McMurtrey Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McMurtrey Site clear sky chart from 2005-11-06 to 2024-11-28.

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	22% 9% 3 6% 4 4 4 3 4 6% 36%	105599
	Night	25% 10% 2 6% 4 3 3 3 4 5% 36%	102612
Transparency	Day	26% 48% 6% 9% 8% 3	105342
	Night	28% 48% 7% 9% 6% 2	102458
Seeing	Day	24% 9% 21% 26% 16% 4	14656
	Night	28% 5% 19% 32% 14% 1	23625

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	43% 18% 3 7% 4 4 3 3 3 4 8%	2539
	11th to 20th	Day	41% 15% 3 6% 4 3 2 2 3 3 18%	2540
	21st to end	Day	44% 12% 3 7% 4 4 3 2 4 4 14%	3022
February	1st to 10th	Day	37% 12% 3 6% 4 4 4 3 5% 5% 17%	1710
	11th to 20th	Day	35% 14% 4 8% 5% 4 3 2 3 4 18%	1665
	21st to end	Day	36% 15% 3 7% 4 3 3 2 3 4 19%	1494
March	1st to 10th	Day	26% 16% 4 8% 6% 5% 4 3 5% 6% 17%	3262
	11th to 20th	Day	24% 11% 3 6% 4 3 3 2 3 3 37%	3490
	21st to end	Day	31% 14% 4 8% 6% 4 4 3 4 5% 16%	3917
April	1st to 10th	Day	32% 14% 4 8% 5% 4 3 3 4 5% 18%	2280
	11th to 20th	Day	27% 14% 3 7% 4 4 4 3 4 5% 25%	2249
	21st to end	Day	25% 12% 3 7% 5% 5% 5% 4 5% 6% 24%	2456
May	1st to 10th	Day	23% 10% 2 6% 4 4 4 3 4 5% 34%	4380
	11th to 20th	Day	14% 9% 2 6% 4 4 4 3 5% 8% 41%	4377
	21st to end	Day	27% 9% 3 7% 5% 5% 5% 4 5% 6% 24%	4813
June	1st to 10th	Day	25% 8% 3 7% 5% 4 5% 3 5% 6% 30%	2660
	11th to 20th	Day	23% 9% 3 7% 5% 5% 5% 5% 5% 7% 27%	2660
	21st to end	Day	11% 5% 2 6% 4 3 5% 4 5% 6% 49%	2660
July	1st to 10th	Day	6% 3 1 3 2 2 3 2 4 5% 69%	4420
	11th to 20th	Day	5% 3 1 2 2 2 2 2 3 4 74%	4420
	21st to end	Day	7% 4 1 3 2 2 2 2 4 5% 67%	4790
August	1st to 10th	Day	4 4 2 4 2 3 2 3 4 5% 66%	4163
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 7% 51%	4026
	21st to end	Day	11% 6% 2 5% 4 4 3 3 4 6% 53%	4287
September	1st to 10th	Day	10% 4 2 4 2 3 3 3 5% 6% 59%	2280
	11th to 20th	Day	15% 7% 3 7% 4 4 4 3 5% 6% 43%	2280
	21st to end	Day	18% 6% 2 6% 3 4 4 3 5% 6% 44%	2002
October	1st to 10th	Day	19% 6% 2 4 3 3 3 3 5% 7% 45%	3180
	11th to 20th	Day	23% 7% 2 7% 4 5% 5% 3 6% 7% 30%	3161
	21st to end	Day	32% 9% 3 7% 4 4 3 3 5% 7% 23%	3339
November	1st to 10th	Day	36% 11% 4 8% 5% 4 3 3 5% 6% 15%	1659
	11th to 20th	Day	39% 11% 3 6% 3 3 4 3 5% 5% 18%	1600
	21st to end	Day	34% 12% 4 7% 4 3 3 3 4 4 22%	1576
December	1st to 10th	Day	32% 14% 3 7% 3 3 3 2 4 4 24%	1520
	11th to 20th	Day	37% 12% 3 9% 4 3 3 3 5% 5% 17%	1928
	21st to end	Day	35% 15% 4 9% 4 4 4 3 4 5% 13%	2794
January	1st to 10th	Night	42% 15% 3 7% 4 4 4 3 3 3 12%	4406
	11th to 20th	Night	40% 15% 3 6% 3 3 3 2 3 4 17%	4420
	21st to end	Night	43% 13% 3 7% 4 3 3 2 3 4 15%	4637
February	1st to 10th	Night	33% 13% 3 7% 5% 4 4 3 3 5% 19%	2470
	11th to 20th	Night	34% 13% 2 6% 4 4 3 3 3 4 25%	2402
