Cruz Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cruz 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	15% 6% 2 5% 5% 3 5% 5% 6% 8% 41%	74704
	Night	15% 6% 2 5% 5% 3 4 5% 5% 7% 42%	75012
Transparency	Day	41% 5% 12% 24% 15% 3	74417
	Night	41% 5% 12% 23% 16% 3	74833
Seeing	Day	18% 9% 25% 25% 16% 7%	13785
	Night	18% 7% 27% 33% 13% 1	23270

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	13% 8% 3 5% 5% 3 5% 5% 5% 7% 42%	1517
	11th to 20th	Day	15% 8% 3 5% 6% 3 5% 6% 6% 6% 36%	1568
	21st to end	Day	13% 9% 4 8% 5% 4 5% 5% 5% 6% 37%	1782
February	1st to 10th	Day	16% 8% 3 6% 4 4 4 6% 6% 7% 36%	1620
	11th to 20th	Day	17% 6% 3 5% 6% 3 4 5% 5% 7% 40%	1645
	21st to end	Day	20% 9% 4 5% 4 3 4 4 4 5% 37%	1480
March	1st to 10th	Day	13% 6% 2 5% 6% 3 5% 4 5% 7% 45%	1873
	11th to 20th	Day	16% 7% 3 4 5% 3 5% 5% 7% 8% 37%	1980
	21st to end	Day	20% 7% 2 5% 5% 3 4 3 5% 6% 38%	2252
April	1st to 10th	Day	16% 6% 3 6% 5% 2 5% 4 6% 7% 41%	2160
	11th to 20th	Day	16% 5% 2 4 5% 3 4 5% 5% 7% 44%	2134
	21st to end	Day	18% 5% 2 5% 5% 2 5% 6% 6% 7% 40%	2160
May	1st to 10th	Day	17% 6% 2 6% 5% 3 5% 5% 6% 7% 37%	2313
	11th to 20th	Day	20% 6% 3 5% 5% 2 5% 4 5% 7% 37%	2478
	21st to end	Day	14% 6% 2 4 5% 3 4 5% 6% 10% 39%	2725
June	1st to 10th	Day	12% 5% 2 4 6% 3 6% 5% 7% 10% 40%	2520
	11th to 20th	Day	13% 6% 2 6% 4 3 6% 6% 5% 9% 39%	2520
	21st to end	Day	10% 4 2 5% 5% 3 5% 6% 6% 10% 44%	2520
July	1st to 10th	Day	10% 5% 2 5% 5% 3 7% 7% 7% 9% 40%	2520
	11th to 20th	Day	7% 4 2 5% 5% 3 7% 7% 8% 10% 43%	2520
	21st to end	Day	9% 6% 3 6% 6% 4 6% 6% 7% 8% 38%	2608
August	1st to 10th	Day	10% 6% 2 6% 6% 4 6% 6% 7% 9% 38%	2327
	11th to 20th	Day	11% 6% 2 5% 6% 4 4 5% 7% 9% 40%	2192
	21st to end	Day	12% 5% 2 4 6% 3 5% 5% 6% 8% 43%	2376
September	1st to 10th	Day	16% 6% 2 6% 4 4 4 4 4 6% 44%	2160
	11th to 20th	Day	13% 4 2 4 4 2 4 4 5% 8% 50%	2160
	21st to end	Day	19% 6% 2 5% 4 3 5% 4 6% 7% 40%	1920
October	1st to 10th	Day	18% 6% 1 5% 4 2 4 4 5% 7% 44%	1800
	11th to 20th	Day	18% 6% 2 4 5% 2 4 3 4 6% 45%	1800
	21st to end	Day	17% 7% 3 4 4 2 4 4 5% 7% 43%	1980
November	1st to 10th	Day	12% 6% 2 4 4 2 3 4 4 9% 50%	1676
	11th to 20th	Day	16% 4 2 4 4 3 4 4 5% 6% 47%	1700
	21st to end	Day	16% 5% 3 5% 5% 2 5% 4 4 7% 44%	1520
December	1st to 10th	Day	18% 8% 3 5% 5% 3 4 5% 6% 7% 36%	1520
	11th to 20th	Day	19% 5% 2 4 5% 3 4 4 6% 8% 39%	1906
	21st to end	Day	17% 7% 2 7% 7% 4 5% 6% 6% 6% 33%	2772
January	1st to 10th	Night	14% 7% 3 5% 5% 3 4 5% 5% 7% 42%	2649
	11th to 20th	Night	19% 7% 3 5% 5% 3 4 4 4 7% 38%	2429
	21st to end	Night	13% 8% 2 6% 5% 3 4 4 6% 7% 43%	2574
February	1st to 10th	Night	18% 8% 4 5% 5% 3 4 4 4 8% 37%	2340
	11th to 20th	Night	17% 7% 3 6% 4 3 4 4 5% 6% 41%	2199
	21st to end	Night	17% 8% 3 5% 5% 4 4 5% 5% 5% 40%	1776
March	1st to 10th	Night	14% 5% 2 4 4 3 4 4 6% 7% 47%	2070
