St. Andrews
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Andrews clear sky chart from 2005-11-06 to 2025-03-12.

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	19% 8% 2 6% 4 4 4 4 5% 7% 36%	108452
	Night	19% 9% 3 7% 4 4 4 3 5% 7% 34%	110611
Transparency	Day	46% 6% 13% 21% 12% 2	112026
	Night	48% 6% 12% 21% 12% 2	115202
Seeing	Day	23% 15% 31% 22% 8% 1	16607
	Night	25% 13% 30% 24% 7% 1	24781

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	16% 11% 3 8% 5% 5% 5% 4 5% 7% 31%	3267
	11th to 20th	Day	14% 12% 4 8% 5% 4 3 3 4 5% 39%	3570
	21st to end	Day	12% 10% 3 8% 4 4 5% 4 5% 7% 38%	4300
February	1st to 10th	Day	17% 11% 3 8% 4 6% 4 4 5% 9% 29%	1800
	11th to 20th	Day	15% 10% 3 8% 5% 6% 4 4 6% 7% 33%	1753
	21st to end	Day	13% 10% 3 6% 5% 5% 4 4 6% 7% 37%	1650
March	1st to 10th	Day	24% 10% 3 7% 4 4 4 3 5% 6% 31%	3367
	11th to 20th	Day	17% 10% 3 7% 6% 6% 5% 4 6% 7% 28%	3491
	21st to end	Day	28% 10% 3 6% 4 4 4 4 5% 5% 26%	3816
April	1st to 10th	Day	21% 10% 2 7% 6% 3 5% 3 5% 6% 32%	2214
	11th to 20th	Day	20% 7% 2 6% 4 3 4 3 5% 7% 39%	2442
	21st to end	Day	22% 7% 2 6% 4 4 4 4 5% 6% 37%	2470
May	1st to 10th	Day	20% 8% 2 7% 5% 5% 5% 4 6% 7% 31%	4106
	11th to 20th	Day	18% 7% 2 7% 5% 4 4 3 5% 7% 37%	4191
	21st to end	Day	17% 7% 2 6% 5% 4 5% 4 6% 9% 35%	4963
June	1st to 10th	Day	23% 8% 2 7% 6% 4 4 3 5% 8% 30%	2850
	11th to 20th	Day	20% 8% 2 6% 4 4 3 3 5% 7% 37%	2850
	21st to end	Day	26% 7% 3 6% 5% 4 5% 4 5% 7% 28%	2850
July	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 5% 6% 8% 31%	4760
	11th to 20th	Day	18% 8% 2 6% 4 4 4 4 5% 8% 37%	4602
	21st to end	Day	20% 8% 2 6% 4 4 4 4 6% 8% 35%	4575
August	1st to 10th	Day	21% 9% 3 7% 5% 4 4 4 5% 7% 31%	4105
	11th to 20th	Day	17% 8% 3 7% 4 5% 5% 3 6% 9% 34%	4045
	21st to end	Day	10% 5% 2 5% 3 4 4 4 5% 8% 48%	4532
September	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 8% 50%	2233
	11th to 20th	Day	16% 4 2 5% 3 3 3 4 4 7% 49%	2090
	21st to end	Day	18% 6% 2 5% 4 3 3 3 4 7% 45%	2090
October	1st to 10th	Day	16% 6% 3 6% 3 4 5% 3 6% 6% 41%	1962
	11th to 20th	Day	20% 5% 2 5% 4 4 5% 3 5% 7% 41%	1900
	21st to end	Day	20% 7% 2 7% 4 4 4 4 4 6% 37%	1957
November	1st to 10th	Day	21% 7% 2 6% 4 3 4 3 5% 5% 39%	1761
	11th to 20th	Day	21% 6% 2 5% 4 4 4 3 6% 8% 39%	1600
	21st to end	Day	21% 10% 2 6% 5% 4 4 3 6% 6% 34%	1600
December	1st to 10th	Day	22% 9% 2 4 4 3 4 4 7% 8% 34%	2596
	11th to 20th	Day	24% 9% 2 6% 4 4 4 3 4 5% 36%	2618
	21st to end	Day	23% 10% 2 4 3 4 4 4 5% 7% 35%	3476
January	1st to 10th	Night	13% 12% 4 9% 5% 4 4 3 6% 8% 32%	6690
	11th to 20th	Night	15% 10% 4 9% 5% 5% 4 3 5% 6% 34%	6315
	21st to end	Night	11% 10% 3 7% 5% 4 5% 4 6% 6% 40%	6654
February	1st to 10th	Night	18% 12% 3 5% 4 5% 5% 4 6% 8% 30%	2600
	11th to 20th	Night	16% 9% 4 8% 6% 5% 5% 3 6% 5% 32%	2530
	21st to end	Night	17% 11% 3 6% 5% 4 3 3 5% 6% 37%	1980
