st to 10th	Night	10% 3 22% 40% 23% 1	570
	11th to 20th	Night	6% 2 25% 47% 19% 1	570
	21st to end	Night	4 2 28% 43% 21% 2	570
July	1st to 10th	Night	11% 1 14% 36% 33% 6%	555
	11th to 20th	Night	6% 3 15% 44% 28% 3	552
	21st to end	Night	8% 2 16% 41% 28% 4	624
August	1st to 10th	Night	7% 2 14% 40% 34% 3	567
	11th to 20th	Night	7% 2 16% 39% 33% 3	564
	21st to end	Night	7% 3 14% 35% 35% 6%	627
September	1st to 10th	Night	14% 2 13% 36% 29% 5%	570
	11th to 20th	Night	11% 2 13% 37% 30% 6%	570
	21st to end	Night	12% 7% 23% 30% 25% 4	755
October	1st to 10th	Night	9% 5% 28% 33% 23% 4	760
	11th to 20th	Night	18% 13% 30% 27% 11% 1	760
	21st to end	Night	25% 14% 32% 23% 6%	836
November	1st to 10th	Night	14% 11% 39% 26% 10% 1	755
	11th to 20th	Night	21% 16% 32% 24% 7%	760
	21st to end	Night	25% 16% 34% 20% 5%	817
December	1st to 10th	Night	25% 15% 34% 21% 5%	950
	11th to 20th	Night	25% 13% 35% 23% 4	950
	21st to end	Night	26% 19% 36% 17% 2	1045

Page generated at 2024-11-01 21:47:36. Version ac07

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