	11th to 20th	Night	17% 7% 2 4 6% 2 4 4 5% 4 44%	1979
	21st to end	Night	18% 6% 4 6% 4 2 4 4 5% 6% 41%	2102
April	1st to 10th	Night	18% 6% 3 5% 5% 3 5% 4 5% 9% 38%	1800
	11th to 20th	Night	14% 6% 2 6% 5% 3 4 4 6% 8% 43%	1775
	21st to end	Night	19% 6% 2 5% 4 2 3 4 4 7% 44%	1800
May	1st to 10th	Night	19% 6% 2 5% 5% 4 4 5% 5% 7% 38%	1660
	11th to 20th	Night	19% 6% 3 6% 5% 3 5% 4 5% 7% 38%	1415
	21st to end	Night	13% 6% 2 6% 5% 3 4 4 5% 11% 41%	1550
June	1st to 10th	Night	14% 5% 2 4 4 3 5% 4 5% 8% 44%	1440
	11th to 20th	Night	13% 4 2 5% 6% 3 4 6% 6% 11% 40%	1440
	21st to end	Night	10% 4 3 5% 5% 2 5% 6% 8% 10% 43%	1440
July	1st to 10th	Night	9% 6% 3 7% 8% 4 6% 4 8% 9% 38%	1440
	11th to 20th	Night	5% 3 2 5% 6% 4 6% 6% 7% 10% 46%	1440
	21st to end	Night	10% 4 2 4 6% 3 5% 7% 6% 11% 42%	1707
August	1st to 10th	Night	10% 4 2 4 7% 3 6% 5% 7% 9% 42%	1611
	11th to 20th	Night	11% 5% 3 6% 7% 4 4 6% 7% 7% 40%	1691
	21st to end	Night	12% 5% 2 5% 5% 4 5% 5% 6% 8% 44%	1980
September	1st to 10th	Night	14% 5% 2 6% 5% 3 4 4 5% 7% 46%	1800
	11th to 20th	Night	13% 5% 2 3 4 2 3 4 5% 7% 52%	1800
	21st to end	Night	17% 6% 2 4 5% 4 5% 4 4 6% 43%	2037
October	1st to 10th	Night	21% 6% 3 5% 4 2 4 3 4 5% 44%	2160
	11th to 20th	Night	15% 5% 2 5% 4 3 3 4 4 7% 48%	2160
	21st to end	Night	18% 5% 2 4 4 2 3 4 4 7% 46%	2376
November	1st to 10th	Night	11% 5% 2 4 4 2 4 4 5% 6% 53%	2402
	11th to 20th	Night	15% 7% 2 5% 4 2 4 4 4 7% 46%	2478
	21st to end	Night	14% 5% 2 5% 5% 3 4 4 5% 7% 45%	2660
December	1st to 10th	Night	19% 7% 3 5% 5% 3 4 5% 5% 6% 37%	2660
	11th to 20th	Night	18% 5% 2 5% 5% 3 5% 5% 5% 7% 41%	3288
	21st to end	Night	18% 6% 3 6% 5% 3 5% 6% 6% 8% 34%	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	43% 3 13% 23% 14% 4	1518
	11th to 20th	Day	46% 4 14% 21% 13% 2	1568
	21st to end	Day	48% 4 9% 23% 13% 2	1782
February	1st to 10th	Day	46% 4 13% 24% 11% 2	1620
	11th to 20th	Day	44% 3 10% 27% 13% 3	1654
	21st to end	Day	50% 3 9% 22% 13% 4	1480
March	1st to 10th	Day	40% 3 12% 28% 15% 2	1876
	11th to 20th	Day	44% 3 11% 19% 17% 5%	1980
	21st to end	Day	48% 4 8% 20% 17% 3	2252
April	1st to 10th	Day	42% 4 11% 20% 17% 5%	2160
	11th to 20th	Day	39% 4 11% 24% 17% 5%	2134
	21st to end	Day	42% 5% 10% 21% 17% 4	2160
May	1st to 10th	Day	44% 5% 10% 22% 15% 3	2313
	11th to 20th	Day	47% 5% 12% 21% 13% 2	2482
	21st to end	Day	39% 7% 16% 23% 13% 2	2725
June	1st to 10th	Day	38% 7% 14% 25% 15% 2	2520
	11th to 20th	Day	41% 6% 13% 24% 13% 3	2520
	21st to end	Day	34% 6% 16% 26% 16% 1	2520
July	1st to 10th	Day	38% 7% 13% 24% 15% 3	2450
	11th to 20th	Day	31% 7% 15% 28% 16% 3	2445
	21st to end	Day	40% 6% 12% 23% 16% 3	2462
August	1st to 10th	Day	41% 6% 12% 25% 15% 1	2314
	11th to 20th	Day	39% 6% 14% 23% 17% 2	2192
	21st to end	Day	38% 6% 13% 25% 16% 4	2376