March	1st to 10th	Night	25% 11% 3 7% 4 4 5% 3 4 4 29%	3692
	11th to 20th	Night	17% 12% 3 8% 6% 5% 6% 4 6% 8% 25%	3497
	21st to end	Night	31% 9% 2 5% 4 4 4 4 5% 5% 27%	3816
April	1st to 10th	Night	25% 10% 3 7% 5% 4 3 3 3 5% 32%	1962
	11th to 20th	Night	22% 9% 3 6% 4 4 3 5% 5% 6% 34%	1688
	21st to end	Night	26% 8% 2 6% 4 4 4 3 5% 7% 32%	1710
May	1st to 10th	Night	23% 6% 3 6% 3 3 4 3 5% 8% 36%	2834
	11th to 20th	Night	19% 8% 3 7% 5% 5% 6% 5% 7% 7% 30%	2704
	21st to end	Night	17% 9% 3 7% 4 5% 4 3 6% 8% 33%	2637
June	1st to 10th	Night	25% 8% 2 8% 4 4 4 4 5% 7% 28%	1330
	11th to 20th	Night	21% 8% 3 6% 4 5% 3 3 6% 7% 34%	1330
	21st to end	Night	29% 8% 3 7% 5% 4 4 3 5% 5% 28%	1330
July	1st to 10th	Night	18% 9% 3 8% 6% 5% 6% 4 7% 8% 27%	2210
	11th to 20th	Night	21% 8% 3 6% 4 4 4 4 6% 8% 30%	2359
	21st to end	Night	21% 8% 3 6% 4 4 4 3 6% 7% 34%	2945
August	1st to 10th	Night	22% 12% 3 8% 4 4 4 4 5% 6% 27%	2825
	11th to 20th	Night	16% 10% 3 7% 4 5% 5% 4 6% 9% 32%	2782
	21st to end	Night	11% 5% 2 6% 4 4 4 3 5% 8% 48%	3124
September	1st to 10th	Night	16% 7% 2 4 4 4 3 3 5% 7% 45%	1944
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 6% 48%	2090
	21st to end	Night	19% 6% 2 4 3 3 4 3 5% 7% 44%	2090
October	1st to 10th	Night	19% 7% 2 5% 4 4 4 3 5% 7% 40%	2218
	11th to 20th	Night	19% 7% 2 6% 3 4 4 3 4 6% 42%	2280
	21st to end	Night	24% 7% 2 6% 4 3 4 3 5% 5% 36%	2638
November	1st to 10th	Night	20% 6% 2 5% 4 4 4 4 5% 7% 39%	2537
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 4 6% 38%	2798
	21st to end	Night	22% 7% 2 6% 4 4 4 3 5% 8% 34%	2800
December	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 5% 7% 36%	4580
	11th to 20th	Night	24% 9% 2 5% 3 3 4 3 5% 5% 37%	5590
	21st to end	Night	22% 11% 3 6% 4 4 4 3 5% 6% 33%	7502

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	53% 5% 14% 21% 6%	3268
	11th to 20th	Day	49% 4 11% 24% 10% 2	3600
	21st to end	Day	47% 5% 11% 16% 19% 3	4300
February	1st to 10th	Day	53% 6% 10% 17% 13% 2	1800
	11th to 20th	Day	50% 6% 12% 19% 12% 1	1761
	21st to end	Day	45% 5% 12% 22% 11% 4	1650
March	1st to 10th	Day	55% 5% 10% 16% 10% 4	3371
	11th to 20th	Day	52% 9% 13% 14% 10% 2	3491
	21st to end	Day	58% 5% 8% 15% 12% 2	3816
April	1st to 10th	Day	53% 5% 11% 15% 13% 3	2214
	11th to 20th	Day	45% 6% 12% 23% 13% 2	2442
	21st to end	Day	47% 6% 10% 19% 14% 4	2470
May	1st to 10th	Day	48% 8% 16% 20% 7% 1	4120
	11th to 20th	Day	45% 8% 17% 21% 8% 1	4195
	21st to end	Day	44% 9% 16% 17% 12% 1	4963
June	1st to 10th	Day	52% 7% 14% 17% 9% 1	2850
	11th to 20th	Day	46% 6% 12% 20% 14% 1	2850
	21st to end	Day	55% 8% 11% 15% 10% 1	2850
July	1st to 10th	Day	48% 9% 12% 19% 11% 1	4685
	11th to 20th	Day	44% 8% 14% 23% 10% 1	4521
	21st to end	Day	45% 8% 14% 22% 10% 1	4429
August	1st to 10th	Day	51% 7% 13% 22% 7%	4092
