Cypress Point Club
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cypress Point Club 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	6% 9% 3 6% 4 4 4 4 6% 10% 45%	105112
	Night	8% 11% 3 7% 4 3 3 3 4 7% 47%	103191
Transparency	Day	31% 6% 11% 18% 20% 14%	104872
	Night	36% 7% 9% 15% 19% 14%	103021
Seeing	Day	9% 5% 17% 32% 30% 7%	15620
	Night	11% 4 13% 28% 33% 11%	25047

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	12% 8% 2 6% 5% 5% 4 3 5% 7% 43%	2795
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 4 6% 6% 36%	2860
	21st to end	Day	18% 10% 3 7% 5% 5% 3 3 6% 7% 32%	3146
February	1st to 10th	Day	13% 7% 2 5% 3 4 4 4 5% 6% 46%	1710
	11th to 20th	Day	11% 7% 2 7% 3 4 4 4 6% 8% 44%	1784
	21st to end	Day	10% 7% 2 6% 5% 4 3 3 4 7% 47%	1570
March	1st to 10th	Day	10% 9% 3 7% 6% 6% 6% 5% 8% 9% 31%	3384
	11th to 20th	Day	8% 6% 2 6% 3 3 3 3 5% 7% 53%	3490
	21st to end	Day	10% 10% 3 7% 5% 5% 5% 4 6% 9% 36%	3884
April	1st to 10th	Day	9% 7% 3 5% 4 4 4 4 6% 8% 45%	2280
	11th to 20th	Day	8% 9% 3 5% 4 3 4 3 5% 8% 49%	2249
	21st to end	Day	6% 9% 2 6% 3 3 3 4 7% 10% 48%	2280
May	1st to 10th	Day	6% 6% 1 3 2 2 3 2 4 7% 64%	3821
	11th to 20th	Day	3 7% 2 6% 3 3 3 4 6% 12% 51%	4087
	21st to end	Day	7% 15% 4 7% 3 3 4 3 5% 7% 44%	4813
June	1st to 10th	Day	5% 10% 3 6% 4 4 4 3 6% 11% 43%	2660
	11th to 20th	Day	4 7% 2 4 3 3 4 4 6% 13% 50%	2660
	21st to end	Day	3 11% 3 7% 3 3 5% 4 5% 10% 46%	2660
July	1st to 10th	Day	1 7% 2 4 2 3 4 4 6% 14% 53%	4329
	11th to 20th	Day	2 10% 4 8% 4 4 5% 4 7% 16% 37%	4110
	21st to end	Day	1 20% 6% 9% 4 4 4 5% 6% 11% 31%	4455
August	1st to 10th	Day	3 13% 7% 15% 6% 5% 5% 4 8% 11% 23%	4091
	11th to 20th	Day	1 10% 3 8% 4 3 4 3 7% 12% 44%	3762
	21st to end	Day	1 10% 3 7% 3 3 4 4 7% 15% 42%	4180
September	1st to 10th	Day	3 5% 2 4 4 3 4 3 7% 12% 54%	2280
	11th to 20th	Day	4 9% 4 5% 3 4 4 4 8% 11% 45%	2280
	21st to end	Day	3 5% 2 4 3 3 4 4 6% 8% 60%	2024
October	1st to 10th	Day	2 4 1 4 3 3 3 2 5% 9% 64%	3180
	11th to 20th	Day	3 13% 3 6% 3 3 3 3 4 7% 52%	3161
	21st to end	Day	4 5% 2 4 4 3 4 3 5% 8% 59%	3448
November	1st to 10th	Day	6% 4 2 4 4 4 4 4 6% 7% 55%	1954
	11th to 20th	Day	7% 7% 2 6% 3 4 4 4 5% 6% 53%	1800
	21st to end	Day	10% 6% 1 4 4 3 4 3 5% 6% 54%	1683
December	1st to 10th	Day	9% 7% 3 6% 5% 4 4 3 7% 9% 44%	1520
	11th to 20th	Day	17% 8% 3 8% 4 5% 4 4 5% 8% 34%	1928
	21st to end	Day	14% 8% 3 7% 5% 5% 4 3 5% 8% 38%	2794
January	1st to 10th	Night	11% 8% 3 6% 4 4 3 3 5% 6% 47%	4155
	11th to 20th	Night	14% 8% 3 8% 5% 4 5% 4 6% 8% 34%	4110
	21st to end	Night	19% 11% 4 8% 5% 4 4 3 5% 7% 32%	4521
February	1st to 10th	Night	14% 8% 2 5% 4 3 4 3 4 6% 48%	2470
	11th to 20th	Night	12% 9% 3 7% 4 4 4 3 5% 7% 43%	2283
	21st to end	Night	12% 8% 2 6% 4 3 4 3 6% 7% 46%	1884
March	1st to 10th	Night	15% 13% 3 8% 6% 5% 5% 3 5% 6% 31%	3475