	21st to end	Night	33% 16% 3 6% 4 3 4 2 4 5% 20%	1960
March	1st to 10th	Night	26% 17% 3 8% 5% 3 4 3 4 6% 20%	3595
	11th to 20th	Night	24% 10% 2 5% 3 2 3 2 3 5% 41%	3490
	21st to end	Night	34% 15% 3 8% 5% 5% 4 3 3 4 17%	3736
April	1st to 10th	Night	29% 14% 3 8% 6% 4 4 2 4 4 22%	1900
	11th to 20th	Night	24% 12% 3 7% 4 4 3 3 4 6% 31%	1879
	21st to end	Night	29% 9% 3 6% 5% 3 3 3 4 6% 28%	1720
May	1st to 10th	Night	27% 10% 3 5% 3 3 3 2 3 3 39%	2579
	11th to 20th	Night	13% 5% 2 4 3 2 3 3 5% 5% 55%	2516
	21st to end	Night	18% 9% 3 6% 5% 5% 4 4 6% 7% 33%	2762
June	1st to 10th	Night	21% 8% 3 6% 5% 3 3 3 3 6% 38%	1520
	11th to 20th	Night	19% 9% 3 7% 4 4 3 2 4 6% 39%	1520
	21st to end	Night	9% 6% 2 4 3 3 3 2 4 5% 59%	1520
July	1st to 10th	Night	4 2 1 2 2 2 2 1 2 3 80%	2540
	11th to 20th	Night	4 2 1 3 2 2 1 1 2 3 77%	2540
	21st to end	Night	5% 2 1 1 1 1 2 1 3 4 79%	2761
August	1st to 10th	Night	3 2 1 2 2 1 2 2 4 5% 76%	2771
	11th to 20th	Night	9% 5% 1 4 3 3 4 3 4 6% 57%	2812
	21st to end	Night	13% 4 2 4 2 3 2 2 4 5% 60%	3388
September	1st to 10th	Night	9% 3 1 3 2 3 3 2 3 6% 66%	1900
	11th to 20th	Night	15% 6% 2 6% 4 3 4 2 5% 4 49%	1900
	21st to end	Night	14% 6% 2 5% 3 3 3 3 4 7% 50%	2175
October	1st to 10th	Night	15% 5% 2 4 3 3 4 3 5% 7% 50%	3800
	11th to 20th	Night	21% 8% 2 6% 3 3 4 4 6% 8% 36%	3777
	21st to end	Night	30% 12% 3 7% 4 3 4 3 4 5% 25%	4224
November	1st to 10th	Night	36% 10% 3 6% 4 4 4 3 4 5% 20%	2636
	11th to 20th	Night	41% 9% 3 6% 3 3 3 3 3 4 21%	2800
	21st to end	Night	32% 10% 3 6% 4 4 4 3 4 5% 26%	2758
December	1st to 10th	Night	33% 11% 3 5% 4 3 3 3 4 5% 25%	2660
	11th to 20th	Night	35% 14% 3 6% 4 3 3 3 4 6% 19%	3276
	21st to end	Night	35% 14% 4 9% 4 4 3 3 3 4 17%	4862

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	52% 41% 4 2 1 1	2539
	11th to 20th	Day	48% 38% 3 8% 2	2540
	21st to end	Day	51% 42% 3 3 1	3022
February	1st to 10th	Day	28% 66% 3 2 1	1710
	11th to 20th	Day	30% 65% 2 2 1	1674
	21st to end	Day	26% 66% 3 2 3	1494
March	1st to 10th	Day	39% 45% 8% 6% 2	3262
	11th to 20th	Day	25% 43% 4 21% 6%	3490
	21st to end	Day	40% 46% 7% 5% 3	3917
April	1st to 10th	Day	23% 67% 5% 3 1	2280
	11th to 20th	Day	21% 67% 4 4 4	2252
	21st to end	Day	23% 67% 3 5% 2	2456
May	1st to 10th	Day	29% 44% 9% 10% 9%	4380
	11th to 20th	Day	19% 47% 14% 15% 5%	4380
	21st to end	Day	36% 48% 10% 6% 1	4814
June	1st to 10th	Day	14% 66% 7% 9% 3	2660
	11th to 20th	Day	17% 67% 5% 7% 4	2660
	21st to end	Day	13% 65% 6% 11% 4 1	2660
July	1st to 10th	Day	8% 36% 4 10% 16% 27%	4351
	11th to 20th	Day	6% 36% 3 12% 30% 13%	4345
	21st to end	Day	13% 37% 8% 17% 22% 3	4639
August	1st to 10th	Day	11% 37% 4 16% 22% 10%	4147
	11th to 20th	Day	20% 43% 13% 16% 5% 3	4065
	21st to end	Day	21% 39% 7% 14% 12% 7%	4286