September	1st to 10th	Day	42% 3 11% 21% 18% 5%	2160
	11th to 20th	Day	33% 4 10% 27% 22% 4	2160
	21st to end	Day	43% 4 9% 21% 20% 3	1920
October	1st to 10th	Day	40% 4 13% 24% 17% 2	1800
	11th to 20th	Day	41% 4 10% 24% 19% 2	1800
	21st to end	Day	41% 3 10% 27% 18% 2	1980
November	1st to 10th	Day	34% 3 13% 26% 20% 4	1676
	11th to 20th	Day	38% 4 11% 23% 20% 2	1700
	21st to end	Day	41% 4 11% 31% 11% 2	1520
December	1st to 10th	Day	47% 4 16% 20% 12% 1	1520
	11th to 20th	Day	44% 3 12% 30% 10% 1	1906
	21st to end	Day	49% 5% 15% 20% 10% 1	2772
January	1st to 10th	Night	41% 4 10% 26% 15% 4	2649
	11th to 20th	Night	46% 4 13% 23% 12% 2	2429
	21st to end	Night	40% 4 10% 28% 16% 3	2574
February	1st to 10th	Night	46% 4 14% 24% 11% 2	2340
	11th to 20th	Night	44% 4 10% 22% 17% 3	2201
	21st to end	Night	47% 3 9% 21% 16% 4	1776
March	1st to 10th	Night	36% 4 12% 26% 19% 3	2080
	11th to 20th	Night	44% 4 10% 22% 17% 4	1980
	21st to end	Night	44% 3 9% 21% 18% 5%	2102
April	1st to 10th	Night	43% 4 11% 20% 17% 4	1800
	11th to 20th	Night	39% 4 10% 23% 20% 4	1775
	21st to end	Night	41% 5% 9% 22% 18% 5%	1800
May	1st to 10th	Night	44% 6% 10% 20% 16% 4	1660
	11th to 20th	Night	46% 6% 12% 15% 18% 3	1416
	21st to end	Night	39% 7% 17% 20% 15% 3	1551
June	1st to 10th	Night	38% 7% 11% 23% 19% 2	1440
	11th to 20th	Night	38% 6% 16% 21% 17% 2	1440
	21st to end	Night	34% 6% 14% 26% 17% 2	1440
July	1st to 10th	Night	43% 7% 14% 19% 14% 3	1400
	11th to 20th	Night	33% 7% 16% 26% 16% 2	1392
	21st to end	Night	33% 8% 18% 21% 18% 2	1612
August	1st to 10th	Night	37% 6% 16% 26% 15% 1	1600
	11th to 20th	Night	40% 6% 15% 20% 16% 2	1691
	21st to end	Night	37% 5% 15% 22% 18% 3	1980
September	1st to 10th	Night	39% 5% 11% 22% 20% 4	1800
	11th to 20th	Night	32% 4 9% 26% 25% 4	1800
	21st to end	Night	42% 5% 8% 20% 21% 3	2037
October	1st to 10th	Night	44% 3 10% 24% 17% 2	2160
	11th to 20th	Night	38% 3 11% 28% 18% 2	2160
	21st to end	Night	38% 4 10% 29% 18% 2	2376
November	1st to 10th	Night	32% 4 11% 27% 21% 5%	2402
	11th to 20th	Night	39% 4 9% 25% 18% 5%	2478
	21st to end	Night	38% 4 13% 28% 16% 2	2660
December	1st to 10th	Night	47% 4 12% 24% 13% 1	2660
	11th to 20th	Night	43% 4 13% 23% 15% 3	3288
	21st to end	Night	47% 5% 15% 23% 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	26% 16% 33% 21% 3 2	180
	11th to 20th	Day	24% 22% 34% 17% 3	216
	21st to end	Day	22% 17% 38% 16% 6% 1	264
February	1st to 10th	Day	26% 17% 35% 18% 4	240
	11th to 20th	Day	25% 18% 43% 14% 1	312
	21st to end	Day	29% 13% 33% 22% 3	346
March	1st to 10th	Day	17% 20% 31% 28% 5%	420
	11th to 20th	Day	21% 16% 36% 20% 5% 2	420
	21st to end	Day	23% 13% 32% 23% 6% 2	462
April	1st to 10th	Day	21% 12% 35% 22% 9% 2	420
	11th to 20th	Day	16% 15% 29% 22% 13% 6%	412
	21st to end	Day	25% 10% 30% 21% 11% 2	420