	11th to 20th	Night	8% 8% 4 7% 3 3 4 3 3 6% 52%	3490
	21st to end	Night	14% 13% 3 7% 5% 4 4 4 5% 7% 34%	3767
April	1st to 10th	Night	9% 10% 4 6% 4 3 3 4 5% 7% 45%	1900
	11th to 20th	Night	10% 9% 3 5% 3 4 3 3 5% 6% 49%	1879
	21st to end	Night	6% 8% 3 5% 3 4 3 3 7% 9% 50%	1900
May	1st to 10th	Night	4 9% 3 5% 2 2 3 2 3 5% 62%	3155
	11th to 20th	Night	4 16% 6% 9% 3 3 2 2 4 8% 42%	2813
	21st to end	Night	6% 17% 3 4 2 2 2 2 4 5% 54%	2762
June	1st to 10th	Night	6% 11% 3 7% 3 4 4 4 4 8% 47%	1520
	11th to 20th	Night	3 10% 3 6% 4 2 2 3 4 9% 55%	1520
	21st to end	Night	3 12% 3 7% 4 3 2 2 5% 7% 52%	1520
July	1st to 10th	Night	1 9% 3 8% 6% 3 3 2 3 6% 57%	2628
	11th to 20th	Night	1 16% 4 10% 4 3 2 2 3 7% 47%	2860
	21st to end	Night	2 30% 5% 13% 4 3 3 2 4 7% 28%	3109
August	1st to 10th	Night	3 25% 10% 19% 6% 2 2 2 2 5% 23%	2848
	11th to 20th	Night	2 12% 4 10% 4 2 2 2 3 4 55%	3085
	21st to end	Night	2 16% 4 7% 3 3 3 2 3 7% 50%	3498
September	1st to 10th	Night	3 9% 2 5% 4 4 3 3 5% 9% 55%	1900
	11th to 20th	Night	4 13% 3 8% 3 2 2 2 4 7% 51%	1900
	21st to end	Night	3 8% 2 4 3 3 3 2 3 8% 61%	2152
October	1st to 10th	Night	3 4 2 4 2 3 3 2 4 5% 68%	3800
	11th to 20th	Night	5% 14% 3 9% 3 3 3 2 3 5% 51%	3777
	21st to end	Night	5% 8% 2 5% 3 3 3 3 4 6% 58%	4121
November	1st to 10th	Night	8% 7% 2 5% 3 3 3 4 4 7% 53%	2343
	11th to 20th	Night	7% 7% 3 5% 4 4 4 3 4 8% 52%	2600
	21st to end	Night	7% 6% 2 5% 3 3 4 3 5% 6% 57%	2648
December	1st to 10th	Night	11% 7% 2 5% 4 3 4 3 5% 7% 49%	2660
	11th to 20th	Night	16% 9% 4 8% 5% 5% 4 4 6% 7% 33%	3276
	21st to end	Night	15% 7% 3 7% 5% 4 5% 4 5% 7% 38%	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	40% 10% 12% 26% 9% 3	2795
	11th to 20th	Day	47% 8% 13% 15% 10% 8%	2860
	21st to end	Day	49% 9% 10% 13% 14% 5%	3146
February	1st to 10th	Day	37% 7% 9% 17% 19% 11%	1710
	11th to 20th	Day	39% 7% 11% 17% 18% 8%	1793
	21st to end	Day	37% 8% 9% 22% 17% 6%	1570
March	1st to 10th	Day	45% 15% 13% 13% 12% 3	3384
	11th to 20th	Day	30% 7% 13% 24% 16% 11%	3490
	21st to end	Day	41% 13% 16% 19% 9% 2	3884
April	1st to 10th	Day	36% 8% 14% 22% 16% 4	2280
	11th to 20th	Day	34% 7% 13% 24% 17% 5%	2252
	21st to end	Day	29% 8% 16% 25% 17% 4	2280
May	1st to 10th	Day	20% 6% 12% 27% 26% 10%	3821
	11th to 20th	Day	17% 4 19% 25% 24% 11%	4090
	21st to end	Day	28% 7% 15% 22% 22% 6%	4814
June	1st to 10th	Day	31% 8% 14% 20% 22% 6%	2660
	11th to 20th	Day	22% 4 15% 24% 24% 12%	2660
	21st to end	Day	24% 3 10% 18% 25% 21%	2660
July	1st to 10th	Day	16% 2 5% 11% 17% 49%	4262
	11th to 20th	Day	26% 4 6% 12% 17% 35%	4041
	21st to end	Day	39% 2 7% 16% 20% 16%	4315
August	1st to 10th	Day	47% 3 5% 12% 22% 12%	4076
	11th to 20th	Day	28% 2 7% 13% 34% 17%	3798
	21st to end	Day	26% 1 7% 17% 31% 18%	4179
September	1st to 10th	Day	18% 2 7% 16% 27% 31%	2280
