Prettyboy Reservoir
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prettyboy Reservoir clear sky chart from 2005-11-06 to 2024-11-30.

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% 6% 2 6% 4 4 5% 4 6% 7% 40%	101510
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	98037
Transparency	Day	41% 6% 12% 24% 15% 3	105147
	Night	40% 5% 11% 24% 16% 3	102540
Seeing	Day	20% 15% 29% 22% 11% 3	15019
	Night	18% 14% 30% 26% 11% 1	24456

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	16% 7% 2 5% 4 4 4 4 5% 5% 45%	2517
	11th to 20th	Day	14% 10% 3 10% 6% 5% 4 3 5% 5% 36%	2790
	21st to end	Day	18% 10% 3 7% 5% 5% 5% 4 5% 6% 34%	3124
February	1st to 10th	Day	13% 9% 3 7% 4 5% 4 5% 7% 6% 37%	1710
	11th to 20th	Day	16% 8% 2 6% 5% 5% 5% 4 5% 6% 40%	1729
	21st to end	Day	19% 10% 3 6% 4 4 3 3 4 6% 38%	1570
March	1st to 10th	Day	23% 6% 2 6% 4 3 4 4 5% 6% 37%	3306
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 7% 7% 45%	3480
	21st to end	Day	20% 8% 3 6% 4 3 3 3 4 5% 41%	3928
April	1st to 10th	Day	15% 8% 2 5% 5% 5% 4 5% 6% 7% 38%	2280
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 5% 7% 44%	2254
	21st to end	Day	18% 5% 2 6% 4 4 4 4 5% 7% 40%	2280
May	1st to 10th	Day	28% 6% 2 6% 4 4 4 4 5% 7% 29%	4003
	11th to 20th	Day	23% 8% 3 7% 5% 5% 5% 4 5% 7% 28%	4393
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	4823
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 7% 10% 39%	2660
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	2660
	21st to end	Day	10% 4 2 5% 6% 4 5% 5% 7% 10% 43%	2660
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 6% 7% 11% 41%	4440
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 38%	4440
	21st to end	Day	10% 8% 3 8% 6% 5% 5% 5% 7% 8% 35%	4476
August	1st to 10th	Day	18% 5% 2 6% 5% 5% 6% 5% 7% 8% 32%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 8% 40%	3858
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 8% 46%	4180
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 4 5% 6% 45%	2280
	11th to 20th	Day	12% 5% 1 5% 4 3 4 3 6% 8% 48%	2280
	21st to end	Day	20% 8% 2 6% 5% 4 4 3 5% 7% 37%	2014
October	1st to 10th	Day	17% 7% 2 5% 3 4 3 3 5% 6% 45%	1900
	11th to 20th	Day	16% 6% 2 5% 4 4 3 3 5% 6% 48%	1900
	21st to end	Day	14% 6% 3 6% 3 3 4 3 5% 6% 46%	2090
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 51%	1799
	11th to 20th	Day	15% 7% 1 4 3 3 4 4 5% 6% 48%	1782
	21st to end	Day	15% 6% 2 5% 4 4 5% 3 4 6% 45%	1598
December	1st to 10th	Day	17% 8% 3 6% 5% 5% 4 3 5% 7% 37%	1520
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 3 6% 7% 40%	1910
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 5% 8% 35%	2772
January	1st to 10th	Night	14% 7% 2 6% 4 4 5% 4 5% 7% 41%	4429
	11th to 20th	Night	18% 9% 2 5% 3 3 3 3 5% 6% 43%	4154
	21st to end	Night	21% 9% 3 6% 4 4 4 2 5% 6% 38%	4532
February	1st to 10th	Night	15% 8% 4 6% 4 4 3 3 6% 7% 40%	2470
