If you’ve ever gone for a swim in the sea during springtime, you may have noticed that the water feels colder in March and April than it did even in the depths of winter!
But why does this happen?
The answer lies in a fascinating natural phenomenon called seasonal lag.
The Science of Seasonal Lag
During the winter months, the air temperature drops significantly, cooling the surface of the sea. However, water has a high heat capacity, meaning it takes much longer to warm up or cool down compared to the air.
1. Winter Cooling: Throughout winter, the sea gradually loses heat to the colder atmosphere.
2. Delayed Minimum Temperature: By late February or early March, the sea reaches its coldest point, despite air temperatures starting to rise.
3. Springtime Warming Delay: As spring progresses, the sun’s energy increases, but the sea takes time to absorb this heat. While the air may feel warmer, the water lags behind, remaining at its lowest temperatures well into March and April.
This seasonal lag isn’t unique to the UK—it happens worldwide in oceans, seas, and even large lakes due to the slow warming and cooling properties of water.
Why Do Lakes and Rivers Warm Up Differently?
Unlike the sea, lakes and rivers experience temperature changes in a slightly different way. While they are also influenced by seasonal lag, several other factors play a role in how quickly they warm up.
Key Factors Affecting Lake & River Temperatures:
• Size Matters: Larger bodies of water retain heat longer and take longer to warm up compared to smaller lakes or shallow rivers.
• Depth Effects: Deeper water is more insulated from temperature fluctuations, while surface water responds more quickly to changes in air temperature.
• Water Circulation & Turnover: In winter, colder, denser water sinks, causing a mixing effect known as turnover. This continues into spring, delaying warming as surface water mixes with cooler, deeper layers.
• Sunlight Penetration: In deeper lakes, sunlight only heats the upper layers, meaning deeper water remains significantly cooler throughout the year.
How Does This Affect Wild Swimming?
If you’re an open-water swimmer, these variations in water temperature mean that:
✅ Rivers and shallow lakes tend to warm up faster in spring than the sea.
✅ Deeper lakes and reservoirs can stay cold well into early summer.
✅ The sea will still feel freezing in spring—even if the air temperature is rising.
Understanding seasonal water temperature changes can help plan your swims and ensure you’re prepared for the coldest dips of the year!
Spring Swim Essentials – Stay Warm & Comfortable
As the sea reaches its coldest temperatures in early spring, having the right gear can make all the difference in keeping you comfortable and extending your time in the water.
Check out our top picks for spring swimming essentials:
✔️ Thermal Swimsuits & Wetsuits – Extra insulation for those chilly dips.
✔️ Neoprene Gloves, Socks & Hoods– Keep your hands, feet and head toasty.
✔️ Changing Robes – Dry off and warm up quickly post-swim.
Browse our collection and stay warm on your spring swims! 🏊♀️❄️
🔗 SHOP HERE