S. Parrett Mtn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the S. Parrett Mtn Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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	21% 8% 3 7% 6% 3 4 4 4 6% 33%	78809
	Night	23% 9% 3 6% 5% 3 4 4 4 5% 33%	81344
Transparency	Day	52% 9% 12% 16% 10% 2	78512
	Night	52% 9% 12% 15% 9% 3	81162
Seeing	Day	21% 9% 20% 26% 19% 5%	13806
	Night	24% 7% 17% 31% 20% 2	22930

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	39% 12% 5% 11% 7% 4 5% 3 3 3 7%	2539
	11th to 20th	Day	39% 14% 3 7% 6% 3 3 3 2 3 15%	2540
	21st to end	Day	35% 12% 4 8% 8% 3 4 4 4 4 14%	3082
February	1st to 10th	Day	30% 11% 4 8% 6% 4 5% 5% 5% 7% 15%	1710
	11th to 20th	Day	30% 11% 5% 10% 7% 3 5% 4 4 5% 17%	1665
	21st to end	Day	30% 12% 4 7% 7% 3 4 4 4 6% 18%	1484
March	1st to 10th	Day	27% 12% 4 8% 8% 4 5% 4 6% 6% 17%	2000
	11th to 20th	Day	29% 11% 4 7% 6% 3 4 4 3 6% 23%	2090
	21st to end	Day	28% 11% 6% 8% 7% 4 6% 5% 4 6% 15%	2342
April	1st to 10th	Day	27% 10% 4 8% 7% 3 6% 4 5% 6% 19%	2185
	11th to 20th	Day	25% 10% 4 7% 6% 4 5% 5% 5% 6% 25%	2129
	21st to end	Day	20% 9% 3 8% 6% 5% 6% 5% 6% 7% 26%	2336
May	1st to 10th	Day	15% 9% 4 7% 7% 3 5% 4 5% 5% 36%	2467
	11th to 20th	Day	19% 7% 3 7% 6% 3 5% 5% 5% 8% 32%	2477
	21st to end	Day	23% 7% 3 6% 6% 4 5% 6% 6% 7% 29%	2723
June	1st to 10th	Day	20% 8% 4 6% 6% 3 4 5% 5% 7% 33%	2520
	11th to 20th	Day	18% 8% 3 6% 7% 3 6% 6% 6% 8% 28%	2520
	21st to end	Day	8% 5% 3 4 4 3 4 4 4 7% 53%	2520
July	1st to 10th	Day	6% 3 2 3 4 2 4 4 4 6% 60%	2520
	11th to 20th	Day	6% 3 2 3 3 2 3 3 5% 6% 63%	2520
	21st to end	Day	4 3 1 2 2 1 3 2 3 5% 73%	2741
August	1st to 10th	Day	3 2 1 4 3 3 4 4 5% 7% 64%	2338
	11th to 20th	Day	5% 3 1 3 3 2 3 3 4 6% 66%	2298
	21st to end	Day	6% 3 2 4 4 3 4 4 6% 8% 56%	2452
September	1st to 10th	Day	6% 3 2 3 4 3 3 4 4 7% 62%	2160
	11th to 20th	Day	11% 7% 3 6% 5% 3 4 5% 5% 7% 44%	2156
	21st to end	Day	16% 5% 2 5% 6% 2 3 5% 5% 6% 44%	1895
October	1st to 10th	Day	19% 8% 3 6% 6% 3 5% 4 5% 6% 37%	1800
	11th to 20th	Day	20% 8% 4 9% 6% 3 4 4 5% 6% 31%	1800
	21st to end	Day	27% 10% 4 8% 5% 3 4 3 3 5% 28%	1948
November	1st to 10th	Day	31% 12% 5% 9% 6% 3 5% 4 4 6% 15%	1570
	11th to 20th	Day	30% 11% 3 10% 7% 4 4 5% 3 5% 18%	1520
	21st to end	Day	27% 11% 5% 9% 7% 3 4 4 4 4 21%	1520
December	1st to 10th	Day	29% 11% 5% 8% 6% 3 3 4 4 5% 22%	1520
	11th to 20th	Day	30% 11% 5% 9% 6% 3 5% 4 6% 7% 14%	1928
	21st to end	Day	33% 14% 5% 13% 9% 3 5% 4 4 3 7%	2794
January	1st to 10th	Night	35% 14% 5% 9% 8% 4 5% 4 3 3 11%	4406
	11th to 20th	Night	39% 13% 4 7% 5% 2 3 3 3 4 17%	4420
	21st to end	Night	38% 12% 4 8% 6% 3 4 3 4 4 16%	4581
February	1st to 10th	Night	28% 12% 4 7% 6% 4 5% 4 5% 6% 20%	2470
	11th to 20th	Night	30% 11% 3 7% 7% 3 4 4 3 4 25%	2402
	21st to end	Night	28% 11% 4 6% 6% 4 4 5% 4 5% 22%	1969
