Measurement · Physical Oceanography · kg/m³ · one key across 2 datasets

Potential density (σθ)

Potential density (Sigma Theta)

NERC P01 SIGTEQ01P06 units UKMC kg/m3key sigma_theta series sigma_theta · sigma_theta_1

1,533,267
values: 2,189
flagged: 44,466
sampling events: 1949–2026
years: 2
datasets: 0–5,351 m
depth: every month
seasons

Heads-up 2,189 of these 1,533,267 values (0.1 %) are flagged by their provider as questionable or bad. The Explorer, the climatology and its anomalies leave them out; do the same when you read the data yourself (cc_qual_ok_sql() in calcofi4r, qual_ok_sql() in calcofi4py).

Measured in

Hydrographic Bottle 842,669 values · 34,839 sampling events · 1949–2021 · 0–5,351 m · 213 grid cells · 650 cruises

series sigma_theta · source column s_theta · flag column s_thtaq 2,158 flagged · 0.3 %
CTD Cast Files 690,598 values · 9,627 sampling events · 1993–2026 · 1–3,630 m · 97 grid cells · 134 cruises

series sigma_theta_1 · source column sig_theta_ts1 · flag column sig_theta_ts1q 31 flagged
Potential density sensor 1

One row per series, one cell per year, darker with more values — the same strip the front door draws per dataset and a species page draws per taxon. Each row is one dataset's own measurement_type; the label is written once, in the title above.

By depth

Values per depth band at the release grain, one thin bar per series. The bands are the record's own (0-10 … 2000+ m).

By month

Values per calendar month, one row per series: CalCOFI's quarterly cruises show as peaks in the months the ships sail.

Range & quality

bounds: The registry declares 15 … 35 kg/m³ for sigma_theta · sigma_theta_1; a value outside it is dropped at the ingest (drop_out_of_bounds()).
observed: Hydrographic Bottle sigma_theta 17 · median 25.99 · 28.14 kg/m³ (5th–95th percentile 24.12–27.19)
CTD Cast Files sigma_theta_1 15.02 · median 26.34 · 34.3 kg/m³ (5th–95th percentile 24.33–26.97)
flags: Hydrographic Bottle sigma_theta s_thtaq on 23,722 of 842,669 rows: 6 21,564 · 8 2,155 · 9 3; qual_ok keeps 840,511. The codes are this dataset's own vocabulary, uninterpreted.
CTD Cast Files sigma_theta_1 sig_theta_ts1q on 31 of 690,598 rows: 8 31; qual_ok keeps 690,567. The codes are this dataset's own vocabulary, uninterpreted.
baseline: A monthly climatology exists for this measurement (the release's climatology table: 1993–2013, dataset × grid cell × calendar month × 10 m bin), so the Explorer can draw anomalies against it rather than values.

Ways in

Explorermaps, sections and time series of this measurement — opens prefilled
https://calcofi.io/explore/?var=sigma_theta
Explorer · a depth section vs normalthe newest cruise's section against the climatology baseline
https://calcofi.io/explore/?lens=section&var=sigma_theta&anom=1
db-querySQL in your browser — the shell opens with this query
https://calcofi.io/db-query/?sql=SELECT+%2A+FROM+__TBL%3Aobs_env__+WHERE+measurement_type+IN+%28%27sigma_theta%27%2C+%27sigma_theta_1%27%29+LIMIT+100%3B
ERDDAP · Hydrographic BottleCalCOFI's own server, the Hydrographic Bottle table constrained to this series
https://erddap.calcofi.io/erddap/tabledap/calcofi_bottle.html?&measurement_type=%22sigma_theta%22
ERDDAP · CTD Cast FilesCalCOFI's own server, the CTD Cast Files table constrained to this series
https://erddap.calcofi.io/erddap/tabledap/calcofi_ctd-cast.html?&measurement_type=%22sigma_theta_1%22

Parquetobs_env is partitioned by measurement_type in the content-addressed store — resolved through the release catalog, never a path built by hand

SELECT * FROM read_json('…/v2026.09.10/catalog.json')
-- objects[] WHERE table = 'obs_env'
--   AND partition_value = 'sigma_theta'
--                    | 'sigma_theta_1'

library(calcofi4r)
con <- cc_get_db()                      # the promoted release
tbl(con, "obs_env") |> filter(measurement_type %in% c("sigma_theta", "sigma_theta_1"))

Python

import calcofi4py as cc
con = cc.cc_get_db()
con.sql("SELECT * FROM obs_env WHERE measurement_type IN ('sigma_theta', 'sigma_theta_1')").df()

This page as data

JSONthis measurement's entry of the release record, verbatim
https://calcofi.io/measurements/sigma_theta.json

Read from measurements.json of release v2026.09.10, written at release by calcofi4db::build_measurements_catalog() beside taxa.json. The catalog's index is /measurements/; the JSON-LD on this page is a schema.org/DefinedTerm in the NERC P01 term set with a PropertyValue per series.

Cite

Cite the release; each dataset's own citation, licence and DOI are on its page — Hydrographic Bottle, CTD Cast Files. The Explorer's Share → Cite writes it for the view you make.