	11th to 20th	Night	17% 8% 3 6% 4 3 4 4 4 5% 41%	2337
	21st to end	Night	16% 8% 3 7% 4 4 4 3 5% 6% 40%	1884
March	1st to 10th	Night	20% 9% 2 6% 4 3 4 3 4 5% 38%	3542
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 6% 50%	3480
	21st to end	Night	14% 9% 3 6% 4 3 3 2 3 5% 48%	3704
April	1st to 10th	Night	15% 8% 2 6% 5% 4 5% 3 6% 7% 38%	1900
	11th to 20th	Night	14% 5% 2 6% 4 4 4 3 5% 9% 43%	1875
	21st to end	Night	17% 8% 2 4 3 3 4 3 6% 7% 42%	1900
May	1st to 10th	Night	25% 8% 3 6% 4 3 4 2 5% 7% 32%	2920
	11th to 20th	Night	24% 10% 4 7% 5% 4 3 3 5% 6% 28%	2496
	21st to end	Night	12% 6% 2 5% 4 4 5% 4 7% 10% 42%	2732
June	1st to 10th	Night	11% 6% 2 6% 4 4 3 4 7% 8% 45%	1520
	11th to 20th	Night	11% 6% 2 5% 5% 5% 5% 5% 7% 9% 41%	1520
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 7% 10% 43%	1520
July	1st to 10th	Night	6% 4 3 8% 5% 6% 6% 6% 7% 10% 39%	2520
	11th to 20th	Night	9% 5% 3 6% 4 5% 6% 5% 8% 10% 39%	2520
	21st to end	Night	12% 7% 2 7% 4 4 4 4 6% 8% 41%	3073
August	1st to 10th	Night	20% 5% 2 6% 4 5% 4 4 7% 9% 34%	2825
	11th to 20th	Night	11% 8% 2 7% 5% 5% 5% 4 6% 7% 39%	2939
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 7% 48%	3476
September	1st to 10th	Night	13% 6% 3 5% 4 3 3 3 6% 8% 46%	1900
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 4 8% 52%	1900
	21st to end	Night	20% 7% 2 6% 4 4 4 3 4 6% 41%	2162
October	1st to 10th	Night	19% 7% 2 6% 3 3 3 3 4 5% 44%	2280
	11th to 20th	Night	15% 5% 2 5% 3 2 4 3 5% 6% 51%	2280
	21st to end	Night	16% 5% 2 4 3 3 4 3 5% 6% 49%	2508
November	1st to 10th	Night	11% 5% 2 5% 4 3 4 3 5% 6% 53%	2497
	11th to 20th	Night	15% 8% 2 5% 4 3 4 3 4 6% 47%	2615
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 7% 46%	2798
December	1st to 10th	Night	17% 8% 2 7% 5% 4 4 4 5% 6% 38%	2660
	11th to 20th	Night	17% 6% 2 5% 4 4 4 4 6% 7% 41%	3285
	21st to end	Night	16% 7% 2 7% 5% 5% 6% 5% 7% 8% 33%	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	39% 6% 10% 29% 12% 4	2518
	11th to 20th	Day	50% 4 9% 21% 13% 4	2790
	21st to end	Day	51% 5% 11% 23% 9% 2	3124
February	1st to 10th	Day	43% 6% 13% 25% 11% 2	1710
	11th to 20th	Day	44% 5% 10% 24% 14% 3	1738
	21st to end	Day	49% 4 9% 21% 13% 4	1570
March	1st to 10th	Day	45% 6% 11% 24% 12% 3	3306
	11th to 20th	Day	35% 7% 9% 28% 18% 4	3480
	21st to end	Day	46% 4 9% 20% 17% 3	3928
April	1st to 10th	Day	42% 7% 12% 20% 16% 4	2280
	11th to 20th	Day	39% 6% 11% 22% 18% 5%	2254
	21st to end	Day	42% 7% 9% 21% 18% 3	2280
May	1st to 10th	Day	53% 5% 10% 20% 10% 1	4040
	11th to 20th	Day	52% 8% 14% 17% 7% 1	4397
	21st to end	Day	34% 7% 14% 26% 17% 2	4837
June	1st to 10th	Day	38% 7% 14% 24% 15% 2	2660
	11th to 20th	Day	40% 8% 14% 23% 13% 2	2660
	21st to end	Day	34% 8% 14% 26% 16% 2	2660
July	1st to 10th	Day	36% 9% 16% 22% 13% 4	4370
	11th to 20th	Day	37% 8% 15% 27% 11% 1	4365
	21st to end	Day	44% 8% 11% 18% 17% 1	4333
