READ THE DIAGRAM
Layering bay, fog, and mountains
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Schematic diagram showing the relationship between valley floor, slopes, and fog position. Does not depict actual cross-sections, scales for elevation or distance, or a fixed fog height. To the supporting reference ↓
FIELD NOTE 01
Southern side near the bay
The southern part of Napa is affected by cold air and fog from San Pablo Bay. The idea that it is always hotter in the south simply due to latitude does not apply. In addition to the north-south axis of the valley, also consider distance from water, wind corridors, and slope aspects.
FIELD NOTE 02
Valley floor, slopes, mountains
Shift your focus from areas around Oakville and Rutherford to the higher vineyards of Howell Mountain and Atlas Peak. In the mountains, some vineyards sit above the fog layer, making it insufficient to simply describe higher altitude as uniformly cooler.
FIELD NOTE 03
The range of Cabernet
Cabernet Sauvignon is an important grape, but there are differences even within Napa as a large AVA. Looking at smaller AVAs, specific vineyards, vintages, and producers’ choices in extraction and barrel usage, you can describe a wine in detail rather than ending with a single phrase like “Napa style.”
FIELD NOTE 04
Not only the north-south position, but also layer valley floors with slopes.
Napa’s southern area introduces the role of cold air and fog from San Pablo Bay. But simply drawing a temperature gradation from south to north cannot explain mountain vineyards. The time spent under fog and the differences between daytime and nighttime temperatures both vary between the valley floor and the slopes above.
The Napa Valley Vintners map of the region distinguishes between mountain and valley floor AVAs. Even for two vineyards that are close together on the map, adding details such as elevation, orientation, and relationship to fog allows for concrete comparison of ripening conditions.
Basis for this section:Napa Valley Vintners｜Climate, soils and terrain ↗ ／ Napa Valley Vintners｜Napa Valley appellation map ↗
FIELD NOTE 05
How to consider vineyards located above the fog layer
Howell Mountain is characterized in official regional explanations as having vineyards above the fog layer. It’s best to avoid oversimplifications such as 'high elevation always means cooler than the valley floor at any hour.' The critical point is which time of day is being compared, considering fog and temperature inversions.
The slope in the diagram is not an actual cross-section. It is a schematic used to understand the relationship between valley floor, fog, and slope, and to help you check the location of vineyards using the relevant AVA or producer's materials.
Basis for this section:Napa Valley Vintners｜Climate, soils and terrain ↗ ／ Napa Valley Vintners｜Napa Valley appellation map ↗
FIELD NOTE 06
Additional column for the Cabernet comparison
If you are comparing Cabernet Sauvignon from mountains to that from valley floor, line up the AVA, vintage, original vineyard, harvest information, blend, and vessel for maturation. A bottle labeled Napa Valley alone does not mean it comes from a single vineyard or is 100% Cabernet. Note down your hypothesis based on geography, and for label origin and varietal declaration, check the rules in the article about appellation labelling separately.
Basis for this section:Napa Valley Vintners｜Climate, soils and terrain ↗ ／ Napa Valley Vintners｜Napa Valley appellation map ↗
Because of the cool air and fog coming in from the bay, climate cannot be determined by latitude alone.
PAUSE & REFLECT
Check your understanding.
Is southern Napa always warmer than northern Napa?
Read the explanation ＋
Because of the cool air and fog coming in from the bay, climate cannot be determined by latitude alone.
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View saved notes →Courses related to this question
Cabernet Sauvignon: Interpreting ripening and blend ratios →
Robert Mondavi: The meaning and substance of Fumé Blanc’s name →
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Sources and reference materials
These are the reference materials for the main text. For details of production regulations, consult the current documents. For individual wine specifications, refer to materials for the relevant vintage.
- Napa Valley Vintners｜Climate, soils and terrain ↗
- Napa Valley Vintners｜Cabernet Growing Conditions ↗
- Napa Valley Vintners｜Napa Valley appellation map ↗