	11th to 20th	Day	25% 4 13% 23% 24% 12%	2280
	21st to end	Day	17% 3 12% 20% 32% 16%	2024
October	1st to 10th	Day	17% 3 6% 11% 27% 36%	3180
	11th to 20th	Day	31% 2 8% 18% 25% 15%	3161
	21st to end	Day	24% 3 7% 22% 27% 17%	3448
November	1st to 10th	Day	26% 8% 11% 23% 21% 11%	1954
	11th to 20th	Day	30% 6% 14% 18% 23% 10%	1800
	21st to end	Day	31% 5% 10% 16% 25% 13%	1683
December	1st to 10th	Day	34% 8% 11% 17% 17% 12%	1520
	11th to 20th	Day	50% 8% 10% 13% 15% 5%	1928
	21st to end	Day	43% 9% 12% 21% 11% 4	2794
January	1st to 10th	Night	37% 12% 14% 21% 12% 4	4155
	11th to 20th	Night	46% 10% 14% 13% 9% 8%	4110
	21st to end	Night	52% 8% 8% 12% 13% 7%	4521
February	1st to 10th	Night	39% 6% 9% 15% 18% 13%	2470
	11th to 20th	Night	40% 7% 10% 15% 20% 7%	2285
	21st to end	Night	37% 9% 11% 23% 14% 6%	1884
March	1st to 10th	Night	52% 12% 9% 11% 12% 3	3475
	11th to 20th	Night	30% 9% 10% 19% 21% 11%	3490
	21st to end	Night	46% 14% 12% 16% 10% 3	3767
April	1st to 10th	Night	37% 10% 13% 20% 15% 5%	1900
	11th to 20th	Night	36% 8% 12% 23% 15% 7%	1879
	21st to end	Night	30% 9% 13% 24% 17% 6%	1900
May	1st to 10th	Night	24% 15% 7% 18% 22% 14%	3155
	11th to 20th	Night	22% 5% 23% 19% 20% 13%	2814
	21st to end	Night	30% 12% 9% 17% 24% 8%	2762
June	1st to 10th	Night	34% 8% 10% 18% 20% 9%	1520
	11th to 20th	Night	27% 4 11% 14% 25% 18%	1520
	21st to end	Night	25% 3 5% 17% 24% 26%	1520
July	1st to 10th	Night	20% 10% 3 7% 13% 47%	2586
	11th to 20th	Night	32% 2 4 10% 14% 38%	2806
	21st to end	Night	51% 3 5% 10% 13% 17%	3011
August	1st to 10th	Night	61% 3 3 7% 11% 15%	2839
	11th to 20th	Night	30% 1 4 7% 34% 24%	3115
	21st to end	Night	32% 1 5% 10% 30% 22%	3498
September	1st to 10th	Night	21% 1 6% 13% 24% 35%	1900
	11th to 20th	Night	29% 4 7% 19% 25% 16%	1900
	21st to end	Night	18% 4 8% 20% 31% 20%	2152
October	1st to 10th	Night	18% 2 6% 12% 24% 38%	3800
	11th to 20th	Night	36% 3 5% 18% 22% 16%	3777
	21st to end	Night	26% 3 6% 21% 26% 18%	4121
November	1st to 10th	Night	31% 5% 9% 20% 21% 14%	2343
	11th to 20th	Night	32% 5% 12% 19% 21% 10%	2600
	21st to end	Night	29% 6% 9% 19% 21% 16%	2648
December	1st to 10th	Night	35% 7% 9% 17% 18% 14%	2660
	11th to 20th	Night	52% 8% 9% 13% 14% 5%	3276
	21st to end	Night	44% 13% 12% 15% 11% 4	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	22% 10% 18% 27% 19% 5%	332
	11th to 20th	Day	14% 7% 19% 30% 27% 3	370
	21st to end	Day	14% 6% 20% 29% 27% 5%	407
February	1st to 10th	Day	14% 11% 17% 28% 23% 8%	370
	11th to 20th	Day	13% 12% 22% 32% 20% 1	423
	21st to end	Day	10% 11% 23% 36% 19% 2	382
March	1st to 10th	Day	16% 11% 24% 34% 14% 2	460
	11th to 20th	Day	15% 9% 22% 31% 19% 4	460
	21st to end	Day	11% 11% 28% 32% 15% 2	506
April	1st to 10th	Day	14% 8% 23% 32% 19% 3	460
	11th to 20th	Day	10% 9% 27% 30% 21% 3	452
	21st to end	Day	7% 8% 28% 29% 22% 7%	460
