Forecast

How Barometric Pressure over the Ocean Affects Weather on Land

Barometric pressure over the ocean is vital to weather prediction on land. The Saildrone Forecast weather app for iOS now includes global air pressure as an animated overlay.

By
Saildrone
,
on
July 16, 2020

Barometric pressure is the force exerted on the Earth’s surface by the mass of the air above it. The amount of pressure in any given location on the surface of the planet is related to temperature, elevation, and gravity. Warmer air is less dense than colder air. Colder air masses have higher pressure and warmer air masses have lower pressure. The atmosphere is warmed by solar radiation, more so near the equator and less so at the poles. The atmospheric circulation results from relieving this heating imbalance and is further complicated due to the Earth’s rotation.

In the Northern Hemisphere, winds around a low-pressure system rotate counter-clockwise; in a high-pressure system, the winds rotate clockwise (this is reversed in the Southern Hemisphere). These pressure systems are one of the largest contributors to weather prediction models. 

Much of the data that goes into weather prediction comes from satellites, but satellites can’t measure pressure directly. Modelers instead have to rely on measurements from small drifting buoys that offer little positioning control. Saildrone unmanned surface vehicles (USVs) record in situ meteorological and oceanic observations including wind speed and direction, humidity, sea and air temperature, chlorophyll, wave height and direction—and of course, barometric pressure. 

Saildrone data is used to better understand changes taking place in the ocean ecosystem, validate other sampling platforms, and improve weather forecasting. The Saildrone Forecast app for iOS is the first weather tool to integrate data collected by saildrones into ultra-high-resolution forecasts, and now includes global air pressure as a map overlay.

Saildrone Forecast weather app for iPad
The Saildrone Forecast app for iPad showing predicted wind and pressure in the Pacific Ocean over a 12-hour time period.

Anyone who’s ever watched the weather report on the local news has had at least a casual introduction to barometric pressure (sometimes also referred to as atmospheric pressure or simply air pressure). Meteorologists frequently point out high-pressure systems that suggest fine weather and low-pressure systems that, depending on where you live, may mean thunderstorms, blizzards, hurricanes, or tornadoes.

Solar radiation heats the planet; some of that heat is then transferred to the atmosphere. Oceans, which cover 72% of the Earth’s surface, absorb most of the solar radiation that reaches the planet, and ocean currents also help to distribute that heat from the warmer tropics to the poles (the ocean and atmosphere each do about half of that job). Air pressure differences can be amplified as air moves across warmer and cooler areas of the ocean or across land/ocean boundaries. Most rainfall over land results from moisture introduced into air masses over the ocean. In addition to temperature, winds, and currents, atmospheric pressure affects how much gas the surface water can hold, and is also used in the calculation of the partial pressures of CO2 in the atmosphere and surface ocean. 

The most common instrument used to measure barometric pressure is an aneroid barometer, which consists of a small, air-tight box that compresses or flexes as air pressure rises and falls moving a needle along an easy-to-read dial. In the ocean, the Global Drifter Program (GDP) deploys surface drifters, small round balls attached to a drogue and equipped with a thermometer and barometer. Once deployed, the devices drift along with ocean currents recording temperature, pressure, and location for approximately 18 months (the average lifespan of the D-cell batteries that power the sensors), before becoming flotsam.

Saildrone collecting data in the ocean
Saildrone USVs collect a variety of atmospheric and oceanographic measurements including barometric pressure, air and water temperature, humidity, radiation, salinity, chlorophyll, wind and current strength and direction, and wave height and period.

As part of the standard sensor package, Saildrone USVs carry a capacitive absolute pressure sensor, which uses a thin piece of metal sandwiched between two rigid plates. As air pressure changes, the plate changes shape, therefore increasing or decreasing the capacitance, which is converted into an electrical signal that can be measured. Saildrones are wind and solar-powered and navigate autonomously according to prescribed waypoints. They can be launched and retrieved from any dock, and designed to perform multiple missions in the harshest ocean conditions.

Saildrone Forecast visualizes air pressure as isobars—lines drawn on the map that connect points of equal pressure. Pressure values are indicated on the isobars in millibars; the average pressure at sea level is 1013.25 millibars. Isobars are useful for locating strong pressure gradients (identifiable by tight packing of the isobars). Larger gradients in pressure indicate strong winds while smaller, more gradual gradients indicate lighter winds.

Saildrone Forecast for iPhone showing pressure and temperature
Saildrone Forecast for iPhone showing the Clouds and Temperature layers with Pressure overlaid.

The Air Pressure overlay is located on the layers panel and can be viewed with any other map layer—try turning it on over the Wind layer for a visual explanation of how pressure gradients indicate wind strength. (Note: Air pressure is best viewed zoomed out; if you are zoomed in too close on the map you may not see the isobar gradients.) On the Temperature layer, move the scrubber back and forth or play a Time Loop and watch how changes in pressure affect the weather in your area. With the Cloud layer and Air Pressure overlay selected, you can watch how weather that originates in the ocean is carried over land.

Recent app updates also include improved rendering of the Cloud layer using higher resolution data as well as enhanced wind barbs and current arrows. An optimized model ingestion system presents new forecast data in the app quicker.

Download Saildrone Forecast for free from the App Store for iPhone and iPad. Questions? Read the Saildrone Forecast FAQ. As always, we welcome your feedback as we continue to re-invent the weather forecast experience.

Resources: 

Luca Centurioni, András Horányi, et al., “A Global Ocean Observing System for Measuring Sea Level Atmospheric Pressure: Effects and Impacts on Numerical Weather Prediction,”Bulletin of the American Meteorological Society, February 2017