There are some places we sail where just one look at the sky, or the way the air feels, lets us know that we’re probably going to be in for a thunderstorm shellacking sometime that day—the summertime Chesapeake Bay comes to mind. But if we’re looking for a more precise way of determining the potential for convention, as well as how severe the wind it produces might be, there are several tools available to sailors that bear exploring and many environmental factors to consider.
Sailors and weather forecasters rely on weather models to help them predict what’s coming, but computer models have difficulty predicting weather events that occur on small scales of time or distance (see “Weather Window: Forecast Models,” May 2023). Convective storms and the wind they generate are a good example of this.
Wind due to convection is caused by temperature differences; air that is warmer than its surrounding air mass rises, and air that’s cooler than its surrounding air mass falls. A group of air molecules moving up or down in the atmosphere due to temperature differences does so for only minutes, and on a scale from a few yards to a few miles. Most computer models cannot resolve events on this scale, so most models do a poor job of predicting wind due to convection.
Let’s break down the science to better understand the environmental factors at play.
Air: Pressure and Temperature
If we read a barometer—a measure of the weight of all air molecules in a column of air—at the bottom of a mountain, then quickly take the barometer up the mountain, the indicated pressure will drop, because there are fewer molecules (less atmospheric weight) above the barometer.
So, as air rises, the pressure falls. And a thermodynamic property of gasses is that as pressure falls, temperature also falls. Temperature of the atmosphere typically falls about 5°F per 1,000 feet of altitude gain, but this varies.
Light colors tend to reflect more of the sun’s energy than dark colors, which absorb and convert more of the sun’s energy into heat. Recently on a 120°F day in Phoenix, the surface temperature of dark pavement was 170°F. Air near the superheated pavement is also warmed to a higher temperature than surrounding air that’s not adjacent to hot pavement. Warmer than its surrounding air mass, this superheated air is lighter, begins to rise, and will continue rising as long as it is warmer than its surrounding air. This transports energy (heat) upward in the atmosphere.
Water: Evaporation and Condensation
Water is heavy. But water vapor is light; it may seem counterintuitive when you’re feeling it on your skin, but moist air is lighter than dry air. The most abundant atmospheric gasses are heavy (nitrogen (N2), oxygen (O2), and carbon dioxide (CO2). The molecular weight of water vapor (H2O) is much lower. So, the more water vapor displaces heavier gasses from a given volume of air, the lighter that volume of air is.
Water vapor has one more unique and crucial property: It contains latent, or potential, heat. We regularly turn liquid water into water vapor on a stove. But evaporation—changing liquid water into water vapor—also happens at cooler temperatures, just more slowly. If you leave a glass of water on the counter, in a few days or weeks the water will be gone; it evaporates, transforming into water vapor, and this also requires heat, causing air near the glass of water to cool very slightly.
Condensation is the opposite process; when air cannot hold all the water in vapor form, some water vapor condenses, forming visible water droplets. This process releases heat—the opposite of evaporation, which requires heat.
Condensation occurs because cooler air cannot hold as much water vapor as warmer air. When drinking a bottle of cold beer on a hot, humid day, water forms on the outside of the beer bottle. That’s because the air near the beer bottle is so cool that it can’t keep all the water in vapor form, so some water condenses on the bottle.
Building a Thunderstorm
Now let’s put this all together. If we take a volume of air which is warmer than its surrounding air, this volume of air rises (let’s also say this volume of air is very moist, with lots of water vapor). As this volume of air rises, it cools, but so does the surrounding air mass. As long as our volume of air remains warmer than its surrounding air mass, it continues rising and cooling, but remaining warmer than surrounding air.
At some point, our volume of air cools to such an extent that it can no longer hold all the water vapor in vapor form, and some water vapor condenses into visible water droplets (clouds). This condensation process releases heat into our volume of air, and now our volume of air is even warmer, so it rises faster. The more water vapor condenses into visible water droplets, the more our volume of air warms, offsetting the cooling which occurs as the atmospheric pressure drops, and the more our volume of air rises.
This is how powerful thunderstorms are made. Once the volume of air is no longer warmer than its surrounding air mass, then our volume of air stops rising and cloud or thunderstorm growth halts.
Lightning is, of course, an obvious concern in thunderstorms, but as sailors, wind is also foremost in our minds. When the large volumes of warm, moist air can no longer rise, they fall, and the downward rush of moving air can reach 50 knots or more. When this air hits the surface of water (or ground), it can’t keep moving down, so its downward motion is converted into outward motion, creating very strong wind at the surface.
