Roger Twitchell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Roger Twitchell Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	18% 7% 3 5% 5% 3 5% 5% 5% 7% 37%	74598
	Night	18% 8% 3 5% 5% 3 4 4 5% 7% 38%	75097
Transparency	Day	45% 7% 11% 21% 14% 2	74304
	Night	45% 6% 11% 22% 13% 2	74926
Seeing	Day	24% 15% 30% 20% 9% 2	13797
	Night	22% 12% 30% 27% 8% 1	22811

By Month

Percentage Cloud Cover NE (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	15% 8% 3 6% 5% 3 5% 4 6% 7% 38%	1517
	11th to 20th	Day	17% 9% 3 6% 6% 3 4 5% 5% 7% 35%	1454
	21st to end	Day	12% 9% 4 6% 6% 3 5% 5% 5% 5% 40%	1584
February	1st to 10th	Day	17% 9% 3 6% 5% 3 6% 5% 5% 6% 34%	1571
	11th to 20th	Day	12% 10% 3 6% 6% 4 5% 5% 6% 6% 37%	1748
	21st to end	Day	16% 9% 3 7% 7% 3 7% 5% 4 6% 33%	1480
March	1st to 10th	Day	15% 9% 3 6% 6% 4 6% 5% 5% 7% 35%	1797
	11th to 20th	Day	15% 8% 4 6% 5% 3 4 4 4 7% 40%	1921
	21st to end	Day	17% 8% 3 7% 5% 3 4 4 4 5% 41%	2258
April	1st to 10th	Day	19% 11% 4 6% 5% 3 3 4 4 6% 35%	2160
	11th to 20th	Day	17% 6% 3 6% 7% 3 5% 5% 5% 7% 38%	2208
	21st to end	Day	24% 9% 3 6% 6% 3 4 5% 4 5% 32%	2340
May	1st to 10th	Day	24% 8% 3 6% 5% 3 5% 5% 5% 6% 31%	2340
	11th to 20th	Day	20% 6% 3 7% 6% 3 4 4 5% 6% 37%	2373
	21st to end	Day	17% 7% 2 5% 6% 3 5% 5% 6% 7% 37%	2725
June	1st to 10th	Day	24% 7% 3 5% 6% 4 5% 5% 6% 7% 29%	2520
	11th to 20th	Day	18% 6% 3 6% 4 3 6% 5% 5% 7% 39%	2520
	21st to end	Day	22% 6% 3 6% 6% 3 6% 5% 6% 7% 29%	2520
July	1st to 10th	Day	17% 6% 2 5% 5% 2 5% 6% 6% 9% 37%	2520
	11th to 20th	Day	13% 7% 3 6% 6% 5% 6% 6% 7% 9% 33%	2520
	21st to end	Day	15% 5% 2 5% 5% 3 6% 6% 7% 9% 36%	2744
August	1st to 10th	Day	14% 5% 2 5% 5% 4 5% 6% 7% 9% 37%	2377
	11th to 20th	Day	14% 5% 3 6% 6% 4 5% 6% 6% 9% 37%	2304
	21st to end	Day	12% 5% 2 4 4 3 5% 5% 6% 8% 45%	2448
September	1st to 10th	Day	13% 4 2 4 4 3 4 5% 6% 8% 46%	2138
	11th to 20th	Day	13% 5% 2 5% 4 2 3 4 3 6% 53%	1980
	21st to end	Day	16% 4 2 4 5% 2 4 4 6% 8% 45%	1980
October	1st to 10th	Day	20% 7% 3 5% 5% 3 4 4 4 6% 39%	1980
	11th to 20th	Day	19% 6% 2 4 5% 3 4 4 5% 7% 41%	1722
	21st to end	Day	25% 8% 2 5% 5% 3 4 4 4 6% 34%	1782
November	1st to 10th	Day	17% 6% 3 4 3 2 3 4 6% 7% 44%	1671
	11th to 20th	Day	21% 4 2 3 4 3 4 5% 7% 7% 40%	1710
	21st to end	Day	22% 6% 3 4 4 2 3 4 5% 7% 39%	1695
December	1st to 10th	Day	21% 12% 3 6% 4 2 4 4 5% 6% 33%	1395
	11th to 20th	Day	23% 8% 3 5% 5% 2 4 4 5% 5% 37%	1824
	21st to end	Day	24% 12% 3 5% 5% 3 4 5% 5% 5% 29%	2772
January	1st to 10th	Night	15% 9% 3 5% 5% 3 5% 5% 5% 7% 37%	2649
	11th to 20th	Night	17% 12% 4 7% 6% 3 5% 4 4 7% 32%	2546
	21st to end	Night	12% 8% 4 7% 5% 3 5% 5% 5% 7% 38%	2772
February	1st to 10th	Night	14% 10% 4 6% 6% 5% 5% 6% 6% 6% 33%	2385
	11th to 20th	Night	12% 10% 3 6% 5% 3 5% 5% 5% 5% 40%	2098
	21st to end	Night	13% 9% 3 6% 5% 4 5% 4 5% 7% 37%	1776