	11th to 20th	Day	46% 7% 17% 20% 9%	4084
	21st to end	Day	33% 6% 15% 26% 19% 1	4532
September	1st to 10th	Day	33% 6% 14% 29% 17% 1	2233
	11th to 20th	Day	35% 6% 12% 26% 19% 3	2090
	21st to end	Day	40% 5% 11% 28% 16% 1	2090
October	1st to 10th	Day	45% 6% 13% 26% 10%	3261
	11th to 20th	Day	36% 4 13% 34% 13%	3140
	21st to end	Day	36% 4 18% 23% 17% 2	3207
November	1st to 10th	Day	47% 5% 11% 22% 13% 1	1761
	11th to 20th	Day	43% 6% 13% 20% 15% 2	1600
	21st to end	Day	51% 5% 15% 20% 7% 1	1600
December	1st to 10th	Day	47% 5% 18% 21% 9%	2596
	11th to 20th	Day	52% 3 15% 15% 11% 5%	2618
	21st to end	Day	48% 6% 14% 17% 13% 2	3476
January	1st to 10th	Night	50% 6% 12% 24% 7% 1	6690
	11th to 20th	Night	51% 5% 12% 22% 9% 1	6360
	21st to end	Night	44% 5% 10% 21% 16% 4	6654
February	1st to 10th	Night	51% 7% 12% 15% 13% 2	2600
	11th to 20th	Night	52% 7% 9% 18% 12% 2	2533
	21st to end	Night	49% 5% 10% 20% 14% 3	1980
March	1st to 10th	Night	59% 5% 7% 15% 12% 2	3701
	11th to 20th	Night	53% 9% 14% 13% 9% 2	3498
	21st to end	Night	59% 4 8% 16% 11% 2	3816
April	1st to 10th	Night	54% 4 10% 15% 13% 4	1962
	11th to 20th	Night	50% 5% 11% 17% 16% 1	1688
	21st to end	Night	53% 5% 9% 16% 14% 2	1710
May	1st to 10th	Night	46% 6% 20% 18% 9% 1	2840
	11th to 20th	Night	48% 11% 16% 18% 7% 1	2705
	21st to end	Night	46% 10% 17% 19% 8% 1	2638
June	1st to 10th	Night	54% 7% 14% 18% 7% 1	1330
	11th to 20th	Night	49% 6% 12% 21% 11% 2	1330
	21st to end	Night	58% 5% 10% 17% 7% 3	1330
July	1st to 10th	Night	54% 6% 13% 15% 11%	2175
	11th to 20th	Night	51% 8% 13% 20% 8%	2317
	21st to end	Night	49% 6% 11% 20% 13% 1	2851
August	1st to 10th	Night	58% 7% 10% 17% 8%	2813
	11th to 20th	Night	48% 6% 16% 21% 8%	2809
	21st to end	Night	34% 6% 13% 26% 18% 2	3124
September	1st to 10th	Night	38% 7% 10% 28% 15% 1	1944
	11th to 20th	Night	37% 5% 13% 24% 19% 1	2090
	21st to end	Night	40% 4 10% 28% 16% 1	2090
October	1st to 10th	Night	43% 5% 12% 26% 12% 1	3699
	11th to 20th	Night	36% 4 12% 28% 20%	3779
	21st to end	Night	43% 3 13% 24% 14% 2	4338
November	1st to 10th	Night	44% 7% 13% 22% 14% 1	2537
	11th to 20th	Night	47% 5% 12% 21% 14% 1	2798
	21st to end	Night	49% 6% 14% 19% 10% 1	2800
December	1st to 10th	Night	48% 5% 10% 28% 9%	4580
	11th to 20th	Night	49% 5% 11% 20% 14% 2	5591
	21st to end	Night	51% 5% 12% 17% 12% 3	7502

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	28% 19% 35% 15% 3 1	200
	11th to 20th	Day	31% 23% 30% 12% 4 1	200
	21st to end	Day	21% 19% 39% 18% 3	365
February	1st to 10th	Day	27% 21% 33% 17% 3 1	400
	11th to 20th	Day	23% 25% 37% 12% 2	396
	21st to end	Day	23% 22% 35% 17% 3	414
March	1st to 10th	Day	24% 24% 33% 17% 2	500
	11th to 20th	Day	24% 23% 31% 15% 6%	464
	21st to end	Day	28% 21% 30% 14% 5% 1	506
April	1st to 10th	Day	30% 22% 31% 13% 3 1	460
	11th to 20th	Day	25% 21% 32% 18% 4 1	452