A Convection Toolbox
There are a few forecasting tools we can use to predict whether the atmosphere on a given day is likely to support moist air rising high enough (and cooling enough) to create thunderstorms. Lifted Index is a fairly simple predictor; it’s the difference in temperature between the air mass at about 18,000 feet (500 millibar altitude) versus the temperature of our volume of air rising from the surface when it reaches that altitude. If Lifted Index is negative, then in theory our rising volume of air could reach this altitude. But Lifted Index assumes a constant rate of temperature drop with altitude, which is often not the case.
A better tool to try to gauge thunderstorm risk is the forecast for CAPE (Convective Available Potential Energy). This accounts for variations in temperature drop with altitude and is a better predictor of updraft strength in condensing air.
Typically, CAPE values under 1,000 (measured as Joules of energy per kilogram of air) indicate modest risk for thunderstorms and suggest any thunderstorms will probably not be strong. CAPE values of much above 1,000 suggest there’s potential for strong to severe thunderstorms. Unfortunately, the CAPE value necessary for severe thunderstorms varies depending on the environment; in the tropics, we may need CAPE of 3,000 for severe thunderstorms, but in New England, CAPE of 1,500 is a better threshold.
One big caution when using CAPE to predict likelihood and intensity of thunderstorms: I like to think of CAPE as a match; it will light a fire if there’s something to burn. Moisture is the necessary fuel. We often see very high CAPE values when the air is very dry. On these days, we won’t have thunderstorms. But when we have high CAPE values and lots of atmospheric moisture, we have the match and the fuel, and we’ll have strong thunderstorms.
In practice, examine the forecast to see if rainfall is likely, and if it is, then look at the CAPE forecast. If the CAPE is over 1,000, then you might see thunderstorms. And the higher the CAPE, the stronger those thunderstorms may be.
A final thunderstorm forecasting tool is the K-index, which takes into account both instability (like CAPE) and moisture. K-index of 20 or higher may support thunderstorms, but strong thunderstorms are not likely until K-index is 30 or higher. If K-index reaches 40, then there’s a very high probability of severe thunderstorms.
While forecast models (especially global models including ECMWF and GFS) lack resolution to properly handle convective wind, Lifted Index, CAPE, and K-index are larger-scale parameters, which models predict fairly well.
Rain can also provide us with some clues. Convective rain frequently is accompanied by higher wind, while stratiform rain usually doesn’t have increased wind. A forecast source with really good graphics can help you discern the difference between stratiform rain and convective precipitation.
For example, the top image above (from WeatherBell) shows precipitation off New England. Note the difference in texture. The multicolored precipitation has bullseyes of yellow, orange, and red, each showing increasing intensity. This is convective precipitation along the leading edge of a cold front. The uniform green is light, stratiform rainfall, behind the cold front. Also, note the mostly uniform rain just off New Jersey associated with a weak secondary, non-convective cold front.
The bottom image shows the wind forecast for one hour later and provides a clearer indication of what is going on in the atmosphere. Notice the areas of strong southwest to west wind to 35 knots or so (yellow and orange) in the area with convective precipitation, versus the light west wind about 10 knots plus or minus (gray and dark blue) in the area just after the cold front with stratiform rain. The weak secondary front, now moved off Maryland, is marked by a brief shot of modestly strong north-northwest to north wind, just above 20 knots.
You can find the forecast for precipitation (QPF) and CAPE from many sources. NOAA’s forecast model website (mag.ncep.noaa.gov/) and Windy (windy.com) are a few of the free websites offering model guidance including CAPE and precipitation.
Our favorite forecast resource with superb graphics for all forecast parameters (including K-index) is WeatherBell: maps.weatherbell.com/ (subscription $270/year).
Incorporating these tools and gaining better understanding of the environmental forces at work can help you keep a more knowledgeable weather eye out when thunderstorms are in the neighborhood.
After operating David Jones’ Caribbean Weather Center from 2004-2010 while a full-time liveaboard, Chris Parker in 2010 started Marine Weather Center (mwxc.com) to provide routing and forecasting for private yachts. MarineWX now has five full-time forecasters and serves thousands of sailors, including providing forecasting for the annual Salty Dawg rallies. He also teaches weather forecasting at boatersuniversity.com/courses/weather-101-basics.