Reading path: Coastal water observation →
A hypothetical two-day contrast
Suppose a notebook contains a surface temperature of 14°C on Monday and 12°C on Tuesday at the same beach. These are hypothetical readings, not station data. The two-degree difference is an observation; “upwelling caused it” is an explanation that still needs evidence. Upwelling occurs when winds move surface water away from a coast and deeper, typically colder, nutrient-rich water rises to replace it [1]. That mechanism makes cooling a reasonable possibility, but two readings alone do not show that it happened at this location.
Make the comparison repeatable
A useful record would include date and exact time, thermometer or sensor, measurement depth, position, and whether the reading was taken from shore or a vessel. Note the tide stage and the water’s condition, too. Tides are long-period waves driven by the gravitational pull of the Moon and Sun; their regular rise and fall can change local water levels [2]. A hypothetical reading taken near a shallow inlet at low tide may sample a different mix of water than one taken farther offshore at high tide. Without consistent locations and depths, the temperatures are difficult to compare.
Separate mechanism from coincidence
Wind direction and duration matter because winds blowing along a coast can help drive water offshore and support upwelling [1]. But a thermometer records temperature, not the force that moved the water. Weather changes, waves, sun-warmed shallows, and mixing can also affect a local measurement. For the hypothetical 14°C and 12°C readings, a careful account would say that the water was cooler on the second day, then list upwelling among explanations to investigate. It would not claim a cause until supporting conditions and repeated measurements made that explanation more convincing.
Build a stronger small record
To improve the comparison, take readings at the same marked position, depth, and times across multiple days, and note the instrument used. Pair those records with wind observations and tide predictions or measured water levels. If possible, compare a shallow nearshore reading with one from a consistent deeper location. A pattern of cooling that coincides with suitable coastal winds and appears across repeated measurements would fit the upwelling mechanism better than one isolated change. Even then, it is evidence for an explanation, not proof from temperature alone. These readings also cannot establish whether a site is safe for swimming; that is a separate question requiring relevant conditions and information.
Sources & further reading
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