September	1st to 10th	Day	13% 62% 6% 10% 7% 3	2280
	11th to 20th	Day	18% 62% 5% 6% 5% 3	2280
	21st to end	Day	17% 61% 7% 8% 6% 1	2002
October	1st to 10th	Day	20% 38% 5% 20% 17% 1	3180
	11th to 20th	Day	35% 40% 8% 10% 7%	3161
	21st to end	Day	38% 40% 5% 11% 4	3339
November	1st to 10th	Day	32% 58% 4 5% 1	1659
	11th to 20th	Day	30% 59% 3 4 4 1	1600
	21st to end	Day	26% 58% 2 4 9% 1	1576
December	1st to 10th	Day	28% 62% 3 4 4 1	1520
	11th to 20th	Day	35% 51% 7% 5% 1	1928
	21st to end	Day	46% 44% 6% 3 1	2794
January	1st to 10th	Night	50% 40% 4 2 2 1	4406
	11th to 20th	Night	47% 40% 3 8% 2	4420
	21st to end	Night	50% 41% 4 4 1	4637
February	1st to 10th	Night	28% 66% 2 3 1	2470
	11th to 20th	Night	28% 65% 2 3 2	2404
	21st to end	Night	23% 66% 4 4 2 1	1960
March	1st to 10th	Night	38% 45% 7% 8% 2	3595
	11th to 20th	Night	21% 44% 9% 18% 7% 1	3490
	21st to end	Night	44% 44% 5% 7%	3736
April	1st to 10th	Night	23% 66% 7% 3 1	1900
	11th to 20th	Night	19% 67% 5% 5% 5%	1879
	21st to end	Night	20% 66% 6% 6% 2 1	1720
May	1st to 10th	Night	29% 42% 8% 14% 7%	2579
	11th to 20th	Night	12% 42% 18% 22% 5%	2516
	21st to end	Night	26% 50% 18% 5% 1	2762
June	1st to 10th	Night	14% 66% 7% 11% 3	1520
	11th to 20th	Night	13% 65% 6% 11% 3	1520
	21st to end	Night	10% 64% 7% 13% 4 1	1520
July	1st to 10th	Night	5% 35% 4 14% 17% 25%	2500
	11th to 20th	Night	9% 35% 3 13% 28% 13%	2492
	21st to end	Night	9% 35% 9% 19% 26% 2	2674
August	1st to 10th	Night	7% 36% 4 21% 25% 6%	2763
	11th to 20th	Night	20% 41% 18% 15% 4 2	2839
	21st to end	Night	20% 38% 10% 14% 12% 6%	3388
September	1st to 10th	Night	11% 62% 7% 11% 6% 3	1900
	11th to 20th	Night	18% 62% 5% 7% 4 4	1900
	21st to end	Night	15% 62% 7% 9% 6% 2	2175
October	1st to 10th	Night	17% 39% 8% 21% 15%	3800
	11th to 20th	Night	31% 40% 11% 12% 7%	3777
	21st to end	Night	40% 40% 5% 10% 4 1	4224
November	1st to 10th	Night	30% 57% 4 6% 2	2636
	11th to 20th	Night	30% 59% 4 4 3 1	2800
	21st to end	Night	24% 59% 3 4 9% 1	2758
December	1st to 10th	Night	28% 60% 3 4 4 1	2660
	11th to 20th	Night	35% 51% 5% 5% 4	3276
	21st to end	Night	45% 44% 6% 4 1	4862

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	48% 11% 13% 18% 8% 2	180
	11th to 20th	Day	46% 9% 15% 18% 7% 4	180
	21st to end	Day	41% 9% 17% 18% 12% 3	335
February	1st to 10th	Day	44% 9% 16% 19% 9% 2	370
	11th to 20th	Day	43% 10% 22% 15% 9% 1	365
	21st to end	Day	39% 14% 20% 19% 7% 1	346
March	1st to 10th	Day	37% 12% 22% 21% 7% 1	460
	11th to 20th	Day	30% 13% 23% 19% 10% 5%	460
	21st to end	Day	34% 14% 22% 19% 10% 1	506
April	1st to 10th	Day	33% 14% 20% 21% 9% 4	460
	11th to 20th	Day	28% 17% 24% 19% 8% 4	452
	21st to end	Day	27% 10% 29% 19% 14% 1	460
May	1st to 10th	Day	18% 16% 25% 24% 14% 3	463
	11th to 20th	Day	27% 12% 22% 21% 15% 4	488