March	1st to 10th	Night	14% 10% 3 5% 6% 3 5% 6% 5% 6% 36%	2150
	11th to 20th	Night	16% 8% 4 6% 5% 3 5% 5% 5% 6% 37%	2038
	21st to end	Night	18% 7% 3 5% 5% 3 5% 3 5% 5% 40%	2095
April	1st to 10th	Night	21% 10% 3 5% 6% 3 5% 3 4 6% 35%	1800
	11th to 20th	Night	17% 7% 4 6% 5% 3 5% 4 4 7% 37%	1700
	21st to end	Night	25% 7% 2 5% 5% 3 5% 4 4 7% 33%	1620
May	1st to 10th	Night	21% 9% 3 6% 5% 3 4 5% 4 5% 34%	1620
	11th to 20th	Night	21% 7% 3 6% 5% 3 5% 4 5% 8% 34%	1535
	21st to end	Night	17% 8% 4 6% 5% 3 5% 5% 6% 6% 36%	1550
June	1st to 10th	Night	24% 7% 3 6% 5% 2 5% 4 5% 6% 34%	1440
	11th to 20th	Night	19% 6% 3 4 5% 2 5% 4 6% 8% 40%	1440
	21st to end	Night	20% 6% 3 5% 5% 3 5% 5% 6% 8% 35%	1440
July	1st to 10th	Night	14% 5% 3 4 5% 4 5% 5% 7% 8% 40%	1440
	11th to 20th	Night	15% 6% 2 5% 5% 3 4 5% 6% 9% 41%	1440
	21st to end	Night	14% 5% 2 4 5% 4 5% 6% 6% 7% 43%	1574
August	1st to 10th	Night	13% 5% 2 6% 4 3 5% 5% 5% 8% 44%	1543
	11th to 20th	Night	14% 6% 2 5% 6% 3 5% 5% 6% 10% 38%	1596
	21st to end	Night	15% 5% 2 4 4 3 4 4 5% 8% 46%	1906
September	1st to 10th	Night	13% 5% 2 3 4 3 4 4 5% 6% 50%	1822
	11th to 20th	Night	16% 5% 2 3 3 2 3 4 5% 6% 52%	1980
	21st to end	Night	16% 6% 2 3 4 3 3 4 5% 8% 46%	1980
October	1st to 10th	Night	19% 7% 2 5% 5% 3 5% 5% 5% 6% 39%	1980
	11th to 20th	Night	17% 7% 2 5% 5% 3 5% 5% 6% 7% 38%	2235
	21st to end	Night	25% 8% 2 4 4 3 4 4 5% 5% 35%	2574
November	1st to 10th	Night	17% 7% 2 5% 4 3 4 4 5% 6% 43%	2407
	11th to 20th	Night	18% 7% 2 4 4 3 5% 4 5% 6% 42%	2470
	21st to end	Night	16% 7% 3 5% 5% 3 3 4 5% 7% 41%	2483
December	1st to 10th	Night	21% 9% 3 7% 5% 3 4 5% 5% 6% 32%	2774
	11th to 20th	Night	21% 8% 3 5% 5% 3 4 4 5% 6% 37%	3355
	21st to end	Night	23% 10% 3 6% 5% 3 4 4 4 6% 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	44% 8% 10% 23% 15% 1	1518
	11th to 20th	Day	49% 6% 13% 19% 11% 2	1454
	21st to end	Day	45% 6% 10% 20% 17% 2	1584
February	1st to 10th	Day	49% 6% 12% 19% 12% 2	1571
	11th to 20th	Day	48% 5% 12% 19% 13% 3	1757
	21st to end	Day	50% 7% 10% 17% 13% 5%	1480
March	1st to 10th	Day	48% 7% 11% 16% 14% 4	1800
	11th to 20th	Day	45% 5% 10% 19% 16% 5%	1921
	21st to end	Day	46% 5% 8% 17% 19% 5%	2258
April	1st to 10th	Day	50% 4 9% 18% 15% 4	2160
	11th to 20th	Day	45% 8% 9% 21% 15% 3	2208
	21st to end	Day	55% 5% 8% 16% 12% 4	2340
May	1st to 10th	Day	53% 6% 7% 19% 13% 2	2340
	11th to 20th	Day	47% 6% 8% 18% 17% 3	2377
	21st to end	Day	44% 8% 12% 21% 13% 2	2725
June	1st to 10th	Day	53% 6% 13% 18% 9%	2520
	11th to 20th	Day	44% 8% 10% 20% 16% 2	2520
	21st to end	Day	53% 8% 11% 17% 10% 1	2520
July	1st to 10th	Day	41% 8% 14% 24% 13% 1	2450
	11th to 20th	Day	44% 10% 12% 24% 9% 1	2445
	21st to end	Day	40% 9% 15% 24% 11% 1	2591
August	1st to 10th	Day	40% 10% 15% 25% 10%	2363
	11th to 20th	Day	42% 8% 14% 25% 11% 1	2304
