Actually, that's not all. Notice there are no contrails. But the sky of London is continuously criscrossed by planes. That means (as I learned from The Cloudspotter's Guide, obtainable from the Cloud Appreciation Society and all good bookshops), that there was little moisture in the upper atmosphere, suggesting fine weather would continue for some time - as it has, so far. Contrails - condensation trails - aren't fumes, they're clouds, the planes just seed them. If there isn't moisture, they don't really get going, they're short and they fade quickly.