March	1st to 10th	Night	24% 11% 4 8% 6% 3 5% 5% 4 5% 25%	2179
	11th to 20th	Night	25% 7% 4 6% 7% 3 4 4 6% 7% 27%	2090
	21st to end	Night	25% 11% 4 7% 8% 4 5% 4 5% 6% 20%	2256
April	1st to 10th	Night	27% 12% 4 7% 7% 3 4 4 4 6% 24%	1820
	11th to 20th	Night	21% 9% 4 6% 5% 3 4 5% 4 8% 31%	1779
	21st to end	Night	23% 9% 4 6% 5% 3 4 3 5% 7% 31%	1620
May	1st to 10th	Night	12% 9% 3 6% 4 3 5% 4 4 6% 42%	1491
	11th to 20th	Night	16% 7% 3 6% 6% 4 5% 5% 5% 7% 37%	1416
	21st to end	Night	17% 8% 3 5% 6% 4 5% 4 7% 7% 33%	1552
June	1st to 10th	Night	17% 6% 3 7% 5% 4 6% 4 5% 6% 37%	1440
	11th to 20th	Night	14% 9% 4 7% 6% 3 5% 4 4 7% 37%	1440
	21st to end	Night	6% 5% 3 4 4 3 4 3 4 5% 60%	1440
July	1st to 10th	Night	6% 3 1 2 3 2 3 3 3 4 68%	1440
	11th to 20th	Night	5% 2 1 3 3 2 3 4 4 4 70%	1440
	21st to end	Night	2 2 1 2 2 1 1 2 3 4 80%	1576
August	1st to 10th	Night	3 2 2 2 3 2 2 3 4 6% 71%	1598
	11th to 20th	Night	4 2 1 2 2 2 2 3 3 5% 73%	1602
	21st to end	Night	6% 3 2 3 3 2 2 3 4 5% 67%	1903
September	1st to 10th	Night	7% 4 1 3 3 1 2 3 4 5% 68%	1800
	11th to 20th	Night	10% 5% 2 5% 5% 4 4 5% 5% 5% 50%	1800
	21st to end	Night	12% 4 2 4 5% 3 4 4 5% 7% 51%	2065
October	1st to 10th	Night	17% 6% 3 4 4 3 4 4 4 5% 45%	2160
	11th to 20th	Night	19% 6% 3 5% 4 3 4 4 4 7% 42%	2160
	21st to end	Night	24% 8% 3 5% 4 2 4 3 3 5% 38%	2407
November	1st to 10th	Night	29% 9% 3 7% 6% 3 5% 4 5% 5% 24%	2504
	11th to 20th	Night	34% 9% 4 7% 5% 3 4 4 4 5% 22%	2660
	21st to end	Night	26% 7% 4 8% 7% 4 5% 4 4 5% 27%	2660
December	1st to 10th	Night	30% 10% 3 7% 6% 3 4 3 4 7% 24%	2660
	11th to 20th	Night	31% 12% 3 7% 6% 4 5% 4 4 7% 18%	3276
	21st to end	Night	29% 13% 5% 10% 8% 4 5% 4 4 3 14%	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	79% 10% 5% 3 2 1	2539
	11th to 20th	Day	76% 5% 6% 9% 3 2	2540
	21st to end	Day	71% 10% 6% 7% 5%	3082
February	1st to 10th	Day	67% 11% 10% 8% 4	1710
	11th to 20th	Day	68% 10% 7% 8% 4 2	1674
	21st to end	Day	68% 8% 10% 7% 5% 2	1484
March	1st to 10th	Day	65% 10% 11% 9% 5%	2000
	11th to 20th	Day	64% 8% 8% 14% 6% 1	2090
	21st to end	Day	70% 10% 10% 9% 2	2342
April	1st to 10th	Day	65% 10% 12% 11% 3	2185
	11th to 20th	Day	60% 9% 10% 12% 6% 3	2132
	21st to end	Day	57% 10% 12% 11% 8% 1	2336
May	1st to 10th	Day	49% 8% 15% 18% 9% 1	2467
	11th to 20th	Day	50% 11% 14% 18% 6% 1	2480
	21st to end	Day	52% 12% 16% 17% 3	2724
June	1st to 10th	Day	50% 9% 18% 17% 5%	2520
	11th to 20th	Day	52% 11% 18% 13% 5%	2520
	21st to end	Day	32% 8% 18% 27% 10% 4	2520
July	1st to 10th	Day	25% 8% 15% 25% 22% 5%	2451
	11th to 20th	Day	21% 7% 11% 26% 29% 6%	2445
	21st to end	Day	15% 4 10% 31% 34% 6%	2589
August	1st to 10th	Day	19% 7% 17% 32% 20% 5%	2321
	11th to 20th	Day	20% 5% 16% 31% 22% 7%	2298
	21st to end	Day	25% 8% 13% 29% 22% 4	2452
September	1st to 10th	Day	23% 6% 13% 27% 19% 12%	2160