	21st to end	Day	22% 11% 22% 24% 16% 5%	529
June	1st to 10th	Day	24% 9% 19% 27% 18% 3	490
	11th to 20th	Day	18% 11% 30% 28% 10% 2	490
	21st to end	Day	8% 7% 24% 33% 23% 4	490
July	1st to 10th	Day	11% 6% 22% 36% 21% 5%	486
	11th to 20th	Day	5% 5% 23% 40% 20% 6%	484
	21st to end	Day	6% 3 18% 40% 28% 5%	531
August	1st to 10th	Day	7% 4 17% 40% 24% 7%	487
	11th to 20th	Day	4 3 17% 36% 32% 8%	477
	21st to end	Day	10% 6% 24% 33% 21% 7%	539
September	1st to 10th	Day	9% 4 22% 40% 20% 5%	490
	11th to 20th	Day	15% 7% 18% 32% 23% 5%	490
	21st to end	Day	17% 8% 24% 29% 18% 3	490
October	1st to 10th	Day	22% 7% 20% 27% 20% 4	490
	11th to 20th	Day	27% 6% 19% 26% 18% 4	490
	21st to end	Day	34% 10% 22% 19% 11% 4	479
November	1st to 10th	Day	36% 11% 22% 18% 9% 3	247
	11th to 20th	Day	37% 13% 21% 15% 11% 4	200
	21st to end	Day	40% 11% 13% 23% 12% 1	194
December	1st to 10th	Day	41% 8% 21% 16% 13% 2	180
	11th to 20th	Day	48% 9% 19% 17% 6% 1	180
	21st to end	Day	39% 13% 17% 20% 10% 1	198
January	1st to 10th	Night	44% 9% 16% 21% 9%	760
	11th to 20th	Night	42% 6% 16% 27% 9%	760
	21st to end	Night	38% 6% 16% 27% 12% 1	836
February	1st to 10th	Night	37% 6% 19% 28% 9% 1	760
	11th to 20th	Night	39% 7% 16% 25% 12% 1	744
	21st to end	Night	39% 8% 17% 28% 8%	628
March	1st to 10th	Night	34% 9% 18% 26% 12% 1	760
	11th to 20th	Night	32% 8% 20% 29% 12% 1	760
	21st to end	Night	35% 8% 22% 24% 11% 1	836
April	1st to 10th	Night	34% 8% 20% 25% 13% 1	760
	11th to 20th	Night	27% 7% 24% 28% 12% 1	751
	21st to end	Night	31% 4 23% 28% 12%	580
May	1st to 10th	Night	18% 6% 28% 34% 13%	406
	11th to 20th	Night	20% 3 21% 38% 16% 1	374
	21st to end	Night	19% 7% 21% 34% 18% 1	411
June	1st to 10th	Night	23% 4 22% 38% 14%	380
	11th to 20th	Night	19% 4 26% 40% 10%	380
	21st to end	Night	9% 2 28% 48% 13%	380
July	1st to 10th	Night	6% 2 20% 55% 16% 1	370
	11th to 20th	Night	6% 4 25% 48% 17%	368
	21st to end	Night	3 2 22% 56% 17% 1	416
August	1st to 10th	Night	4 2 16% 46% 30% 2	516
	11th to 20th	Night	5% 1 12% 44% 35% 4	564
	21st to end	Night	8% 4 19% 45% 22% 2	765
September	1st to 10th	Night	10% 3 18% 46% 21% 2	760
	11th to 20th	Night	15% 3 14% 35% 29% 3	760
	21st to end	Night	14% 3 19% 43% 19% 2	760
October	1st to 10th	Night	21% 3 19% 33% 21% 3	760
	11th to 20th	Night	25% 3 15% 32% 22% 3	760
	21st to end	Night	32% 4 21% 30% 11% 2	836
November	1st to 10th	Night	37% 6% 17% 31% 9% 1	780
	11th to 20th	Night	44% 7% 15% 25% 9% 1	800
	21st to end	Night	34% 6% 18% 31% 11%	788
December	1st to 10th	Night	36% 6% 20% 28% 9% 1	760
	11th to 20th	Night	44% 6% 21% 22% 7%	760
	21st to end	Night	46% 7% 23% 17% 7%	836

Page generated at 2024-11-24 21:22:19. Version ac07

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