	21st to end	Day	27% 16% 32% 19% 5% 1	460
May	1st to 10th	Day	26% 19% 32% 19% 4	463
	11th to 20th	Day	21% 15% 34% 22% 6% 1	545
	21st to end	Day	24% 15% 32% 20% 8% 1	735
June	1st to 10th	Day	25% 13% 33% 21% 7% 1	680
	11th to 20th	Day	23% 11% 27% 27% 11% 1	680
	21st to end	Day	31% 7% 27% 25% 9% 1	680
July	1st to 10th	Day	23% 5% 30% 30% 11% 1	670
	11th to 20th	Day	22% 5% 28% 31% 13% 2	575
	21st to end	Day	18% 7% 33% 32% 10% 1	532
August	1st to 10th	Day	16% 7% 31% 33% 11% 1	486
	11th to 20th	Day	20% 6% 25% 32% 14% 1	479
	21st to end	Day	17% 9% 26% 29% 16% 3	539
September	1st to 10th	Day	13% 6% 32% 37% 10% 1	490
	11th to 20th	Day	21% 11% 25% 25% 14% 4	490
	21st to end	Day	18% 11% 28% 27% 13% 3	490
October	1st to 10th	Day	19% 15% 25% 23% 15% 2	423
	11th to 20th	Day	21% 14% 29% 25% 10% 2	390
	21st to end	Day	25% 16% 32% 22% 5% 1	429
November	1st to 10th	Day	25% 16% 31% 22% 5% 2	395
	11th to 20th	Day	26% 21% 31% 16% 6%	400
	21st to end	Day	28% 21% 29% 18% 3 1	400
December	1st to 10th	Day	28% 18% 35% 15% 5%	399
	11th to 20th	Day	36% 25% 25% 11% 4	200
	21st to end	Day	27% 27% 25% 15% 6% 1	220
January	1st to 10th	Night	25% 20% 32% 20% 3	873
	11th to 20th	Night	28% 18% 33% 18% 4	800
	21st to end	Night	17% 23% 39% 18% 3	880
February	1st to 10th	Night	27% 20% 33% 17% 3	800
	11th to 20th	Night	24% 24% 32% 16% 2 1	783
	21st to end	Night	25% 17% 34% 20% 3	660
March	1st to 10th	Night	24% 18% 37% 19% 2	800
	11th to 20th	Night	23% 19% 30% 24% 4	767
	21st to end	Night	24% 17% 34% 19% 5% 1	836
April	1st to 10th	Night	31% 15% 32% 18% 3	703
	11th to 20th	Night	26% 10% 37% 22% 5%	564
	21st to end	Night	30% 10% 33% 21% 7%	570
May	1st to 10th	Night	26% 8% 33% 26% 8%	570
	11th to 20th	Night	27% 7% 30% 30% 6%	562
	21st to end	Night	24% 8% 35% 26% 6% 1	556
June	1st to 10th	Night	28% 6% 24% 33% 8%	380
	11th to 20th	Night	23% 6% 26% 30% 14% 1	380
	21st to end	Night	34% 2 21% 34% 8% 1	380
July	1st to 10th	Night	24% 4 22% 38% 11%	370
	11th to 20th	Night	24% 5% 19% 38% 13% 1	368
	21st to end	Night	25% 3 21% 38% 13%	533
August	1st to 10th	Night	18% 5% 24% 41% 12%	568
	11th to 20th	Night	23% 4 20% 37% 15%	562
	21st to end	Night	17% 4 23% 37% 18% 1	627
September	1st to 10th	Night	20% 3 30% 32% 14% 2	643
	11th to 20th	Night	20% 6% 25% 28% 20% 2	760
	21st to end	Night	21% 7% 27% 29% 14% 1	760
October	1st to 10th	Night	23% 7% 27% 29% 14% 1	760
	11th to 20th	Night	21% 12% 32% 23% 11% 1	760
	21st to end	Night	27% 17% 29% 21% 6%	836
November	1st to 10th	Night	24% 13% 32% 22% 8% 1	780
	11th to 20th	Night	25% 17% 31% 19% 7% 1	900
	21st to end	Night	29% 19% 29% 18% 4	900
December	1st to 10th	Night	33% 14% 33% 15% 4	900
	11th to 20th	Night	29% 23% 31% 14% 2 1	900
	21st to end	Night	30% 20% 30% 16% 4	990

Page generated at 2025-03-09 09:08:44. Version ac07

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