	21st to end	Day	35% 7% 14% 24% 19% 1	2448
September	1st to 10th	Day	34% 8% 14% 26% 17% 1	2138
	11th to 20th	Day	33% 5% 10% 29% 20% 3	1980
	21st to end	Day	37% 6% 12% 28% 17% 1	1980
October	1st to 10th	Day	46% 6% 10% 22% 15% 1	1980
	11th to 20th	Day	43% 6% 13% 27% 11% 1	1722
	21st to end	Day	51% 6% 10% 18% 13% 2	1782
November	1st to 10th	Day	39% 4 15% 24% 14% 3	1671
	11th to 20th	Day	41% 7% 12% 20% 17% 3	1710
	21st to end	Day	45% 5% 17% 21% 11% 1	1695
December	1st to 10th	Day	51% 6% 12% 19% 10% 2	1395
	11th to 20th	Day	48% 5% 9% 19% 17% 2	1825
	21st to end	Day	56% 6% 11% 17% 9% 2	2772
January	1st to 10th	Night	45% 6% 12% 23% 12% 2	2649
	11th to 20th	Night	53% 7% 11% 17% 10% 1	2546
	21st to end	Night	44% 6% 9% 21% 17% 3	2772
February	1st to 10th	Night	49% 7% 13% 16% 13% 3	2385
	11th to 20th	Night	45% 6% 10% 21% 15% 3	2100
	21st to end	Night	45% 6% 10% 21% 14% 4	1776
March	1st to 10th	Night	47% 6% 9% 21% 14% 3	2160
	11th to 20th	Night	47% 6% 12% 18% 14% 4	2039
	21st to end	Night	45% 5% 8% 18% 19% 4	2095
April	1st to 10th	Night	51% 4 8% 18% 16% 3	1800
	11th to 20th	Night	46% 5% 9% 23% 17% 1	1700
	21st to end	Night	52% 6% 11% 16% 11% 4	1620
May	1st to 10th	Night	51% 7% 7% 19% 14% 2	1620
	11th to 20th	Night	49% 6% 10% 20% 12% 4	1536
	21st to end	Night	48% 7% 13% 20% 11% 1	1551
June	1st to 10th	Night	51% 7% 11% 20% 11% 1	1440
	11th to 20th	Night	42% 7% 14% 22% 14% 2	1440
	21st to end	Night	47% 8% 15% 20% 10% 1	1440
July	1st to 10th	Night	40% 9% 15% 22% 13% 1	1400
	11th to 20th	Night	39% 8% 16% 27% 10%	1392
	21st to end	Night	37% 10% 16% 25% 12% 1	1486
August	1st to 10th	Night	38% 9% 14% 28% 11%	1533
	11th to 20th	Night	41% 9% 15% 23% 13%	1596
	21st to end	Night	37% 7% 12% 27% 16% 1	1906
September	1st to 10th	Night	34% 7% 13% 31% 15%	1822
	11th to 20th	Night	34% 5% 11% 30% 19% 2	1980
	21st to end	Night	36% 6% 12% 29% 15% 2	1980
October	1st to 10th	Night	44% 7% 11% 22% 14% 1	1980
	11th to 20th	Night	42% 8% 12% 26% 10% 1	2235
	21st to end	Night	50% 6% 11% 19% 12% 2	2574
November	1st to 10th	Night	42% 6% 11% 26% 14% 2	2407
	11th to 20th	Night	42% 6% 11% 23% 16% 2	2470
	21st to end	Night	42% 7% 14% 23% 13% 1	2483
December	1st to 10th	Night	52% 6% 10% 21% 10% 1	2774
	11th to 20th	Night	48% 5% 10% 19% 17% 1	3355
	21st to end	Night	53% 5% 11% 21% 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	32% 16% 27% 20% 5%	180
	11th to 20th	Day	28% 18% 31% 19% 2 2	164
	21st to end	Day	19% 25% 33% 18% 6%	176
February	1st to 10th	Day	32% 17% 32% 17% 2	281
	11th to 20th	Day	26% 25% 33% 13% 3	416
	21st to end	Day	24% 22% 31% 20% 3	346
March	1st to 10th	Day	25% 19% 35% 16% 4	420
	11th to 20th	Day	28% 20% 33% 14% 4 1	420
	21st to end	Day	26% 22% 28% 14% 7% 2	462
April	1st to 10th	Day	34% 20% 24% 15% 5% 2	420
	11th to 20th	Day	29% 21% 28% 14% 7% 2	412