August	1st to 10th	Day	47% 7% 13% 24% 9%	4091
	11th to 20th	Day	38% 7% 11% 28% 14% 1	3897
	21st to end	Day	35% 6% 12% 28% 18% 2	4180
September	1st to 10th	Day	39% 6% 11% 19% 20% 4	2280
	11th to 20th	Day	34% 5% 10% 27% 21% 3	2280
	21st to end	Day	47% 5% 8% 18% 19% 2	2014
October	1st to 10th	Day	38% 4 7% 20% 27% 5%	3160
	11th to 20th	Day	27% 5% 10% 26% 22% 10%	3140
	21st to end	Day	26% 3 10% 29% 28% 4	3428
November	1st to 10th	Day	33% 4 13% 25% 21% 4	1799
	11th to 20th	Day	36% 5% 11% 25% 21% 3	1782
	21st to end	Day	40% 5% 11% 29% 13% 2	1598
December	1st to 10th	Day	46% 5% 14% 23% 11% 1	1520
	11th to 20th	Day	42% 6% 10% 31% 10% 1	1906
	21st to end	Day	46% 5% 16% 21% 11% 1	2772
January	1st to 10th	Night	42% 5% 11% 26% 13% 3	4429
	11th to 20th	Night	42% 4 11% 27% 14% 2	4154
	21st to end	Night	48% 4 7% 28% 12% 2	4532
February	1st to 10th	Night	43% 4 14% 25% 13% 1	2470
	11th to 20th	Night	44% 4 9% 21% 18% 4	2339
	21st to end	Night	45% 4 10% 20% 15% 5%	1884
March	1st to 10th	Night	48% 5% 8% 26% 12% 1	3542
	11th to 20th	Night	33% 6% 10% 33% 15% 2	3480
	21st to end	Night	41% 3 8% 22% 21% 5%	3704
April	1st to 10th	Night	44% 5% 10% 20% 17% 4	1900
	11th to 20th	Night	38% 5% 12% 21% 19% 5%	1875
	21st to end	Night	40% 6% 10% 22% 17% 5%	1900
May	1st to 10th	Night	52% 6% 9% 19% 12% 2	2949
	11th to 20th	Night	55% 6% 13% 14% 10% 1	2497
	21st to end	Night	35% 9% 17% 25% 12% 3	2739
June	1st to 10th	Night	35% 8% 11% 26% 18% 2	1520
	11th to 20th	Night	38% 7% 14% 24% 16% 2	1520
	21st to end	Night	35% 7% 14% 26% 16% 2	1520
July	1st to 10th	Night	38% 10% 19% 17% 11% 4	2480
	11th to 20th	Night	36% 10% 16% 25% 11% 1	2472
	21st to end	Night	41% 7% 14% 21% 17% 1	2975
August	1st to 10th	Night	45% 7% 18% 21% 8% 1	2814
	11th to 20th	Night	42% 6% 10% 28% 13% 1	2966
	21st to end	Night	34% 5% 11% 30% 16% 2	3476
September	1st to 10th	Night	37% 6% 12% 21% 20% 5%	1900
	11th to 20th	Night	32% 6% 9% 25% 25% 3	1900
	21st to end	Night	45% 5% 9% 19% 19% 3	2162
October	1st to 10th	Night	43% 4 10% 19% 21% 3	3800
	11th to 20th	Night	24% 4 11% 27% 28% 7%	3779
	21st to end	Night	25% 3 11% 32% 24% 5%	4120
November	1st to 10th	Night	32% 5% 10% 27% 22% 5%	2497
	11th to 20th	Night	39% 4 8% 25% 20% 5%	2615
	21st to end	Night	36% 5% 12% 26% 18% 2	2798
December	1st to 10th	Night	45% 5% 12% 24% 13% 1	2660
	11th to 20th	Night	41% 5% 11% 23% 17% 3	3288
	21st to end	Night	45% 7% 15% 22% 10% 1	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	28% 28% 29% 11% 4 1	180
	11th to 20th	Day	35% 23% 31% 10% 1	258
	21st to end	Day	23% 31% 32% 11% 3	297
February	1st to 10th	Day	26% 24% 38% 10% 2	270
	11th to 20th	Day	34% 26% 34% 4 1	336
	21st to end	Day	33% 23% 32% 11% 2	382
March	1st to 10th	Day	25% 27% 30% 17% 2	460
	11th to 20th	Day	27% 24% 31% 14% 3 2	460
	21st to end	Day	25% 23% 32% 15% 5% 1	506
April	1st to 10th	Day	24% 23% 35% 13% 5% 1	460
	11th to 20th	Day	19% 23% 29% 18% 9% 2	452