May	1st to 10th	Day	8% 8% 23% 30% 25% 5%	463
	11th to 20th	Day	12% 4 21% 33% 23% 7%	488
	21st to end	Day	14% 5% 19% 32% 23% 7%	529
June	1st to 10th	Day	6% 5% 17% 39% 25% 8%	490
	11th to 20th	Day	3 3 23% 36% 31% 4	490
	21st to end	Day	5% 2 13% 38% 37% 5%	490
July	1st to 10th	Day	6% 1 12% 35% 40% 6%	486
	11th to 20th	Day	5% 6% 29% 49% 10%	484
	21st to end	Day	5% 1 6% 34% 43% 11%	531
August	1st to 10th	Day	7% 2 11% 38% 37% 5%	487
	11th to 20th	Day	4 1 9% 36% 40% 10%	477
	21st to end	Day	3 2 10% 32% 45% 8%	539
September	1st to 10th	Day	6% 1 11% 29% 41% 12%	490
	11th to 20th	Day	8% 3 14% 33% 34% 8%	490
	21st to end	Day	3 3 12% 29% 42% 10%	490
October	1st to 10th	Day	3 3 15% 31% 39% 9%	490
	11th to 20th	Day	7% 5% 12% 28% 33% 14%	490
	21st to end	Day	7% 3 14% 31% 33% 11%	539
November	1st to 10th	Day	6% 5% 18% 27% 33% 11%	490
	11th to 20th	Day	9% 7% 15% 30% 29% 11%	300
	21st to end	Day	10% 4 20% 28% 31% 7%	247
December	1st to 10th	Day	14% 5% 22% 22% 29% 8%	180
	11th to 20th	Day	12% 8% 21% 23% 27% 9%	180
	21st to end	Day	14% 12% 22% 28% 18% 6%	198
January	1st to 10th	Night	17% 7% 13% 32% 25% 7%	760
	11th to 20th	Night	12% 6% 14% 31% 29% 9%	760
	21st to end	Night	17% 6% 11% 31% 30% 5%	836
February	1st to 10th	Night	16% 6% 16% 28% 26% 8%	760
	11th to 20th	Night	15% 8% 16% 31% 25% 6%	744
	21st to end	Night	14% 8% 18% 32% 24% 4	628
March	1st to 10th	Night	18% 8% 17% 29% 23% 5%	760
	11th to 20th	Night	15% 6% 15% 30% 27% 7%	760
	21st to end	Night	14% 8% 19% 29% 25% 4	836
April	1st to 10th	Night	13% 6% 18% 29% 29% 5%	760
	11th to 20th	Night	13% 9% 16% 28% 27% 7%	751
	21st to end	Night	8% 6% 17% 31% 28% 10%	760
May	1st to 10th	Night	9% 5% 15% 32% 31% 9%	748
	11th to 20th	Night	14% 4 13% 22% 35% 12%	554
	21st to end	Night	14% 4 15% 28% 27% 12%	411
June	1st to 10th	Night	9% 2 13% 30% 35% 11%	380
	11th to 20th	Night	6% 2 18% 27% 40% 8%	380
	21st to end	Night	6% 1 8% 24% 44% 18%	380
July	1st to 10th	Night	5% 2 10% 23% 43% 17%	417
	11th to 20th	Night	7% 1 7% 15% 42% 29%	552
	21st to end	Night	12% 9% 20% 39% 20%	624
August	1st to 10th	Night	13% 1 11% 22% 40% 14%	567
	11th to 20th	Night	7% 1 8% 24% 42% 19%	723
	21st to end	Night	8% 8% 19% 44% 20%	836
September	1st to 10th	Night	5% 1 4 19% 49% 21%	760
	11th to 20th	Night	9% 1 8% 24% 42% 14%	760
	21st to end	Night	5% 1 6% 21% 43% 23%	760
October	1st to 10th	Night	8% 2 6% 30% 39% 16%	760
	11th to 20th	Night	7% 3 11% 24% 38% 17%	760
	21st to end	Night	7% 4 8% 28% 34% 19%	836
November	1st to 10th	Night	10% 3 11% 30% 36% 11%	780
	11th to 20th	Night	11% 3 11% 30% 37% 8%	800
	21st to end	Night	9% 3 13% 37% 31% 7%	788
December	1st to 10th	Night	14% 5% 13% 36% 26% 6%	760
	11th to 20th	Night	17% 6% 17% 31% 25% 5%	760
	21st to end	Night	17% 8% 18% 32% 22% 4	836

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