Missions run by the pride of the RAF, top guns from No 1 Fighter Squadron, have returned to base in their £20m Harrier GR7s with a full load of "smart" air-to-ground missiles. In fact, in a week of raids, the Harriers only carried out two successful missions.

Some would say these "smart" weapons cannot be that intelligent if they can be stopped by smoke.

The Ministry of Defence is adamant that these raids are "not a failure" - so what is making the top guns hold their fire?

Harriers fly from the Gioia del Colle base in Italy

For those in the know, military chiefs, politicians and pilots, there is neither surprise nor failure. In fact, returning to their base in Gioia del Colle ordnance intact because of smoke obscuring their targets or low cloud cover is deemed a success.

The Ministry of Defence says the aim, quite literally, of these missions is to precisely pick off military targets while causing a minimum amount of collateral damage.

For that they use laser-guided air-to-ground bombs such as the Paveway II and III - costing about £40,000 each. But even in this age of high-technology a pilot must still be able to see the target in order to guide the bomb home.

Laser-guidance process:

• Target is locked on to by laser
• The laser reflects a cone back into the sky
• Paveway is launched and "funnelled" onto the target down the laser cone.

But heavy rain can cause the laser beam to scatter, meaning the target cannot be pinpointed and locked. Equally, if fog, smoke or flames mean the pilot who is using the laser equipment to "designate" the target cannot see what is being aimed at - even with infra-red equipment - then he cannot lock on the laser beam and will call off the attack.

The bottom line is that pilots will not risk dropping a bomb which loses lock and lands in a non-military site such as a village, causing civilian casualties.

Nato is working hard to bring in aircraft like the B-1 bomber that can deliver weapons whatever the weather - bombs such as the Jdam, a bomb that relies on satellite guidance.

This is the first time that modern precision-guided weapons - "smart" bombs - have been used in such an extensive operation in a European theatre of operations.

Public expectations about the effectiveness of air power are to a large extent based on the experience of the Gulf War. But Kosovo, even in spring, is not Kuwait or Iraq. Indeed it is often forgotten that the weather too played a factor in the early stages of the Gulf War air campaign.

At the UK Ministry of Defence briefing the day after a second set of Harriers returned to base fully loaded, Chief of the Defence Staff Sir Charles Guthrie underlined the goal: "What we do not want to do is to create any more collateral damage than we have to - we do not want to kill innocent civilians.

"What we really do need is to have visual contact between the pilot and the target he is going to attack."

The Paveway II - ready for action if the weather is good

Defence Secretary George Robertson also brought out the point of international law. If the Allied forces use indiscriminate bombing where many civilians are killed they could be in breach of protocols banning the targeting of civilians in military conflict.

"We behave in accordance with international law unlike Slobodan Milosevic. We are constrained by the need to be precise," he said.

But the pilots of No 1 Fighter Squadron know what their job is. As one Scottish pilot said: "In peace time if you get it wrong you get a hard time from the rest of the formation.