	21st to end	Day	25% 16% 37% 14% 7% 1	460
May	1st to 10th	Day	27% 19% 28% 17% 8% 2	463
	11th to 20th	Day	24% 14% 31% 21% 8% 2	487
	21st to end	Day	15% 7% 27% 28% 19% 3	529
June	1st to 10th	Day	14% 13% 31% 21% 15% 4	490
	11th to 20th	Day	16% 9% 30% 29% 12% 3	490
	21st to end	Day	12% 9% 33% 29% 14% 3	490
July	1st to 10th	Day	12% 4 22% 33% 20% 9%	485
	11th to 20th	Day	6% 4 23% 37% 24% 6%	484
	21st to end	Day	12% 5% 30% 32% 18% 4	532
August	1st to 10th	Day	12% 6% 23% 31% 21% 7%	486
	11th to 20th	Day	10% 6% 24% 34% 19% 7%	479
	21st to end	Day	11% 5% 24% 32% 18% 10%	539
September	1st to 10th	Day	17% 10% 21% 22% 20% 10%	490
	11th to 20th	Day	12% 6% 24% 30% 20% 8%	490
	21st to end	Day	27% 13% 23% 23% 12% 3	490
October	1st to 10th	Day	23% 9% 25% 22% 14% 6%	490
	11th to 20th	Day	19% 16% 27% 25% 11% 3	490
	21st to end	Day	15% 17% 37% 20% 8% 2	539
November	1st to 10th	Day	17% 15% 33% 23% 10% 2	415
	11th to 20th	Day	19% 22% 34% 17% 8%	382
	21st to end	Day	30% 16% 36% 15% 5%	200
December	1st to 10th	Day	26% 23% 35% 12% 3 1	180
	11th to 20th	Day	31% 23% 29% 13% 4	180
	21st to end	Day	25% 21% 34% 14% 7%	198
January	1st to 10th	Night	20% 21% 39% 17% 3	760
	11th to 20th	Night	25% 19% 33% 19% 3	760
	21st to end	Night	18% 21% 42% 18% 1	836
February	1st to 10th	Night	21% 19% 37% 21% 2	760
	11th to 20th	Night	26% 25% 34% 14% 1	743
	21st to end	Night	25% 21% 37% 14% 3	628
March	1st to 10th	Night	21% 20% 37% 19% 3	760
	11th to 20th	Night	21% 21% 35% 18% 5%	760
	21st to end	Night	21% 19% 39% 18% 4 1	836
April	1st to 10th	Night	21% 24% 34% 18% 4	760
	11th to 20th	Night	17% 16% 37% 23% 7%	751
	21st to end	Night	22% 16% 32% 23% 7%	760
May	1st to 10th	Night	21% 10% 31% 27% 9% 1	640
	11th to 20th	Night	23% 7% 31% 31% 8% 1	375
	21st to end	Night	16% 5% 22% 37% 18% 1	411
June	1st to 10th	Night	11% 3 32% 33% 20%	380
	11th to 20th	Night	13% 4 27% 40% 15% 1	380
	21st to end	Night	11% 4 26% 44% 15%	380
July	1st to 10th	Night	8% 4 20% 39% 25% 4	370
	11th to 20th	Night	8% 3 19% 40% 28% 3	368
	21st to end	Night	10% 3 20% 39% 26% 2	610
August	1st to 10th	Night	12% 4 13% 46% 23% 2	567
	11th to 20th	Night	12% 5% 17% 39% 25% 2	653
	21st to end	Night	14% 3 17% 37% 25% 4	836
September	1st to 10th	Night	16% 4 22% 33% 22% 4	760
	11th to 20th	Night	12% 4 19% 36% 25% 4	760
	21st to end	Night	23% 6% 22% 30% 17% 2	760
October	1st to 10th	Night	22% 8% 22% 26% 18% 4	760
	11th to 20th	Night	17% 12% 28% 30% 11% 1	760
	21st to end	Night	20% 13% 32% 23% 12%	836
November	1st to 10th	Night	14% 12% 35% 28% 10% 1	780
	11th to 20th	Night	19% 19% 31% 23% 7% 1	800
	21st to end	Night	17% 18% 35% 22% 7% 1	800
December	1st to 10th	Night	23% 18% 36% 17% 7%	760
	11th to 20th	Night	19% 17% 40% 20% 4	760
	21st to end	Night	21% 19% 34% 25% 2	836

Page generated at 2024-11-27 17:30:50. Version ac07

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