The HYREX Intense Observing Periods (IOPs)

Summary of the IOPs

IOP1   Period:  18 Oct 94 (21Z) - 19 Oct(21Z) - 'Aircraft' Warm Wave
IOP2   Period:  04 Dec 94 (21Z) - 05 Dec(21Z) - Trailing cold front
IOP3   Period:  06 Dec 94 (12Z) - 07 Dec(12Z) - Triple point over Brue catchment area.
IOP4   Period:  14 Feb 95 (06Z) - 14 Feb(18Z) - 6 mm rain
IOP5   Period:  26 Feb 95 (21Z) - 27 Feb(21Z) - No rain
IOP6   Period:  10 Jul 95                     - Hailstorm/Thunderstorm
IOP7   Period:  26 Jul 95                     - Thundery trough from south
IOP8   Period:  23 Aug 95                     - Front from the south-west
IOP9   Period:  06 Sep 95 (09Z) - 07 Sep(03Z) - `IRIS'

IOP1: 18 Oct 94 (21Z) - 19 Oct(21Z)

A slow - moving low was situated to the west of Ireland with a long trailing front which made slow progress eastwards over the period. Rainfall was greater over northern Britain but a band of precipitation passed over Southern England with rainfall over the Brue early on the 19th October. There was evidence of a short warm front close to the more notable occluded (or cold) front, complicating the interpretation of rainfall patterns over southern UK. Rainfall was mainly stratiform in character but with embedded small-scale (probably) convective showers. Over the Brue a total accumulated rainfall of about 6 mm was recorded.

IOP2: 04 Dec 94 (21Z) - 05 Dec(21Z)

Early on the 5th Dec a low centre deepened by 12 mb in 12 hours as it approached western Europe. The low centre tracked north of Ireland to lie between Iceland and Scotland early on the 6th Dec. A long trailing cold front passed over the UK rapidly during the afternoon of the 5th Dec. There were squally blustery showers behind the cold front and it was notably windy over this period. The front was oriented north-south and progressed steadily eastwards over the Brue catchment. The daytime rainfall (over 12 hours ) was typically 12 mm in the south-west peninsula. Over the Brue catchment the accumulated mean rainfall was 8 mm.

IOP3: 06 Dec 94 (12Z) - 07 Dec(12Z)

Late on the 6th Dec a low deepened rapidly (22 mb in 12 hours) as it approached western Europe. At 12 Z on the 7th Dec the central pressure was 940 mb and it lay to the south of Iceland. The triple point passed over the Channel, lying just south of the Brue at 00 Z on the 7th Dec. During the night of the 6/7th Dec rainfall passed over the Brue catchment. It was heavy in places in the UK and was accompanied by gales. Places in Cornwall recorded over 20 mm rainfall over a 24 hour period. There were blustery showers following the frontal passage. Over the Brue catchment the accumulated mean rainfall was 9 mm.

IOP4: 14 Feb 95 (06Z) - 14 Feb(18Z)

A small area of low pressure progressed towards western Scotland during the 14th Feb. There was little development over this period. Southern England experienced the passage of a weak warm front followed by a more marked cold front. Rainfall in the south-west peninsula began at 12 Z with daytime rainfall (over 12 hours) in places amounting to 17 mm. It was an exceptionally mild day over England (maxima around 12 degrees C). Over the Brue catchment the accumulated mean rainfall was 6 mm.

IOP5: 26 Feb 95 (21Z) - 27 Feb(21Z)

No rainfall over the UK. This IOP was used for some flights of the C-130 aircraft.

IOP6: 10 Jul 95

Overnight 9/10 July much of the country had a fine night with some low cloud and mist. In the south west of England there was more cloud with thunderstorms in parts of Dorset and Devon. During the day the southwest continued to have thundery showers, heavy in places and these gradually extended into Wales and parts of central England in the afternoon of the 10th July. In Clwyd 48 mm were recorded over a 24 hour period. Near the Brue area, places recorded 7 mm over 12 hours of the daytime of the 10th July. These showers were associated with troughs passing over England. These were associated with an extended and complex area of low pressure stretching from west of Ireland into France.

IOP7: 26 Jul 95

Thunderstorms started around 0 Z on 26 July over the Channel Islands and then spread northwards reaching Devon and Cornwall before dawn. These storms continued to spread to the Bristol Channel soon after dawn and then moved eastwards reaching southern England by mid-morning and London by midday. In the afternoon the showers in the west were dying out although the southwest stayed cloudy with the odd shower. These storms were associated with the northward progression of a warm front linked to a low lying to the south of Ireland. Behind the front there were two troughs which passed over north-west France.

IOP9: 6 Sep 95 (09Z) - 7 Sep(03Z)

During 6th Sept a compact low approached the south-western tip of the UK deepening rapidly (16 mb in 12 hours). By 18 Z it lay to the south-west of Ireland. Rainfall over the south-west was in association with the approaching warm front and triple point. Rainfall was heavy during the evening of the 6th Sept.