May	1st to 10th	Day	24% 12% 26% 21% 13% 4	423
	11th to 20th	Day	21% 9% 26% 24% 14% 7%	447
	21st to end	Day	20% 5% 17% 24% 22% 12%	485
June	1st to 10th	Day	15% 4 25% 27% 19% 10%	450
	11th to 20th	Day	16% 7% 22% 27% 20% 8%	450
	21st to end	Day	13% 3 23% 31% 20% 10%	450
July	1st to 10th	Day	13% 2 12% 26% 29% 18%	445
	11th to 20th	Day	5% 2 12% 32% 33% 17%	444
	21st to end	Day	15% 2 16% 31% 27% 8%	488
August	1st to 10th	Day	11% 3 14% 28% 29% 14%	446
	11th to 20th	Day	12% 4 12% 28% 27% 17%	439
	21st to end	Day	13% 3 13% 30% 25% 16%	495
September	1st to 10th	Day	19% 4 16% 20% 24% 16%	450
	11th to 20th	Day	14% 4 16% 26% 25% 16%	450
	21st to end	Day	22% 8% 19% 26% 17% 8%	450
October	1st to 10th	Day	23% 4 19% 25% 21% 8%	450
	11th to 20th	Day	18% 9% 25% 28% 18% 3	450
	21st to end	Day	18% 7% 33% 30% 9% 3	495
November	1st to 10th	Day	15% 9% 28% 27% 17% 4	368
	11th to 20th	Day	16% 15% 30% 28% 11% 1	360
	21st to end	Day	19% 17% 32% 19% 11% 2	180
December	1st to 10th	Day	24% 15% 33% 19% 6% 2	180
	11th to 20th	Day	24% 14% 37% 16% 8% 1	180
	21st to end	Day	21% 12% 35% 22% 10% 1	198
January	1st to 10th	Night	17% 11% 41% 27% 4	760
	11th to 20th	Night	24% 11% 30% 30% 4	728
	21st to end	Night	18% 10% 40% 29% 2	792
February	1st to 10th	Night	21% 12% 36% 28% 2	720
	11th to 20th	Night	21% 15% 39% 23% 2	703
	21st to end	Night	21% 15% 39% 22% 3	592
March	1st to 10th	Night	18% 11% 39% 27% 5%	720
	11th to 20th	Night	21% 12% 35% 25% 6% 1	720
	21st to end	Night	21% 7% 37% 29% 6%	792
April	1st to 10th	Night	21% 12% 37% 26% 5%	720
	11th to 20th	Night	18% 8% 32% 33% 10%	711
	21st to end	Night	21% 7% 30% 33% 8% 1	720
May	1st to 10th	Night	21% 6% 25% 37% 10% 1	580
	11th to 20th	Night	20% 5% 28% 35% 12%	354
	21st to end	Night	14% 2 17% 42% 23% 2	389
June	1st to 10th	Night	15% 3 19% 40% 22% 1	360
	11th to 20th	Night	14% 1 18% 46% 21%	360
	21st to end	Night	11% 3 14% 53% 18% 1	360
July	1st to 10th	Night	11% 3 14% 41% 28% 3	350
	11th to 20th	Night	7% 1 9% 47% 33% 3	348
	21st to end	Night	11% 2 12% 46% 27% 2	527
August	1st to 10th	Night	11% 1 5% 48% 31% 3	537
	11th to 20th	Night	13% 2 10% 40% 32% 3	627
	21st to end	Night	14% 2 11% 40% 30% 4	792
September	1st to 10th	Night	16% 3 16% 35% 25% 4	720
	11th to 20th	Night	14% 4 10% 38% 30% 4	720
	21st to end	Night	19% 3 17% 37% 22% 2	720
October	1st to 10th	Night	23% 3 18% 33% 19% 4	720
	11th to 20th	Night	18% 6% 25% 36% 15% 1	720
	21st to end	Night	21% 5% 32% 30% 12% 1	792
November	1st to 10th	Night	13% 4 31% 37% 15% 1	740
	11th to 20th	Night	18% 11% 28% 34% 9% 1	760
	21st to end	Night	16% 10% 33% 32% 10%	760
December	1st to 10th	Night	23% 10% 33% 27% 6% 1	760
	11th to 20th	Night	18% 9% 38% 29% 6%	760
	21st to end	Night	20% 8% 36% 33% 3	836

Page generated at 2024-01-09 16:24:27. Version ac07

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