	11th to 20th	Day	39% 9% 14% 18% 14% 7%	2156
	21st to end	Day	40% 9% 13% 18% 15% 5%	1895
October	1st to 10th	Day	47% 8% 10% 18% 14% 3	1800
	11th to 20th	Day	53% 8% 13% 12% 12% 2	1800
	21st to end	Day	59% 7% 9% 11% 12% 1	1948
November	1st to 10th	Day	70% 8% 9% 9% 3	1570
	11th to 20th	Day	68% 7% 10% 8% 6% 2	1520
	21st to end	Day	66% 7% 7% 9% 11% 1	1520
December	1st to 10th	Day	65% 6% 9% 10% 8% 1	1520
	11th to 20th	Day	67% 11% 11% 9% 2	1928
	21st to end	Day	75% 14% 6% 3 2	2794
January	1st to 10th	Night	75% 10% 6% 5% 3 1	4406
	11th to 20th	Night	71% 8% 6% 10% 4 1	4420
	21st to end	Night	70% 9% 8% 7% 5% 1	4581
February	1st to 10th	Night	65% 12% 9% 10% 4 1	2470
	11th to 20th	Night	62% 8% 13% 9% 6% 2	2404
	21st to end	Night	63% 9% 11% 9% 5% 2	1969
March	1st to 10th	Night	60% 9% 11% 14% 6%	2179
	11th to 20th	Night	55% 11% 12% 14% 7% 1	2090
	21st to end	Night	64% 11% 14% 10% 1	2256
April	1st to 10th	Night	63% 8% 12% 12% 5%	1820
	11th to 20th	Night	51% 11% 17% 12% 6% 3	1779
	21st to end	Night	54% 9% 13% 15% 6% 2	1620
May	1st to 10th	Night	42% 10% 16% 22% 8% 2	1491
	11th to 20th	Night	45% 10% 18% 18% 5% 4	1416
	21st to end	Night	47% 12% 18% 18% 5%	1552
June	1st to 10th	Night	47% 8% 18% 22% 5%	1440
	11th to 20th	Night	47% 9% 20% 19% 5%	1440
	21st to end	Night	28% 7% 19% 30% 12% 5%	1440
July	1st to 10th	Night	20% 7% 14% 29% 23% 7%	1400
	11th to 20th	Night	18% 5% 12% 27% 31% 7%	1392
	21st to end	Night	9% 3 11% 34% 32% 10%	1488
August	1st to 10th	Night	14% 5% 16% 37% 22% 6%	1590
	11th to 20th	Night	16% 5% 16% 36% 19% 7%	1602
	21st to end	Night	20% 6% 13% 36% 20% 5%	1903
September	1st to 10th	Night	19% 6% 15% 28% 20% 11%	1800
	11th to 20th	Night	33% 8% 14% 23% 13% 9%	1800
	21st to end	Night	32% 8% 16% 19% 17% 8%	2065
October	1st to 10th	Night	40% 7% 11% 21% 16% 4	2160
	11th to 20th	Night	42% 9% 13% 17% 15% 3	2160
	21st to end	Night	49% 9% 10% 14% 16% 2	2407
November	1st to 10th	Night	60% 10% 9% 14% 6%	2504
	11th to 20th	Night	64% 9% 9% 8% 8% 2	2660
	21st to end	Night	58% 10% 9% 10% 12% 2	2660
December	1st to 10th	Night	60% 8% 10% 11% 10% 1	2660
	11th to 20th	Night	65% 10% 10% 11% 4	3276
	21st to end	Night	66% 15% 10% 6% 2	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	43% 14% 16% 13% 12% 2	180
	11th to 20th	Day	41% 6% 22% 17% 13% 2	180
	21st to end	Day	32% 11% 18% 20% 15% 5%	369
February	1st to 10th	Day	35% 11% 19% 17% 14% 3	370
	11th to 20th	Day	34% 13% 24% 17% 11% 2	365
	21st to end	Day	34% 15% 22% 17% 11% 1	341
March	1st to 10th	Day	30% 15% 22% 23% 8% 1	460
	11th to 20th	Day	27% 12% 23% 21% 14% 3	460
	21st to end	Day	29% 18% 23% 18% 11% 1	506
April	1st to 10th	Day	31% 15% 20% 18% 11% 5%	429
	11th to 20th	Day	28% 16% 25% 19% 7% 5%	412
	21st to end	Day	22% 14% 25% 23% 14% 2	420
May	1st to 10th	Day	18% 15% 21% 26% 16% 4	423