	21st to end	Day	34% 19% 27% 12% 7% 2	420
May	1st to 10th	Day	35% 18% 25% 17% 3 2	423
	11th to 20th	Day	22% 20% 35% 15% 6% 2	447
	21st to end	Day	21% 16% 34% 21% 8% 1	485
June	1st to 10th	Day	23% 15% 34% 18% 8% 2	450
	11th to 20th	Day	22% 14% 31% 21% 10% 2	450
	21st to end	Day	22% 12% 32% 22% 10% 2	450
July	1st to 10th	Day	18% 9% 31% 28% 10% 3	445
	11th to 20th	Day	16% 7% 34% 28% 11% 4	444
	21st to end	Day	14% 8% 35% 27% 13% 3	488
August	1st to 10th	Day	13% 9% 29% 31% 15% 4	446
	11th to 20th	Day	16% 10% 28% 26% 15% 5%	439
	21st to end	Day	14% 11% 29% 27% 13% 5%	495
September	1st to 10th	Day	17% 8% 26% 25% 18% 6%	450
	11th to 20th	Day	19% 12% 25% 22% 13% 9%	450
	21st to end	Day	17% 13% 29% 24% 11% 6%	450
October	1st to 10th	Day	20% 17% 22% 21% 16% 3	450
	11th to 20th	Day	25% 13% 28% 19% 13% 1	374
	21st to end	Day	34% 14% 24% 19% 7% 1	396
November	1st to 10th	Day	25% 14% 28% 21% 10% 2	365
	11th to 20th	Day	30% 14% 30% 19% 7% 1	370
	21st to end	Day	33% 16% 24% 19% 7% 1	355
December	1st to 10th	Day	27% 20% 29% 16% 8%	180
	11th to 20th	Day	34% 15% 33% 13% 4	180
	21st to end	Day	34% 18% 27% 15% 5% 1	198
January	1st to 10th	Night	22% 16% 35% 23% 3	760
	11th to 20th	Night	26% 18% 30% 23% 4	728
	21st to end	Night	19% 17% 36% 24% 3	792
February	1st to 10th	Night	22% 18% 33% 25% 3	720
	11th to 20th	Night	20% 17% 39% 20% 3	703
	21st to end	Night	23% 16% 33% 24% 4	592
March	1st to 10th	Night	25% 16% 36% 21% 2	720
	11th to 20th	Night	23% 16% 34% 21% 6% 1	720
	21st to end	Night	25% 16% 30% 22% 5% 1	792
April	1st to 10th	Night	28% 15% 29% 23% 3	720
	11th to 20th	Night	22% 13% 36% 23% 5%	636
	21st to end	Night	28% 15% 31% 21% 5%	540
May	1st to 10th	Night	26% 11% 30% 27% 5% 1	540
	11th to 20th	Night	24% 12% 31% 27% 6%	474
	21st to end	Night	21% 7% 26% 36% 8% 1	389
June	1st to 10th	Night	25% 6% 29% 31% 9%	360
	11th to 20th	Night	19% 5% 28% 34% 14%	360
	21st to end	Night	22% 3 25% 39% 11%	360
July	1st to 10th	Night	16% 2 26% 40% 16%	350
	11th to 20th	Night	18% 1 24% 39% 18% 1	348
	21st to end	Night	16% 3 19% 43% 19% 1	394
August	1st to 10th	Night	13% 3 22% 42% 19% 1	469
	11th to 20th	Night	16% 4 27% 35% 16% 1	532
	21st to end	Night	15% 7% 22% 36% 19% 2	718
September	1st to 10th	Night	15% 5% 29% 32% 18% 1	720
	11th to 20th	Night	19% 7% 26% 26% 19% 4	720
	21st to end	Night	17% 10% 27% 28% 17% 1	720
October	1st to 10th	Night	22% 10% 21% 33% 13% 1	720
	11th to 20th	Night	21% 11% 29% 26% 12% 1	720
	21st to end	Night	30% 12% 29% 24% 5%	792
November	1st to 10th	Night	22% 12% 30% 28% 7% 1	740
	11th to 20th	Night	23% 13% 30% 26% 7% 1	760
	21st to end	Night	23% 16% 29% 26% 6%	760
December	1st to 10th	Night	27% 13% 35% 20% 5%	814
	11th to 20th	Night	25% 17% 31% 23% 4	792
	21st to end	Night	28% 16% 31% 23% 3	836

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