This column is prompted by a very happy memory – my first visit to Portugal's Douro Valley, one of the most visually spectacular wine regions in the world.

The wines are spectacular, too, and if you haven't yet made holiday plans I'd strongly recommend a trip there.

But back to the bottles and that memorable tour. The first afternoon I sat on a shady patio overlooking the river and its startlingly steep vineyard terraces. It was hot – a thunderstorm threatened ­– but the wine in my glass was cool, fresh and utterly drinkable.

It surprised me that such pleasurable white wine came from so harsh an environment, but in nearby vineyards previously red vine stocks were being regrafted with white varieties, to produce more of these unexpected wines. Drinking them, and others later in the same trip, completely overturned my view on cool wines from hot places.

What's crucial is not daytime temperature, but the difference between what the mercury shows at midday and in the early hours of the morning. The bigger that gap, the better the chance of ripening grapes retaining acidity and introducing freshness into the resulting wine. That happens most often in inland places, and in high ones.

In the Douro Valley this summer diurnal temperature variation is considerable, switching between close to 30 degrees celsius to around 14. Elsewhere, especially in regions such as Pyrenean Spain, parts of California, Central Otago in New Zealand or Argentina's Uco Valley, the difference can be bigger, even exceeding 20 degrees celsius.

There are plenty of wines available that will let you enjoy this effect, including Douro whites from the vineyard I visited – São Luiz, owned by respected port producer Kopke. Both São Luiz Douro White, super fresh and seeming to reflect the stony landscape, and its Reserva sister, complex with subtle oak and even stronger minerality, (£12 and £17.50, ocado.com) show off the tempting character of blended indigenous Portuguese white grapes such as viosinho, gouveio and arinto. They're delicious.

More hot days/cool night expressions come from not too far away, the high Spanish plain close to Vallodolid (the town is, remarkably, the nation's tapas capital, where white wine is preferred to sherry as partner to extraordinary, appetising inventiveness). This time the wines are the single-variety verdejos of Rueda, Spain's largest white wine appellation.

In the general quality uplift there, big national names are increasingly joining smaller local growers. Torres is one, offering aromatic lemony, herbal pleasure in night-harvested Céleste (£11-£13, winedirect.co.uk, sohowine.co.uk); Beronia is another, with a fresh and summery sipper (£7-£10, Waitrose, Ocado). And classy examples come from Ramón Bilbao – the Edición Limitada Verdejo is a tropical fruit-tinged delight (£17.50, greatwine.co.uk).

It's time to dig out the climate charts and broaden your white wine horizons.

Eight wines to try this month

The feel-good factor in drinking is always important, and for me these bottles offer something more than enjoyment. To start, pleasures from The Wine Society, which celebrates its "value with values" message. "Value isn't about the lowest possible price," it argues, "but finding a wine you'll love, at a price you're happy to pay, knowing that the grower who made it has been treated well."

Appreciate this with the the salt-spray lime flavours of The Society's Exhibition Santorini Assyrtiko (thewinesociety.com, £15), fruit and minerality perfectly balanced. Or travel virtually to high in Chile's Colcagua valley, for organic, biodynamic Viña Koyle Cerro Basalto Garnatxa (£15) where complex aromatics build into serious cherry fruit, classy and entrancing.

These are among many highlights in a 1,400-plus selection, including a tempting choice below £10 a bottle, with no minimum spend and free delivery. Consider, too, a producer's policy towards people and community: Ridgeview, maker of prime English fizz for more than 25 years, epitomises good practice, caring for employees and paying them properly, putting women centre stage and giving £1 to a hospitality/food education charity for every website bottle sold. Toast that with classic Ridgeview Bloomsbury (£30, ridgeview.co.uk, and many retailers).

Big-brand Torres in Spain remains a family – and very green – business despite its size. Alongside Céleste Rueda (see main article), red partner Céleste Ribera del Duero (£13, Waitrose and independents) showcases aromatic fruit of fine tempranillo wrapped in gentle oak.

More bottles with happy credentials: crisp, fruit-full Frappato Terre Siciliane is organically grown and bottled in 100 percent recycled glass. Fluette gamay, from innovative winemaker Jean-Noël Barrau in south-west France, is also light, pretty and juicy warm-weather drinking, again best served cool (both £12, laithwaites.co.uk)

From classic gamay country, Beaujolais Villages Lache Moi la Grappe (£12, getir.uk) offers darker, more serious flavours. No-added-sulphur winemaking allows luscious fruit to shine, as it does also in the impressive Mathilde Chapoutier Côtes du Rhône Sans Sulfites Ajoutés £17.50, thewinereserve.co.uk).