	11th to 20th	Day	27% 10% 15% 27% 18% 4	448
	21st to end	Day	17% 8% 18% 29% 21% 6%	485
June	1st to 10th	Day	21% 7% 20% 26% 22% 4	450
	11th to 20th	Day	18% 12% 25% 26% 16% 3	450
	21st to end	Day	8% 6% 17% 36% 27% 6%	450
July	1st to 10th	Day	6% 4 17% 35% 30% 7%	446
	11th to 20th	Day	5% 3 19% 42% 25% 6%	444
	21st to end	Day	3 3 15% 37% 34% 9%	487
August	1st to 10th	Day	4 3 10% 38% 35% 10%	447
	11th to 20th	Day	3 1 12% 35% 36% 13%	437
	21st to end	Day	7% 4 18% 36% 28% 7%	495
September	1st to 10th	Day	8% 3 24% 30% 28% 6%	450
	11th to 20th	Day	14% 5% 16% 28% 29% 6%	450
	21st to end	Day	14% 9% 22% 30% 20% 4	450
October	1st to 10th	Day	20% 7% 22% 25% 18% 8%	450
	11th to 20th	Day	21% 7% 18% 27% 20% 7%	450
	21st to end	Day	29% 12% 22% 19% 11% 6%	463
November	1st to 10th	Day	33% 12% 23% 19% 11% 2	221
	11th to 20th	Day	26% 16% 22% 15% 14% 6%	180
	21st to end	Day	28% 12% 20% 18% 17% 4	180
December	1st to 10th	Day	37% 10% 14% 21% 12% 6%	180
	11th to 20th	Day	36% 13% 22% 19% 8% 2	180
	21st to end	Day	41% 12% 21% 11% 12% 3	198
January	1st to 10th	Night	40% 9% 18% 20% 12% 1	760
	11th to 20th	Night	35% 10% 15% 23% 15% 1	760
	21st to end	Night	33% 7% 15% 29% 14% 2	836
February	1st to 10th	Night	31% 10% 18% 25% 15% 2	760
	11th to 20th	Night	31% 10% 18% 23% 17% 1	744
	21st to end	Night	32% 12% 17% 30% 9%	628
March	1st to 10th	Night	28% 10% 21% 27% 13% 1	760
	11th to 20th	Night	24% 9% 20% 31% 14% 1	760
	21st to end	Night	28% 10% 22% 28% 12% 1	836
April	1st to 10th	Night	31% 8% 18% 26% 16% 1	728
	11th to 20th	Night	23% 9% 21% 29% 16% 1	711
	21st to end	Night	24% 9% 20% 30% 16% 1	540
May	1st to 10th	Night	15% 7% 24% 34% 19% 1	411
	11th to 20th	Night	17% 5% 19% 32% 24% 2	354
	21st to end	Night	17% 6% 20% 35% 21% 1	389
June	1st to 10th	Night	18% 5% 21% 33% 23% 1	360
	11th to 20th	Night	14% 6% 23% 42% 13% 1	360
	21st to end	Night	7% 2 21% 48% 22% 1	360
July	1st to 10th	Night	6% 2 12% 50% 29% 1	350
	11th to 20th	Night	7% 1 16% 48% 27% 1	348
	21st to end	Night	3 2 10% 52% 33% 1	394
August	1st to 10th	Night	4 1 8% 39% 41% 6%	524
	11th to 20th	Night	5% 1 7% 36% 47% 4	534
	21st to end	Night	7% 3 14% 43% 31% 3	715
September	1st to 10th	Night	7% 3 16% 41% 31% 2	720
	11th to 20th	Night	11% 3 12% 31% 37% 5%	720
	21st to end	Night	13% 4 15% 37% 27% 3	720
October	1st to 10th	Night	19% 3 19% 28% 27% 3	720
	11th to 20th	Night	21% 5% 13% 30% 27% 4	720
	21st to end	Night	27% 7% 20% 25% 18% 4	792
November	1st to 10th	Night	30% 8% 17% 31% 14%	740
	11th to 20th	Night	36% 10% 16% 23% 13% 1	760
	21st to end	Night	28% 7% 19% 30% 16% 1	760
December	1st to 10th	Night	32% 8% 17% 27% 15% 2	760
	11th to 20th	Night	37% 9% 17% 22% 14% 1	760
	21st to end	Night	37% 11% 20% 22% 9% 1	836

Page generated at 2024-03-31 03:08:56. Version ac07

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